Snvllton Geol (2006) 49: 413-429 DOl 10.1007/s00254-005-0089-9
N. Subba Rao
Received: 25 February 2005 Accepted: 30 August 2005 Published online: 9 November 2005 © Springer-Yerlag 2005
N. Subba Rao Department of Geology, Andhra University, Yisakhapatnam 530 003, India E-mail:
[email protected] Tel.: +91-891-2844716 Fax: +91-891-2525611
Seasonal variation of groundwater quality in a part of Guntur District, Andhra Pradesh, India
Abstract The area in Guntur district, Andhra Pradesh, Indi~, is selected to discuss the impact of seasonal variation of ground water quality on irrigation and human health, where the agriculture is the main livelihood of rural people and the groundwater is the main source for irrigation and drinking. Granite gneisses associated with schists and charnockites of the Precambrian Eastern Ghats underlie the area. Groundwater samples collected seasonally, pre- and post-monsoons, during three years from forty wells in the area were analyzed for pH, EC, TDS, TA, TH, Ca2+, Mg2+, Na +, K +, CO~-, HCO)", Cl-, SO~-, NO)"and F-. The chemical relationships in Piper's diagram, Chebotarev's genetic classification and Gibbs's diagram suggest that the ground waters mainly belong to noncarbonate alkali type and Cl- group, and are controlled by evaporationdominance, respectively, due to the influence of semi-arid climate, gentle
Introduction
The development of agriculture is a key factor in the economic development of a country like India, as the agriculture is the main source of sustenance for the majority of the population (about 700 million) in the country and contributes 46% to the gross national product. Exploitation of groundwater has increased greatly, particularly for agricultural purpose, because large parts of the country have little access to rainfall
slope, sluggish drainage conditions, greater water-rock interaction, and anthropogenic activities. A comparison of the ground water quality in relation to drinking water quality standards proves that most of the water samples are not suitable for drinking, especially in post-monsoon period. US Salinity Laboratory's and Wilcox's diagrams, and %Na + used for evaluating the water quality for irrigation suggest that the majority of the ground water samples are not good for irrigation in postmonsoon compared to that in premonsoon. These conditions are caused due to leaching of salts from the overlying materials by infiltrating recharge waters. A management plan is suggested for sustainable development of the area. Groundwater quality· Agricultural area· Management plan· Guntur district· Andhra Pradesh . India Keywords
due to frequent failures of monsoon and variable flow of surface water sources (rivers, lakes and artificial basins). Groundwater irrigation started with only 6.5 million hectares (Mha) in 1950-1951 (CGWB 1992), which was increased to 46.5 Mha in 2000-2001 (Sivanappan 2002), meeting about 70% of the irrigation water requirements of the country. This clearly indicates the growing pressure on groundwater resources. Groundwater quality is as important as the quantity. Poor quality of water adversely affects the plant growth
414
and human health (Wilcox 1948; Thorne and Peters on 1954; US Salinity Laboratory Staff 1954; Holden 1971; Todd 1980; ISI 1983; WHO 1984; Hem 1991; Karanth 1997). Adverse conditions increase investment in irrigation and health, and decrease agricultural production, which, in turn, reduces agrarian economy and retards improvement in the living conditions of rural people. Unsustainable development is the result. A number of studies on ground water quality with respect to drinking and irrigation purposes have been carried out in the different parts of the country (Ourvey et al. 1991; Agrawal and Jagetia 1997; Niranjan Babu et al. 1997; Subba Rao et al. 1999; Majumdar and Gupta 2000; Khurshid et al. 2002; Sreedevi 2004; Subba Rao and John Oevadas 2005), but little work on this aspect has so far been done in the rural area of Guntur district, Andhra Pradesh, India. The area, covering three administrative units (mandals) of Phirangipuram, Medikonduru and Muppalla in the district (Fig. I), is no exception from this. Earlier studies carried out in this area (Subba Rao 2002; 2003a, b) have evaluated ground water chemistry, groundwater prospecting and management, and groundwater quality with respect to fluoride concentration. These studies, however, have not attempted to assess the seasonal variation of groundwater quality. Groundwater samples collected seasonally, pre-monsoon and post-monsoon, during 1999-2001 from the area are selected for the present study.
Study area
Location and climate The study area covers 510 sq km and lies between latitudes 16°15'N-16°22'30"N and longitudes 8003'20"E80°22' 13"E (Fig. I) in the Survey of India topographical Fig. 1 Location and geology map of the study area, Guntur district, Andhra Pradesh, India
8 1,000 mg/1). The critical value of -1.65 for the variables, such as EC, rest of the samples are fresh water (TDS < 1,000 mg/l) in TDS, Mg2+, Na+, K+, CO~-, Cl-, NO)" and F-, the the respective seasons. The TH is from 180 to I, 100 mg/I difference of means between pre- and post-monsoons in pre-monsoon (mean: 422 mg/I) and post-monsoon is significant at 5% level. Hence, the results clearly (mean: 449 mg/1), suggesting the increased concentraindicate that there is evidence of seasonal effect on the tion towards post-monsoon. According to the TH mean values of groundwater quality. classification (Sawyer and McCarty 1967), the groundResults and discussion
420
Table 4 Season-wise combined chemical composition CV 19.64 19.96 11.04 16.97 er er -2.23 220.06 192.36 11.54 14.11 -0.54 5.16 159.81 448.80 36.13 0.43 1.19 131.44 479.17 34.65 33.93 -0.10 63.42 65.06 40.76 -1.79 -2.52 36.44 33.80 35.55 26.96 27.55 128.14 125.96 13.33 379.10 73.98 34.14 29.60 50.68 30.53 69.87 68.12 -4.28 -0.43 -5.74 37.85 38.02 39.16 38.85 145.26 15.75 142.90 0.35 370.97 566.44 142.15 95.55 7.44 21.79 46.22 -1.51 5.43 35.61 38.67 0.43 757.88 .95 37.61 -1.81 0.41 30.45 28.97 754.08 737.18 -1.78 39.69 38.62 143.35 340.30 1.16 490.15 -2.08 Mean 163.03 96.83 2,603.05 1,691.98 significant Chemical Not constituents significant Not Not Significant significant Significan t (mg/I)
of ground water samples collected from the study area during 1999-200 I
Pre-monsoon Post-monsoon 377.50 7.92 1.09 Remark 505.91 38.98 361.16 421.60 2,429.18 1,578.97 354.31 21.02 57.61 31.08 67.59 136.76
er standard deviation, CV coefficient of variation,
t computed value of test of significance at 5% level
Geochemical classification To assess the geochemical evaluation in groundwater flow systems, a graphical representation of Piper's diagram (Piper 1944) is extensively used. The chemical data of the groundwater samples collected from the Fig. 7 Spatial distribution of TDS concentration (mg/\): a pre-monsoon and b post-monsoon
a
0
I
80 S
b
0 I
80 S
study area are plotted in the Piper's diagram (Fig. 8). The chemical data of the sample points, as shown as a cluster, fall in the subdivisions of I, 2, 3, 4, 5, 7 and 9 (Table 5), indicating that the alkalies (Na + and K +) and strong acids (CI-, SO~-and NO) mainly dominate the chemical character of the groundwater from
I
o 80 20
o 80 20
I
421
Fig. 8 Geochemical classification of ground waters (After Piper 1944): a pre-monsoon and b post-monsoon. All sample values fall in the cross-hatched area
a
C. roy" ~.,.
I
80
60
40
o
100
~
180
~4t; 60 40
100 6040
20
20
0
40
q- ea2+
60
80
Cl --I>
b
100
,I ()
... ~.,.
80 60
"
40
o
o
40 ~O"'IOO 0
~
6Oroy"
20
a'" 80; 100
80
60
q- ea2+
pre-monsoon to post-monsoon. Results froth subdivision 9 (no one cation-anion pair type) trend towards subdivision 7 (non-carbonate alkali type) along with the flow path of groundwater due to increased concentration from additional sources of Na +, CI-, SO~and N03" ions, following the monsoon (Table 4) with leaching of salts from overlying materials and infiltration of recharge waters. The contour lines of groundwater table and iso-concentration lines of TDS drawn seasonally correspond to each other (Figs. 5 and 7), which show an increasing trend of TDS along
40
60
40
20
with a decreasing water level contour line from pre- to post-monsoon periods, supporting the inference of leaching activity. Statistical analysis further confirms the role of seasonal effect on the contribution of ions, except SO~-, to the groundwater quality (Table 4). Within the study area, soils and aquifer materials are the major sources of cations and anions (Eq. 3), whereas the fertilizers and irrigation-return-flows are additional contributors of Mg2 + , Na +, K +, Cl-, SO~and N03" ions, in addition to F- (Subba Rao 2002, 2003a).
422
Table 5 Distribution of groundwater samples (%) in the subdivisions of Piper's diagram (Piper 1944)
Area
Pre-monsoon
I 2
3 4 5
6 7 8 9
Cations (silicates)
Samples fall (%) in
Subdivisions
Alkaline earths exceed alkalies Alkalies exceed alkaline earths Strong acids exceed weak acids Weak acids exceed strong acids Carbonate hardness (secondary alkalinity) exceeds 50% Non-carbonate hardness (secondary salinity) exceeds 50% Non-carbonate alkali (primary salinity) exceeds 50% Carbonate alkali (primary alkalinity) exceeds 50% No one cation-anion pair exceeds 50%
+ H2C03 = H2Si04 + HC03" + cations + clays.
(3)
According to the hydrogeochemical genetic classification (Chebotarev 1955), the groundwaters are dominated by Type IV facies. They belongs to major group of CI-, which have class III of Cl- - HC03 and class IV of Cl-SO~- in the Type IV facies; the latter class being most dominant. This suggests the prevalence of inadequate flushing of ground waters, because of gentle slope and sluggish drainage conditions, which lead to longer contact of water with aquifer material and consequently higher solubility of minerals. The CI-- SO~class dominates most of the post-monsoon groundwaters (Tables I, 2, 3 and 4), which further supports the release of additional sources of salts from the anthropogenic activities into the groundwater body during rainy season. Mechanisms controlling groundwater chemistry Gibbs's diagrams, representing the ratios of Na +: (Na + + Ca2+) and CI-: (Cl- + HC03) as a function of TDS, are widely employed to assess the functional sources of dissolved chemical constituents, such as precipitation-dominance, rock-dominance and evaporation-dominance (Gibbs 1970). The chemical data of ground water sample points of the area are plotted in Gibbs's diagrams (Fig. 9). The distribution of sample points, as shown as a cluster, suggests that the chemical weathering of rock-forming minerals and evaporation are influencing the groundwater quality. Evaporation increases salinity by increasing Na + and Cl- with relation to increase of TDS. Semi-arid climate, gentle slope, lack of good drainage conditions and longer residence time of groundwater also contribute to the groundwater quality. Evaporation greatly increases the concentrations of ions formed by chemical weathering, leading to higher salinity. Kankar results from evaporation activity and occurs as intercalation in the soil. As a result, the water sample points move from the zone of rock-dominance towards
10 90 92 8 I 88
Post-monsoon 10
90 99
I I
98
II
the zone of evaporation-dominance. Anthropogenic activities (agricultural fertilizers and irrigation-returnflows) also influence the evaporation by increasing Na + and CI-, and thus TDS. Semi-arid climate also trends to evaporation-dominance ground water systems. Similar environmental conditions also occur during post-m onsoon, when salinity increases (Table 4) due to increased leaching.
Verification of leaching activity Seasonal variation in the concentrations of chemical constituents (Table 4) is mainly due to leaching of salts by recharge waters during monsoon. For example, variation in the concentrations of Na + (354-379 mg/l) and CI- (506-566 mg/I) from pre- to post-monsoons is more prominent by virtue of their higher solubility. The concentration of Ca2+ is constant (58 mg/I) in both seasons, whereas the concentration of Mg2 + varies from 68 to 74 mg/I, being dominant in postmonsoon. This is caused by ion exchange between Na + and Ca2+ and/or precipitation of CaC03 (Subba Rao 2002), suggesting an enrichment of Na + relative to Ca2 + and attaining higher concentration of Mi + than Ca2+ in the groundwater. The content of K + is more in pre-monsoon (21 mg/I) than in post-monsoon (17 mg/I), because of absorption onto the c1al- horizons. In the case of CO~- plus HC03 and S04 - ions, variation in the latter ion is significant in post-monsoon (142 mg/l) compared to that in pre-monsoon (137 mg/l) and opposite in the former ion in pre-monsoon (409 mg/l) compared to that in post-monsoon (393 mg/ I) due to amphoteric exchange. There is a considerable variation of N03 from pre-monsoon (39 mg/I) to postmonsoon (46 mg/l), reflecting the involvement of anthropogenic sources. Similar variation in the concentration of F- (I.l-I.2 mg/I), if any, indicates the role of leaching activity (Subba Rao 2003a). Thus, the ground waters with concentrations in both seasons are characterized by Na+ >Mg2+ >Ca2+ >K+:CI-> CO~- + HC03 > SO~- > N03 > F- facies. These varia-
423
a
Fig. 9 Mechanisms controlling groundwater quality (After Gibbs 1970):a pre-monsoon and b post-monsoon. All sample values fall in the crosshatched area
104
104
o
o
0.2
0.4
Na+: (Ne +
'" /1' ",/' Rock.dom.iJumcl(
"
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C,r) 102, (Ne + 101
,~/'. /f:"I
I I I 0."'......
'J ~.
./~/",
I
\
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-.
101
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I ,;'" 0.4 0.6 0.8 0.2 102 ~
o
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0.8
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cif)
', , -, "'",/'-,
~~ . I' ,/ ~$f 0 104" 7
b "
",/
tions are also reflected clearly in the concentrations of TA, TH and TDS. For instance, the concentration of TH (422-449 mg/I) is more than the concentration of TA (340-361 mg/I) in the groundwaters; this relation suggests that the non-carbonate hardness (TH > TA) is more significant character in post-monsoon (l09 mg/I) than in pre-monsoon (60 mgfl) due to increase of CI-, SO~- and NO)" ions during rainy season. Similarly, the TDS shows higher concentration in post-monsoon (1,692 mgfl) than in pre-monsoon (l,579 mg/I), because of leaching of various salts into the post-monsoon groundwaters by infiltrating recharge waters. As per
o~"QO ~4'fJ.~
......
.••..
"/'/~
0.2
0.4
0.6
0.8
CI-: (CI- + HCO;)
~~. \ " \ \ {/j "0.••.• .••.. ""","" I .I ,~i (CI+ IHCO;) ock-domjnane( 103 ......CI-:
'J
,
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the statistical analysis, concentrations of TDS, NO)" and F-, however, cant in the groundwater
the effect of season on the Mg2+, Na +, K +, CO~-, CI-, is observed to be more signifiquality (Table 4).
Groundwater quality
Quality criteria for drinking purpose The pH (6.9-8.5) in groundwater samples in pre- and post-monsoons during 1999-2001 (Table 6) is within the
424
safe limits (6.5-8.5) prescribed for drinking water by ISI (1983) and WHO (1984). The concentration ofTDS (750 to 3,360 mg/I) is more than the recommended limit of 500 mg/I allowed (lSI 1983; WHO 1984) in all groundwater samples in both seasons, causing a gastrointestinal irritation in the consumers. Concentrations of Ca2+ and/or Mg2+ ions increase TH. Groundwaters collected from 73% (1999), 92% (2000) and 95% (2001) in pre-monsoon, and 92% (1999) and 97% (2000 and 2001) in post-monsoon samples (Table 6) have TH beyond the safe limit of 300 mg/l suggested for drinking water by ISI (1983). Hard water leads to incidence of urolithiosis (WHO 1984), anencephaly, parental mortality, some types of cancer (Agrawal and Jagetia 1997) and cardio-vascular disorders (Durvey et al. 1991). Such waters can also develop scales in water heaters, distribution pipes and well pumps, boilers and cooking utensils, and require more soap for washing clothes (Todd 1980; Hem 1991; Karanth 1997). The relationship of TH with TA indicates that the ground waters contain non-carbonate hardness. Such hardness cannot be removed easily from the waters, as in the case of carbonate hardness (TH < TA; Chow 1964), by boiling. In drinking water, the concentration of Na + should not exceed 200 mg/1. A sodium-restricted diet is recommended to patients suffering from hypertension or congenial heart diseases and also from kidney problems. For such people, extra intake ofNa + through drinking water may prove critical (Holden 1971). In 85% (1999),92% (2000) and 95% (2001) of the groundwater samples observed during pre-monsoon, and in 92% (1999 and 2000) and 95% (2001) observed during postmonsoon contain Na + higher than the limit of 200 mg/1. In the area, 54% (1999), 67% (2000) and 77% (2001) of the ground water samples in pre-monsoon, and 59% (1999),49% (2000) and 90% (2001) of the groundwater samples in post-m on soon have exceeded the safe limit (300 mg/l) of HC03" recommended for drinking water by ISI (1983; Table 6). The content of HC03" has no known adverse health effects, but still should not exceed 300 mg/1. On the other hand, 87% (1999), 92% (2000) and 97% (2001) of the pre-monsoon groundwater samples, and 95% (1999), 92% (2000) and 95% (2001) of the post-monsoon groundwater samples have concentration of CI- exceeding the required limit of 250 mg/l suggested for drinking water (ISI 1983). Excess concentration of CI- in drinking water gives a salty taste and has a laxative effect in people not accustomed to it. The chemical data show that 33% (1999),44% (2000) and 46% (2001) in the pre-monsoon, and 44% (1999) and 46% (2000 and 2001) in the post-monsoon ground water samples (Table 6) have SO~- content exceeding the tolerance limit of 150 mg/I prescribed for drinking water (lSI 1983). Higher concentration of SO~- in drinking water is associated with respiratory problems (Maiti 1982; Subba Rao 1993). In combination with Na + and
Mg2 +, SO~- also exerts a cathartic effect on digestive tracts. In 36% (1999), 18% (2000) and 44% (2001) of the groundwater samples belonging to pre-monsoon, and in 49% (1999), 46% (2000) and 62% (2001) of the groundwater samples belonging to post-monsoon period, the concentration of N03" is higher than the recommended level of 45 mg/I suggested for drinking water (lSI 1983; WHO 1984). Excessive N03" in drinking water can cause a number of health disorders, such as methemoglobinemia, gastric cancer, goitre, birth malformations and hypertension (Majumdar and Gupta 2000). Fluoride is an essential element for maintaining normal development of healthy teeth and bones. Deficiency of F- in drinking water below 0.6 mg/I contributes to tooth caries. An excess of over 1.2 mg/I causes fluorosis (ISI 1983). Groundwater samples observed in 5% (1999 and 2000) and 3% (2001) during pre-monsoon, and those in 15% (2000) during post-monsoon have less than the limit of 0.6 mg/l allowed (Table 6). In 41% (1999), 31% (2000), and 33% (2001) of the ground water samples during pre-monsoon, and those in 49% (1999), 44% (2000) and 46% (2001) during postmonsoon have more than the limit of 1.2 mg/1. Health disorders from fluorosis have been reported in the study area (Subba Rao 2003a). Quality criteria for irrigation purpose EC and Na + play a vital role in suitability of water for irrigation. Higher salt content in irrigation water causes an increase in soil solution osmotic pressure (Thorne and Peterson 1954). Since plant roots extract water osmosis, the water uptake of plants decreases. The osmotic pressure is proportional to the salt content or salinity hazard. The salts, besides affecting the growth of plants directly, also affect the soil structure, permeability and aeration, which indirectly affect the plant growth. The total concentrations of soluble salts in irrigation water can be classified into low (Cl), medium (C2), high (C3) and very high (C4) salinity zones. These zones (Cl to C4) have the value of EC less than 250 j.1S/cm,250 to 750 j.1S/cm,750 to 2,250 j.1S/cmand more than 2,250 j.1S/cm, respectively. Higher EC in water creates a saline soil. Another important chemical parameter for judging the degree of suitability of water for irrigation is sodium content or alkali hazard, which is expressed in sodium adsorption ratio (SAR). The SAR is computed, where the ion concentrations are expressed in meq/I, as shown below:
=
Na+. (4) {[Ca2+ + Mg2+] /2} 0.5 There is a close relationship between SAR values In irrigation water and the extent to which Na + is
SAR
425
Table 6 Distribution of groundwater Chemical constituents (mg/l)
WHO (1984)
ISI (1983)
samples (%) exceeding the drinking water standards 1999 Pre-monsoon
2000 Post-monsoon
Pre-monsoon
2001 Post-monsoon
Pre-monsoon
Post-monsoon
I
15.38 250 45 53.85 35.90 43.59 46.15 17.95 48.72 46.15 92.31 94.87 97.44 43.59 46.15 48.72 61.54 58.97 275 92.31 75 15.38 15.38 17.95 20.51 17.95 -46.15 -48.72 15.38-43.59 2.56-33.33 5.13-30.77 300 6.5-8.5 500 72.79 100 92.31 100 97.44 100 100 100 97.44 7.5-8.5 500 100 33.33 87.18 150 66.67 76.92 43.59 94.87 89.74 84.62 4200 500 limits300 30 5.13-41.03 Within 92.31 the safe 30 pH (units)0.6-1.2
absorbed by soils. If water used for irrigation is high in Na + and low in Ca2+, the ion-exchange complex may become saturated with Na + , which destroys soil structure, because of dispersion of clay particles. As a result, the soils tend to become deflocculated and relatively impermeable. Such soils can be very difficult to cultivate. The sodium hazard is expressed in terms of classification of irrigation water as low (SI: < 10), medium (S2: 10 to 18), high (S3: 18 to 26) and very high (S4: > 26). The values of SAR in the study area range from about 2 to 14 of pre- and post-monsoon groundwater samples during 1999-2001 presented in Tables 1,2 and 3. The SAR with the value of 2 indicates that the waters have higher concentration of Na + relative to the concentration of Ca2+. The SAR with the value of 14 indicates a very higher content of Na +. The higher the SAR values in the water, the greater the risk of sodium. The US Salinity Laboratory's diagram (US Salinity Laboratory Staff 1954) is used widely for rating the irrigation waters. SAR is plotted against EC. The plot of chemical data of the ground water samples of the area in the US Salinity Laboratory's diagram is illustrated in Fig. 10. Distribution of percentage of water samples in the diagram is given in Table 7. The groundwater sample points, as shown as a cluster, fall in C3S1 (5-18% of waters waters 7.69 20.09 56.40 5.13 53.84 17.95 87.18 79.19 2C4S2 0.52 20.81 12.82 23.09 64.09 66.66 12.82 Pre-monsoon 28.20 30.77 41.0317.95 2.57 43.5915.38 30.77 41.03 33.32 > 69.23 71.79 74.36 79.91 20.52 160% 28.21 43.59 30.77 23.09 2.82 5.13 12.82 51.29 53.84 38.46 17.95 7.69 USgroundwater Moderate Permissible
