scholars at BHU, Sri Avadhesh Kumar, Sri Shivaji Saha, Sri Tapas Mahajan, Sri Amiya. Kumar Samal, Sri ..... LIST OF ABBREVIATIONS ..... approachable by National Highway no. 30 and no. ...... grade downward to channel sand at the base (Page and Mowbray, 1982). Thus, the ...... Colombo, Sri Lanka: International Water.
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UNDERTAKING
I, Sudarsan Sahu, certify that the work embodied in this Ph.D. thesis titled “Hydrogeological conditions and geogenic pollution in parts of western Bihar” is my own bonafide work carried out by me under the guidance of Dr. Vaibhava Srivastava (Supervisor), Department of Geology and Dr. Dipankar Saha (Cosupervisor), Central Ground Water Board, MER, Patna. The whole responsibility of my research work is mine and I am always ready for making improvement at any time if it required.
Date: Place: Varanasi
(SUDARSAN SAHU)
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BANARAS HINDU UNIVERSITY VARANASI CANDIDATE’S DECLARATION I, Sudarsan Sahu, certify that the work embodied in this Ph. D. thesis is my own bonafide work carried out by me under the supervision of Dr. Vaibhava Srivastava and the co-supervision of Dr. Dipankar Saha, for a period of six years from July 2007 to June 2013 at Banaras Hindu University and Central Ground Water Board, MER, Patna. The matter embodied in this Ph. D. thesis has not been submitted for the award of any other degree/diploma. I declare that I have faithfully acknowledged, given credit to and referred to the research workers wherever their works have been cited in the text and the body of the thesis. I further certify that I have not wilfully lifted up some other’s work, para, text, data, results, etc. reported in the journals, books, magazines, reports, dissertations, theses, etc., or available at web-sites and included them in this Ph. D. thesis and cited as my own work. Date: ……………….. Place: Varanasi (Sudarsan Sahu) -------------------------------------------------------------------------------------------------------
Certificate from the Supervisor/Co-supervisor This is to certify that the above statement made by the candidate is correct to the best of my/our knowledge.
(Dr.Dipankar Saha) Regional Director CGWB, MER, Patna (Co-supervisor)
(Dr. Vaibhava Srivastava) Associate Professor (Supervisor)
(Signature of the HOD with seal)
4
PRE-SUBMISSION SEMINAR COMPLETION CERTIFICATE This is to certify that Sri. Sudarsan Sahu, a bonafide research scholar of this department/school, has satisfactorily completed the course work/ comprehensive examination/ pre-submission seminar requirement which is a part of his Ph. D. programme.
Date: ……………… Department) Place: Varanasi
(Signature of the Head of the
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COPYRIGHT TRANSFER CERTIFICATE Title of the Thesis: HYDROGEOLOGICAL CONDITIONS AND GEOGENIC POLLUTION IN PARTS OF WESTERN BIHAR Candidate’s Name: SUDARSAN SAHU
Copyright Transfer The undersigned hereby assigns to the Banaras Hindu University all rights under copyright that may exist in and for the above thesis submitted for the award of the Ph. D. degree.
(Sudarsan Sahu)
6
ACKNOWLEDGEMENTS The author wishes to express his profound gratitude to Dr. Vaibhava Srivastava, Department of Geology, Banaras Hindu University, Varanasi, under whose supervision this work has been completed. His constant guidance had been a great source of inspiration throughout the study. The author also extends his profound gratitude to Dr. Dipankar Saha, Central Ground Water Board, MER, Patna, who, as co-supervisor of this works, constantly encouraged enthusiastically and enriched the understanding in various aspects of the subject. The author is highly grateful to the present Head of Department Prof. Satyendra Singh and former Head of Department Prof. H. B. Srivastava for extending the facilities at BHU. Sincere thanks are extended to Research Progress Committee (RPC) members Prof. B.P. Singh, Prof.U.K. Shukla and Late Prof. A.S. K. Murthy. The author is indebted to Dr. N. Janardana Raju, earlier at Department of Geology, Banaras Hindu University, Varanasi, under whose supervision this work took shape in the formative initial years. Permission accoreded and encouragement extended by the Chairman, Central Ground Water Board, Ministry of Water Resources, Govt. of India is gratefully acknowledged. The author is also grateful to Dr. R. S. Singh (Retd.) and Dr. P. C. Chandra (Retd.), who, as Regional Directors of CGWB, Patna, during the period of this research work, extended their valuable suggestions and useful comments at various stages of the study. Special thanks goes to Sri A. K. Agrawal, Scientist ‘D’, earlier at CGWB, MER, Patna, for his constant encouragement throughout the study. The author also extends his sincere thanks to S. N. Sinha, G. K. Roy, T. B. N. Singh, D. G. Dastidar, R. R. Shukla, S. N. Dwivedi, S, Upadhyay and K. G Bhartaria, who, through discussions helped in understanding at various stages of the work. Thanks are extended to Dr. A. K. Singh, Geological Survey of India, Patna, for his suggestions and kind advice. The author is indebted to Dr. K Rajan, ICAR, Patna, for his help in analysis of soil samples. The author gratefully acknowledges the help extended by PHED, Govt. of Bihar, during the period of the research work. Thanks are extended to the people working the chemical laboratories at ISM, Dhanbad, and CGWB, Patna, for their invaluable help in the analysis of groundwater samples. The author is highly thankful to Dr. A. S. Nayak, Department of Geology, Banaras Hindu University, Varanasi, for his invaluable help. The help of the Research scholars at BHU, Sri Avadhesh Kumar, Sri Shivaji Saha, Sri Tapas Mahajan, Sri Amiya Kumar Samal, Sri Ravindra Kumar, Ms Vibha Katiyar and Ms Gargi Sharma is gratefully acknowledged. The endless blessings of the beloved parents, to which the author is indbted, have remained a constant source of courage during the research work. Last but not the least, the author is deeply indebted to the wife Ms Sasmita Sahu, son Soumyakant and daughter Samridhi, who, painstakingly co-operated during this arduous journey.
(Sudarsan Sahu)
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LIST OF TABLES Table No.
Title
Page
Table 1.1
Salient climatological data of the study area in parts of Bhojpur and Buxar districts, Bihar.
7-7
Table 1.2
Mean monthly normal rainfall (in mm) distribution.
7-7
Table 1.3
Mean monthly rainfall distribution in the year 2007.
9-9
Table 2.1
The general stratigraphic succession in the North Bihar Plains.
21-21
Table 2.2
Quaternary morpho-/lithostratigraphy of the study area.
21-21
Table 3.1
Monthly mean silt load of Ganga at Buxar and Patna.
35-35
Table 4.1
Standard mesh used for sediment grain-size analysis.
69-69
Table 4.2
Grain size parameters for the sand samples in the four boreholes; (1) Bharauli, (2) Nargada Narayanpur, (3) Karnamepur and (4) Paharpur.
77-79
Table 4.3
Percentage of grain size fractions in four boreholes.
79-79
Table 4.4
Sedimentary facies identified in the area within 30 m below ground.
80-80
Table 4.5
Stratigraphic facies in the flood plain of Ganga within 30 m below ground.
83-84
Table 4.6
Sedimentary facies in craton derived sediments in boreholes.
85-85
Table 5.1
Grain Size Ratio for different depth zones in boreholes.
101-101
Table 5.2.a
Details of key wells of water level measurement from shallow aquifer.
111-111
Table 5.2.b
Details of water levels measured from shallow aquifer.
112-114
Table 5.3
Piezometric level in the deeper aquifer (AG-II) in the study 123-123 area.
Table 6.1
Hydrogeological details of the deep tube wells in which 142-142 pumping tests were carried out.
Table 6.2
Aquifer hydraulic parameters of AG-II worked out by Theis’ 148-148 (1935) method.
Table 6.3
Aquifer hydraulic parameters of AG-II worked out by Jacob’s 148-148 (1946) method
Table 6.4
Aquifer hydraulic parameters of AG-II worked out by Theis’ 149-149 Recovery method.
Table 6.5
Aquifer hydraulic parameters worked of AG-II by Hantush Curve fitting method (1960).
150-150
Table 6.6
Aquifer hydraulic parameters of AG-II worked out by Walton Curve Fitting Method (1962).
150-150 8
Table 6.7
Vertical hydraulic conductivity (K’) of the middle clay determined by pumping test.
151-151
Table 6.8
Grain size parameters and the hydraulic conductivity (K) values obtained through empirical equations for different depth zones in four boreholes.
153-157
Average K values and estimated transmissivity (T) of AG-I through empirical equations.
158-158
Table 6.9
Table 6.10 Average K and T values of AG-II through empirical equations. 158-158 Table 6.11 Comparative values of T of AG-II obtained through pumping 159-159 test and empirical equations. Table 6.12 (A) Average transmissivity (T) and hydraulic conductivity (K) values of AG-II determined by conducting pumping test. (B) Average 160-160 storativity (S) values of AG-II in the Newer Alluvium.
Table 6.13 Reported values of T and S for AG-II in the Newer Alluvium.
161-161
Table 7.1
Methods adopted, instruments used and detection limits for the chemical constituents analysed.
168-169
Table 7.2.a
Results of chemical analysis of groundwater samples from shallow aquifer (AG-I) in the Newer Alluvium.
173-175
Table 7.2.b
Results of chemical analysis of groundwater samples from the shallow aquifer in the Older Alluvium.
176-177
Table 7.3
Salient statistical parameters of chemical quality of groundwater in shallow aquifer (both the newer and older alluvium).
177-178
Table 7.4.a Table 7.4.b
Chemical parameters in percentage distribution to total ions 193-194 and hydrogeochemical facies of AG-I in the Newer Alluvium. Chemical parameters in percentage distribution to total ions and hydrogeochemical facies of shallow aquifer in the Older 195-195 Alluvium.
Table 7.5
Hydrogeochemical facies of shallow aquifer and their areal coverage.
196-196
Table 7.6
Mean concentration of the chemical constituents of hydrogeochemical facies groups in shallow aquifer.
196-196
Table 7.7
Results of chemical analysis of groundwater samples from the 200-200 deeper aquifer (AG-II) in both the newer and older alluvium.
Table 7.8
Salient statistical parameters of the chemical constituents of groundwater in AG-II.
201-201
Table 7.9
Chemical parameters in percentage distribution to total ions and hydrogeochemical facies in AG-II.
203-203
the
Table 7.10 Block-wise ground water arsenic level distribution in the study area.
206-206
Table 8.1
Details of groundwater samples tested for arsenic and iron.
220-224
Table 8.2
Organic Carbon content of various sedimentary facies.
229-229
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LIST OF FIGURES Figure No
Title
Page
Figure 1.1
Location and administrative map of the study area covering parts of Bhojpur and Buxar districts in western Bihar.
5-5
Figure 1.2
Variation of climatic parameters in the study area.
8-8
Figure 1.3
Normal annual rainfall distribution superimposed over local drainage.
9-9
Figure 1.4
Topographic elevation contour in m above mean sea level.
10-10
Figure 1.5
Map showing the study area as a part of the Sone megafan.
11-11
Figure 2.1
The map of a part of Ganga Basin showing various basement structural elements and major geomorphic divisions.
19-19
Section across the Ganga Plain showing cratonward pinch out of various stratigraphic units of foreland deposits. ADisposition of sediments derived from the craton and Himalayan in the Sone-Ganga alluvial tract.
22-22
Basement contour and structural elements in and around the study area. The X-Y section represents the basement depth profile.
24-24
Figure 2.4
Bouger gravity anomaly map of the study area and its adjoining parts.
25-25
Figure 2.5
Geological map depicting the distribution pattern of morphostratigraphic and lithostratigraphic units.
27-27
Figure 2.6
Photograph showing the development of carbonate concretions (calcretes or kankars) in the soil horizon in the Older Alluvium.
28-28
Photograph depicting the fine gray sand deposits with ~2.0 m mud cover at top in point bars in the Older Flood Plain (OFP) of Ganga
29-29
Figure 2.8
Photograph depicting the Active Flood Plain (AFP) deposits of Ganga.
29-29
Figure 3.1
Map showing the drainage in the study area.
31-31
Figure 3.2
(A) The smaller channels like Kumhari (in photo), Gangi and Banas often display meandering pattern in the Older Alluvium. (B) The typical yazoo pattern shown by the Gaur Bhagar river alongside the Ganga. (C) The incised Banas River in the Older Alluvium. (D) The Thora River cuts across the levee of Ganga before joining it at upstream from Buxar.
32-33
Figure 3.3
Photograph showing the southern bank of the Ganga at Buxar.
33-33
Figure 3.4
Slope-discharge diagram for the Ganga River at Allahabad,
34-34
Figure 2.2
Figure 2.3
Figure 2.7
10
Patna, Saidpur and Rajmahal. Figure.3.5
Normalized mean monthly discharge in relation to mean annual discharge (Qmonthly/Qannual mean) for the Ganga River.
35-35
Figure 3.6
Drainage basin of the Sone River.
36-36
Figure 3.7
A small rivulet joining the Sone River from western side at Bikramganj.
36-36
Figure 3.8
The Sone River near Sandesh depicting its braided character.
37-37
Figure.3.9
Normalized mean monthly discharge in relation to mean annual discharge (Qmonthly/Qannual mean) for the Sone River.
37-37
Figure 3.10 The eastern bank of the Sone River at Bishunpur between Sandesh and Koilwar showing its sandy nature.
38-38
Figure 3.11
A comparison of the channels of Ganga during the years 1963 and 2009.
39-39
Figure 3.12 River bend chute cut-offs of the Ganga due to its northward migration.
40-40
Figure 3.13 Schematic model for the development of scroll bar complexes in the flood plain of Ganga.
41-41
Figure 3.14 Scroll-bar geometry in the flood plain of Ganga.
42-42
Figure 3.15 Typical old abandoned cross bar channels of Ganga.
42-42
Figure 3.16 The rates of channel migration and floodplain topography.
43-43
Figure 3.17 Distribution of palaeochannels of the Sone River with the oldest bed of the Ganga River.
44-44
Figure 3.18 A segment of the active channel of Sone at Bishunpur with the root of palaeochannels towards Ara, Maner, and Patna.
45-45
Figure 3.19 (a) Soil excavation exposing the palaeochannel sand of the Sone River. (b) (i) A mis-fit channel occupying a braid channel course of Sone palaeochannel. (b) (ii) The Sone palaeochannel braid bars forming undulatory mounds. (c) Stratified Ganga palaeochannel sand at Bariswan. (d) A mis-fit channel follows the curvilinear tract of Ganga palaeochannel.
48-48
Figure 3.20 Schematic map of Ganga Plain showing the location of megafans. Figure 3.21 Nature of channel movement over the Sone megafan. Figure 4.1 Figure 4.2 Figure 4.3
SRTM image showing part of the study area with the location of boreholes used for sedimentological studies. Topographical sheets of 1932 (a) and 1972 (b), showing north/northwestward migration of the stem channel. A and B- Channel fill sequence, C- Point bar platform sequence, D- Point bar sequence, E-the bottom part of B
50-50 51-51 54-54 56-56 58-58
11
showing the alternate sequence of Coarser Sone sand and the finer Ganga sand. Figure 4.4
Photographs depicting (A) the typical Sone sand, (B) typical grayish fine Ganga sand, (C) the black lacustrine clay with coarser particles of quartz, feldspars and kankars and (D) the pure lacustrine clay.
59-59
Figure 4.5
A segment of the Ganga River showing the nature of river banks.
60-60
Figure 4.6
Schematic diagram displaying two cycles of deposition by the Ganga.
62-62
Figure 4.7
A part of the study area with one of the palaeochannels of Ganga, the associated geomorphic features and the nature of depositions.
63-63
Figure 4.8
The depositional pattern on the braid island.
64-64
Figure 4.9
Borehole lithology at Bharauli.
66-66
Figure 4.10 Schematic diagram showing the Sone megafan and encroachment of the Ganga River from north over the megafan surface.
67-67
Figure 4.11 The major clay beds at deeper levels.
68-68
Figure 4.12 Geological section (X-Y) across the length of the Sone megafan showing more clay content in western part in comparison to the eastern part.
68-68
Figure 4.13 Bar diagram for the percentage distribution of different size grades of sand particles in the borehole samples.
70-70
Figure 4.14 Frequency distribution curves of sand samples obtained from boreholes.
71-71
Figure 4.15 Depth wise variation of mean/median size of sand particles.
73-73
Figure 4.16 Plotting of Mean grain size (Mz) vs sorting (σ), skewness.(Sk) and kurtosis (KG).
76-76
Figure 4.17 Stratigraphic facies identified in the area within 30 m below ground.
82-82
Figure 4.18 Schematic diagram depicting the disposition of Himalayan (Ganga) and craton (Sone) derived sediments.
86-86
Figure 4.19 Bar diagram depicting the sand particle size distribution in Sone and Ganga sand.
88-88
Figure 4.20 A comparison between the mean grain size of borehole sands of Sone origin and that of the bed load sand in the active Sone.
89-89
Figure 4.21 The shape and extent of Sone megafan in MGP based on existing data.
90-90
Figure 4.22 Schematic diagram illustrating the northward extent of the cratonic alluvial wedge in the Ganga Basin and the course of Ganga River in MGP during the Late Pleistocene.
91-91
12
Figure 5.1
Hydrogeological transects, (a) A-A’, (b) B-B’, (c) C-C’ and (d) D-D’.
93-99
Figure 5.2
Litho-facies and grain-size triangle (modified after Pettijohn and Randich, 1966).
100-100
Figure 5.2
Litho-facies distribution within (a) 0-50 m, (b) 51-100 m, (c) 101150 m, (d) 151-200 m and (e) 201-250 m in the study area.
102-105
Figure: 5.3
Hydrographs based on monthly water levels from Buxar and Milki, plotted against the monthly rainfall of 2011.
108-108
Figure 5.4
Plot depicting last ten years rainfall in Bhojpur and Buxar districts in the study area and their variation from the normal rainfall.
109-109
Figure 5.5
(a) Pre-monsoon and (b) post-monsoon depth to water level contours (shallow aquifer) in m bgl for the year 2010 with 115-116 sub-normal rainfall. (c)Annual water level fluctuation contour map for the year 2010.
Figure 5.6
(a) Pre-monsoon and (b) post-monsoon depth to water level contours (shallow aquifer) in m bgl. (c) Annual water level 117-118 fluctuation contour map for 2002 with normal rainfall.
Figure 5.7
(a) Pre-monsoon and (b) post-monsoon water table contours 119-119 in m amsl (shallow aquifer) for the year 2010.
Figure 5.8
(a) Pre-monsoon and (b) post-monsoon water table 120-120 contours in m amsl (shallow aquifer) for the year 2002.
Figure 5.9
Long term water level trend in the shallow aquifer.
121-121
Figure 5.10 (a) Pre-monsoon and (b) post-monsoon piezometric level contours in m bgl. (c) Annual piezometric level fluctuation 124-124 contour (in m) of deeper aquifer for the year 2010. Figure 5.11 (a) Pre-monsoon and (b) post-monsoon piezometric level 125-125 contours in m amsl (deeper aquifer) for the year 2010. Figure 5.12 Schematic diagram illustrating the pattern of water level 129-127 distribution in different aquifer groups. Figure 5.13 Sand isopach contours (in meter) within 0-250 m depth 129-129 below ground. Figure 5.14 Isopach contour map of the top aquitard (in meter) in the 129-129 study area. Figure 6.1
Borehole lithologs of the pumping test wells with the design 140-140 of tube wells.
Figure 6.2
Photo showing the pumping test at Karnamepur.
141-141
Figure 6.3
Semi-log plot of time vs drawdown data for the main wells (MW) at Karnamepur, Bariswan, Paharpur, Nargada Narayanpur and Bharauli pumping test sites.
144-144
Log-log plot and match points for Theis (1935), Walton (1962) and Hantush’s (1960) curve fittinging methods for four pumping test sites.
145-146
Semi-log plot of residual drawdown vs log t/t’ for Theis Recovery Method for four pumping test sites.
147-147
Figure 6.4
Figure 6.5
13
Figure 6.6
Range of typical hydraulic conductivity (K) values for geological materials (based on Driscoll 1986 and Todd 153-153 1980).
Figure 6.7
(a) Plot of the T values of AG-II obtained through different 162-162 pumping test data interpretation techniques.
Figure 6.8
Plot of the S values of AG-II obtained through different 163-163 pumping test data interpretation techniques.
Figure 6.9
(a) The variation of transmissivity (T) and (b) storativity (S) 164-164 in the western parts of the study area.
Figure 7.1
Index map showing groundwater sample locations from the 168-168 shallow aquifer in newer (1) and older (2) alluvium.
Figure 7.2
Distribution of pH in groundwater in shallow aquifer in the 170-170 area.
Figure 7.3
Electrical conductivity contour map for shallow aquifer.
Figure 7.4
(a) The mean of major chemical constituents of groundwater (in mg/L) in shallow aquifer in the newer and older alluvium 172-172 and (b) the constituents in percentage of total ions.
Figure 7.5
Areal distribution of (a) bicarbonate (HCO3̅) in mg/L & (b) HCO3̅ concentration in percentage of total anions in shallow 179-180 aquifer.
Figure 7.6
Areal distribution of (a) sulphate (SO42-) in mg/L, (b) in 181-181 percentage of total ions in shallow aquifer.
Figure 7.7
Areal distribution of (a) sodium (Na in mg/L & (b) Na+ concentration in percentage of total cations+) in shallow 182-182 aquifer.
Figure 7.8
(a) The average distribution of major chemical parameters in the shallow aquifer in mg/L (after excluding the sub183-183 normal distributions. (b) Triangular plot of Ca2+, Mg2+ and + Na (in % of total cations).
Figure 7.9
(a) Areal distribution of calcium (Ca2+) in mg/L. (b) in 184-184 percentage of total ions in shallow aquifer.
171-171
Figure 7.10 (a) Areal distribution of chloride (Cl̅) in mg/L, (b) in 185-185 percentage of total anions in shallow aquifer. Figure 7.11 (a) Areal distribution of Magnesium (Mg2+) in mg/L. (b) in 187-187 percentage of total cations in shallow aquifer. Figure 7.12 Scatter plots of chemical parameters for shallow aquifer.
189-191
Figure 7.13 Trilinear diagram (year 2010) showing the chemical composition of groundwater in the shallow and deeper 192-192 aquifers. Figure 7.14 Areal distribution of hydrogeochemical facies in the shallow 198-198 aquifer. Figure 7.15 Scatter plots of chemical parameters for deeper aquifer.
201-201
Figure 7.16 Relation between the concentration levels of groundwater 204-204 14
arsenic and iron in the arsenic affected shallow aquifer in Newer Alluvium. Figure 7.17 Arsenic contaminated part of the study area.
206-206
Figure 7.18 (a) Plot of depth of source aquifer vs the arsenic level in groundwater (in µg/L). (b) Depth-wise variation of 207-207 groundwater arsenic in aquifers in borehole at Bariswan. Figure 7.19 Seasonal variation of groundwater arsenic.
208-208
Figure 7.20 Average concentration of major ions in groundwater in 209-209 shallow and deeper aquifer. Figure 7.21 Na/Ca ratio, depicting Na+ rich Older Alluvium and Ca2+ rich groundwater in areas adjoining the Ganga River in 210-210 Newer Alluvium. Figure 7.22 (a) Ca/Mg ratio in the groundwater samples of shallow aquifer in the newer and older alluvium. (b) Plot between 213-213 the bicarbonate concentration and the Ca/Mg ratio of shallow aquifer. Figure 7.23 Areal distribution of the Ca/Mg ratio in the shallow aquifer.
214-214
Figure 8.1
(1A) Location map of the area under detailed study. (1B) Details of the observation points made in the flood plains of 215-215 the Ganga.
Figure 8.2
Sample locations and abundance of arsenic in the detailed 225-225 study area.
Figure 8.3
Relationship between frequency of water samples and abundance of arsenic in four major geomorphic units in the 226-226 flood plain of Ganga.
Figure 8.4
Schematic section showing groundwater arsenic distribution 228-228 in different stratigraphic facies.
Figure 8.5
The thickness of surface mud cap overlying the SAS vs groundwater arsenic concentration plot. (a) Groundwater arsenic concentration and the thickness of mud cap within 229-229 4.5 m, (b) Groundwater arsenic concentration with greater thickness of mud cap.
Figuer 8.6
Photograph showing accumulation of run-off/flood water in 230-230 the palaeo-channel depressions in the flood plain of Ganga.
Figure 8.7
(a) A schematic model showing the geomorphic surfaces in the MGP and the arsenic contaminated flood plains. (b) Depicts a model of distribution of channel plugs of various 232-232 dimensions in the flood plains and the associated variability in the groundwater arsenic concentration.
15
LIST OF ABBREVIATIONS MGP: NBP: SBP: AFP: OFP: AG: MAP: CAP: PZ: SRTM: EPF: WPF: MSRF: RF: PCH: LS: PBS: CBS: SAS: DW: DTW: TDS: EC: pH: OC: BH: MW: EW: OW:
Middle Ganga Plain North Bihar Plains South Bihar Plains Active Flood Plain Older Flood Plain Aquifer Group Marginal Alluvial Plain Central Alluvial Plain Piedmont Zone Shuttle Radar Tomography Mission East Patna Fault West Patna Fault Munger Saharsa Ridge Fault Rapti Fault Palaeochannels Levee sequence Point bar sequence Channel bar island sequence Shallow aquifer sand Dug well Depth to water level Total dissolved solids Electrical conductivity Hydrogen potential Organic carbon Borehole Main well Exploratory well Observation well
LIST OF SYMBOLS Q: T: S: K: D: D’: s: s’: d10: Uc: N: Md: Mz: Σ:
Discharge in tube wells Transmissivity of aquifers Storativity of aquifers Hydraulic conductivity of aquifers Thickness of aquifer Thickness of aquitard Drawdown Residual drawdown Effective grain size in sediment Uniformity corfficient Porosity Median size Inclusive graphic mean Standard deviation (Sorting) 16
Sk: KG :
Skewness Kurtosis
17
PREFACE The Bihar state, located in the Middle Ganga Plain (MGP), is covered by alluvium in ~89% of its geographical area. The hard rocks of Chhotanagpur Granite Gneiss and Vindhyan Supergroup cover the southern fringes of the state adjoining to the state of Jharkhand. The Ganga River forms the axial drainage, dividing the state into almost two halves. The state bears an agrarian economy with about 90% of the state population living in rural areas and depending on agricultural pursuits. Fertile soil and abundant water resources in the state sustains intensive cultivation. The state is endowed with potential alluvial aquifers embedded in the Quaternary deposits. The shallow aquifers within 50-60 m depth are exploited for irrigation and drinking in rural areas, whereas in urban areas, the aquifers up to 200 m depth are exploited for drinking. However, the aquifer in large alluvial tracts along the Ganga River and patches in northern plains of the state are affected by elevated concentration of arsenic (>50 µg/L, the state regulatory limit) in groundwater, making it unfit for human consumption. The pollution zone covers a geographical area of 9,104 km2 falling mostly in the rural parts of 15 districts (out of total 38). More than 10 million people (~10% of state populace) reside in the risk zone (Census 2001) with the dependency on the contaminated aquifer for their drinking need. If 10 ppb limit (WHO 1993) is considered, this number can be many-folds. In this scenario, it is important to understand the hydrogeological conditions and the groundwater potential of the affected parts. Clarity in understanding with sound scientific knowledge of the prevailing groundwater quality, the reasons of arsenic release to groundwater and availability of alternate arsenic safe aquifers may help in planning for sustainable groundwater development and management plans for those areas. The study area was chosen in parts of Bhojpur and Buxar districts, covering 2132 km2 in the South Ganga Plain bordering the Ganga River. It comprises Quaternary deposits unconformable overlying the Precambrian basement rocks. The interplay of fluvial activity of the rivers Ganga and Sone, driven by the extrinsic factors like climate and tectonics have shaped the geomorphological and stratigraphic architecture in the area. In and around the area, there are number of palaeochannels of Sone forming a megafan in the marginal parts of the Ganga Basin. 18
The Ganga River, in due course of its northward migration over the megafan surface, has created a wide (5-20 km) flood plain, which is made up of Newer Alluvium comprising unoxidised fine sand (micaceous), silt and clay (gray to dark gray in colour). The flood plain is marked with geomorphic features like abandoned channels and meander scars of Ganga bearing. The frequent floods in the Active Flood Plain (AFP, Diara Formation) rework and redistribute the sediments. The Older Flood Plain (OFP, Fatwa Formation) in the Newer Alluvium often remains above the flood level. It is characterized by additional ‘clay’ sedimentary facies of lacustrine origin (organic matter rich) deposited in the channel cut off lakes as clay plugs. The Older Alluvium (the Mohanpur Formations) is topographically elevated and occupies the southern parts of the study area beyond the flood plains of Ganga. It is rather free from flood and water-logging and is presently undergoing postdepositional mild erosion. The master slope of the entire area is towards the Ganga at north. In the Newer Alluvium, the top ~30 m of the sequence is dominated with finer Himalayan derived sediments. It is followed by largely the coarser zones of Sone origin constituting prolific aquifers down to the depth of 250 m below ground level (bgl). Between the bottom lying craton derived sediment and the overlying Himalayan derived sediment, often there exists a transition zone of 15-20 m (~30-50 m depth range) thickness, comprising alternation of sediments derived from both the sources interleaved with some mixed layers. This may be reflecting the boundary between the Newer Alluvium and the Older Alluvium. The Sone megafan possesses stacked columns of multiple sand zones intervened with minor clay lenses. The aquifers in the megafan with the grain size of more than 80 % of the aquifer materials as coarse to very coarse sand warrants a good groundwater potential. Calcium carbonates or kankars (often in bulk) and sometimes carbonate cemented sands are observed in Sone sand. In the Newer Alluvium, two aquifer groups (AG-I & AG-II) have been identified within the explored depth of 300 m bgl. Wide pervasive middle clay (aquitard), lying at the depth range of ~87-135 m separates the two aquifer groups. Groundwater in AG-I exists in an unconfined condition due to the pervious character of the mud cover
19
at the top of the flood plain sequence, while that in the AG-II exists under semiconfined to confined condition due to limited permeability of the middle clay. The two-tier aquifer system is however not discernible in the Older Alluvium. There are multiple aquifer systems of low to moderate potentiality separated by clay zones within the depth of ~100 m bgl in the Older Alluvium. Further below, the AG-II appears and seems to be reasonably continuous up to the bottom clay at ~273 m bgl. Within the depth range of 0-100 m bgl, the area is predominantly ‘clayey sand’ in litho-facies character as defined by Pettijohn and Randich (1966). In the depth range of 100-150 m bgl, the presence of middle clay makes ‘sandy clay’ and ‘clay’ as the most prevailed facies types. However, in the depth ranges of 151-200 and 201-250 m bgl, the ‘sand’ facies covers major parts of the area. In the hydrological year 2010, the depth to water levels of shallow aquifer for preand post-monsoon periods ranged between 2.6-8.76 m bgl and 1.62-9.3 m bgl respectively. With respect to mean sea level, the water levels for the seasons fall in the ranges of 43.7-61.4 and 45.5-62.8 m amsl respectively. During the year, decline in water level was noticed at certain localities (range: -0.10 to -1.21) due to sub-normal rainfall in the year. The piezometric level in the deeper aquifer (AG-II) stands higher (0.10-3.54 m) in comparison to that of the shallow aquifer (AG-I). Both the aquifer groups possess hydraulic gradient towards north/northeast. Pumping test in tube wells tapping the granular zones within the depth range of 94-250 m bgl of AG-II in the Newer Alluvium with discharges in the range of 183-195 m3/hour indicates good potential of the aquifer group. The average transmissivity (T) values have been assessed within the range of 5163-10,527 m2/day. Grain size based empirical equations predict T values in the range of 2647-14,932 m2/day with a mean value of 5979 m2/day, which is close to the values determined through pumping tests. For the AG-I, the equations assess the T values in the range of 2867-9123 m2/day, indicating its less transmissive character in comparison to AG-II. The storativity (S) values are largely 50 µg/L) in groundwater is posing threat to the people living in the plain. Drinking water is one of the pathways of arsenic ingestion to human body. Arsenic poisoning causes life-threatening diseases like gangrene, cancer of the intestines, liver, kidneys and the bladder. The most widespread contamination is detected in Bengal Delta Plain (BDP), covering eastern parts of West Bengal, India and Bangladesh, affecting about 40 million inhabitants (Mandal et al. 1996; Nickson et al 1998; BGS and DPHE 2001). In recent years, high incidence of arsenic in groundwater has been reported from the Middle Ganga Plain (MGP) (Chakraborty et al 2003; Shah 2008), covering parts of Bihar and Uttar Pradesh. In Bihar, the worst affected districts are Bhojpur, followed by Buxar, Patna, Vaishali, Bhagalpur, Samastipur, Khagaria, Chapra, Munger and Darbhanga. Chakraborty et al (2003) recorded an arsenic level of 1654 µg/L in groundwater in 27
Semaria Ojhapatti, a village in Bhojpur district. The World Health Organisation guideline for a safe limit is 10 µg/L, while the Indian government's guideline is 50 µg/L. Management plans for the optimum use of the resource has been imperative for overall development of the society and economy. Any management plan to become successful requires the study of hydrogeological conditions controlling the potential and assessment of groundwater quality is very important. It is essential to understand the aquifer disposition and its geometry, mode of occurrence of groundwater, aquifer parameters, and chemical quality of groundwater. Special attention is to be given to delineate the location and extent of groundwater quality deterioration in vulnerable areas, and the cause and source of contamination with the recommendations of possible remediation measures. This requires understanding of the palaeo-climatological and environmental parameters under which the sediments containing polluted groundwater were deposited. The geomorphic processes, driven by extrinsic factors like climate, tectonics and sea level changes, play significantly in carving out the hydrogeological conditions. Keeping the above aspects in view, an attempt has been made to study the hydrogeological conditions and geogenic pollution in western Bihar covering parts of Bhojpur and Buxar districts in the Ganga-Sone interfluve area. 1.2 SELECTION OF THE STUDY AREA The Ganga Plain is the most densely populated part of India. Its fertile soil and abundant water resources help in surplus food production. The prime states are the Uttar Pradesh (238,000 km2 area, 199 million populations) in the Upper Ganga Plain (UGP), and Bihar (94,163 km2 area, 104 million populations, Census 2011) in the Middle Ganga Plain (MGP). It is essential to have a proper understanding of the hydrogeological conditions and status of groundwater quality before planning for groundwater development and management. Various workers have studied the the Recent-Quaternary alluvium in different parts of the Ganga Basin for its hydrogeological characteristics. Still, there exist large gaps in scientific information on the groundwater system, particularly in MGP. It needs for building up systematic scientific understanding of the nature of the aquifers, their geometry and configuration and their hydraulic characteristics for their proper development and management. The chosen study area extends over vast stretches of Gangetic alluvium ranging in age from Recent to Middle Pleistocene. The area is 28
intensely cultivated; almost the entire irrigation need is met from groundwater. The groundwater also caters the entire domestic demand of the area. The study area, falling in parts of Bhojpur and Buxar districts in the western parts of Bihar state, is situated on the southern bank of the Ganga River. Geologically, the area comprises of unconsolidated Quaternary sediments. Except few patches in the northern plains of the state, the reported arsenic contaminated areas mostly fall along both the bank of Ganga River. All are low lying flood plains of the river. The area chosen for present study forms the widest flood plain of Ganga in MGP and seemed to be ideal for hydrogeological investigations in relation to geogenic pollution. Elevated concentrations of arsenic (>50 µg/L, the state regulating limit) in groundwater were reported from various parts of the area The presence of typical palaeochannels of the rivers Ganga and Sone, and the nature of shifting of rivers are few of the geomorphological phenomenons, which are not observed in other parts of the state along Ganga. 1.3 LOCATION, AND HISTORICAL IMPORTANCE The present study area is located in the Ganga-Sone interfluve (Fig 1.1) in MGP, surrounded by the Ganga River in the north and west, the Sone River in the east, and Bhabhua and Rohtas (Sasaram) districts of Bihar in the south. Further south to the area lays the Vindhyan Highlands. The area is spread over 2,132 km2 and occupies parts of Survey of India toposheet nos. 72 C/ 2, 3, 6, 7, 10, 11, 14 and 15 and 63 O/14, 15. The area includes 13 northern blocks of the districts; 9 blocks from Bhojpur (Koilwar, Shahpur, Behea, Ara, Barhara, Udwantnagar, Jagdispur, Garhani and Behea) and 4 blocks from Buxar district (Simri, Buxar, Brahmpur and Chausa). The district headquarter of Buxar is Buxar town itself, while that of Bhojpur is at Ara town. These two towns are the main hub of trading activity and marketing of agricultural products. The study area is located about 80 km west of Patna and approachable by National Highway no. 30 and no. 31. The area is also connected by rail with Howrah-Patna–Delhi broad gauge route.
29
showing
the
Ganga
interfluve
with
41 '
Bihar.
(d)
Detail
Buxar
00 '
Roads Rails
Bihar state.
0 84 00 '
Canals
New Bhojpur
dr
0
84
0
0
84 15 '
kilometers
10
Braid bars/islands
20
. rN he Ch
.
30 '
Gaura Bhagar N.
Ga ng aR .
0
84
Jagdispur
0 84 30 '
Behea
Shahpur
Semaria
Dehra N.
rN aga Bh
Karnamepur
N. awati Dharm
Brahmpur
15 '
B h ag ar N.
Study area boundary
an Ch
. aN
Neazipur
Few important locations Drainage
District HQ
Block boundary
study area covering parts of 25 ' Bhojpur and Buxar districts of 24 0
Chausa
. N
administrative map of the
of
study area in the western parts
drainage condition and the
Sone-Gnag
Sone-Ganga interfluve. (c) The
and south Bihar plains and the
0 The Bihar state with the north 25
Basin and the Bihar state. (b)
India
0
84
Gan gi N .
Figure1.1: (a) Map of
Ara
Water bodies (channel cut-off lakes)
Garhani
Koilwar
Babura
45 '
0 84 45 '
0
84
Sandesh
Udwantnagar
Basantpur
.
gi N . Gan
GANGA BASIN
So ne R.
N
R. asa rm n Ka
s N. Ban a
a or
N hari Kum
Th
30
0
25 24 '
41 '
0
25
The district Buxar has witnessed few important fights between the foreign invaders and the countrymen, which decided the fate of India afterwards. During the Mughal period, the historic battle between Humayun and Sher Shah was fought at Chausa in this district in 1539 A.D. The British forces under Sir Heoter Munro defeated the Muslim army of Mir Qasim, Shuja-ud-Daulah and Shah Alam-II on 23rd June 1764 on the grounds of Katkauli situated at about 6 km from Buxar town. The stone memorial erected by Britishers at Katkauli bears testament to the fight even today. The district Bhojpur played a major role in India's struggle for independence. Veer Kunwar Singh of Jagdishpur was the leader of the mutineers during the first war of independence in 1857, called the Sepoy Mutiny by the British. The fighting was so severe that two of the five Victoria Crosses ever awarded to civilians by the British were awarded during this battle. Similarly, the Buxar district bears strong historical records. 1.4 DEMOGRAPHY During 2001 census, total population of the 13 blocks in the study area was at 22, 36,842 with a population density of 1049 persons per km2. About 84% of the population lived in rural areas depending on agricultural pursuits as their main livelihood. Agricultural laborers accounted 10% of the population. As per the 2011 census, the Bhojpur and Buxar districts, the parts of which make the present study area, have registered a decadal population growth rate of ~21%. 1.5 CLIMATE The climate of the district is of moderately extreme type. The hot weather begins at about the middle of March, when hot westerly winds begin to blow during the day. The months of April and May are extremely hot. In a normal year, the monsoon sets in by the second week of June and the rains continue with intermissions till the end of September or the early part of October. July, August and September are the most humid months (Table 1.1) when the climate remains hot and sultry. The cold weather begins from the month of November and lasts till the beginning of March. January is the coldest month when temperature comes down to as low as 10 0C. From the month of April till the first break of monsoon, the area experiences occasional thunder-storms also. In December and January the average minimum temperature comes down to less than 10 0C (Govt. of Bihar, 1994). The lowest temperature recorded at Patna is 2.2 0C in February 2, 1905. Humidity varies from 24.7% to 83.45% (Table 1.1). 31
Summer stretches from March till the onset of monsoon i.e., June. The peak summer months are April and May. May is the hottest month with maximum average temperature exceeding 39 0C. The average minimum temperature is also highest in May. The humidity remains lowest during these months. Table 1.1: Salient climatological data of the study area in parts of Bhojpur and Buxar districts, Bihar. Month
Factors
Jan. Feb. Mar. Apr. May Jun. Jul. Aug.
Sept.
Oct.
Average Temprature Max 23.4 26.6 32.9 38.2 39.8 37.4 33.2 32.5 (0C) Min 9.1 11.5 16.5 22.5 27.3 27.1 26.4 26.0
Nov.
Dec.
32.4
32.0 28.7 24.7
25.3
21.5 14.3
9.7
Relative Humidity Max 75.6 63.4 48.5 41.1 49.7 66.0 81.9 88.7 81.8 75.1 70.9 73.6 (%) Min 55.0 43.4 29.1 23.9 30.4 53.1 74.0 77.0 75.4 65.6 56.1 55.0 Mean Wind Velocity 3.98 4.0 5.1 6.2 8.3 10.0 9.7 8.3 9.7 7.0 4.7 3.5 (km/hr) Source: Govt. of Bihar,1994
1.5.1 Rainfall During the monsoon season, the area receives rainfall from southwest monsoon. As per the records available from Indian Meteorological Department (IMD) the normal date of arrival of monsoon is 11th June and normal date of withdrawal is 3rd October. The normal annual rainfall (70 years) for the Bhojpur and Buxar districts stand at 1081 and 991 mm respectively (Table 1.2). Table 1.2: Mean monthly normal rainfall distribution in Bhojpur and Buxar districts (70 years normal in mm from 1901 to 1970). Month District Jan.
Feb.
Mar.
Apr.
May
Jun.
Jul.
Aug.
Sept.
Oct.
Nov.
Dec.
Total
Bhojpur
19.3
17.8
8.4
6
17.9
99
290.6
314.2
219.9
51.4
7.5
3.8
1080.9
Buxar
15.1
17.7
5.4
11.7
20.6
103.3
278.3
292.8
182.7
50.9
7.6
5.4
991.1
Source: Govt. of Bihar 2002
Rains set in by the 2nd week of June accompanied by a fall in temperature and increase in humidity. The area experiences maximum rains during the months of July and August (Fig 1.2). The average rain-fall, in the normal conditions, recorded in these months is in the proximity of 300 mm. The district gets easterly winds from June to September, which brings rains. From October, the direction of the winds is reversed and westerly winds blow till May. There is slight rainfall in October, but
32
November and December are quite dry. Some winter rain occurs in January and February. 100
350 300
80 250
70 60
200
50 150
40 30
Average rainfall (mm)
Average temparature (0C) Average relative humidity (%)
90
100
20 50
10 0
0 Jan.
Feb. Mar. April May June
July
Aug. Sept. Oct.
Nov. Dec.
Month Montly mean rainfall distribution in mm (Bhojpur) Monthly mean rainfall distribution in mm (Buxar) Daily maximum temparature (monthly mean) Daily minimum temparature (monthly mean) Maximum relative humidity (monthly mean)
Figure 1.2: Variation of climatic parameters in Bhojpur and Buxar districts in the study area. The rainfall represents the 70 years normal rainfall in the area.
The average annual rainfall stood at 1405 mm for the year 2007, during which the present study was started (Table 1.3). Monsoon rainfall accounted 90% of total annual rainfall. In month-wise distribution, the month July receives maximum rainfall, constituting 50 % of the monsoon rainfall and about 44 % of the entire annual rainfall. Three months viz: November, December and January have recorded no rainfall during the year. During non-monsoon period, highest rainfall is observed in the month of May (3.4 % of the annual rainfall). The isohyetal map has been prepared on the basis of 70 years normal annual rainfall of local available five rainfall stations (Fig 1.3). It reveals higher (>1000 mm/year) rainfall in the eastern part of the area around Ara, Sinha, Babura, Udwantnagar and Sandesh. In the central parts around Shahpur and Bariswan, there is rainfall 600 m at the northern and northeastern parts of the area. . It indicates a basement dip of about 1:84 towards north and northwest. X-Y section represents the basement depth profile (see the basement depth contour map for the location).
49
0
'
0
'
0
84 00
83 00 N
rm Ka
Recorded earthquakes
R. G an ga
R.
Jagdispur
Ara (Bhojpur)
Fatuha
Sandes
-70
-60
0 -6
25
Patna
Buxar
50 km
0 -5 -40
0
a nas
-80
.
Channel bar islands
- 90
R dak
Drainage
Gh -110 PF ag W hra Li R. ne am en Babura t
0
26' 00
Gan
Bouger Gravity Anomaly (mGal) Study area Fault
'
85 00
Nasriganj
.
-60
25' 00
F EP
-50
L in ea m
en t
ne So
0
Daudnagar
R
-40 -50
-5 0 a S ou Sone Narmad
ault th F
-40
INDIAN CRATON
-40 -30
0
24 ' 00
Figure 2.4: Borger gravity anomaly map of the area and its adjoining parts (GSI 2000). Along with figure 2, it indicates a break in the slope pattern in the basement with one depression, aligned approximately along the present course of the Sone River.
2.4.2 Morphostratigraphy The Quaternary alluvial deposits, comprising alternate successions of brownish yellow sand (fine to coarse) and yellowish clay, are mostly derived from the peninsular highlands. The thin veneer at the top (from few centimetres to up to ~ 50m) is made up of Himalayan derived gray to grayish black finer sediments. In the absence of any diagnostic fossil and absolute age data, various authors have applied morphostratigraphic criteria for mapping the geomorphic surfaces in the area (Roy et al 1991; Ghosh and Roy 1992). The criteria includes the break in landform slope, degree of dissection, degree of pedogenic transformations, degree of compaction of the alluvial fill, state of preservation of landform elements and the nature and degree of oxidation of the constituent sediments and the alluvial landscape of the area. Early descriptions divided the Quaternary alluvial deposits of the Ganga Plain into (a) Older Alluvium (‘Bhangar’) corresponding to Middle to Upper Pleistocene, and (b) Newer Alluvium (‘Khadar’) (Fig 3) having an age of Upper Pleistocene to 50
Recent (Pascoe 1950). The Bhangar deposits form elevated surfaces located much beyond the reach of flooding by the present day rivers, whereas the younger (Khadar) deposits occupy lower elevations areas. The Newer Alluvial Group (NAG), forming younger terraces, has been divided into two units (Om Prakash et al 1990; Dayal, 1997): (i) Active Flood Plain, (ii) Older Flood Plains (Fig 2.5). In the following sections, a broad synthesised morphostratigraphic classification has been elaborated for the area. 2.4.2.1 Active Flood Plain (Diara Surface) This youngest surface of the Quaternary deposits, exhibiting unstable depositional landscape covers the northern part (~92 km2) of the study area (Fig 2.5). It comprises the sediments within the meander belt of the Ganga stem and braided channels of the Sone. The sediments are unoxidised fine sand (micaceous), silt and clay. The landform elements are point bar, channel bar, natural levee, crevasse splays, backswamps etc. It is also denoted as Diara Surface (Roy et al 1991). 2.4.2.2 Older Flood Plain (Fatwa Surface). Channel entrenchment and migration of the Ganga River further north has left behind the Older Flood Plain (OFP), forming major parts of the area (~869 km2). The area is inundated only during extreme flood events. This surface represents older meander belts of the Ganga. The landforms include natural levee, meander scar with point bars, abandoned/cut-off channel, backswamp etc. The OFP is also referred as Fatwa Surface (Roy et al 1991). The contact between the Diara Surface and Fatwa Surface is not well defined on the ground. 2.4.2.3 Older Alluvial Plain (Mohanpur Surface) The OFP surface gradually merges with this oldest depositional land unit to the further south, occupying the topographically elevated parts, covering major part of the study area (~1171 km2). It is also denoted as Mohanpur Surface, which is equivalent to Nawada Surface (Roy et al 1991) in the eastern parts of the state. This surface has a gently northward master slope as indicated from the general trend of the channels. The land unit is dissected by sparsely populated sub-parallel drainage system. The drainage channels are deep in the southern part and become shallower in the northern part showing aggrading nature. The Mohanpur Surface is rather free from flood and water-logging and is presently undergoing post-depositional mild erosion. A number of fluvial landforms in the form of elongated sandy ridges (trending nearly north-south) are observed near Dumraon. 51
Figure 2.5: Geological map depicting the distribution pattern of morphostratigraphic and lithostratigraphic units in the study area.
52
2.4.3 Lithostratigraphy Three lithostratigraphic units, which are given the status of “Formation”, have been identified in the present area (Roy et al. 1991). In order of decreasing antiquity the units are (1) Mohanpur formation (2) Fatwa formation (3) Diara formation. 2.4.3.1 Mohanpur Formation This unit is made up of multiple alluvial fills of coarse sand with minor gravel following upwards into medium to fine sand and finally yellowish sticky clay at the top. Such repetitive sequences in borehole logs indicate deposition of these sediments in fluvial point bar environment. Calcareous and ferruginous concretions are commonly observed within the clayey-top as well as within the sand horizons (Fig 2.6).
Figure Photograph
2.6: showing
development of carbonate concretions (calcretes or kankars) in the soil horizon in
Older
(Mohanpur
Alluvium Formation)
specifically exposed along the river banks.
2.4.3.2 Fatwa formation The deposits representing this unit are comparatively less oxidized. The lack of matured oxidized sediments is indicated by the absence of brown and yellow soil within the constituent elements. The OFP deposit of river representing of the Fatwa Formation comprises of gray coloured fine sand and gray to dark gray silty to clayey sediments. Thin soil horizon with preserved root lets and burrows are observed in these flood plain sediments (Fig 2.7). Bedding structures like horizontal parallel lamination, wavy, ripple and cross laminations, tabular and trough cross bedding and small scale penecontemporaneous deformation structures are common.
53
Figure 2.7: Fine gray sand deposits with ~2.0 m mud cover at top in point bars in the Older Flood Plain of Ganga.
2.4.3.3 Diara formation The present day flood plain and associated active channel deposits of the Ganga River considered as Diara Formation, exhibiting white to grayish white, dominantly loose micaceous silty sand to fine-grained clayey sediments (Fig 2.8). The lighology essentially exhibits a wide range of sedimentary structures indicating fluctuating hydrodynamic condition. The structures include (l) parallel lamination of silt and clay overlain by cross laminated silt and sand; (2) fining upward point bar deposit with basal channel lag conglomerate and ripple drift cross lamination. These are noted mostly in levee part of the river bank sections; there is no remarkable break between the Diara and Fatwa sediments. Diara sediment can be differentiated from that of Fatwa formation by its freshness of sediments, lighter colour, absence of ferrugineous nodules and Kankar etc. As the process of alluviation is still going on, soil formation has not yet taken place.
(a)
(b)
Figure 2.8: Active Flood Plain deposits of Ganga. (a) Deposits in point bars with fine to very fine sand interleaved with minor flood plain mud. (b) Flood plain mud deposits interleaved with centimeter scale sand and silt of sheet flow and crevasse splay origin.
54
CHAPTER 3 GEOMORPHOLOGY
3.1
INTRODUCTION Geomorphology is the branch of geology dealing with the origin, evolution and
configuration of natural features of the earth's surface. It originates from the Greek words geo, which means earth, and morphology, which refers to the scientific study of form and structure. There exists considerable overlap between the geomorphology and hydrogeology disciplines. These two have much to offer each other in several fields such as river-groundwater interactions, the location and evolution of aquifers and landform evolution etc. There is increasing realization of the utility of geomorphological techniques and ideas in the analysis of groundwater systems. This chapter deals with the study of drainage characteristics and geomorphological landforms in the study area and the processes that shape them. A systematic approach is made on the study of geomorphic setting. The role of tectonics in the evolution of the terrain has been assessed. The work encompasses the entire Sone River activity area even beyond the present area undertaken for research, since the palaeochannels of Sone are observed on both sides of the active channel. It documents the tectonic influence on Ganga and Sone, a southern tributary of the Ganga. 3.2 GEOMORPHOLOGIC SETTING The study area in the Sone-Ganga interfluve is located in the southern part of the MGP at the north of exposed peninsular craton. It forms the widest marginal plain in the entire Ganga Basin (Fig 2.1) and lies sufficiently upstream of the coastline. It has been argued to remain out of the reach of marine influence (Goodbred 2003; Tandon et al 2008). Fluvial regimes controlled by climate (discharge and sediment supply) and tectonics has largely been responsible for its geomorphological evolution during Quaternary. 3.2.1 Drainage setting The west to east flowing Ganga River forms the master drainage forming the northern and western boundary of the study area. The Sone River is the important peninsular drainage situated at the eastern boundary of the study area. 55
Buxar
Chausa
0
840 00 '
Boundary between NA & OA
District HQ Few important locations Drainage
Study area boundary
N.
840 15 '
hh
N. awati
20
N er
.
.
ga R.
GauraBhagar N.
Ga n
Jagdispur
Behea
Shahpur
Semaria
Dehra N.
rN aga Bh
Karnamepur
BrahmpurC
Dharm
kilometers
10
Braid bars/ islands
New Bhojpur
dra an Ch
Neazipur . B h ag ar N
' 840 30
840 30 '
Ara
Water bodies (channel cut-off lakes)
Garhani
Koilwar
840 45 '
Sandesh
Udwantnagar
Basantpur
Babura
840 45 '
N.
250 24 '
250 41 '
56
Figure 3.1: Map showing the drainage in the study area in Sone-Ganga interfluves in the western parts of Bihar state.
250 24 '
250 41 '
Ka r m nas aR .
N.
Gan gi
' 840 15
Ban as N .
' 840 00
.
Gan gi N. hari Kum
N.
So ne R
a or Th
Besides the major rivers Ganga and the Sone, the other important streams in the area are Gangi, Banas, Dharmawati, Chher, Dehra, Gaur Bhagar and Thora (Fig 3.1), which are mostly 2nd or 3rd order streams forming tributaries to Ganga. They are ephemeral (Govt. of Bihar 2007) and plains-fed in nature. The drainage pattern is subdendritic to dendritic with the streams in particularly Older Alluvium showing meandering pattern (Fig 3.2.A). These rivers are east to northeast flowing, except the Thora, which flows westward. The streams Dharmawati and Gaura Bhagar in the area show Yazoo pattern (Sinha and Friend 1994) (Fig 3.2.B) in a considerable distance along the southern bank/ levee of river Ganga. A few of the rivers like Banas and Gangi in some instances cut across the meander scars to join river Ganga, but in many cases follow the same semi-circular path of the old meanders of Ganga at their convex sides (outer swale). Most of the streams coming from the Older Alluvial area take a rightward turn before entering the Newer Alluvium (Fig 3.1). The smaller streams are low to moderately incised and show bank heights of the order of 4-5 m (Fig 3.2.C). When they approach Ganga, it increases to the tune of 6-8 m (Fig 3.2.D).
B
A
Figure 3.2: (A) The smaller channels like Kumhari (in photo), Gangi and Banas often display meandering pattern in the Older Alluvium. (B) The typical yazoo pattern shown by the Gaur Bhagar river alongside the Ganga in the study area. Arrows indicate flow direction in rivers.
57
D
C
Figure 3.2: Contd. (C) The incised Banas River in the Older Alluvium. (D) The Thora River cuts across the levee of Ganga before joining it at upstream from Buxar. The rivers flow towards east. Arrows indicate flow direction in rivers.
3.2.1.1 Ganga River, its Channel pattern and hydrology The Ganga River at the western boundary of the study area flows northeasterly, whereas, at the northern boundary, the river flows in an almost West to East direction (Fig 3.1). The river displays sinuous (P=1.29) character with distorted meanders in a narrow valley before its confluence with Karmnasa River at the southwest of Buxar. This has been related to vertical uplift in the area (Singh 2004; Swamee et al 2003).
Thereafter,
the
river
is
moderate to highly incised (w/d = 42–70) and follows an almost straight and single channel course (P=1.00-1.03) to about 15 Km
Figure 3.3: Southern bank (~15 m high) of the Ganga River at Buxar.
downstream of Buxar with bank heights 10-15 m (Fig 3.3).
58
It is succeeded by a moderately meandering (P=1.10-1.30) and braided morphology, which continues up to Fatuha at ~15 downstream of Patna. However, the braid bars are visible only during the lean flow periods. The gradient of the river in this reach is very low, varying between 5 to 6 cm/km. Fig 3.4 depicts discharge-slope data as depicted by Leopold and Wolman (1957), and Balek and Kolar (1959) for the River Ganga at Allahabad, Saidpur, Patna and Rajmahal. It indicates that the data at Allahabad and Saidpur fall well inside the field of meandering pattern, whereas the plot for Patna falls very close to the discriminating line between the two fields and the plot for Rajmahal falls well inside the braided pattern. Thus, it seems that the reach of river Ganga from Saidpur to Maner (~30 km upstream of Patna) represents a transitional phase between meandering in its upstream and braided in its downstream. Unlike the upper reaches, the Ganga River has developed few stabilized islands in the MGP between Buxar to Patna. 1.0
Explanation Braided Straight Meandering
Channel slope
10 -1
S = 0.06 Q-44 4
10 -2
10 -3
3 102
103
104
105
2
1 106
Bankful discharge (ft³/s) Ganga river slope and mean discharge at various locations 1- Allahabad
2- Patna
3- Saidpur
4- Rajmahal
Figure 3.4: Slope-discharge diagram for the river Ganga at Allahabad, Patna, Saidpur and Rajmahal (after Leopold and Wolman 1957, and Balek and Kolar 1959).
The average annual mean discharge of Ganga at Patna stands at ~11600 m3/s (Latrubesse et al 2005), 76% of which flows during the monsoon months only (Fig 3.5). The bankfull discharges of the river at Buxar and Patna are 31020 and 35135 m3/s respectively. Stream power per unit width (W/m2) reduces from ~83 at Buxar to ~9 at Patna. Within this stretch, the river possesses flood plains attached with both the
59
banks. Average of 20 years annual flooding discharge of the river at Patna is 43744 m3/s. The Ganga River
Qmonthly/Qmean
4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 Jan Feb
Mar Apr May Jun Jul Aug Sept Oct Nov Dec Time (month)
Figure.3.5: Normalized mean monthly discharge in relation to mean annual discharge (Qmonthly/Qannual mean) for the Ganga River (After Latrubesse et al 2005). Monthly average sediment load data (Table 3.1) in monsoon period for the Main Ganga Stem channel at Buxar shows that fines constitute more than 90 % of the total load except the month of August in peak discharge during which 18 % of the total load is medium sand. Out of total load, coarse sediment constitutes only 1 to 2 % (GFCC, 1990). On an average, out of the total sediment load, fines constitute 91.3% at Buxar and 83.6% at Patna during the monsoon. Table 3.1: Monthly average silt data in monsoon period for the Main Ganga Stem channel at Buxar and Patna for the years from 1971 to 1987 (source: GFCC 1990).
Sl. No
Month
Av. monthly sediment load in Mt Buxar
Patna
Graded %age of sediment to total Coarse in %
Medium in %
Fine in %
Buxar
Patna
Buxar
Patna
Buxar
Patna
1
Ju
0.84
8.77
0.00
2.63
1.33
9.38
98.67
87.99
2
Jul
154.31
283.48
1.81
3.58
5.85
13.32
92.34
83.10
3
Aug
1132.74
1066.82
1.99
4.64
17.80
14.67
80.20
80.70
4
Sept
480.72
982.53
1.71
3.61
7.60
16.29
90.69
80.10
5
Oct
190.58
231.40
1.23
2.76
4.01
11.24
94.76
86.00
Total
1959
2573
In response to the rainfall pattern in the basin, 80% of the discharge of the Ganga and 95% of its sediment load are delivered to the margin only during the rainy 60
season from June to September (Goodbred 2003). In other seasons of the year the river possesses low discharges with minimal dynamic activity and sediment transfer. 3.2.1.2 Sone River, its channel pattern and hydrology Sone
River
originates from the Maikals Range
of
Sone River Basin
Amarkantak
Ri ve r
The
So ne
highlands in the elevated plateau of central India. After flowing
northerly
Rohtasgarh
River enters in alluvium
and
easterly directions for about 592 km in a hilly terrain, it debouches into the Gangetic alluvial plains of central Bihar after a sharp turn around
the
0
50
100 km
Rohtasgarh
Figure 3.6: Drainage basin of the Sone
plateau and finally flows in a
River. The basin is elongated in NE-SW direction.
north-easterly direction along
The river possesses a narrow valley in the Ganga
the eastern flank of Rohtasgarh Ridge to join the Ganga River on its southern bank at Babura near Patna. The river is ephemeral in nature with formidable bankfull discharges in the monsoon months (June to September), but in the remaining parts of the year the river turns into a number of disconnected pools of water bodies with minor flow channels. It receives all its tributaries in the high lands (catchment area of 68916 km2) and not a single stream joins the river in its ~ 200 km long alluvial reaches (catchment area of 1312 km2 only) other than a few local minor rivulets (Fig 3.6 & 3.7). Figure 3.7: A small rivulet joining the Sone River from western side at Bikramganj. Arrow indicates flow towards Sone. 61
Unlike the Ganga River, the Sone hardly possesses any flood plain. It flows in an entrenched channel in the alluvial plain and displays a braided character (Fig 3.8) with many braid channel sand bars and islands throughout its journey up to Babura. The river has a steep gradient (~1:1300) in the stretch from Rohtasgarh to Dehri in which it gradually enters into Bihar
Figure 3.8: The Sone River near Sandesh depicting the braided character of the river.
plains, and thereafter, it flows with an average gradient of ~1:2500 up to Babura where it meets the Ganga. The channel of the Sone has been very wide (about 5 km at Dehri) and shallow (high width-depth ratio) in the plains. However, downstream of Doudnagar up to Koilwar, throughout the entire eastern boundary of the study area, the river is moderately incised having 8-16 m bank cliffs. In the upstream reaches up to Daudnagar, the river flows in an almost flat and straight course (P=1.05-1.10) with very wide channel (4–5 km, w/d=500-700). There is marked increase in the sinuosity (P=1.10-1.20) downstream of Daudnagar within the study area. Average annual mean discharge in the Sone River stands at ~1000 m3/s. The discharge in the river during monsoon periods is high (Fig 3.9) and normally remains within 5000-6000 m3/s. The bankfull discharge at Koilwar stands at 5052 m3/s. The Sone River
Qmonthly /Qmean
5.00 4.00 3.00 2.00 1.00 0.00 Jan Feb Mar Apr May Jun
Jul
Aug Sep Oct Nov Dec
Time (Month)
Figure.3.9.Normalized mean monthly discharge in relation to mean annual discharge (Qmonthly/Qannual mean) for the Sone River. 62
The unit stream power increases from ~11 W/m2 at Daudnagar to ~19 W/m2 at Koilwar. The average annual sediment load of the river channel increases from 24.3 million tones at Japla (further upstream station to Daudnagar) to 33.27 million tones at Koilwar, despite the fact that the river does not receive any tributary between Daudnagar and Koilwar contributing significant discharge other than a few local minor rivulets. The river is degrading nature of the river and flows channel
in
an (Sahu
2010) (Fig 3.10).
incised et
al
Figure 3.10: The eastern bank of the Sone River at Bishunpur between Sandesh and Koilwar showing its sandy nature.
3.2.2 Flood Plain Morphology As mentioned earlier, the Sone River does not possess any regular flood plain. The study area encompasses a wide flood plain (5-20 km) of Ganga attached to its southern bank. The river is variously deep (min. 4 m to max. 17 m) and southern bank stands at elevation of 53 to 63 m (amsl). During the lean periods the river flows within the incised channel. But during peak discharge seasons the river overtops the channel in its central and eastern parts and reaches floodplain unlike the major Himalayan and cratonic incised rivers in the West Ganga Plain (WGP) which seldom reach their left out flood plains thousands of years ago except during the exceptional events (Singh et al 1997; Sinha et al 2002; Sinha et al 2005; Gibling et al 2005; Tandon et al 2006; Srivastava et al 2003). The overall topography in the surrounding region in the Ganga-Yamuna interfluves in the WGP is of degradational nature. All smaller rivers in the WGP are also incised, and in some areas, a strong development of dissected topography is manifested as badlands (Singh et al 1999).
On the
contrary, the present study area of the middle Ganga plains is characterized by a rapidly filling and aggradational regime.
63
R E R
G
A
N
G
I
V
A
Channel cut-offs
2009
1963
Bar islands
River channels
Bar deposits
Figure 3.11: A comparison of the channels of Ganga River during 1963 (from topographical sheet) and 2009 (traced from satellite imagery). It shows both downstream and transverse components of movements.
The distribution of meander scars in the flood plain of Ganga River indicates that the river has shifted by about 5–20 km towards north within its reach between Buxar and Patna. Such large shifting has not been reported from any other reach of the river in MGP. Shifting takes place with both downstream and transverse components of movements. A comparison between the maps of 1963 (topographical sheet of Survey of India) and satellite imagery of 2009 (IRS LISS-III) indicates ~78 m/year and ~52 m/year of downstream and transverse movement rates respectively (Fig 3.11). 3.2.2.1
Meander scars: channels cut-offs and point bars
The flood plain is dotted with a number of small and large channel cut- off lakes, which are partially of fully infilled (serving in many cases as low-lying mudflats), point bars with typical ridge (scroll bar) and swale topography (Fig 3.12). These features make the flood plain little bit undulating. Most of the cut-offs of the Ganga are "chute cut-offs" as defined by Allen (1965). Chute cut-offs are expected only where lateral migration is active, and where loop development and floodplain erodibility during brief over-bank flows are such as to allow the creation of new short-circuiting channels (Lewin 1983). Many of these cutoffs have been clay plugged and a few in the western part of the area towards Buxar town are still holding water all the year round, forming small and large shallow water bodies and indicating their late separation from the main stream. During the peak discharge season these areas are flooded with water. After the flood recedes many of
64
the partially sediment filled channel cut-off depressions (central portions) and disrupted drainage systems hold water forming ponds for few post-monsoon months. 0
0
84 00 ' N
84 45 ' Ganga R.
Ghaghra R.
0
0
25 41'
So ne R.
25 41'
Channel cut-off lake
PF W
0
25 24 '
Bar deposits Cut-off meander scars with ridges and depressions Area of Channel migration
0
15
30 km
0
0
25 24 '
Area of channel avulsion and Migration
F EP
0
84 00 '
84 45 '
Figure 3.12: River bend chute cut- offs of the Ganga River due to progressive lateral migration (northward) drawn from satellite image (modified after Sahu et al 2010). Note that all the abandoned bends possess the convex orientation opposite to the direction of migration of the Ganga. The exceptions (hachured ones) in the eastern part may be due to avulsion of the river channel being closure to EPF at this part (see the text later on). The rectangle inset has been produced as Figure 3.15.
Though, the northern bank of river Ganga within this reach also possesses adjoining flood prone low-lying areas, the typical meander scars have been asymmetrically preserved only along the southern bank of the river. Except one (hatched one in Fig 3.12, located on the eastern side) all the meander scars present on this belt lie with their convex sides facing southward. These types of flood plain features with the above described characteristics are associated only with actively migrating river channels (MiaIl, 1996; Leeder and Alexander 1987). The scroll shaped ridges (scroll bars) alternating with depressions known as swales (Reineck and Singh 1980) are well developed in the point bars. The scroll bars are around 2 to 3 m higher from the ground of the swale depressions. Swales are filled with fine-grained muddy sediments and in many cases marshes have developed in them. Repeated migration during floods might have produced these scroll bars. Many of the scroll bars are never subdued even during very high floods and for this reason habitations have been developed on them.
65
The typical scroll pattern flood plain topography has been formed as a result of sequential formation of cross bar channels as the stem channel migrates (McGowan and Gardner, 1970).
Figure schematic
Cross bar channels
3.13
depicts
model
for
a the
development of the scroll-bar that covers the flood plains in the
Main channel
B
meander
loops.
As
the
main
channel migrates, bar deposits are
A
left as a series of scroll bars on the scale of the main channel (Fig 3.13.A). As time passes, the bend migrates
farther,
and
floodplain/cross bar channels also migrate, reworking the deposits of
C
D
Flow path
the large bend and leaving behind smaller-scale scroll bars (Fig 3.13.
Scroll bar Channel filling
B). Eventually
Figure 3.13: Schematic model for the
the
main
development of scroll bar complexes in the flood
channel bend is cut off by a more
plain of Ganga.
direct channel, and the old channel begins
filling
with
sediment.
Subsequent to the cut off, water continues to flow through floodplain channels in the bend area, and they rework the surface deposits (Fig 3.13.C & D). If enough time passes, most of the surface features from the larger-scale bend can be erased, and the smaller-scale floodplain channel scroll bars will dominate the surface morphology of the floodplain. The resulting scroll-bar complexes show arcs of two distinct sizes, outer arcs on the scale of the main channel and inner arcs on the scale of the floodplain/cross bar channels (Fig 3.14).
66
Figure 3.14: Scroll-bar geometry in the Ganga River flood plain in its middle reaches in Bihar, traced from the satellite imagery. The decreasing radius of curvature of the bars towards the centre suggests that the outer ones were deposited by the bends of the main channel, whereas the inner ones were deposited by smaller scale bends of flood plain channels.
The abandoned cross bar channels serve as conduits for flood water to the plain. When approached by the second cycle of migration, the channel seems to remain hanging in respect to the stem channel. In such cases, the abandoned channel carries water from the main channel during high flood stages.
B
A
Ganga R.
Figure 3.15: (A) A typical old abandoned cross bar channel of Gangain its flood plain. Arrows indicate the old banks of the channel. (B) A cross bar channel of Ganga often conduiting water to flood plain. Its bed is sandy. The dotted arrow indicates flow direction in the cross bar channel. 67
3.2.2.2 Natural Levee On its southern bank within the stretch from Buxar up to its confluence with Sone River, the levee formed by river Ganga is not as prominent as those formed at Banaras, or at downstream at Patna. This may be due to various reasons such as high rate of migration of river channel and finer silt load within this stretch of the river (Table 3.1). In laterally active meandering rivers, rates of lateral migration (Fig 3.16) represent a control on the size of natural levees (Melton 1936; Hudson 2004). If migration rate and sedimentation rate remain constant through time, the height and morphology of the levee should reach a steady state or equilibrium form (Törnqvist and Bridge 2002). Exhaustion or paucity of coarse suspended sediment load generally leads to decline in size of the natural levee (Hudson et al 2003, Kolb 1963). Downstream of Maner, the size of natural levees has increased, may be due to increase in sediment load and sediment grain size in the axial Ganga River.
Flood plain bedforms
Levee
Channel/ Point bar Older flood plain
A
Figure
3.16:
Relationship between rates of channel migration and floodplain topography: (A) higher HIGH RATES OF CHANNEL MIGRATION
B
rates
of
lateral
migration result in smaller natural levees because of the reduction in the time for sediment to accumulate; (B) lower
rates
of
lateral
migration results in a larger and more stable natural LOW RATES OF CHANNEL MIGRATION
levee (redrawn from Hudson 2004).
3.2.2.3 Palaeo-/abandoned channels The meander scars in the Ganga flood plain as described in the above section have resulted by the process of channel migration and abandonment through chute cut-off. Along with these cut-offs of main Ganga channel take place the abandonment of few cross bar channels of small dimension. The scars tapering southward indicate 68
northward migration of Ganga. In this process, the meander loops growing southward only are preserved and those lying in the north of the migrating channel are eroded and reworked upon. However, the northward tapering scars at the eastern parts of the area might be indicating channel avulsion in Ganga. The river Sone has undergone avulsions in geologic past as indicated from the abandoned channel beds lying on both the sides of the present active channel course. In the present work, 11 palaeochannels (PCHs) of the river has been identified and mapped including the 9 numbers (PCH III to XII in Fig 3.17) as earlier reported by Sahu et al (2010). Though, the river enters the Ganga plain in Bihar at Rohtasgarh, the avulsions have taken place in the downstream parts at some particular points. In the satellite maps, braiding is clearly discernible in those long abandoned channels. Similar to the active channel, most of these palaeochannels were lowly sinuous. Sinuosity is observed only in few of them lying to the east of the active channel. 0
0
84 00'
N
85 30'
CHANGES IN THE COURSE OF SONE RIVER Gh ag Li hra ne Chapra am R. en t
Koilwar
PF W
o Th
Ara X (Bhojpur)
ra R.
Jagdispur
II
Bihta
VI
Bishunpur
V
IV
R.
Y Ba na s
I
0
VIA
0
25 ' 41
Fatuha
Barh
IIIA Mokama
Sandes
Barkagaon 84 15 '
30 km
Hajipur XII IX XI Maner Khagaul VII Patna VIII
Babura Buxar
15
0
Gandak R.
G ang a R.
0
25 ' 41
0
84 45'
Scale
Arwal
0
0
25 ' 07
VI
Nasriganj
R.
X
III
25 ' 07
Daudnagar
So ne Pu
Nalanda
np
un
R.
F EP
INDEX
Dehri
Old beds of Sone River Indrapuri barrage
0
24 ' 33
Old bed of Ganga River Active river channels Lineament
Rohtas Garh
Fault
Gaya
Japla
Peninsular exposure (hard rocks)
0
24 ' 33
Study area See caption
INDIAN 0
84 00
'
CRATON 0
84 45'
0
85 30'
Figure 3.17: Distribution of palaeochannels of the Sone River with the oldest bed of the Ganga River (modified after Sahu et al 2010) with respect to the faults/lineaments. The shaded area in square and ellipse are produced as Fig 3.18.
69
The channels of the Sone towards Patna (out of study area) were moderately meandered (P= 1.30–1.40) displaying moderate slope of the plain (Fig 3.18). But the channel towards Ara in the study area was straight, laterally extensive (5.80 km in an average) and highly braided with a number of large braid bars (Fig 3.18), on which villages have long been established. The river must have followed a very steep slope (Twidale 1966; Schumm and Khan 1972; Schumm 1986) with high discharge and coarser bed-load transport (Ouchi 1985). At a constant discharge and load, though a river follows a meandering pattern on a steep slope, it straightens and turns braided if the slope increases further and crosses a critical limit (Schumm and Khan 1972). The river pattern might have been aided by the trend of a lineament that coincides with the channel course.
So n e Ri
ver
N
~6 .0 Km
t en am ne Li
Bishunpur Palaeo-channel towards Ara
Palaeo-channel towards Patna
Sandes
Figure 3.18: A segment of the active channel of Sone River at Bishunpur with the root of palaeochannels towards Ara, Maner and Patna. The channel towards Ara was straight, wide and highly braided and those towards Maner and Patna were moderately meandered.
3.3 DISCUSSION The study area forms basically a fluvial terrain in the MGP. The sediments have been brought to the area from the north lying Himalayas and the south lying peninsular highlands. The sinuous Ganga River in WGP shows moderate braiding 70
with low sinuosity in MGP due to lower slope and increased discharge and sediment load contributed laterally by the tributaries like the Sone, Karmnasa and Punpun from south and the Ghaghra, Gandak, Baghmati, Kosi etc from north. Lateral gradient from south towards north is more and the Sone River follows a relatively straight and braided course to join the Ganga. 3.3.1 Tectonic geomorphology Geomorphology of a fluvial terrain depends on the hydrology and morphology of the river channels and their dynamics. These in turn depend on the external factors like climate, tectonics, base level of erosion and not the least anthropogenic activities. The rivers in the study area have remained active as indicated by the palaeochannels of Sone, Ganga and the smaller stream Banas. Tectonics has been cited as the driving force for the river channel dynamism in the area (Sahu et al 2010). Sahu et al (2010) have argued for a tilting of the block bounded by EPF and WPF; uplift at the SE end along the EPF and subsidence at NW end along WPF. The net result has been a structural inclination of the block towards north and northwest. The response of alluvial rivers to this kind of lateral basin tilting has been discussed by many earlier workers (Cotton 1941; Nanson 1980; Holbrook 1992; Schumm et al 2000; Holbrook and Schumm 1999; Peakall et al 2000). In general, lateral basin tilting causes a deviation in the normal downstream migration of the meanders. The flow velocities of the river in general are augmented in the down-tilt direction, thereby resulting in the channel migration and avulsions in that direction (Cotton 1941). 3.3.1.1 Avulsions and migrations in rivers In addition to the set of palaeochannels of Sone mapped by Sahu et al (2010), the present work identifies two more palaeochannels of the river in the study area (PCH-I & II in Fig 3.17). Both these channels (PCH-I along Dehri-Pirro-Jagdispur and PCH-II along Dehri-New Bhojpur) have a single trunk channel with its root at Dehri (Fig 3.17). On the basis of the visibility of the channels on the surface and the extent of obliteration of the palaeochannel features by natural processes, the channel through Dehri-Pirro-Jagdispur seems to be older than even PCH-III. This channel got deflected westward towards New Bhojpur, which might have been due to the same reason of northwestward tilting of the basin. Later on, the river took the course along the lineament of EPF (PCH-III). Sahu et al (2010) have suggested that owing to close location of this palaeochannel to EPF, it experienced maximum ground tilting (highest lateral gradient) during uplift along the fault and consequently deflected its channel 71
several times westward, down in a NW direction from its earlier ENE direction of flow (PCH-III to XII in Fig 3.17). The last old course (PCH-X) of the river was towards Ara town in a NW direction. The Ganga River, being located at the distant parts of the hanging wall block, away from the loci of uplift along EPF, has experienced lower tilt rates causing migration of river channel (Sahu et al 2010). It has been revealed that fault-induced tilting of the basin floor forces the meander-belts to preferentially migrate in the direction of tilt (producing an asymmetric meander-belt). In this case loops growing opposite to the ground tilt are preferentially preserved and those formed in the downtilt direction are reworked upon and destroyed due to reoccupation and passage of active channels (Leeder and Alexander 1987; Peakall 1998). However, the preservation of loops pointing in the down-tilt direction in the flood plain of the Ganga indicates that, the channel course shifted northward by channel avulsion at some point upstream to the growing loop. Thus the loop has probably escaped from being reworked and destroyed. 3.3.1.2 Palaeochannels and their hydrogeological significance Palaeochannles are the drainage/rivers / streams which were flowing either ephemeral or perennial during past but now these are lost due to tectonic activities, climatic changes and geomorphic activities. Palaeochannels are important hydrogeological units with great potential for groundwater exploitation (Singh 1995). The palaeochannels of the Sone River are ~2.0-4.5 km wide and consists of braid channel deposits of fine to coarse sand. On the surface, braid channel depressions (forming temporary water bodies) and the elevated mounds of bars are often visible (Fig 3.19). Few centimeters to few meters of finer clayey sediments of younger age cover the palaeochannels. The curvilinear palaeochannels of the Ganga River are of similar width as the active Ganga. In contrary to the braided Sone palaeochannels, the Ganga palaeochannels are associated with point bar deposits, which often lye exposed or possess a thin finer clayey sediment cover. However, human activities have exposed the channel sand at many localities of both the Sone and Ganga rivers. Often, the palaeochannels of Sone are occupied by small mis-fit channels. These features probably form the important groundwater recharge/discharge boundaries.
72
Figure 3.19: (a) Soil excavation exposing the palaeochannel sand (mottled) of the Sone River at New Bhojpur. (b) (i) Shows a mis-fit channel occupying a braid channel course of Sone palaeochannel. (b) (ii) Shows the Sone palaeochannel braid bars forming undulatory mounds on the surface. It also depicts the depressions associated with the braid channels. (c) Stratified Ganga palaeochannel sand at Bariswan. (d) A mis-fit channel follows the curvilinear tract of Ganga palaeochannel in the flood plain active the Ganga River.
3.3.2 Sone megafan: the less talked megafan in the Ganga Basin The large fan-shaped sediment body deposited at the mountain exit of the river is referred to as fluvial megafan. The fan deposits, in many cases, consist of cycles of megasequences, tens to hundreds of meters thick. They trap the bulk of the coarse
73
sediments delivered from the mountain catchments (Harvey et al 1999). These in many cases provide good groundwater storage spaces and thus forming regional aquifers of high discharge. Coarse-grained, permeable fan sediments promote groundwater recharge and in cases the flow in the fan-forming river ceases during the lean period. The distribution pattern of palaeo-/abandoned channels of Sone, drainage setting in the area and the sub-surface lithology are quite suggestive of the existence of Sone megafan. The Gangetic Plain in India serves excellent examples of (mega) fan deposits; at the Himalayan mountain exit, many rivers have generated prominent megafans; from east to west lye the Kosi, the Gandak, the Ghaghra, the Sarda and the Yamuna-Ganga megafans (Geddes 1960; Gole and Chitale 1966; Wells and Dorr 1987; Mohindra et al 1992; Singh, 1996). In contrast, the craton origin rivers, exiting the peninsular foot-hills at the south in the Ganga Basin are not reported to form a ‘fan’, except the Sone River (Fig 3.20), which, Geddes (1960), first reported to be forming a megafan after entering the Ganga plain at Rohtas Garh, though; a detail account of it is still lacking. 3.3.2.1
Sone megafan in view of the global distribution of megafans The global distribution of modern megafans is primarily restricted to 15°–35°
latitude in the Northern and Southern hemispheres, corresponding to climatic belts that fringe the tropical climatic zone. Formation of fluvial megafans requires particular climatic conditions, and it is observed that modern fluvial megafans in actively aggrading basins are produced by rivers that undergo moderate to extreme seasonal fluctuations in discharge owing to highly seasonal precipitation patterns (Leier et al 2005). The Sone River, lying between 240 N – 260 N latitudes, is ephemeral and its discharge varies significantly in different seasons in a year (Fig 3.9). During Pleistocene also the peninsular India could not experience glaciations (Hora 1951). However, there were climatic fluctuations alternating with pluvial and arid periods, indicating seasonal nature of the peninsular Sone during Pleistocene also. Such a climate is suitable for weathering and generation of loose sediment material. The average slope of the South Ganga Plains is low and the drainage lines are short.
74
0
0
80
76
84
0
0
88
YG-Fan: Yamuna-Ganga Megafan 0
Sd-Fan: Sarda Megafan
32
G-Fan: Gandak Megafan K-Fan: Kosi Megafan
YG-Fan
P F:
Piedmont Fan Surface
Sn-Fan:
Sone egafan
PF
Delhi
Sd-Fan
Gh a ghra
Ya m una
PF
G-Fan
Ko si
28
ak nd Ga
0
K-Fan
Ganga
Patna
So ne
Sn-Fan
0
100
200 km
0
24
Figure 3.20: Schematic geomorphic map of Ganga Plain showing the location of megafans. The Gandak megafan might have overlapped the Sone megafan due to southward migration of Ganga River in the basin (modified after Singh 1996).
Alluvial fans often occupy rapidly subsiding extensional and foreland basins, where faults controlled basin margin relief (Nanson and Gibling 2003). Tectonics control may influence sediment production in the source area, and together with gross topography, appear primarily to control fan location, fan setting, and gross fan geometry (Harvey 2002). Tectonics appears to influence fan morphology and sedimentary sequences primarily through an influence on accommodation space (Silva et al 1992; Viseras et al 2003). The Sone megafan (Sone-Ganga alluvial tract) area is traversed by few most active faults in the region such as EPF and WPF. These faults have played larger role in creating and consuming space of sedimentation accommodation through uplift and subsidence. 3.3.2.2 Some necessary pre-conditions of megafan formation Leier et al (2005) identifies fluvial megafans on the following bases: (1) existence of a distinguishable fan-shaped body of sediment; (2) the sediment fan is larger than 30 km from apex to toe (the range of alluvial fans) (3) the river associated with the megafan has distributary characteristics, bifurcating into smaller channels, or at least maintaining discharge levels in the main channel (i.e. no tributaries join the megafan river once it exits the topographic front); and (4) existence of abandoned channels whose trends are in a divergent or arcuate disposition (indicating a radial 75
sediment dispersal). The author has studied data of 202 rivers worldwide to find out the qualities that discriminate the megafan-forming rivers from rivers that fail to create megafans. He finds that with sufficient aggradation rate and discharge, a river will form a fluvial megafan if it undergoes large seasonal fluctuations in discharge, provided the river gets enough space for migration and the channel outlets are widely spaced after exiting the topographic front. The Sone River satisfies all the criteria as cited above: (1) it has formed a distinguishable fan-shaped body of sediment; (2) the sediment fan is at least 200 km from apex to toe; (3) no tributaries join the river in its entire 200 km journey in the alluvial stretch; (4) abandoned channels exist broadly with diverging character. Again, as discussed earlier (section 3.2.1.2), the river possesses highly seasonal flow pattern and its sediment load also remains high probably since Pleistocene. There has been ample space available for the river to migrate in the alluvial tract. The river conveys to the Ganga, a flow of about 10760 m3/s (average of 1977-1986: Central Water Commission), which is much larger than the minimum river discharge (~ 20 m3/s) required to form a fluvial megafan (Leier et al 2005). 3.3.2.3
Nature of channel movement over Sone megafan In general, the channel in a megafan sweep both ways though channel
avulsion from the apex of the fan as is seen on the Kosi megafan in MGP. However, in Sone megafan, the channel has changed its course in the downstream parts at few particular nodal points (Fig 3.15).
Figure 3.21: Nature of channel movement over Sone megafan. Channel avulsions have taken place at three nodal points A, B and C in the downstream parts. 76
Based on the nature of obliteration of surface expressions such as burying of channel pools (water bodies) and leveling of braid bars of the river, the trunk channel in the western side of the active one seems to be the oldest channel of the river. Later on, the channel had shifted straightway to the eastern side of the megafan. In the initial phases of the fan building, autocyclic processes such as river bed building (sedimentologic) might have controlled the channel movement (avulsion). But, the trend pattern in the distribution of palaeochannels thereafter suggests allogenic (tectonic) factor responsible for the channel avulsions. 3.3.2.4
Important issues to address The Sone River is a large cratonic river and bears a huge sediment load of
coarser character. Despite of its unusual location in a marginal alluvial set-up, the river has accumulated thick sediment columns in a wide area. However, the following issues need to be addressed before concluding about the megafan: 1. Whether the sediments in the area considered for megafan are craton derived? 2. How a peninsular river could gather such an enormous volume of sediment? 3. What is the exact areal extent of the megafan?
77
CHAPTER 4 SEDIMENTOLOGY
4.1 INTRODUCTION Over the past few decades, several workers have shown that heterogeneity in hydraulic conductivity (K) and other aquifer parameters (e.g., porosity and storage coefficients) are controlled largely by the nature and distribution pattern of sediment in the aquifer (e.g., Fogg 1989; Davis et al., 1997; Hornung and Aigner 1999; Ritzi et al 2000). It is important to have knowledge regarding the physical and textural character of the sediment constituting the aquifer. The spatial distribution in different sedimentary facies and the range of grain size in them determine the potential of the aquifers, particularly in case of Quaternary unconsolidated alluvial aquifers. In this chapter a systematic attempt has been made to present the disposition of sediments as observed in outcrops (sand mines, cuts and trenches) and from the sediments obtained in the form of drill cut samples from few shallow as well as deeper boreholes drilled in the area. The study tries to identify different sedimentary facies on the basis physical characters of sediments and particle size distribution. The study further tries to delineate the probable areal extent of the Sone megafan in MGP on basis of sedimentological and geomorphological evidences obtained around the area. At the end, an attempt has been made to put light on the long standing debate (Sinha et al 2009) on the craton sediment input in filling the Ganga Basin and its significance in shaping up the hydrogeological characteristics of the area. 4.2 DATA USED AND METHODOLOGY Drill cut samples from 10 deep boreholes (Fig 4.1) down to a maximum depth of 350 m bgl, were studied in the present work in addition to 4 shallow boreholes to cover a wide range of area under investigation. Drill cut samples from 10 deep boreholes (Fig 4.1) down to a maximum depth of 350 m bgl, were collected under personal supervision during the drilling operation. The study also makes use of the lithologs obtained from 4 shallow borehole (max. 50 depth), out of which, author attended 3, during the drilling through manual hammering. After visual inspection of the sediment samples for their physical and textural properties, lithologs were prepared. Moreover, lithologs for few other 78
locations in the area were collected from different agencies namely Geological survey of India and Central Ground Water Board from their archives. Further, seven existing trenches (2–3 m deep and 1.5–2.7 km long) excavated for laying telephone cables and 14 sand mines (4–7 m deep) located in the area were studied for sediment disposition. Details of the locations of sand mines and trenches are given Fig 8.1 in chapter 8. Sand samples (51 no) from four boreholes, active river beds (5 no) and from sub-surface exposures like sand mines and trenches (10 no) have been subjected to grain size analysis for the determination of their particle size distribution. For the purpose, sediment samples of 250 gm by weight were taken out of the bulk from each sample by repeated process of coning and quartering. Samples were also collected from the existing sand mine heads for particle size distribution analysis. The analysis was carried out using standard set of sieves of sizes 2000, 1000, 500, 250, 180, 125 and 70 µm. Sediment fractions of size 2 mm, granule and pebble
2
100
1-2
0 to -1
1 to 2 mm, very coarse sand
3
50
0.5-1.0
1 to 0
0.5 to 1 mm, coarse sand
4
25
0.25-0.5
2 to 1
0.25 to 0.5 mm, medium sand
5
12
0.12-0.25
3 to 2
0.12 to 0.25, fine sand
6
7
0.063
7
Pan
0.12 4
0.07 to 0.12 mm, very fine sand silt and clay
4.4.1 Graphical representation The grain size bar diagrams (Fig 4.13) as well as frequency distribution curves (Fig 4.14) have been prepared in order to understand the nature of size distribution in the borehole sediments. From the figures it is evident that the sand is coarser in greater thickness of boreholes in the north/northwest than in the southern parts.
94
Weight (%) retained
Karnamepur 70.0 60.0 50.0 40.0 30.0 20.0 10.0 0.0
Weight (%) retained
4.5 3.5 2.5 1.5 0.5 -0.54.5 3.5 2.5 1.5 0.5 -0.54.5 3.5 2.5 1.5 0.5 -0.5 4.5 3.5 2.5 1.5 0.5 -0.54.5 3.5 2.5 1.5 0.5 -0.54.5 3.5 2.5 1.5 0.5 -0.5 4.5 3.5 2.5 1.5 0.5 -0.5 4.5 3.5 2.5 1.5 0.5 -0.5
40.0
19-31 m
31-38 m
38-44 m
44-50 m
50-68 m
68-90 m
90-100 m
130-135 m
Grain size (phi-)
30.0 20.0 10.0 0.0
4.5 3.5 2.5 1.5 0.5 -0.5 4.5 3.5 2.5 1.5 0.5 -0.5 4.5 3.5 2.5 1.5 0.5 -0.5 4.5 3.5 2.5 1.5 0.5 -0.5 4.5 3.5 2.5 1.5 0.5 -0.5 4.5 3.5 2.5 1.5 0.5 -0.54.5 3.5 2.5 1.5 0.5 -0.5 4.5 3.5 2.5 1.5 0.5 -0.5
135-156 m
156-159 m
159-203 m
222-228 m
222-228 m
228-234 m
234-237 m
237-250 m
Paharpur Weight (%) retained
50.0 40.0 30.0 20.0 10.0 0.0
Weight (%) retained
4.5 3.5 2.5 1.5 0.5 -0.54.5 3.5 2.5 1.5 0.5 -0.54.5 3.5 2.5 1.5 0.5 -0.5 4.5 3.5 2.5 1.5 0.5 -0.54.5 3.5 2.5 1.5 0.5 -0.5 4.5 3.5 2.5 1.5 0.5 -0.5 4.5 3.5 2.5 1.5 0.5 -0.5 4.5 3.5 2.5 1.5 0.5 -0.5
60.0 50.0 40.0 30.0 20.0 10.0 0.0
4-13 m
13-28 m
28-50 m
50-69 m
69-75 m
75-82 m
82-113
125-140 m
Grain size (phi-)
4.5 3.5 2.5 1.5 0.5 -0.5 4.5 3.5 2.5 1.5 0.5 -0.54.5 3.5 2.5 1.5 0.5 -0.5 4.5 3.5 2.5 1.5 0.5 -0.5 4.5 3.5 2.5 1.5 0.5 -0.5 4.5 3.5 2.5 1.5 0.5 -0.5
140-147 m
147-168 m
168-196 m
196-206 m
213-236 m
236-250 m
Weight (%) retained
Bharauli 40.0 30.0 20.0 10.0 0.0 4.5 3.5 2.5 1.5 0.5 -0.54.5 3.5 2.5 1.5 0.5 -0.54.5 3.5 2.5 1.5 0.5 -0.5 4.5 3.5 2.5 1.5 0.5 -0.54.5 3.5 2.5 1.5 0.5 -0.5 4.5 3.5 2.5 1.5 0.5 -0.5 4.5 3.5 2.5 1.5 0.5 -0.5 4.5 3.5 2.5 1.5 0.5 -0.5
31-37 m
37-43 m
Weight (%) retained
70.0 60.0 50.0 40.0 30.0 20.0 10.0 0.0
Weight (%) retained
Nargada Narayanpur
60.0 50.0 40.0 30.0 20.0 10.0 0.0
56-62 m
62-106 m
138-144 m
144-181 m
181-206 m
215-250 m
Grain size (phi-)
4.5 3.5 2.5 1.5 0.5 -0.54.5 3.5 2.5 1.5 0.5 -0.54.5 3.5 2.5 1.5 0.5 -0.5 4.5 3.5 2.5 1.5 0.5 -0.54.5 3.5 2.5 1.5 0.5 -0.5 4.5 3.5 2.5 1.5 0.5 -0.5 4.5 3.5 2.5 1.5 0.5 -0.5 4.5 3.5 2.5 1.5 0.5 -0.5
19-25 m
25-35 m
40-44 m
44-50 m
50-56 m
56-65 m
65-78 m
92-99 m
Grain size (phi-)
4.5 3.5 2.5 1.5 0.5 -0.5 4.5 3.5 2.5 1.5 0.5 -0.5 4.5 3.5 2.5 1.5 0.5 -0.5 4.5 3.5 2.5 1.5 0.5 -0.5 4.5 3.5 2.5 1.5 0.5 -0.5
134-146 m
146-178 m
178-195 m
195-210 m
210-247 m
Figure 4.13: Bar diagram for the percentage distribution of different size grades of sand grains in the borehole samples obtained from Karnamepur, Paharpur, Bharauli and Nargada Narayanpur in Bhojpur district in the study area.
95
Karnamepur
Paharpur
100
100
90
90
80
80
70
Cumulative Percent
Cumulative Percent
70
60
50
40
60
50
40
30
30
20
20
10
10
0
0
-2.00
-1.00
0.00
1.00
2.00
3.00
4.00
5.00
6.00
-2.00
-1.00
0.00
Grain Size (Phi)
3.00
4.00
5.00
6.00
Nargada Narayanpur
Bharauli
100
90
90
80
80
70
70
Cumulative Percent
Cumulative Percent
2.00
Grain Size (Phi)
100
60
50
40
60
50
40
30
30
20
20
10
10
0 -2.00
1.00
0
-1.00
0.00
1.00
2.00
3.00
4.00
5.00
6.00
-2.00
-1.00
0.00
Grain Size (Phi)
1.00
2.00
3.00
4.00
5.00
6.00
Grain Size (Phi)
Figure 4.14: Frequency distribution curves of sand samples obtained from Karnamepur, Paharpur, Bharauli and Nargada Narayanpur boreholes.
4.4.2 Statistical Measures 4.4.2.1 Median size (Md) and inclusive graphic mean(Mz) All natural sediment samples contain grains having a range of sizes. However, it is frequently necessary to characterize the sample using a single typical grain diameter as a measure of the central tendency of the distribution. The median grain diameter (Md, denoted as φ50 in ‘phi’ scale), is a sample characteristic most often
96
chosen. It is the particle diameter in respect of which half of the particles (by weight) in the sample will have a larger diameter and half will have a smaller. As per the Median values, the sediment samples from the borehole at Bharauli fall in the medium to coarse sand category (Md: 0.24 to 1.23 φ) (see Table 4.2), except the sample from the depth range of 31–37 m, which is of very coarse sand category (Md at – 0.05 φ). However, visually most of the sediment grains within this depth range are granules (Md < -1 φ) of kankars (calcium carbonates) and are not reflected in the grain size fractions due to lack of a mesh size larger than 200 microns. Majority of samples from Nargada Narayanpur are of medium to coarse sand character (Md: 0 to 1.75 φ), with the exception of fine sand (Md: 2.20 φ), at the depth range of 19–25 m and coarse to very coarse sand (Md: -0.20 φ), at 92–99 m depth range. The coarseness of sediments increases at Karnamepur and Paharpur. At the Karnamepur site, except the coarse sand in the depth ranges of 44-50 m and 50-68 m, the remaining analyzed sand zones of the borehole show very coarse sand. At Paharpur, very coarse sand is reflected in the sand zones below 82 m depth up to the drilled depth of the borehole, whereas above 82 m depth the sand zones show an alternation of fine sand, very coarse/coarse sand and coarse sand. The inclusive graphic mean (Mz) suggested by Folk & Ward (1957) is more meaningful than the median size for it is the average of three points on the cumulative size distribution curve. The depth-wise variation of mean and median grain size of sand samples in the four boreholes has been produced in Fig 4.15. Inclusive graphic mean is widely used distribution parameter and is regarded by most authors (Folk and Ward, 1957; Passega 1964) as an indicator of the average energy of the transport and sedimentation agent. This size parameter has been used to characterize the sediment in terms of the Wentworth scale textural terminology (Table 4.1). Since this mean value is obtained taking also the finer tail into consideration with the coarser tail of the size distribution curve, it interprets most of the very coarse sand samples (on the basis of median value) as coarse sand and few medium sand as fine sand (Table 4.2). However, since the sediment samples lack any significant finer tail; in many cases the size interpretations in both the cases of median and inclusive graphic mean remain the same. The percentage of grain size fractions in the analyzed columns of four boreholes has been given in Table 4.3.
97
Karnamepur
Paharpur
Me an/M e dian (m m )
M ean/M e dian (m m )
0-19 19 - 31 31 - 38 38 - 44 44 - 50 50 - 68 68 - 90 90 - 100 100 - 115 115-130 130 - 135 135 - 156 156 - 159 159 - 203 203-222 222 - 228 228 - 234 234 - 237 237 - 250
0.5
1
1.5
0
2 0-4 4. - 13
C la y wit h f ine s a nd le ns e s
1.5
2
C la y wit h f ine s a nd le ns e s
C la y wit h f ine s a nd le ns e s
75 - 82 82 - 113 113-125 125 - 140 140 - 147
C la y wit h f ine s a nd le ns e s
147 - 168 168 - 196 C la y wit h f ine s a nd le ns e s
196 - 206 206-213
C la y wit h f ine s a nd le ns e s
213 - 236 236-250 M edian
M ean
M ean
Nargada Narayanpur
Bharauli
M e an/M e dian (m m )
0-19
1
50 - 69 69 - 75
M edian
0
0.5
13 - 28 28 - 50
Depth range (m)
Depth range (m)
0
0.5
1
Me an/M edian (m m )
0
1.5
C la y wit h f ine s a nd le ns e s
0-31
0.5
1
1.5
C la y wit h f ine s a nd le ns e s
19 - 25 25 - 35
31 - 37
40 - 44
37 - 43
44 - 50 56 - 62
56 - 65 65 - 78 78-92
C la y wit h f ine s a nd le ns e s
92 - 99 99-134
Depth range (m)
Depth range(m)
50 - 56
62 - 106 106-138
C la y wit h f ine s a nd le ns e s
138 - 144
C la y wit h f ine s a nd le ns e s
144 - 181
134-146 146-178
181 - 206
178-195 206-215
195-210 210-247 M edian
C la y wit h f ine s a nd le ns e s
215 - 250 M ean
M edian
M ean
Figure 4.15: Depthwise variation of mean/median grain size of sand samples in the boreholes at Karnamepur, Paharpur, Nargada Narayanpur and Bharauli.
4.4.2.2 Standard deviation (Sorting, σ) Standard deviation measures the sorting of the sediment and fluctuations in the kinetic energy or velocity conditions of the depositing agent (Sahu 1964). For the presently studied 51 sediment samples collected from four boreholes, the standard
98
deviation values vary within 0.46 to 1.39 φ, with an average of 0.97 φ (Table 4.2). Out of 51 sediment samples, 29 (56.86 %) are poorly sorted (σ: 1.01-1.39 φ) and 16 (31.4 %) are moderately sorted (σ: 0.71-0.99 φ). There are also 5 samples (9.8 %) showing moderately well sorted character (σ: 0.61-0.70 φ). Fine sand (micaceous), similar to the present day Ganga sand, occurring at shallow depth (Nargada Narayanpur and Paharpur) are well sorted to moderately well sorted (σ: 0.46-0.70 φ) in nature. In general it is observed that among the sand samples presently studied, better sorting lies towards the coarsest and finest ones. This may be due to the presence of finer admixture in the other sediment samples. Furthermore, inclusive graphic mean plot with the sorting values exhibits a systematic relation that sorting bear with the sand particles (Fig 4.16.A). Within the mean size range of sand, the coarser tail (Mz > 0.5 φ) and the finer tail (Mz < 2.0 φ) show better sorting than the intermediate size ranges (0.5-1.5 φ). 4.4.2.3 Skewness (Sk) The skewness of a size distribution reflects the energy regimen of the depositing medium. The skewness values for the presently studied sediment samples, as measured from the cumulative frequency distribution curves, range between -0.61 and 1.28 (Table 4.2). Out of the total 51 sediment samples collected from the four bore holes, 41 are positively skewed (80.40 %) with 39 of them being fine to very fine skewed (76.50 % of total) and 2 having near symmetrical particle size distribution. The rest 10 samples are negatively skewed (19.60 %) with 9 of them being coarse to very coarse skewed (17.64 %) and 1 having near symmetrical particle size distribution (Fig 4.16.B). The negatively skewed (-0.01 to -0.61) samples are mainly concentrated (60.0 % of them) in a single borehole located at Nargada Narayanpur. Leaving apart this borehole, 34 (89.47 %) out of 38 sediment samples register positive skewness (Sk: 0.01 to 1.28) with two of them having near symmetrical distribution. All the sediment samples collected from the Karnamepur borehole are positively skewed (Sk: 0.091.28) with one of them having near symmetrical particle size distribution (Sk: 0.09). Out of 29 sediment samples collected from the two boreholes at Karnamepur and Paharpur, which show coarser sand, 27 are (93.10 %) positively skewed. Positive skewness indicates an excess of fines and negative skewness indicates an excess of coarse grained material. Though, in general positive skewness indicates that the energy of the depositing agent rarely exceeds a certain threshold, those may also imply the introduction of fine material (Friedman 1961). Usually this happens 99
when fluvial deposits resulting from suspension material, overlaps the sediment transported on the bottom through traction and salting, due to the reduced turbulence of fluvial current. Furthermore, positive skewness of sediments indicates the unidirectional transport (channel) or the deposition of sediments in sheltered low energy environment. It has been indicated that negative skewness has relationship with the intensity and duration of a high energy depositional agent through the removal of fines (Friedman 1961; Sahu 1964). If the energy of the agent remains above a relatively high threshold, material finer than a certain size will not be deposited and the sediment should display negative skewness because of resulting excess of coarse detritus. Duane (1964) elaborated similar reasoning and indicated that winnowing action induced by fluid media is the mechanism producing negative skewness. According to him, negatively skewed curves are attributed to erosion and non-depositional places, whereas positively skewed curves indicate deposition.
Me a n gra in siz e vs S orting
1.5 1.4 1.3 1.2 1.1 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0
P oorly s orted M oderately s orted
-0.5
0
0.5
1
1.5
W ell sorted
2
Fine sand
Coarse sand
Mediumsand
Moderately well sorted Very coarse sand
Sorting
A
2.5
3
M e an gr ain s ize (p h i-)
Figure 4.16: (A) Plotting of mean grain size (Mz) vs sorting in the analysed sediment samples.
100
Mean grain size vs Skewness
Fine sand
Mediumsand
Coarse sand
1.4 1.3 1.2 1.1 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 -0.1 -0.2 -0.3 -0.4 -0.5 -0.6 -0.7 -0.8 -0.5
Very coarse sand
Skewness
B
Strongly fine skewed Fine skewed Nearly symmetrical Coarse skewed Strongly coarse skewed
0
0.5
1
1.5
2
2.5
2
2.5
3
M e an grain size (phi-)
Leptokurtic
Fine sand
Very leptokurtic
Medium sand
2.3 2.2 2.1 2 1.9 1.8 1.7 1.6 1.5 1.4 1.3 1.2 1.1 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 -0.5
Coarse sand
Mean grain size vs Kurtosis
Very coarse sand
Kurtosis
C
Mesokurtic Platukurtic
Very platykurtic
0
0.5
1
1.5
3
M e an grain size (phi-)
Figure 4.16: Contd. (B) Plotting of mean grain size (Mz) vs skewness.(Sk) and (C) mean grain size (Mz) vs kurtosis (KG) in the analyzed sediment samples.
4.4.2.4 Kurtosis (KG) The KG values of the 51 sediment samples collected from the four boreholes range between 0.45-2.1. However, greater number of samples (72.54 %) show KG 500 m depth) of cratonic sediment deposits in the middle parts of the basin. Middle to Late Pleistocene might have remained the building phase of the megafan, when the monsoon was stronger (Prell and Kutzback 1987; Overpeck et al 1996; Waelbroeck et al 2002). The coarser sediment buldge might have forced the Ganga to flow at the foot zones of the megafan (Fig 4.22.A). The Sone River must be traversing a longer 115
distance at that time to confluence with Ganga. During Holocene, the Ganga River has encroached southward, incising on to the Sone megafan (Fig 4.22.B).
Figure 4.22: Schematic diagram illustrating the northward extent of the cratonic alluvial wedge in the Ganga Basin based on the observation of Sinha et al (2009) at KenBetwa sub-basin and many other CGWB water wells in Middle Ganga Plain. (A) The course of Ganga during the initial phases of Late Pleistocene (~128-75 Ka) and after LGM (~18-10 Ka) up to the onset of Holocene, when the strength of Southwest Monsoon was stronger than today (modified after Singh and Singh 1971). (B) The active course of Ganga w.r.t. the cratonic wedge.
116
CHAPTER 5 AQUIFER GEOMETRY AND WATER LEVEL BEHAVIOURS
5.1 INTRODUCTION For a better planning of groundwater development, it is essential to understand the geometry of the aquifer and the occurrence and movement of groundwater in it. Knowledge of the spatial extent of the aquifer is imperative for the estimation of total groundwater reserve. The properties which have a strong bearing on the groundwater development are the groundwater storage and yield. In case of poor groundwater quality affecting a part or the aquifer system in entirety, the knowledge of the aquifer configuration, its spatial extent and groundwater flow regime helps in making quality deterrent plans of groundwater development. Preliminary idea on the nature and extent of the underlying aquifer is obtained from the study of the geology and geomorphology of the area. The sediment architect depends on the nature and type of the operating geomorphic agent. In fluvial landscapes, the efficiency of the drainage system and the provenance determines the dimension and texture of the aquifer system to be generated in due course of time. Moreover, the extrinsic factors such as climate, base level of erosion and tectonics, affect the fluvial processes by modifying the energy and their distribution pattern in the basin, thus also controlling the formation of aquifers. Longitudinal information about the aquifers in any basin is obtained through surface geophysical surveys and analysis of samples from boreholes. 5.2 DATA USED AND METHODOLOGY Sub-meter scale borehole logs (lithologs) of both the shallow (~50 m) and deeper (~250 m) depths have been prepared on the basis of the analysis of drill-cut samples collected from boreholes. The available geophysical logs of the boreholes have been utilized in validating and reconciling the lithologs of the respective boreholes. Hydrogeological sections along the length and breadth of the area have been generated using those lithologs. The clay and sandy clay zones, which separate the granular zones constituting the aquifers, have been identified. Classification of aquifer groups has been undertaken for the Older and Newer alluvial areas. Litho117
facies analysis of the aquifers, based on Pettijohn and Randich (1966), has been carried out for every 50 m depth interval. Depth to water levels have been collected from a set-up of 67 key wells in the area for the periods of pre- and post-monsoon of the year 2010 to study the behavior of water level. During the year, the area received sub-normal rainfall. To assess the groundwater flow regime during a normal rainfall year, the depth to water level data of net work stations set by Central Ground Water Board (CGWB) for the periods of pre- and post-monsoon of the year 2002 has been utilized. The water levels were utilized to prepare depth to water level maps and also referenced to mean sea level to prepare water level contour maps for ascertaining groundwater flow regime. Long term trend in water level has been analyzed for six stations using the data of CGWB. Water level in the tube wells tapping granular zones in deeper levels have also been collected for the study a comparative study with the shallow aquifer water level behavior. 5.3 HYDROGEOLOGICAL TRANSECTS Four hydrogeological transects (Fig 5.1)have been prepared; (1) along E-W, traversing the Newer Alluvium (A-A’), (2) along E-W, traversing the Older Alluvium (B-B’), (3) traversing across the Older and Newer Alluvium with N-S alignment at the eastern parts of the area (C-C’), and (4) traversing across the Older and Newer Alluvium at the central parts of the area with N-S alignment (D-D’). 84° 00'
84° 15'
84° 30'
84° 45'
N GAN GA RIVER
25° 45' 0
5
10
15
25° 45'
D
20 Km
Neazipur
Scale
1
Karnamepur
A
Nat
1
Churawanpur
Buxar
B Kithpura
Maner
Basantpur Ratantola
Brahmpur
2
Bhojpur (Ara)
Udwantnagar
2
Chausa
25° 30'
A'
C
Bariswan Shahpur
Bikram
Jagdispur
1-
Older Flood Plain (Fatwa Surface) 84° 00'
Active channels/cut-off lakes Administrative boundary 84° 15'
Sandesh
RIV ER
Garhani
NE
2Newer Alluvium (NA)
Older Alluvium (OA)/Upland (Mohanpur Surface) Boundary between NA & OA
SO
Channel braid barsActive Flood Islands/point bars Plain (Diara Surface) Over bank deposits
25° 30'
B'
D'
Charpokhri 84° 30'
C'
Sahar
84° 45'
Figure 5.1: Location map of the hydrogeological transects A-A’, B-B’, C-C’ and D-D’. 118
5.3.1 Transect A-A’ The hydrogeological section (Fig 5.1.a) lies in the Newer Alluvium extending from Basantpur at the east to Nat in the west of the area. However, to gain information on the area further east of Basantpur, a borehole outside the area (Maner) has been considered. The boreholes are topped by clay/mud/silt deposits of thickness varying between 10-15 m within the study area. The thickness increases eastward and reaches maximum (~58 m) at Maner, outside the study area. The muddy deposit is underlain by potential aquifer systems down to the borehole depth of 250 m below ground. The aquifers consist of sand of various sizes between very fines to coarse grain with interlayers of gravels. In the entire area, a clayey zone within the depth range of 85-128 m separates the sand sequences into two aquifer groups. The thickness of this intervening clay layer decreases westward. Towards west of the area, the overall grain size decreases consistently. The section is more gravelly in its eastern and central parts. The most remarkable aspect is that, the Ganga fine sand, observed at the top levels of boreholes in the flood plain of active Ganga, is also noticed at deeper levels in the 180-220 m depth range. Though, the gravels are in general composed up of rounded to sub-rounded quartz and feldspar grains, at places these are mixed up of calcium carbonate concretions and less frequently with carbonate cemented sand. The lithology obtained from the shallow boreholes indicates the coarse sand and gravel horizons at the borrom half within 50 m below ground are at places interspersed with centimeter scale clay lenses (refer chapter 4).
119
A
Churamanpur
Very fine to fine sand
Mud/Clay Medium to coarse sand
Fine to medium sand
Karnamepur Bariswan
Figure 5.1.a: Hydrogeological transect in the Newer Alluvium in the study area.
Nat
Neazipur
Coase sand with gravels Coarse sand with interlayers of gravels
Basantpur
A' Maner
120
5.3.2 Transect B-B’ The section (B-B’) with an east-west orientation mostly traverses the Older Alluvium, except its western parts, where the boreholes at Churamanpur and Kithpura are located in the patches of Newer Alluvium (Fig 5.1.b). The borehole at Bikram, which is located beyond the study area at its eastern side, has been included in the transect to elucidate the continuity of the aquifers.
Figure 5.1.b: Hydrogeological transect in the Older Alluvium, lying towards south of the flood plains of Ganga in the study area.
The maximum depth of information in this transects B-B’ is 330 m below ground at Shahpur. There is increase in the thickness of the top clay/mud/silt layer, which reaches 52.15 m to 42.65 m at Garhani and Udwantnagar boreholes in the Older Alluvial parts. However, there seems to be negligible changes in the physical character and configuration of the aquifer systems underlying the top clay. As section approaches the Ganga in the Newer Alluvium, gray coloured fine micaceous sand of
121
Himalayan origin appear at the top of the succession. In a similar fashion to the Newer Alluvial transect (A-A’), the Ganga sand is observed at the deeper levels within the same depth range of 180-220 m below ground. The middle clay in the Newer Alluvial transect (A-A’) seems to be missing in the western parts. However, there exist other clay zones that divide the sand columns into multi-aquifers. It depicts two clay zones of thickness within 4-12 m. The depth to the upper one varies from 55 m at the east to 66 m in the western parts around Sandesh and Kithpura respectively, whereas the deeper one is encountered at depths of 76 and 81 m at the eastern and western parts respectively around the same location. At deeper levels (>120 m), most of the clay zones that advance from the west to eastward, seem to pinch out somewhere in the middle of the area. Similar to transect A-A’, there is systematic appearance of gravelly zones at two particular depth ranges; the first being within 55-110 m and the second one at 150-200 m depth range. These are either topped or interleaved with clay zones. 5.3.3 Transect C-C’ The transect in the eastern parts of the area runs in a north-south direction across the boundary of Newer and Older alluvium (Fig 5.1.c). The section enters the Newer Alluvium at Ratantola, just at the north of Ara town. The transect clearly displays a thick top-clay, which decreases significantly in the Newer Alluvial area at the northern parts. The top clay in the Older Alluvium often contains gravel size carbonate nodules (kankars). The middle clay is encountered within the depth range of 90-140 m below ground. The two clay zones with wide east-west coverage as detected in the transect B-B’ do not extend much beyond the Newer-Older Alluvium boundary and thin out in the middle of Newer Alluvial tract after Ratantola. Though, thicker sand columns are present in the entire area, frequency of coarser materials increases westward.
122
C' Sahar Ratantola Udwantnagar
C Basantpur Garhani
Clay Fine to very fine sand Fine to medium sand Medium to coarse sand Coarse sand with interlayers of gravels
Figure 5.1.c: Hydrogeological transect across the Older and Newer Alluvium in an N-S direction in the study area.
5.3.4 Transect D-D’ Similar to transect C-C’, the transect D-D’ also runs from south to north across the Newer-Older Alluvium boundary in the central parts of the area (Fig 5.1.d). This again displays thicker top-clay in the Older Alluvial part, which is often laden with carbonate nodules. The transition from Older to Newer Alluvium takes place at the north of Shahpur, where after, a decrease in the thickness of top-clay takes place. The
123
two clay zones identified in the earlier transect (C-C’) appear to continue in the northern Newer Alluvial area. However, the clay layers merge southward at Charpokhri (outside the study area). The middle clay appears in the Newer Alluvial stretch within the depth of 105-130 m below ground and seems to continue southward in the Older Alluvium beyond Shahpur. Above the middle clay, the transect depicts multiple aquifers separated by clay layers. Coarse sand and gravels make the bulk of the aquifer, particularly in the Newer Alluvial parts. However, the aquifer underlying the middle clay persists down to the depth of ~270 m without intervention of any major clay. The aquifer is dominated by medium to coarse sand with interlayers of gravels.
D'
D
Karnamepur
Shahpur
Jagdispur
Charpokhri
Clay Fine to very fine sand Fine to medium sand Medium to coarse sand Coarse sand with interlayers of gravels
Figure 5.1.d: Hydrogeological transect across the Older and Newer Alluvium in a NS direction at the central parts of the area in the study area.
124
5.4 LITHO-FACIES ANALYSIS OF THE ALLUVIAL DEPOSITS The technique of Pettijohn and Randich (1966) has been followed (Fig 5.2) to study the litho-facies of the alluvial deposits. The analysis has been carried out for each 50 m thick zone starting from ground level down to the depth of 300 m bgl. The zones are 0 to 50 m bgl, 50 to 100 m bgl, 100 to 150 m bgl, 150 to 200 m bgl, 200 to 250 m bgl and 250 to 300 m bgl. The unit thickness of 50 m has been chosen based on the subsurface lithology of the area and also based on the fact that the top-clay thickness some of the goes up to 50 m. This classification is based on plotting of Grain Size Ratio (GSR) on a triangular diagram having three corners represented by the following litho-units, ‘clay’, ‘sand’ and ‘gravel’ (Fig. 5.2). The GSR is worked out by the following equation: Grain Size Ratio = (Sand + Gravel)/(Clay + Silt)
……………………(5.1)
Where, sand, gravel, clay or silt is represented by their cumulative thickness in m within the particular depth zone.
GRAIN SIZE RATIO = SAND + GRAVEL /CLAY + SILT
Figure 5.2:
Litho-facies and grain-size triangle (modified after Pettijohn and
Randich, 1966).
125
In this equation ‘Sand’ includes all size grades of sand, viz., fine, medium, and coarse, ‘Clay’ includes sandy clay and clay with minor sand. The cumulative thickness of Sand + Gravel or Clay + Silt has been worked out from the lithological log of the bore wells. Table 5.1: Grain Size Ratio for different depth zones in boreholes. Grain Size Ratio
Boreho Sl No
Borehole Location
Lat.
Long.
le
0-
51-
101-
151-
201-
251-
Depth
50
100
150
200
250
300
(m bgl)
m
m
m
m
m
m
301 350 m
BH-1
Babura
25.674
84.776
100
2.3
*
NA
NA
NA
NA
NA
BH-
Sinha
25.676
84.676
50
1.6
NA
NA
NA
NA
NA
NA
BH-2
Ratantola
25.593
84.656
84
4.7
1.13
NA
NA
NA
NA
NA
BH-3
Basantpur
25.635
84.607
224
1.7
2.85
0.52
*
0.33
NA
NA
BH-
Bakhorapur
25.668
84.688
50
1.4
NA
NA
NA
NA
NA
NA
BH-4
Sandesh
25.402
84.740
94
1.9
1.14
NA
NA
NA
NA
NA
BH-5
Garhani
25.425
84.535
141
0.0
9.00
3.56
NA
NA
NA
NA
BH-6
Udwantnag
25.503
84.620
109
0.1
4.00
0.50
NA
NA
NA
NA
BH-7
Paharpur
25.643
84.491
286
1.9
*
*
*
6.14
0.67
NA
BH-8
Amrahi
25.580
84.472
248
1.9
0.79
1.08
*
NA
NA
NA
BH-9
Chakawat
25.572
84.472
91
3.1
4.86
NA
NA
NA
NA
NA
BH-
Jagdispur
25.466
84.420
80
0.5
5.00
NA
NA
NA
NA
NA
BH-
Bariswan
25.628
84.437
250
9.0
6.14
5.25
2.85
6.14
NA
NA
BH-
Nargada
25.604
84.440
250
1.2
2.33
6.14
*
*
NA
NA
BH-
Shahpur
25.600
84.404
330
0.4
2.13
3.17
*
*
2.38
*
BH-
Bharauli
25.634
84.378
250
1.9
*
0.92
24.5
11.5
NA
NA
BH-
Karnamepu
25.655
84.362
245
3.5
5.25
4.00
24.0
3.50
NA
NA
BH-
Brahmpur
25.597
84.306
246
0.5
2.13
3.17
24.0
*
NA
NA
BH-
Neazipur
25.680
84.152
246
1.1
1.78
4.00
*
*
NA
NA
BH-
Nat
25.610
84.038
102
2.6
7.33
NA
NA
NA
NA
NA
BH-
Churamanp
25.581
84.026
246
2.5
2.85
5.25
2.85
*
NA
NA
BH-
Kithpura
25.555
83.953
110
0.0
4.00
*
NA
NA
NA
NA
BH*
Charpokhri
-
-
108
0.4
0.56
0.33
NA
NA
NA
NA
VES
Jagdispur1
24.427
84.411
300
0.9
0.47
0.32
0.35
*
*
NA
VES
Jagdispur2
24.425
84.407
345
1.2
4.75
0.35
0.28
4.00
*
9
BH*
Bikram
25.444
84.868
250
2.1
1.08
3.17
9.00
2.57
NA
NA
BH*
Maner
25.643
84.890
300
0.6
*
1.17
*
1.5
0.25
0.1
Explanation: NA = Grain Size Ratio could not be calculated due to insufficient data in that depth range * = Grain Size Ratio more than 10, BH*- Borehole located outside the study area
126
The GSR for different depth zones is produced in Table 5.1. Though, the analyses have been carried out up to the depth of 330 m bgl, maps have been prepared only up to 250 m bgl for only two bore holes reach the depth of 300 m bgl. Lithofacies analyses based on sand percentage in unconsolidated aquifer have been applied by different workers to understand the geometry of the aquifer like alluvial fan and fan-delta deposits of Caribbean islands (Ranken et al 2000) and deltaic deposits in Bengal Basin (Sikdar 2000). The litho-facies map of 0-50 m bgl (Fig 5.2.a) depicts 66.0% of the total area being covered by the ‘clayey sand’ facies indicating the existence of low to moderately thick clay layers.
The area around Babura, Koilwar, Sandesh, Ara,
Semaria, Neazipur and Buxar in the northern and eastern parts fall in this category. It is followed by the ‘sandy clay’ facies, which covers 22.4% of the area in two patches; (1) the south central parts around Shahpur, Brahmpur and Jagdispur (2) between Buxar and Chausa in the west. There are three ‘clay’ facies zones; (1) around the borehole at Kithpura in the western parts, adjoining to the bank of Ganga River, (2) around Udwantnagar and Garhani in the eastern parts, and (3) in between Brahmpur and Jagdispur (10.8% of the area). The ‘sandy clay’ and the ‘clay’ facies together indicate the predominance of finer sediments. The ‘sand’ facies has been observed at a small patch at Bariswan tube well.
25.7 Neazipur Babura
Rajpur kalan
Semaria
25.6
New Bhojpur
Shahpur Brahmpur
Buxar
Koilwar Ara
25.5
Chausa
Jagdispur
Boundary between NA & OA
Udwantnagar
Location of boreholes
25.3
Sand
83.9
Clayey sand
84
Garhani
0.25
1
8
Lithofacies (0-50 m bgl)
Sandy clay
84.1
84.2
0
25.4
Sandesh
0
10
20 Water bodies (channel cut-off lakes)
Kilometers
Clay
84.3
84.4
84.5
84.6
84.7
84.8
Figure 5.2.a: Litho-facies distribution within 0-50 m below ground level in the Newer and Older Alluvial deposits in the study area.
127
Considerable changes have been observed in the distribution of litho-facies within the depth range of 51-100 m bgl (Fig 5.2.b), particularly in the north-eastern parts around Babura (BH-1), Paharpur (BH-7) and Bharauli (BH-14), where the ‘clayey sand” facies in the depth range of 0-50 m bgl is underlain by the ‘sand’ facies. A second patch of ‘sand’ facies appears at Garhani (BH-5), which was earlier covered by ‘clay’ facies in the zone of 0-50 m bgl. The facies covers in total 9.4% of the study area. The remaining major part i.e 90.6% of the area is marked by ‘clayey sand’ facies. The ‘sandy clay’ and ‘clay’ facies patches in the south-central and western parts in the 0-50 m zone turns to ‘clayey sand’ facies in the current zone. This is indicative of lesser clay zones in the 51-100 m bgl depth range towards the Older Alluvial area in comparison to the overlying zone.
25.7 Neazipur Babura
Rajpur kalan
Semaria
25.6
New Bhojpur
Brahmpur
Buxar
Maner
Shahpur Koilwar Ara
25.5
Chausa
Jagdispur
Boundary between NA & OA
Udwantnagar
Bikram
Location of Bore holes
Sandesh
0
1
8
0.25
Lithofacies (51 -100 m bgl)
25.4
Garhani
0 Sand
Clayey sand
Sandy clay
10
20
Kilometers
Clay
25.3 83.9
84
84.1
84.2
84.3
84.4
84.5
Water bodies (channel cut-off lakes)
84.6
84.7
84.8
Figure 5.2.b: Litho-facies distribution within 51-100 m below ground level in the Newer and Older Alluvial deposits in the study area.
The ‘sand’ facies in the 51-100 m bgl zone shrinks to around Paharpur (BH-7) only in the 101-150 m bgl zone, due to the appearance of clay at Babura and Bharauli (Fig 5.2.c). However, another patch of ‘sand’ facies appears at the western parts around Kithpura (BH-20), which was earlier covered by ‘clayey sand’ facies. The total area covered by ‘sand’ facies remains at 7.2%. There are two patches; (1) between Udwantnagar (BH-6), Ara and Basantpur (BH-3), (2) around Jagdispur (VES), where the ‘clayey sand’ facies in the depth range of 51-100 m turns to ‘sandy clay’ facies. This facies covers 16.5% of the study area. The remaining major 76.3% of the area, covering Sandesh, Babura, Garhani, Shahpur, 128
Neazipur, Neazipur, Brahmpur, Shahpur Semaria, Churawanpur and Buxar is marked by ‘clayey sand’ facies. The zone between 151-200 m bgl (Fig 5.2.d) shows predominance of ‘sand’ facies, covering 71.7% of the study area, indicating coarser nature of the sediments. The ‘clayey sand’ facies forms the next major litho-facies, covering the 27.9% of the study area in three patches; (1) around Garhani and Jagdispur in the south-central parts, (2) around Buxar and Chausa in the western parts, and (3) a small patch around Bariswan in the central parts of the study area. This facies is also possessive of good aquifers with intercalations of minor clay. There is a small patch south to Jagdispur (0.40% of the area), where, ‘sandy clay’ facies is observed.
25.7 Neazipur Babura
Rajpur kalan
Semaria
25.6
New Bhojpur
Brahmpur
Buxar
Maner
Shahpur Koilwar Ara
25.5
Chausa
Jagdispur
Boundary between NA & OA
Udwantnagar Bikram
Location of boreholes/VES
Garhani Sandesh
0
1
8
0.25
Lithofacies (101-150 m bgl)
25.4
0 Sand
Clayey sand
10
Water bodies (channel cut-off lakes)
Kilometers
Clay
Sandy clay
20
25.3 83.9
84
84.1
84.2
84.3
84.4
84.5
84.6
84.7
84.8
25.7 Neazipur Babura
Rajpur kalan
Maner
Semaria
25.6
New Bhojpur
Brahmpur
Buxar
Shahpur Koilwar Ara
25.5
Chausa
Jagdispur
Boundary between NA & OA
Udwantnagar
Bikram
Location of boreholes/VES
Sand
0.25
1
8
Lithofacies (151-200 m bgl)
Clayey sand
Sandy clay
0
25.4
Garhani Sandesh
0
10
20 Water bodies (channel cut-off lakes)
Kilometers
Clay
25.3 83.9
84
84.1
84.2
84.3
84.4
84.5
84.6
84.7
84.8
Figure 5.2: Litho-facies distribution within (c) 101-150 m, and (d) 151-200 m below ground level in the Newer and Older Alluvial deposits in the study area.
129
In the zone between 201-250 m bgl, (Fig 5.2.e) the area covered by ‘sand’ facies decreases to 50.3% of the study area. However, the facies is more confined in the central and western parts of the area around Jagdispur, Shahpur, Brahmpur, Buxar and Chausa, indicating increase in sand content in the sediment towards west. Significant is the fact that the eastern parts around Ara, Sandesh and Karnamepur, which were earlier ‘sand’ facies in the depth range of 151-200 m bgl, now turns to ‘clayey sand’ facies. The ‘clayey sand’ is the other major litho-facies in the depth range, forming 44.3% of the study area in the eastern and north-central parts around Sandesh, Babura, Garhani, Ara, Udwantnagar, Paharpur and Karnamepur. There is a patch of ‘sandy clay’ facies around Basantpur in the north-western parts covering 5.4% of the study area. There is no area of ‘clay’ facies in the depth range of 201-250 m bgl.
25.7 Neazipur Babura
Rajpur kalan
Maner
Semaria
25.6
New Bhojpur
Brahmpur
Buxar
Shahpur Koilwar Ara
25.5
Chausa
Jagdispur
Boundary between NA & OA
Udwantnagar
Location of boreholes/VES 0
Sandesh
0 Sand
Clayey sand
Sandy clay
Clay
10
20
Kilometers
25.3 83.9
84
84.1
84.2
Bikram
Garhani
0.25
8
1
Lithofacies (201-250 m bgl)
25.4
84.3
84.4
84.5
Water bodies (channel cut-off lakes)
84.6
84.7
84.8
Figure 5.2.e: Litho-facies distribution within 201-250 m below ground level in Newer and Older Alluvial deposits in the study area.
5.5 HYDROGEOLOGICAL CLASSIFICATION The sand zones in the alluvium form the potential groundwater repositories in the area. Broadly, the deposits are made up of several fining upward sequences, beginning with coarse sand and gravel at the bottom, grading into fine sand, which is ultimately topped by clay. However, since the basin has remained tectonically affected, certain irregularities are observed, such as the fining upward sequence interrupted by advent of coarser materials. It has resulted in few coarsening upward sequences.
130
Seven lithology types can be identified in the drill-cut samples, viz: (1) fine to very fine sand, (2) fine to medium sand, (3) medium to coarse sand (4) coarse sand with gravels (5) coarse sand with interlayers of gravels, (6) sandy/silty-clay and (7) clay. Hydrogeologically the lithology types can be grouped into three categories, viz: (i) types 1 & 2 form low to moderate potential aquifers, for they possess clay partings at places, (ii) the types 3, 4 & 5 form potential aquifers and (iii) types 6 & 7 form aquitards. 5.5.1 Newer Alluvium The study of the thickness and geometry of different litho-units in the hydrogeological transects revealed that there are clay zones of local extent in the central and western parts of the area. The clay/mud (litho-types 6 & 7) zone forming an aquitard within the depth range of 87 and 139 m bgl is pervasive in the Newer Alluvium. The thickness of the aquitard varies within 8-24 m. The aquitard, hereafter referred as ‘middle clay’ broadly divide the vertical sand sequence into two-tier aquifer systems, particularly in the central and eastern parts of the area; aquifer group I (AG-I) and aquifer group II (AG-II). Brief descriptions of the two aquifer groups are given as below. 5.5.1.1 Aquifer Group I (Shallow aquifer) It ranges from the ground surface down to the depth of 87-126 m bgl. The near-surface fine-grained sequence (lithology groups 6 & 7) contain considerable sand percentage, and thus give the aquifer group I (AG-I) an unconfined character. The aquifer group can be identified with three broadly different assemblages of granular litho-types, particularly in the central and eastern parts of the area (Fig 5.1.a); (1) assemblage-1, dominated by fine to very fine Ganga sand, form low to moderate potential aquifer underlying the mud cover at the top of the sequence (within ~30 m depth), (2) assemblage-2, dominated by coarse Sone sand and gravels (often highly loaded with kankars) with interlayers of fine sand and centimeter scale clay lenses at places, form moderate to high potential aquifer (30-60 m depth range), (3) assemblage-3, the bottom parts (~60m to middle clay) of the aquifer group consisting of mainly medium to coarse Sone sand and at places with lenses of fine to medium sand forms the more potential part of the aquifer group for the entire area. Most of the hand pumps and tube wells in the area remain confined within the assemblage-1 & 2, which are reportedly contaminated with groundwater arsenic in many patches. Both the assemblages are characteristically ‘clayey sand’ in litho-facies in major parts of 131
the Newer Alluvial area (Pettijohn and Randich 1966), except the southcentral parts, where, they form ‘sandy clay’ and ‘clay’ facies, depicting more clay composition (Fig 5.2.a). In the western parts of the area, however, the assemblage-2 is absent in the lithological sequence and the assemblages-1 & 3 are only observed in this part of the area. The assemblage-3 bears less clay, particularly in the eastern and central parts of the Newer Alluvium around Bariswan, Bharauli, Paharpur and Babura, as is evident from its ‘sand’ litho-facies character. In western parts around Brahmpur, Neazipur and Churamanpur and in the southcentral parts around Ratantola, Basantpur, Behea and Shahpur, the assemblage is ‘clayey sand’ in litho-facies character. 5.5.1.2 Aquifer Group II (Deeper aquifer) The aquifer group II (AG-II) starts at various depths ranging from ~116 to 139 m bgl and continues up to the depth of ~250 m below ground. Lying below the middle clay, it is laterally extensive from east to west. Groundwater occurs under semi-confined to confined condition because of low permeability of the middle clay. The overall grain-size of AG-II is coarser and it can be inferred that it would have high hydraulic conductivity. At the eastern and central parts of the area, there lays bottom clay at the base of the AG-II. Thickness of the aquifer group increases westward because of the absence of the bottom clay. The overall grain-size of AG-II is coarser in comparison to AG-I and it can be inferred that it would have high hydraulic conductivity. The aquifer group is broadly uniform in the area, except the variation in sand size at various depths from mediumcoarse to fine-medium at Neazipur (BH-17). Clay content is less and the aquifer group reflects ‘sand’ litho-facies (Fig 5.2.c) within the depth range of 151-200 m bgl in the entire area of Newer Alluvium, except a small patch of ‘sandy clay’ facies in the western parts around Churamanpur. 5.5.2 Older Alluvium The two-tier aquifer system is not discernible in the Older Alluvial area because of different aquifer-aquitard configuration (Fig 5.2.b). Observation of hydrogeological sections C-C’ and D-D’, indicates southward (towards Older Alluvium) continuation of the intervening aquitard zones in the AG-I. The Shahpur borehole records the bottom clay at the depth of 273 m bgl. Between the top clay and the the bottom clay, there are 3-4 clay layers of thickness varying between 3.5 and 15 m, which divide the sand column into different aquifer systems of low to moderate potentiality. Below the depth of ~100 m bgl, the aquifer seems to be reasonably 132
continuous (of >150 m thickness) down to the depth of bottom clay. The lithology of this aquifer system indicates high hydraulic conductivity, where groundwater occurs under semi-confined to confined condition. This aquifer system continues towards north as AG-II in the Newer Alluvial areas. The bottom clay in geologic section-1 continues in Older Alluvium, forming the base of the main aquifer system. 5.6 OCCURRENCE AND MOVEMENT OF GROUND WATER 5.6.1 Seasonal water level regime of shallow aquifer The monthly water level (year 2011) data from the shallow aquifer obtained from the Buxar and Milki indicate that the groundwater system in the plain is strongly influenced by the seasonal variation in the southwest monsoon rainfall (Fig 5.3). 840 00'
0
25 41 '
840 15'
0
84
30 '
840 45 '
250 41 '
Milki
Buxar
250 24'
0
10
250 24'
20
kilometers 840 00 '
Water bodies (channel cut-off lakes) 0 84 15'
(a)
Figure: Location
map
0 84 30'
0 84 45 '
5.3:
(a)
of
well
hydrograph stations at Buxar and Milki in the study area. (b) Hydrographs based on monthly water levels from Buxar and Milki,
plotted
against
the
(b)
monthly rainfall of 2011 (in mm) in the respective blocks.
In the well hydrographs, shallowest depth to water levels (DTWs) has been observed in the month of August. After August, there is a continuous fall of water level till the onset of next monsoon. But the rate of water level recession is not uniform (Fig 5.3). The slope of the recession limb in both the hydrographs is comparatively steep between the months of August and November (0.18 m/month for Milki and 0.30 m/month for Buxar), indicating rapid discharge from the aquifer. The ground water pumping is minimum during this period because of less demand for irrigation due to high soil moisture condition (CGWB 1984; CGWB 1997). But the lowering of water level in this period is due to considerable seepage from aquifer towards the streams draining the area. About 50 percent of the monsoon recharge is lost between August and November. Between November and June the slope of the 133
recession limb is gentle (0.13 m/month for Milki and 0.06 m/month for Buxar). No rise in water levels has been observed in any measurements in non-monsoon months, indicating no significant recharge from non-monsoon rainfall. To study the spatial and seasonal behaviour of groundwater levels, 67 dug wells were monitored during the study (Table 5.2). The wells represent different morphostratigraphic units and cover all the community development blocks. Two sets of water levels were taken; during May 2010 (pre-monsoon) and November 2010 (post-monsoon). The pre-monsoon measurements were taken in the last week of the month, whereas the post-monsoon water levels were collected during the first week of November. Wherever the wells were under regular pumping, the measurements were taken in morning, before the commencement of pumping for the day. It was also ensured that no other pumps were under operation in the vicinity during the measurements. However, the year coincided with a drought period continuing from the year 2009 in the state of Bihar (Fig 5.4) with at least 22% deficiency of rainfall from the normal precipitation (IMD 2010).
Annual norm al/ac tual rainfall (m m )
1400 1200 1000 800 600 400 200
10 20
20
09
08 20
07 20
06 20
05 20
04 20
03 20
02 20
20
01
0
Ye a r Normal-Bhojpur Normal-Bux ar
A c tual-Bhojpur A c tual-Bux ar
Figure 5.4: Plot displaying last ten years rainfall in Bhojpur and Buxar districts in the study area and their variation from the normal rainfall in the districts.
134
Table 5.2 (a): Details of Key wells of water level measurement from shallow aquifer in both the newer and older alluvium. Location
District
Latitude
Longitude
RL (m amsl)
MP (m)
DW-1 DW-2
Kulharia * Chandwa*
Bhojpur do
25.5777 25.5577
84.7618 84.6263
57 60
0.70 0.70
DW-3
Milki*
do
25.6739
84.6379
55
0.65
DW-4
Udwant nagar*
do
25.5014
84.6235
63
0.75
DW-5
Naya Haripur
do
25.6418
84.7916
52
0.80
DW-6
Babura
do
25.6739
84.7888
51
0.80
DW-7
Kusihan
do
25.5048
84.7388
60
0.70
DW-8
Sandesh
do
25.4155
84.7484
65
0.85
DW-9
Amrahi Nawada
do
25.5794
84.4822
58
1.00
DW-10
Bharauli
do
25.6332
84.3861
56
1.15
DW-11
Karnamepur
do
25.6514
84.3765
55
0.85
DW-12
Jagdispur
do
25.4649
84.4216
67
NA
DW-13
Nargada
do
25.602
84.4409
57.5
0.90
DW-13’
Garhani*
do
25.423
84.5540
66
0.65
DW-14
Neazipur*
Buxar
25.6765
84.1468
58
NA
DW-15
Asha Pahari
do
25.6557
84.1554
59
1.00
DW-16
Barkagaon
do
25.6245
84.0728
60
0.45
DW-17
Sahu Para
do
25.5942
84.0363
62
1.25
DW-18
Dumraon*
do
25.5863
84.1458
64
0.60
DW-19
Brahmpur*
do
25.6063
84.3073
60
0.46
DW-20
Buxar*
do
25.5829
83.9969
63
0.60
DW-21
Chausa*
do
25.5074
83.9036
64
0.85
DW-22
Semaria
Bhojpur
25.6151
84.4290
57
0.60
DW-23
Benwalia
do
25.6392
84.4974
57.5
0.85
DW-24
Sharaur
do
25.6047
84.4683
57
0.35
DW-25
Betauthi
do
25.5968
84.4353
58.8
0.74
DW-26
Dhobaha
do
25.6092
84.6006
54.5
1.00
DW-27
Shahpur
do
25.6008
84.4032
56.9
0.45
DW-28
Raghunathpur
do
25.5586
84.3095
60.5
1.25
DW-29
Bariswan
do
25.6310
84.4397
57.5
0.55
DW-30
Dallupur
do
25.6309
84.3190
57.7
0.65
DW-31
Nimej
do
25.6158
84.2943
59.1
0.96
DW-32
Barki Nainijor
do
25.7048
84.3528
58.0
0.80
DW-33
Shivpur
do
25.4797
84.3796
65.8
0.35
DW-34
Bhadwar
do
25.4818
84.3393
67.2
0.66
DW-35
Dalipur
do
25.4190
84.3920
69.6
0.76
DW-36
Tenduni
do
25.4310
84.4175
70.0
1.55
Sl.No
135
DW-37
Hetampur
do
25.5033
84.4012
62.3
0.58
DW-38
Mahuaon
do
25.5360
84.4022
60.1
0.76
DW-39
Kaithi
do
25.5520
84.2817
61.7
0.80
DW-40
Rajpur Kalan
Buxar
25.6753
84.0859
60.8
0.35
DW-41
Dullahpur
do
25.6271
84.0925
62.1
0.90
DW-42
Boksa
do
25.5401
84.0440
63.4
0.45
DW-43
Dumri
do
25.6249
84.1883
59.1
0.74
DW-44
Simri
do
25.6417
84.1143
63.8
1.10
DW-45
Baruna
do
25.5665
84.0752
68.9
0.87
DW-46
Rajapur
do
25.6761
84.1733
61.0
0.77
DW-47
Kamarpur
do
25.8417
83.9383
63.0
1.45
DW-48
Channi
do
25.4966
83.9248
62.2
1.22
DW-49
Mahdah
do
25.5277
84.0200
61.4
0.80
DW-50
Danwan
Bhojpur
25.5306
84.4620
60.4
0.65
DW-51
Kaunra
do
25.5219
84.5580
59.6
0.50
DW-52
Kasap
do
25.4681
84.6101
63.8
0.55
DW-53
Bagwan
do
25.4206
84.5836
63.3
0.85
DW-54
Khopira
do
25.4213
84.6508
62.6
0.44
DW-55
Phulari
do
25.3640
84.6942
65.9
0.77
DW-56
Bichhiaon
do
25.4408
84.7213
62.8
0.67
DW-57
Sarthua
do
25.4741
84.6695
60.6
0.56
DW-58
Karwa
do
25.5181
84.6880
60.5
0.40
DW-59
Chandi
do
25.5354
84.7465
59.9
0.30
DW-60
Bakhorapur
do
25.6676
84.6883
55.0
1.35
DW-61
Semaria
do
25.6712
84.7272
56.0
0.88
DW-62
Balua
do
25.6572
84.5756
55.7
0.60
DW-63
Basantpur
do
25.6386
84.5928
56.9
0.45
DW-64
Birampur
do
25.6057
84.7281
55.8
0.80
DW-65
Saraiya
do
25.6465
84.6362
54.0
0.45
DW-66
Baligaon
do
25.4162
84.4993
66.5
0.90
DW-67
Gajrajganj
do
25.5690
84.5267
57.4
0.48
*Network stations of CGWB for water level measurement.
136
DW-1 DW-2 DW-3 DW-4 DW-5 DW-6 DW-7 DW-8 DW-9 DW-10 DW-11 DW-12 DW-13 DW-13’ DW-14 DW-15 DW-16 DW-17 DW-18 DW-19
Sl.No
Depth to water level 2010 Drought year (m bgl) PrePostmonsoon monsoon 7.4 7.35 4.5 3.6 6.95 5.35 2.6 3.1 6.55 6.47 7.3 5.47 7.8 7.2 7.65 6.05 4.5 4.4 5.65 5.42 4.45 5 7.02 5.8 5.8 5 7.15 6.28 6.6 7.2 8.55 7.85 7.2 7.3 8.76 9.3 4.79 5.19 0.87 -0.6 0.7 -0.1 -0.54 -0.4
0.05 0.9 1.6 -0.5 0.08 1.83 0.6 1.6 0.1 0.23 -0.55 1.22 0.8
Fluctuation (m)
50.85 52.40 51.45 54.80 55.24 55.21
49.60 55.50 48.05 60.40 45.45 43.70 52.20 57.35 53.50 50.35 50.55 59.98 51.70
Water tableMay (m amsl)
51.72 51.80 52.15 54.70 54.70 54.81
49.65 56.40 49.65 59.90 45.53 45.53 52.80 58.95 53.60 50.58 50.00 61.20 52.50
Water tableNov (m amsl)
Depth to water level 2002 Monsoon year (m bgl) PrePostMonsoon monsoon 6.43 5.50 5.35 3.42 5.76 4.74 2.34 1.75 5.35 2.95 4.55 3.29 6.19 5.01 8.67 7.50 4.59 3.80
Table 5.2 (b): Details of water levels measured from shallow aquifer in both the newer and older alluvium.
-
-
-
1.17 0.79
1.26 1.18
2.40
0.93 1.93 1.02 0.59
Fuctuation (m)
-
-
-
55.33 55.41
61.45 51.81
61.65
50.57 54.65 49.24 60.66
Water tableMay (m amsl)
-
-
-
137
56.50 56.20
62.71 52.99
64.05
51.50 56.58 50.26 61.25
Water tableNov (m amsl)
DW-20 DW-21 DW-22 DW-23 DW-24 DW-25 DW-26 DW-27 DW-28 DW-29 DW-30 DW-31 DW-32 DW-33 DW-34 DW-35 DW-36 DW-37 DW-38 DW-39 DW-40 DW-41 DW-42 DW-43 DW-44 DW-45
2.4 1.62 5.25 4.78 4.85 5.1 4.5 5.3 5.6 4.92 5.4 5.35 4.95 5.74 5.9 6.93 7.31 5.11 4.55 6.25 7.14 7.85 6.75 7.48 7.59 8.36
3.57 3.3 5.75 5.8 5.45 5.85 5.55 5.7 5.25 5.85 4.66 4.72 4.45 7.36 7.48
8.7 9.2 6.47 6.3 6.46 8.25 8.43 6.88 6.27 7.94 8.22
1.17 1.68 0.5 1.02 0.6 0.75 1.05 0.4 -0.35 0.93 -0.74 -0.63 -0.5 1.62 1.58 1.77 1.89 1.36 1.75 0.21 1.11 0.58 0.13 -1.21 0.35 -0.14 59.43 60.70 51.25 51.70 51.55 52.95 48.95 51.20 55.25 51.65 53.04 54.38 53.55 58.44 59.72 60.90 60.80 55.83 53.80 55.24 55.25 56.45 61.10 59.75 58.70 59.58
60.60 62.38 51.75 52.72 52.15 53.70 50.00 51.60 54.90 52.58 52.30 53.75 53.05 60.06 61.30 62.67 62.69 57.19 55.55 55.45 55.98 55.99 60.35 60.10 58.75 60.72
5.35 4.95 -
2.35 3.10 -
3.00 1.85 -
57.65 59.05 -
138
60.65 60.90
DW-46 DW-47 DW-48 DW-49 DW-50 DW-51 DW-52 DW-53 DW-54 DW-55 DW-56 DW-57 DW-58 DW-59 DW-60 DW-61 DW-62 DW-63 DW-64 DW-65 DW-66 DW-67
6.33 3.42 3.52 5.66 5.15 3.15 2.7 3.55 3.9 6.32 7.1 3.95 6.22 7.88 7.37 7.68 6.45 6.27 7.7 6.5 5.15 4.1
6.5 1.81 1.95 5.18 4.42 3.61 3.45 3.2 3.85 5.18 5.78 3.92 5.8 7.35 5.61 5.88 5.38 5.26 6.95 5.32 3.68 4.32
-0.17 1.61 1.57 0.48 0.73 -0.46 -0.75 0.35 0.05 1.14 1.32 0.03 0.42 0.53 1.76 1.8 1.07 1.01 0.75 1.18 1.47 -0.22 55.70 56.65 54.28 52.02 47.63 48.32 49.25 50.63 48.10 47.50 61.35 53.30 52.55 53.67 56.52 52.83 55.86 60.68 54.67 59.58 58.68 55.74
57.02 56.68 54.70 52.55 49.39 50.12 50.32 51.64 48.85 48.68 62.82 53.08 53.66 54.25 56.65 51.62 56.21 60.54 54.50 61.19 60.25 56.22
-
-
-
-
139
In the hydrological year 2010, the DTWs map (Fig 5.5. a & b) indicate that the pre-monsoon water levels ranged between 2.6 m bgl and 8.76 m bgl. The shallow water levels (2.6-3.42 m bgl) are observed in two stretches, falling in the Older Alluvium (Mohanpur Formation); one around Udwantnagar and Garhani at the southeastern parts of the area and the second one falls around Buxar and Chausa (DW-20 & 21), located close to the Ganga at the western parts of the area. Water levels of moderate depth (3.52-5.45 m bgl) cover the central parts of the area around Shahpur, Semaria, Brahmpur and Ara, falling in parts of both the Mohanpur Formation and Fatwa Formation (Newer Alluvium). The deeper water levels (5.558.76 m bgl) are marked in two linear stretches; (1) around Jagdispur, New Bhojpur and Rajpur Kalan along SE-NW at the western parts of the area covering parts of both the Mohanpur Formation and Fatwa Formation, (2) along the Sone River up to its confluence with the Ganga at Babura in N-S direction. The water levels during post-monsoon period varied between 1.62 m bgl and 9.3 m bgl (Fig 5.5.b). The distribution pattern of shallow and deeper water levels remained the same as in the pre-monsoon period. Most of the key wells recorded nominal rise varying from 0.03 m to 1.83 m only due to scanty rainfall in the year. Further decline in water level (range: -0.1 to -1.21) during the period was observed in two patches; one around Udwantnagar and other around Brahmpur and New Bhojpur (Fig 5.5.c).
(a) 25.7
Neazipur Babura
Rajpur kalan
Semaria
25.6
New Bhojpur Brahmpur Buxar
Shahpur Koilwar Ara
25.5
Chausa
Jagdispur
Udwantnagar
Boundary between NA & OA
Garhani
Location of key wells
25.4
25.3
2.5
4.0
5.5
7.0
8.5
Index
Sandesh
0
Pre-monsoon depth to water level map in m bgl (May 2010) 83.9
84
84.1
84.2
84.3
10
20 Water bodies (channel cut-off lakes)
Kilometers 84.4
84.5
84.6
84.7
84.8
Figure 5.5: (a) Pre-monsoon depth to water level contours (shallow aquifer) for the year 2010 with sub-normal rainfall in the study area.
140
(b)
25.7
Neazipur Babura
Rajpur kalan
Semaria
25.6
New Bhojpur Brahmpur Buxar
Shahpur Koilwar Ara
25.5
Chausa
Jagdispur
Udwantnagar
Boundary between NA & OA
Garhani
Location of key wells
25.4
25.3
1.5
3.0
4.5
6.0
7.5
9.0
Index
Sandesh
0
Post-monsoon depth to water level map in m bgl (Nov 2010) 83.9
84
84.1
84.2
84.3
10
20 Water bodies (channel cut-off lakes)
Kilometers 84.4
84.5
84.6
84.7
84.8
(c) 25.7
Neazipur Babura
Rajpur kalan
Semaria
25.6
New Bhojpur Brahmpur Buxar
Shahpur Koilwar Ara
25.5
Chausa
Jagdispur
Udwantnagar
Boundary between NA & OA
Garhani
Location of key wells
25.4
25.3
-1.2
-0.6
0.0
0.6
1.2
1.8
Index
Sandesh
0
Annual groundwater level fluctuation map in m (Year 2010)
83.9
84
84.1
84.2
84.3
10
20 Water bodies (channel cut-off lakes)
Kilometers 84.4
84.5
84.6
84.7
84.8
Figure 5.5: Contd. (b) Post-monsoon depth to water level contours (shallow aquifer). (c) Annual water level fluctuation contour map (shallow aquifer) of the study area for the year 2010 with sub-normal rainfall.
To gain an idea of normal water level regime in the area, the water level data of hydrograph stations for the year 2002 (CGWB 2002) have been utilized for study. The rainfall in this year and the years preceding and succeeding it were largely normal (Fig 5.4). The pre-monsoon, post-monsoon depth to water level maps and the annual fluctuation map have been prepared (Fig 5.6. a, b & c). Broadly, the pattern of distribution of shallow and deeper water levels is similar to the drought affected year. The pre-monsoon depth to water level varied in the range of 2.34-8.67 m bgl, which is close to the range as observed during the year 2010. However, the post-monsoon water level (range: 1.72-7.50 m bgl) of the year
141
2002 indicates more recharge during the year in comparison to the year 2010 during which the water level was quite depleted due to scanty rainfall.
(a) Babura
Neazipur
25.65
Semaria
New Bhojpur Buxar
25.55
Brahmpur
Shahpur
Koilwar Ara Udwantnagar
Jagdispur
25.45 Garhani Water bodies (channel cut-off lakes)
Sandesh
Boundary between NA & OA
0
10
2.0
3.0
4.0
5.0
6.0
Location of key wells
7.0
8.0
25.35
Index
20 Pre-monsoon depth to water level contour in m bgl (May 2002)
Kilometers
25.25 83.9
84
84.1
84.2
84.3
84.4
84.5
84.6
84.7
84.8
(b) Babura
Neazipur
25.65
Brahmpur New Bhojpur
Semaria Shahpur Koilwar
Buxar
25.55
Ara
Chausa Udwantnagar
25.45
Jagdispur
Garhani
Water bodies (channel cut-off lakes)
Boundary between NA & OA
0
10
1.6
2.6
3.6
Post-monsoon depth to water level contour in m bgl (Nov 2002)
25.25 84
4.6
7.6
20
Kilometers
83.9
5.6
Index
Location of kew wells
6.6
25.35
Sandesh
84.1
84.2
84.3
84.4
84.5
84.6
84.7
84.8
Figure 5.6: (a) Pre- and (b) post-monsoon depth to water level contours (shallow aquifer) for the year 2002 with normal rainfall in the study area.
142
Babura
Neazipur
25.65
Brahmpur New Bhojpur
Semaria Shahpur
Buxar
25.55
Koilwar Ara
Chausa Udwantnagar
25.45
Jagdispur
Garhani
Water bodies (channel cut-off lakes)
Boundary between NA & OA
0
10
0.6
1.1
2.6
1.6
Index
Location of kew wells
2.1
25.35
Sandesh
20
Kilometers
Annual water level fluctuation map in m (Year- 2002)
25.25 83.9
84
84.1
84.2
84.3
84.4
84.5
84.6
84.7
84.8
Figure 5.6: Contd. (c) Annual water level fluctuation contour map (shallow aquifer) of the study area for 2002 with normal rainfall.
The depth to water levels record an annual fluctuation in the range of 0.59-3.0 m, depicting rise in the water level regime in the entire area (Fig 5.6.c), in contrary to the fall as registered during 2010 (Fig 5.5.c). The rise is 500 µg/L respectively. Among the tested sources with
