J Radioanal Nucl Chem (2014) 302:1391–1398 DOI 10.1007/s10967-014-3566-3
Assessment of environmental radioactive elements in groundwater in parts of Nalgonda district, Andhra Pradesh, South India using scintillation detection methods Tirumalesh Keesari • Hemant V. Mohokar Bijay Kumar Sahoo • G. Mallesh
•
Received: 27 August 2014 / Published online: 11 September 2014 Ó Akade´miai Kiado´, Budapest, Hungary 2014
Abstract The objective of this work was to determine the radioactive element concentrations in groundwater in parts of the Nalgonda district. Results indicate that 222Rn activity is present in significant levels in deep groundwater compared to shallow groundwater and tank water. An increasing 222Rn activity trend is noticed along the well depth while electrical conductivity, uranium, and alkalinity levels showed inverse trends. Environmental tritium data indicates modern recharge to groundwater. Inter-elemental correlations suggest that high dissolved uranium is associated with high alkalinity and high electrical conductivity groundwater. The study also infers recharge sources and mechanisms to shallow and deep groundwater. Keywords Uranium Radon Nalgonda Inter-elemental correlations Groundwater Liquid scintillation Introduction Groundwater resources play a major role in ensuring livelihood security across the world, especially in economies that T. Keesari (&) H. V. Mohokar Isotope Applications & Radiopharmaceuticals Division, Bhabha Atomic Research Centre, Trombay, Mumbai, India e-mail:
[email protected];
[email protected] Present Address: T. Keesari US Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025, USA B. K. Sahoo Radiological Physics & Advisory Division, Bhabha Atomic Research Centre, Trombay, Mumbai, India G. Mallesh DHAN Foundation, Hyderabad, Andhra Pradesh, India
depend on agriculture. Most parts of the south Indian peninsula are prone to droughts because of the specific geoclimatic setting, such as semi-arid climate, hard-rock aquifers, high evapotranspiration, high aridity, and variable rainfall inputs. This situation makes communities directly affected by climatic variability and therefore demands a sustainable water management policy. Despite efforts by agencies, the water issues still stand unresolved due to ever increasing stress on water in terms of quantity and quality. The Nalgonda district of Andhra Pradesh falls in a semiarid climate and the groundwater is mostly confined to hard rock aquifers, which are fractured rock aquifers covered by crystalline basement complex, metamorphic rocks, and extensive intrusive granites. Most of the water supply for irrigation and drinking is met by groundwater in this district. The most compelling issues of this district in terms of groundwater usage are, the alarming rate at which groundwater levels are dropping and the very poor chemical quality, which causes hardship to the communities, especially from rural areas. However, recent studies in India have shown that uranium is also a cause of great concern as in certain parts of India, where groundwater is found to be contaminated by high levels of uranium [1–3]. Uranium levels close to mg l-1 are reported in parts of Punjab state [4]. High levels of uranium in groundwater are also reported in areas near ore formations, hard rock and basement aquifers [5]. Uranium deposits are reported in some parts of this district by Central Ground Water Board [6]. The health effects of uranium can be divided into carcinogenic and non-carcinogenic effects [7] and these classifications are based on the radiological risk by radiation of uranium isotopes and the chemical risk as a heavy metal. For ingested uranium, the main target organ of toxicity is the kidney [7]. It has been reported that an exposure of about 0.1 mgkg-1 of body weight of soluble natural uranium results in transient chemical damages to kidney.
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Apart from uranium, radon is also reported by many researchers as a common groundwater contaminant in Indian, especially from hard rock aquifers [8]. Radon-222 (222Rn) is a noble gas with half life of 3.82 days and it is reportedly the second leading risk factor for lung cancer. 222 Rn is produced from 226Ra as part of the decay series of 238 U. The concentration of Ra in aquifer matrix is several orders of magnitude higher than the dissolved Ra concentrations in surface water; hence groundwater 222Rn activities are commonly two or three orders of magnitude higher than those of surface water. In addition to contamination aspects, both U and Rn are commonly employed by researchers to investigate geochemical processes, oxic or anoxic conditions of aquifer, subsurface groundwater discharge, delineation of fissure and faults, and as a paleoredox indicator [9, 10]. Tritium is a radioisotope of hydrogen with a half-life of 12.43 years and a useful tracer for differentiating old groundwater from current meteoric water. The background (pre-atomic bomb) levels of cosmogenic tritium have been determined to be 5–10 TU (tritium unit = one tritium atom per 1018 protium atoms) in the troposphere; however, thermonuclear testing has produced a peak concentration of tritium of 10,000 TU in precipitation of the northern hemisphere. Subsequent to the ban on nuclear testing, the tritium concentration in precipitation has significantly dropped and returned to background levels. The use of 3H dating of groundwater has been based primarily on models of the long-term trend of 3H content in hydrological systems [11]. To our knowledge there is only limited data available in this district with regard to occurrence of radioactive elements. With this background, an environmental assessment study was carried out in parts of Nalgonda district in order to (i) determine on uranium (total), 222Rn and 3H along with other water quality parameters in different water bearing zones, (ii) assess the vertical distribution and (iii) evaluate the geochemical processes and dynamics controlling the radioisotope availability in groundwater. In addition to serving as a data bank of environmental radioisotopes, this data can also be helpful to water agencies in formulating the safe water usage.
Materials and methods Study site description The study area falls in the Chityal Mandal of the Nalgonda district covering an area of about 50 sq. km between latitudes 17.10° to 17.30° and longitudes 77.50° to 79.60°. The average rainfall is about 700 mm per annum and mostly received during southwest monsoon that occurs from June
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Fig. 1 Location map of the a study area and b sampling points
to September. Since the rainfall is mostly confined to a few intense rain events lasting only for 20–30 days in a year, most of the irrigation requirements are met by groundwater supplies. A few tanks are also located in the study area. These tanks are basically depressions present in the undulating topography and play a major role in confining the surface run-off and enhancing the groundwater recharge in shallow zones. Water from the tanks is also used for irrigation. Ninety percent of the study area is occupied by Archean crystalline rocks, which comprise granites, gneisses, schists and intrusives. The consolidated metasedimentary rocks comprising limestones, quartzites and shales occupy 9 % in the southern part of the district. Unconsolidated deposits comprising alluvial sands and clay occur as isolated and narrow patches along the major rivers and streams, occupying around 1 % of the area [12]. The crystalline rocks are inherently less porous. However, with weathering, the rocks undergo fracturing, fissuring, and jointing over a period of time resulting in secondary porosity, which forms the repository for groundwater. The groundwater occurs under water table conditions in the weathered zone and semi-confined and confined conditions in fractured zone [6].
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Sample collection Most of the samples were collected in and around two villages, Gundrampalli and Aipoor of Chityal Mandal, shown in Fig. 1a. A total of forty water samples were collected from bore wells (BW, 22 nos), dug wells (DW, 4 nos), hand pumps (HP, 5 nos) and tanks (9). The sampling points are shown in Fig. 1b. BWs tap groundwater from depths ranging from 45 to 100 m below ground level (bgl) and represent the deep groundwater. Dug wells and hand pumps range from depths 18 to 45 m bgl and represent shallow groundwater. Tanks are generally 2–5 m deep depending on topographic relief. A water depth of 3 m bgl was taken for data interpretation. Analytical methods The collected water samples were filtered through 0.45 lm filter paper, acidified with 0.01 M nitric acid and stored in 100 ml pre-washed Tarson bottles. Water samples were measured in situ for physico-chemical parameters such as electrical conductivity (EC), pH, temperature and dissolved oxygen (DO) using portable instruments (Orion make). Alkalinity was measured by titration. A few representative samples were measured for dissolved radon using an indigenously developed Smart Radon Monitor (SRM). The radon measurements in SRM are based on detection of alpha particles emitted from radon and its decay products, formed inside a scintillation cell volume [13]. Water samples were collected in leak tight glass bottles (40 ml). While sampling caution was taken to avoid formation of bubbles in the water. Subsequent to sampling, measurements were carried out using the SRM by following the bubbling technique [14]. The radon concentration in water (Cw) from the measured concentration (Cair) with SRM was estimated using following equation Vair Cw ¼ Cair k þ ð1Þ Vw where, Vair and Vw denote air and water volume respectively, k is the distribution factor of radon concentration between water and air. Typical value of k is 0.2 which can be neglected in comparison to Vair/Vw (5 in this case). The method detection limit is 14 Bqm-3 for 1 h counting (95 % confidence) and upper limit is 50 MBq m-3. The operation and calibration was done as described by Gaware et al. (2011) [15]. For tritium measurement, 500 ml water samples were collected in airtight polyethylene bottle. 250 ml of distilled sample was electrolytically enriched at a low temperature of about 1–4 °C and sample—scintillator mixture (8:12 ml) taken in a 20 ml polythene vial was counted in an ultra-low background (0.5 cpm) liquid scintillation
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counter (Quantulus model 1220). The 3H values are expressed in tritium unit (TU). One TU of sample has 3 H/1H ratio equals to 1/1018, which corresponds to 0.12 Bq/kg of water. The minimum detection limit for this method is 0.5 TU (2r) for 500 min counting. The counting efficiency and the calibration factor of the counter were about 25 % and 70 TU/cpm respectively [16]. Uranium was measured using ICP-AES with a measurement error better than 10 %.
Results and discussion Analytical results for physicochemical parameters, uranium, radon and tritium, and the well depths of samples are given in Table 1. Water quality Physico-chemical parameters show wide variation in electrical conductivity in various sources. BW samples have EC in the range of 400–2,870 lScm-1 and a similar range is also shown by DW&HP water samples (580–2,980 lScm-1) while tank water shows lower range of EC: 80–920 lScm-1. About 25 % of the samples measured showed EC [ 1,500 lScm-1, indicating brackish nature of groundwater. The alkalinity values range from 150 to 490, 240 to 575 and 130 to 225 mgl-1 for BW, DW&HP, and tank water respectively. The uranium concentration ranges from 7 to 370 lgl-1 in BW samples while in DW&HP samples it ranges from 3 to 296 lgl-1. About 30 % of the total samples found to exceed the permissible limit of 60 lgl-1 set by AERB [17] and about 80 % of the samples fall above WHO limit of 15 lgl-1 [18]. On the other hand, radon levels measured in selected samples indicate 9.6–31.5 Bql-1 in BW while the same in DW&HP is 4.5–20.5 Bql-1. About 15 % of the total samples showed higher radon levels as compared to USEPA standard of 11 Bql-1. However it can be observed that most of the BW samples showed elevated radon levels (Table 1). In the case of tanks, water samples showed low levels of uranium and radon ranging from 16 to 75 lgl-1 and 0.1 to 0.7 Bgl-1 respectively. Box and Whisker plots were used to understand the distribution of the above data sets. A clear decrease in median values of EC is noted from BW to DW&HP to tank water, however the spread is greater in the case of HP&DW samples (Fig. 2a). In the case of alkalinity the HP&DW showed higher median levels compared to BW samples and followed by tank water (Fig. 2b). It can be observed that the median value for uranium remains more or less same in all the samples indicating a common source. The spread in HP&DW samples compared to other two sources probably
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Table 1 Environmental isotope data along with physico-chemical parameters of water samples collected from Nalgonda district Depth Meter
Temp. °C
PH
DO mg l-1
Alkalinity mg l-1
E.C. lS cm-1
Uranium lg l-1
222
48.8
35.24
7.45
2.52
490
2,879
370
16.1
54.9
34.67
7.12
2.15
61.0
33.66
7.32
1.96
30.06
7.54
3.31
Rn Bq l-1
S.no.
Well owner
Source Unit
1
M. Sathireddy
BW
2
Mohan Reddy
BW
3
S. Yadayya
BW
4
A. Timnecalaiah
BW
70.1
5
G. Shankarayya
BW
61.0
28.3
7.5
4.7
649
6
G. Yadayya
BW
100.6
28.7
7.28
3.4
240
423
7 8
P. Lingareddy B. Yadayya
BW BW
61.0 91.5
29.4 29.1
7.3 7.3
3.3 3.9
410 375
930 837
140 62
18.5
9
Veera Swamy
BW
70.1
27.97
7.24
2.5
330
788
10
10
B. Yadayya
BW
54.9
28.9
7.3
3.2
350
837
47
11
M. Ramulu
BW
61.0
28.9
7.5
3.7
340
1,684
41
12
A. Yadayya
BW
51.8
28.1
7.2
3.5
155
2,299
50
13
Velimedu town
BW
70.1
28.12
7.3
3.5
280
1,562
33
14
M. Lingaiah
BW
61.0
27.9
7.4
3.8
300
1,108
45
15
P. Shivana
BW
45.7
27.8
9.04
7.8
150
448
41
16
O. Sathaiah
BW
45.7
30.06
7.6
4.2
1,347
12
17
S. Maraiah
BW
54.9
29.33
7.28
3.2
1,687
204
18
C. Padarareddy
BW
61.0
27.66
7.25
4.29
644
280
19
B. Rajireddy
BW
45.7
28.81
7.39
4.11
970
72
20
M. Anjaiah
BW
48.8
28.9
7.39
4.75
696
36
21
V. Raj Reddy
BW
61.0
29.69
7.55
4.01
831
20
22 23
S.Palli Town U. P. School
BW HP
54.9 30.5
29.26
7.26
4.3
757
58 17
24
SC COLONY
HP
18.3
28.94
7.27
1.75
475
2,982
13
25
Perepalli
HP
18.3
29.43
7.22
1.47
575
1,926
296
20.5
26
G. Shankarayya
HP
45.7
28.5
7.2
4.56
667
13
13.6
27
Karinga Swamy
HP
30.5
28.29
7.15
4.74
1,868
94
28
V. Egayya
DW
18.3
33.19
7.45
2.95
29
E. C. Reddy
DW
27.4
33.46
7.18
1.81
30
S. Palli town
DW
21.3
28.1
7.33
4.13
31
M. Bhankaraiah
DW
24.4
29.02
7.29
4.85
32
Vittala Cheruvu
Tank
3.0
33.64
8.93
5.68
33
Vittala Cheruvu
Tank
3.0
29.26
8.65
4.81
34
OoraKunta
Tank
3.0
28.83
9.25
8.02
200
35
SC COLONY
Tank
3.0
29.14
7.95
4.08
225
36
Iddaoni cheruvu
Tank
3.0
30.21
7.84
2.53
130
691
16
37
Gudetivari Kunta
Tank
3.0
27.8
7.7
2.8
83
72
0.7
38 39
Bussuvani Kunta Vayilla Kunta
Tank Tank
3.0 3.0
27.9
7.62
0.8
190 175
668
48 36
0.1 0.43
40
Manikunta
Tank
3.0
30.24
8.36
7.12
744
75
1,759 300
295
1,071
18
350
425
76.1
780
3
H T.U.
21 14.2
6.5
7
9.6
5.0
13
31.5
6.0
7.0
6.5 9.8
5.8 5.5
6.0
637
118
1,469
230
4.3
240
581
3
696
45
175
499
47
0.2
549
44
0.33
925
24
0.33
422
55
6.5 6.9 7.0
6.5
EC electrical conductivity, DO dissolved oxygen
indicates other processes influencing the uranium concentration (Fig. 2c). On the contrary, radon levels show a clear decreasing trend from BW to HP&DW to tank water (Fig. 2d).
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Spatial variations It can be noted from the contours that large parts of the study area have lower levels of each parameter (Table 1).
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Fig. 2 Box and Whiskar plots of a electrical conductivity (lScm-1), b alkalinity (mgl-1), c uranium (lgl-1) and d radon (Bql-1)
In the case of alkalinity, high values are noted in the northwest part of the study area, suggesting high partial pressure of CO2 in groundwaters. High alkalinity might be derived from rock weathering or from root zone activities or both. Not much variation is noticed in the case of EC of groundwaters in this region. Small pockets of high fluctuations are observed in the case of alkalinity and U. In the case of Rn and U it can be observed that southeast regions show high uranium but low radon levels. This can be attributed the amount of radium in the vicinity of the groundwater. Radon backgrounds also depend on the emanation efficiency of the solid and geology of the aquifer material. In general, water and soil from granitic regions usually contain more radon than water and soil from sediments such as sandstone; and subsurface water contains more radon than surface water. Uranium is more abundant in oxygen rich groundwaters, which gradually decreases with the lowering dissolved oxygen values.
The inverse trend noted in U and Rn spatial distribution indicates either uranium mineralisation or accumulation of Ra due to earlier geochemical processes such as, leaching, transport and redeposition. It is reported that groundwater in parts of Karnataka showed elevated radon levels [8] especially from granitic terranes. Similar observations were reported by researchers in other granitic regions [19]. Vertical distributions Depth profiles of EC, alkalinity, uranium and radon are presented in Fig. 3a–d. The data points don’t show a systematic correlation with depth in all the plots. However, a general pattern can be discerned, with most of the data points falling on a particular trend. In the case of EC (Fig. 3a), data points show decrease in EC with depth, excepting few samples (shown in box) which are mostly tank water and near surface groundwater. This inverse trend indicates that groundwater is basically a mixture of
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Fig. 3 Depth-wise variation of a electrical conductivity (lScm-1), b alkalinity (mgl-1), c uranium (lgl-1) and d radon (Bql-1), dotted arrows indicate trend line
deep freshwater flowing through fractured hard rocks and high mineral content shallow water. It’s possible that leaching of salts from the soil zone, contribution from evaporated surface waters, atmospheric deposition might lead to high EC in shallow zone water, while faster circulation of deeper waters due to the fractured nature of the rocks allows only limited water–rock interaction and hence low EC. Alkalinity also shows a decreasing trend with depth (Fig. 3b) again indicating presence of soil CO2 in the root zone. It is reported that partial pressure of CO2 in the root zone is typically, 100 times higher than atmosphere. Again in this case, the tank water shows low alkalinity, which is mostly derived from dissolution of atmospheric CO2 with a minor contribution from root zone respiration. Similar trends are noted for uranium. Excepting tank water and a few near surface groundwaters, most of the samples
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show decrease in uranium concentration with depth. Two different trends are noticed in the plot (Fig. 3c), which could be attributed to varying levels of dissolved oxygen (Table 1). Contrary to above trends, radon shows a positive correlation with depth (Fig. 3d). Radon is a daughter product of radium. Even though Ra is highly immobile in the surficial environment, the law of dynamic equilibrium demands that some of it pass into the solution. Since it has a relatively long half-life (1622 years), it can migrate long distances and eventually accumulate on rock surfaces by adsorption–desorption mechanisms. While most of this Ra is adsorbed on surfaces at any one time, the Rn emitted enters the water phase easily. Radon will not diffuse in water beyond small distances, typically 8 m [20], so its escape from deep groundwater is not significant and can be
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Fig. 4 Correlation between uranium (lgl-1) and a electrical conductivity (lScm-1), b alkalinity (mgl-1), and c radon (Bql-1), dotted arrows indicate trend line
accumulated along groundwater flow. In the case of tanks, mechanical agitation by stream turbulence or wind action can allow radon to escape into atmosphere.
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dissolved bicarbonate and carbonates. Hence, uranium can form strong U-CO3 complexes, which do not strongly sorb to minerals and metal oxides and facilitate the transport of uranium [21]. No trend is observed between U and Rn levels of groundwater (Fig. 4c). High levels of Rn ([11 Bql-1) are observed in groundwaters with low U concentration (\75 lgl-1). Even though U leaching is low under reducing conditions, the solubility of radium and iron (Fe2?) is comparatively high. Thus accumulations of Ra are particularly prominent in deep groundwaters, where reducing conditions prevail. Dissolved oxygen levels also suggest near-reducing conditions (\2 mgl-1) at most of the locations (Table 1). As these groundwaters mix with shallow groundwater, which is relatively oxic, iron precipitates as ferric oxide and adsorbed Ra coprecipitates. Therefore, as groundwaters change from oxidising to reducing conditions U leaching decreases but the production of Ra and Fe2? increases, leading to high Rn in groundwaters. Unlike groundwater from granitic rocks, groundwater from metasediments and alluvial plains has low radon activity, which is the case in the shallow groundwaters of the study area. In tanks Rn levels are much lower due to rapid transfer to the atmosphere. The environmental tritium values of the samples range in between 5 and 7 T.U (Table 1). It can be stated that all these waters are recently recharged, during the past 40 years. There is no significant difference seen in the case of shallow and deep groundwaters indicating the groundwater residence times are in the same order of magnitude in both the cases. This fact corroborates with the rapid flow conditions in the fractured hard rock aquifer and interconnection between shallow and deep groundwaters.
Summary Insights into geochemical processes and dynamics Plots of uranium with EC, alkalinity, and radon were produced in order to investigate the inter-elemental correlations and to understand the geochemical properties of groundwater governing the uranium and radon levels in water (Fig. 4a–c). Most of the samples fall along the EC axis with low uranium content (\50 lgl-1), indicating high U samples are not associated with high EC samples, shown by box in Fig. 4a. However, some samples show a positive correlation, shown by dotted arrow in Fig. 4a. At these locations high salt content promotes dissolution of uranium in groundwater. In the case of the alkalinity versus uranium plot (Fig. 4b), it can be seen that most of the samples fall on a positive trend, shown by the dotted arrow, indicating that alkalinity stabilizes the uranium in groundwater. A few samples, shown in the box, fall in a group with low alkalinity and low dissolved uranium. These samples are mostly from tanks. Since the pH of studied waters is in the range of 7–9, the alkalinity can be attributed to
The majority of the samples showed elevated levels of uranium in groundwater, and a few samples showed radon levels above drinking water permissible limits. Decreasing trends are noted along the depth in the case of electrical conductivity, alkalinity and uranium concentration while an inverse trend is noted in the case of radon levels. Groundwater with high alkalinity showed high uranium, indicating U-CO3 complexes are driving the uranium solubility. The probability of Rn diffusion is high in soil zone leading to low Rn levels in shallow zone groundwater compared to deeper ones. Environmental tritium data suggests that groundwater is modern in both shallow and deep zones. Environmental tritium, geochemistry, and geology data suggest that the shallow and deep zones are connected. With frequent failure of regular monsoon and high intense rains during cyclones the better option would be to store rainwater and increase groundwater recharge by the help of rainwater harvesting structures such as
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percolation tanks, check dams, and other suitable structures. Acknowledgments Authors sincerely acknowledge the constant support and encouragement by Dr. K. L. Ramakumar, Group Director (RC & IG) and Dr. Gursharan Singh, Associate Director (I), Radiochemistry and Isotope Group, Bhabha Atomic Research Centre, Mumbai. Thanks are also due to DHAN Foundation, Tamil Nadu, Dr. U. Saravana Kumar, Head, Isotope Hydrology Section and Dr. Sapra (RPAD) of BARC for their help during the course of this study. Authors are thankful to Ms. Thivya, Earth science department, Annamalai University, Tamil Nadu for preparing location map, Mr. Sadasiva and his team, DHAN foundation, Hyderabad for help during field work. Dr. Sahayam, NCCM, BARC, Hyderabad is duly acknowledged for providing uranium analysis.
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