CHAPTER 6 GROUNDWATER HYDROCHEMISTRY AND HYDROCHEMICAL PROCESSES

Transcription

1 69 CHAPTER 6 GROUNDWATER HYDROCHEMISTRY AND HYDROCHEMICAL PROCESSES 6.1 GENERAL Groundwater contains a wide range of dissolved solids and contain small amount of dissolved organic matter and gases. Groundwater, which is always in motion through aquifers and it interacts with the aquifer material in the subsurface environment. During this movement groundwater may dissolve, transport and deposit mineral matter. These changes are mainly based on the surface and subsurface environment. The ionic composition of groundwater is controlled by the chemical composition of rain, composition of infiltrating surface water, properties of soil and rock in which the groundwater moves, contact time and contact surface between groundwater and geological material along its flow path, rate of geochemical (oxidation/reduction ion exchange, dissolution, evaporation, precipitation) process and microbiological process. Generally, the chemical quality of groundwater depends, to a large extent, on the host of rock constituting the aquifers (Eriksson and Khunakassen, 1966). Geologically the Tondiar basin is underlain by rocks of Archean age consisting of granites, gneiss and charnockites. Hydrogeochemical studies of groundwater were carried out to determine the groundwater nature in the Tondiar River Basin, Southern India. Groundwater samples were collected from September 2005 to November 2006, from 45 wells located in the study area. These samples were analysed for concentration of major ions, trace elements and nutrients. In this chapter the interpretation made from the study of major ions are discussed. About

2 70 four hundred groundwater samples of the study area were collected and analysed for EC, ph, Ca 2+, Mg 2+, Na +, K +, HCO 3 -, CO 3 2-, Cl - and SO PHYSICAL PARAMETERS OF GROUNDWATER The most common physical parameters were measured in the field at the time of sampling are EC, ph, Eh provides useful preliminary information of the area. The groundwater is generally colourless, odourless and taste it varies according to locations. The spatial distribution of the groundwater ph during July 2006 is given in Figure 6.1 and it varies from 6.5 to 8.3, with a mean value of 6.9. ph of water is a very important indication of its quality, which is controlled by the amount of dissolved Carbondioxide, carbonates and bicarbonates. Addition of salts to water may cause rapid rise in ph. The CaCo 3 increases the ph of water making it alkaline. Ghandour et al reported (1985) ph decreases with increasing salinity. The ph values of the groundwater samples are within permissible limit (BIS, 2003) in this area. The central part of the study area has relatively high ph. In general the groundwater is alkaline in nature. The EC of groundwater of the study area ranges from 625 to 4688 μs/cm, with the mean value of 1958 μs/cm. The spatial variation of EC (μs/cm) in the months of March 2006 and November 2006 are given in the Figure 6.2 and 6.3. Groundwater of the well situated in Pennagar, Desur, Konagampattu, Rettani, Pelampattu have high EC value. The groundwaters in these locations are slightly saline in nature and this is due to the bedrock formation, agricultural activities and local pollution occurs as isolated patches of this area. There is not much difference in the EC value between March 2006 and November During the monsoon period the EC is slightly reduced due to the rainfall recharge. The minimum EC value is found in the Northeastern part of the study area (Korrakottai) and northern part of the basin (Kottupakkam). The redox potential indicates the oxidation and reduction process in the groundwater. The redox potential (Eh) generally varies from 26mv to 207mv as shown in Figure 6.4.

3 71 July 2006 Figure 6.1 Spatial distribution of ph of groundwater July 2006

4 72 March 2006 Figure 6.2 Spatial distribution of EC( S/cm) of groundwater March 2006

5 73 November 2006 Figure 6.3 Spatial distribution of EC( S/cm) of groundwater November 2006

6 74 November 2006 Figure 6.4 Spatial distribution of Eh (mv) of groundwater November 2006

7 MAJOR ION CHEMISTRY The study of major ion concentration of the groundwater of this area will provide information about the hydrochemical status of an aquifer. The ranges of concentration of major ions in groundwater of the study area are given in Figure 6.5. The concentrations of dissolved major cations and anions in the groundwater vary both regionally and seasonally. The general order of dominance of cations is Na + >Ca 2+ >Mg 2+ >K + - and for anions is HCO 3 >Cl - >SO 2-4 >CO - 3. Thus Na + - and HCO 3 are the dominant ions present in groundwater of this area Mean Minimum Maximum Concentration (mg/l) Na K Ca Mg Cl HCO3 CO3 SO4 Figure 6.5 Range of concentration of major 6.4 SEASONAL VARIATION OF MAJOR IONS In general groundwater quality will change due to the variation in rainfall recharge, exploitation of groundwater, variation in land use, irrigation return flow, geochemical reaction and geological formation. These factors play a major role in seasonal variation of ionic composition in groundwater in this area. The composition of the infiltrating rain water depends on the

8 76 frequency of rainfall, soil environment, agriculture pattern and thickness of vadose zone (Scheytt 1997). The monthly variation of the major ions concentration of groundwater of three representative wells of this area illustrated in the Figures 6.6, 6.7 and 6.8 along with the rainfall and groundwater level. The EC and the concentration of major ions of groundwater of this area vary significantly with respect to time. In the study area rainfall recharge occurs generally from the month of October to January and comparatively high evaporation occurs from the month of March to May. These two factors play a major role for the seasonal variation of major ions. Monthly variation in major ion chemistry of the wells located in hard rock formation respond to rainfall more quickly due to the intensive of weathering and fracturing of hard rocks. The comparison between rainfall and water level indicates the rise in water level when the monthly rainfall exceeds 300mm. The rise in groundwater level in this area during the Northeast monsoon has resulted in decrease in ionic concentrations due to the dilution. The recharge process reduces the ionic concentration of groundwater. However, during the non-monsoon period, increase in major ion concentration is observed, due to the lowering of water level and the evaporation process. The concentration of the most of the major ions follows the water level fluctuation pattern. Similar results were observed in the hard rock aquifers of Guntur district, Andhra Pradesh, southern India (Subba 2005). However, in a few wells there is a slight increase in ionic concentration with rise in water level due to local pollution (animal solid waste storage and human waste) and due the dissolution of precipitated salts. In the hard rock area the water table generally fluctuates within the weathered and fractured rock zone in semi-confined conditions. Major ion concentration increases due to dissolution of precipitates present along flow path during recharge. Thus the seasonal variations of the study area is mainly controlled by the recharge process and strongly influenced by the bedrock geology, but may also be attributed to the impact of agricultural pollution.

9 77 Figure 6.6 Monthly variation of rainfall and concentration of major ions in Pennagar (well no.6)

10 SO4 (mg/l) Figure 6.7 Monthly variation of rainfall and concentration of major ions in Chendur (well no.40)

11 79 Figure 6.8 Monthly variation of rainfall and concentration of major ions in Vallam (well no. 26)

12 SPATIAL VARIATION OF MAJOR IONS The major ion concentrations of groundwater of the study area vary spatially according to groundwater recharge due to variation in amount of precipitation, irrigation return flow, agricultural activities and geochemical reactions of the formation. Sodium, among the alkalis, is a predominant chemical constituent of the natural water. The sodium ion is the dominant cation (Figure 6.9) of the study area and it varies from 10 to 457 mg/l. Maximum concentration of sodium ion is found along the northwestern and southern part of the study area. Potassium range from 1 to 159 mg/l and it varies from season to season. The occurrence of potassium is found less in nature and therefore, it is found at lower concentrations than sodium. Maximum concentration of the potassium is observed in the well nos 16 and 29. High concentration of sodium and relatively low concentration of potassium in the groundwater might be due to the weathering of silicates. This type of low proportion of potassium and high sodium has been reported by few researchers (Mohan et al 2000). Calcium is a common and widespread element and it is distributed widely in soils and rocks. Calcium is the second dominant cation in the groundwater of this region and it ranges from 35 to 218 mg/l (Figure 6.10). Usually the groundwater in the hard rock regions has the higher concentration of calcium. The maximum concentration of calcium is found in the north western and south western parts of this area. Magnesium concentration ranges from 10 to 67 mg/l with the mean value of 28 mg/l (Figure 6.11). There is not much variation in the concentration of the magnesium ion of the groundwater samples. Magnesium content is generally controlled by the presence of CO 2. The primary source of carbonate and bicarbonate ions in groundwater is the dissolved carbondioxide in rainwater (Karnath, 1989). Bicarbonate (Figure 6.12) values ranges from 192 to 665 mg/l and is the dominant anions of the

13 81 study area. Higher concentration of bicarbonate in the study area might be due to the weathering of silicate rocks and bicarbonate in present I the infiltrating rainwater. Almost the entire area has high concentration of the bicarbonate. The alkalinity of the water in this area is caused by dissolved bicarbonate salts. The concentration of bicarbonate ions decrease slightly after the monsoon. Carbonate concentration in groundwater of the study area ranges from 0 to 53 mg/l. Maximum concentration of the carbonate is observed in the well no: 24. In this well carbonate is present during the monsoon season when there is flow in the river and during the rest of the period the carbonate is absent. Thus carbonate is usually present only during the rainy season in two wells of this area. Figure 6.13 shows chloride concentration ranges from 26 to 899 mg/l. Chloride is considered as a strong acid compared to other ions. The maximum concentration is found in the well nos 6 and 33 in northwestern and south western part of the study area. The chloride concentration of groundwater of the wells located in gneiss rock formations is higher than that of the wells located in the Charnockite rock formations. Sulphate is widely distributed in reduced form in both metamorphic and sedimentary rocks as a metallic Sulphide through it is not a major constituent of the earth s outer crust. Sulphate concentration in groundwater of this area ranges from 10 to 400 mg/l. The well no 37 has the higher Sulphate concentration which is located in the southern part of the area. Sulphate concentration in natural water is less than chloride and the same is observed in the groundwater of the study area also. Sulphate concentration in this area is influenced by the agriculture patterns, since the man-made chemical fertilisers are used in this area. In general the regional variations of all major ions behave more or less in a similar manner. Thus the higher concentration is observed in the Northwest and Southwest part of the study area. Low concentration is observed in the northern part and northeastern of the study area. In most of

14 82 the months, as there is no flow in the Tondiar River, the concentration of ions in groundwater is high. The tanks can store water only for 1 or 2 months. Hence the ionic concentration is high most of the months, except during the month of November to January. During the month of October to November there is heavy rain which dilutes the groundwater by the recharge process. Hence, the concentration of certain ions decreases and there is also increase of certain ions like potassium and nitrate due to the applications of fertilizers. May 2006 Figure 6.9 Spatial distribution of Sodium (mg/l) concentration of groundwater May 2006

15 83 May 2006 Figure 6.10 Spatial distribution of Calcium (mg/l) concentration of groundwater May 2006

16 84 May 2006 Figure 6.11 Spatial distribution of Magnesium (mg/l) concentration of groundwater May 2006

17 85 May 2006 Figure 6.12 Spatial distribution of Bicarbonate (mg/l) concentration of groundwater (May 2006)

18 86 May 2006 Figure 6.13 Spatial distribution of Chloride (mg/l) concentration of groundwater May 2006

19 87 May 2006 Figure 6.14 Spatial variation of HCO3/Cl (meq/l) ratio of groundwater (May 2006)

20 VARIATION RATIO BETWEEN HCO 3 /Cl The Figure 6.15 shows the rational variation in the ratio between chloride and bicarbonate. This variation generally follows the direction of groundwater flow. Uphari and Toth (1989) observed the groundwater evolves from bicarbonate dominate facies in the recharge area to Chloride dominate facies in the discharge area. Similar results were observed in the study area. When the groundwater flows towards the discharge area (Tondiar River during summer season) the younger water gets enriched in Chloride. During the groundwater flow, the groundwater becomes more mineralized as it dissolves more aquifer material. This was clearly revealed by the regional variation in HCO 3 /Cl ratio of groundwater. The ratio decreases towards southern part and clearly shows recharge area and groundwater flow towards. The Bicarbonate may be derived from the soil zone CO 2 and at the time of weathering of the parent materials (Hudson 1997, Mohan et al 2000). Bicarbonate may also derive from the dissolution of Carbonates and Silicates present in the study area. The soil zone consists of roots, decay matter, organic matter which in turn combines with the rainwater/infiltrating water to form Bicarbonates by the following reactions CO 2 + H 2 O H 2 CO 3 (6.1) H 2 CO 3 H HCO 3 (6.2) The source of high concentration of Bicarbonates may also be derived from the dissolution of soil CO 2 during the percolation of irrigation as well as rain water and also silicates present in this area. Chloride is considered as a strong acid compared to other ions. However, the chloride concentration is comparatively higher in a few wells located in the gneiss formation. In general the chloride concentration is low in

21 89 the wells in the charnockite areas. Chloride concentration of water samples indicates that the possible sources may be due to the irrigation return flow and rainfall recharge. The HCO 3 /Cl (meq/l) molar ratio is generally less than 4 in groundwater of hard rocks; however in few wells in the hard rock formation is higher (Figure 6.15) HCO3/Cl (meq/l) Wells in Gneiss rock formations Wells in Charnockite rock formations Well No. Figure 6.15 HCO3/Cl ratio groundwater samples 6.7 HDROCHEMICAL FACIES OF GROUNDWATER The geochemical nature of groundwater can be understood by plotting the concentrations of major cations and anions in the piper trilinear diagram. The trilinear diagram of Piper (1953) is very useful in bringing out the chemical relationship in groundwater. This is useful to understand the total chemical character of groundwater samples in terms of cations and anions pairs. The study area is most dominant cations is sodium and the most dominant anions is Bicarbonate. Four major hydrochemical facies have been identified from the Piper diagram (Figure 6.16) based on the major ion chemistry of groundwater of this area. They are:

22 90 i) CaHCO 3 Type ii) NaCl Type iii) Mixed CaMgCl Type iv) Mixed CaNaHCO 3 Type Figure 6.16 Piper diagram for classifying groundwater types

23 91 The CaHCO 3 (Carbonate hardness) type and NaCl (primary salinity) type of water are the dominate type of water in the study area. The above type of water might have been derived from the groundwater recharge, irrigation return flow and ion exchange process. There is no difference in the distribution of various hydrochemical facies between the groundwater of occurring in gneiss and charnockite formation. Schoeller s (1965) diagram (Figure.6.17) also shows that the groundwaters in the study area have similar composition irrespective of geological formation. This Figure also indicates the similarity in the chemical ratios of concentrations, which indicate that the groundwater flow makes the groundwater is more or less similar in nature. Figure 6.17 Scholler diagram for groundwater samples (January 2006)

24 HDROCHEMISTRY AND LAND USE Most part of the study area is under intensive cultivation, human settlement and the other part is isolated hillocks and forest mostly in the northern part of the study area. Hence, the land use variation is reflected in the hydrochemistry of this area as in other place. (Cain et al. 1989). Variation of groundwater quality in an area is a function of physical and chemical parameters that are greatly influenced by geological formations and anthropogenic activities. At several sites the spatial variation between the land use and hydrochemistry was observed. Well nos. 9 and 13 is the well contains the mostly fresh water in the study area, as it is located near the hillocks in the northwestern part of the study area. As there is no human settlement around this well, and the soil is deep red soil and there is very less agricultural cultivation and mostly dry crops cultivation and also somewhat close to the forest area The wells located very close to the human settlement, saline soil and also to the intensive agriculture cultivation have very high salinity. In few wells the aquifer it self in saline nature and this is due the geological formation and the nature of soil is saltiest As almost entire area is being intensively cultivated and irrigated and it is importance of understand its effect on hydrochemistry. This area has been subjected to the application of excessive inorganic fertilisers for almost two decades. In the case of agricultural areas, these activities may generate great quantities the ionic concentration of potassium and nitrate of some wells are generally higher located in agricultural fields than the domestic wells. Generally domestic wells are constructed in this study area inside the Tanks and away from the human settlement.

25 HYDROGEOCHEMICAL PROCESS Hydrogeochemical process occurring within groundwater zone by interaction with aquifer minerals result in the chemical nature of water. Geochemical processes are very important as they control the composition of the groundwater in the aquifer system. The geochemical processes are responsible for the seasonal and regional variation in groundwater quality as discussed earlier. The geochemical process changes the groundwater quality during its flow from the recharge area. The geochemical properties of various groundwater bodies are determined by the chemistry of water in the recharge area as well as the subsurface formation. The various geochemical processes that are responsible for the chemical character of the groundwater of this area are discussed below MECHANISMS CONTROLLING GROUNDWATER QUALITY Gibbs (1970) proposed a diagram to understand the relationship of the chemical components of waters and classified the groundwater chemistry resulting due to three mechanisms as shown in (Figure 6.18). This plot explains the relationship between water chemistry and aquifer lithology. Such a relationship, help to identify the factors controlling the groundwater chemistry. The Fig 6.18 suggests that the chemical weathering of rockforming minerals is influencing the groundwater quality. As most of the points plot in the region of rock water interaction, this is likely to be the dominant process controlling the groundwater chemistry of this area. However, some points also fall in the region near the evaporation, indicating that this process is also responsible for the groundwater chemistry.

26 94 Figure 6.18 Gibbs diagram Evaporation is the natural process that would increase the concentration of the ions in groundwater. Thus evaporation is an important process that increases the concentration of ions, especially during the dry period and the evaporation increases the water will tend move toward salinity. This would cause increase in concentration of ions in surface and subsurface water. When evaporation is a dominant takes place, entire area this would enrich the concentration ions and increases salinity in soil zone, due to decline of the groundwater table level Evaporation is a dominant process in the entire study area as this area fall in semi- arid region, where the ionic concentration increases with lowering of water level. The presence of such linear relationship between sodium-to-chloride ratio vs EC is indicative of concentration by evaporation or evapotranspiration as reported by Jankowski and Acworth The plot shows the Na/Cl vs EC (Figure 6.19) would give a straight line, which would then be an effective indicator of concentrations of

27 95 ions by evaporation or evapotranspiration. Thus, evaporation is an important process that increases the concentration of ions, during dry period and groundwater diluted during subsequent monsoon recharge. Similarly sodium vs chloride plot (Figure 6.20) indicates that most of samples plot above the fresh water evaporation line. This indicates that evaporation may not be the major process controlling groundwater quality. Hence, sodium in the groundwater might have been derived from some other processes. If halite dissolution is responsible for sodium, Na/Cl molar ratio should be approximately equal to 1, where as ratio greater than 1 indicates that Na is released from silicate weathering reaction (Meybeck 1987). Samples having Na/Cl ratio greater than one (Figure 6.19) indicates excess sodium, which might have come from silicate weathering. If Silicate weathering is a probable source of sodium, the water samples would have HCO 3 as the most abundant anions (Rogers 1989).In the present study, bicarbonate is the dominant anions. Hence silicate weathering may be the reason for sodium in groundwater. Samples having Na/Cl ratio approximately less than one indicate the possibility of some other chemical sources. Figure 6.19 Plot of Na/Cl (meq/l) Vs EC (μs/cm)

28 96 Figure 6.20 Plot of Na(meq/l) Vs Cl (meq/l) SILICATE WEATHERING PROCESS Silicate weathering process is an important process that is expected to control the groundwater chemistry in the hard rock formation. The groundwater occurring in the hard rock formation generally has high concentration of major ions due to the weathering of rocks. Groundwater in the Tondiar river basin comprises of hard rocks and these rocks are highly weathered and fractured. Silicate weathering is understood by the relationships between the major ions present in the groundwater. In this area sodium is the dominant cation next to calcium in the groundwater of the study area. A relationship between (Ca+Mg) vs HCO 3 diagram shown in Figure 6.21 indicates that most of the data points fall above the1:1 equiline, although few points below the equiline.

29 97 Figure 6.21 Plot of (Ca+Mg) Vs HCO 3 plot indicating silicate weathering It suggests that an excess of alkalinity of the waters have been balanced alkalies. The excess of alkaline earth elements (Ca + Mg) over HCO 3 in samples reflect an extra source of calcium and magnesium ions. It might have been balanced by Cl - and SO 2-4 or supplied by silicate weathering. The ratio will be close to unity, if the dissolution of calcite, dolomite and gypsum is the dominant reaction in aquifer system. The data points toward the Y axis (Figure 6.21) indicate high concentration of Ca+Mg over HCO 3 which is mainly balanced by ion exchange process.the (Ca+Mg) vs Total cations (TZ) shows that the data lie far below the theroritical line (1:1) as shown in (Figure 6.22) depicating an increasing contribution of alkalies to the major ions. But a few groundwater samples of wells located in the hard rock regions have higher concentration of (Na +K) than (Ca+Mg).The Na+K vs Total cations scatter diagram (Fig. 6.23) of the study area shows sample points

30 98 falling both along and above the Na+K = 0.5 Total cations. This suggests that the cations in the groundwater might have been derived from silicate weathering. Datta and Tyagi (1996) observed that the contribution of cations may be derived from silicate weathering when Na+K = 0.5 Total cations. The slightly lower concentration of (Na+K) is likely to be caused by Ca/Na exchange process, which might have reduced the amount of Na in the groundwater. Since this region comprise of composite gneiss and charnockite, weathering of silicates might be the possible source of ions. Weathering of silicate rocks resulting in high Na and K has also been reported in the hard rock regions Naini Industrial area, Uttar Pradesh by Mohan eta al (2000). Silicate dissolution is the probable source of Na in the study area because water that derives solutes primarily by silicate weathering has high HCO 3, which is the most abundant anion in this area (Equiline 1:1). A Na/Cl ratio approximately equal to 1 is usually attributed to halite dissolution., where as ratio greater than 1 is Na is released from silicate weathering A molar ratio Na/Cl ratio >1 (Figure 6.24) is due to Na is released due to silicate weathering reactions while the molar ratio of Na/Cl <1 are due to the halite dissolution (Meybeck 1987).In the study area the molar ratio of Na/Cl of the groundwater samples generally ranges from 0.04 to 4.72 with an average of Figure 6.24 shows that the value of Cl as a function of Na in the groundwater. The dissolution of halite in water release equal concentrations of Na and Cl into the solution and the figure 6.24 the data point s are clustered around the equiline 1:1 This indicates that silicate weathering is the source of sodium. 2NaAlSi 3 O 8 +2H 2 CO 3 +9H 2 O = Al 2 Si 2 O 5 (OH) 4 +2Na + +4H 4 SiO 4 +2HCO 3 (Albite) (Silicate weathering) (Kaolinite) (6.3)

31 99 Thus it is observed that a silicate weathering is an important process occurring in the study area. Figure 6.22 Plot of (Ca+Mg) Vs Total Cations indicating silicate weathering Figure 6.23 Plot of (Na+K) Vs Total Cation indicating silicate weathering

32 100 Figure 6.24 Plot of Na Vs Cl plots explaining the mixing process ION EXCHANGE PROCESS Under certain conditions, the ions attracted to a solid surface may be exchanged for other ions in aqueous solution. This process is known as ion exchange process, but in some natural soil (Clay) cation exchange is dominant and the clay has high percentage of colloidal sized particle. A plot of Na vs Cl concentration of groundwater of the study area with 1:1 line is given in Figure The sample points fall above and below the 1:1 line. The sample points plotting below the 1:1 line indicate the depletion of sodium with respect to chloride. Similarly the sample points plotting above the 1:1 line indicate the increase of sodium with respect to chloride. Both the process shows the evidence of cation exchange process (Jankowski and Acworth 1997; Salama 1993). Excess of Ca and Mg in groundwater may be due to the exchange of Na in water by Ca and Mg in clay particle. The cation exchange process is explained by the following reaction Ca+2Na (exchanged) 2Na + + Ca (exchanged) (6.4) Mg 2+ +2Na (exchanged) 2Na + +Mg (exchange) (6.5)

33 101 Where (exchanged) denotes the cation exchanged on water or soil. Figure 6.25 plot of Cl Vs Na indicating ion exchange process Figure 6.26 Relations between Ca+Mg and SO 4 +HCO 3

34 102 Figure 6.27 Relations between Ca+Mg-HCO 3 -SO 4 and Na-Cl The plot of SO 4 +HCO 3 vs Ca+Mg (Figure 6.26) shows that the most of the groundwater samples from hard rock formation are clustered around the 1:1 line, if the dissolution of calcite, dolomite and gypsum are dominant reaction in a system. Excess of calcium and magnesium in groundwater of hard rock formation may be due to the exchange of sodium in water by calcium and magnesium in clay material The plot of Na-Cl vs Ca+Mg-HCO 3 -SO 4 (Figure 6.27) also help to identify the ion exchange process in the aquifer system. Na-Cl (meq/l) represents the amount of Na gained or lost relative to that provided the halite dissolution, whereas Ca +Mg-HCO 3 (meq/l) represents the amount of Ca and Mg gained or lost relative to that provided by gypsum, calcite and dolomite dissolution. If ion exchange is a significant composition-controlling process, the relation between should be linear slope (Jankowski et al 1997). In the study area, the groundwater samples plotted in the plot have a slope of -0.31, which indicates certain extent of reverse ion exchange. This confirms that Ca,

35 103 Mg and Na concentration are interrelated to reverse ion exchange process. Further, to discriminate which ion (Ca or Mg) controls the hydrochemical reactions, two indices of Base Exchange (IBE), namely the chloroalkaline indices (CAI1 and CAI2) where estimated and presented below. The ion exchange between the groundwater and its host environment during residence or travel can be understood by studying the chloro-alkaline indices. To know the direction of exchange during the path of groundwater through the aquifer, Schoeller (1965) suggested 2 chloroalkaline indices CAI1 and CaI2 (May 2006) to indicate the exchange of ions between groundwater and its host environment. The ion exchange and reverse ion exchange was confirmed using chloro-alkaline indices. CAI1 = Cl-(Na+K)/Cl (6.6) CAI2 = Cl-(Na+K)/SO 4 + HCO 3 + CO 3 + NO 3 (6.7) (All values are measured in meq/l) When there is an ion exchange between Na or K in groundwater with Mg or Ca in the aquifer material (rock/weathered layer), both of the indices are positive, indicating ion exchange of sodium in groundwater with calcium or magnesium in the weathered material. While in reverse exchange both indices are negative when there is an exchange of Mg or Ca in the waters with Na and K in the rocks. The chloroalkaline indices (CAI1 and CAI2) are used to evaluate the event of base-exchange process during rock water interaction. In the study area, the value of these indices varies between positive and negative values (Figure 6.28, 6.29). There is no systematic seasonal variation in the values of indices. So the ion exchange reactions seem to occur in both the directions depending on the season, groundwater flow path, mixing of water and evaporation process. Thus the cation exchange

36 104 process is one of the important geochemical processes that control the groundwater chemistry of the area. Figure 6.28 Variation in Chloro-alkaline indices (CAI) in all the wells indicates indicating the Ion exchange process May 2006 LEGEND Positive areas Negative areas Figure 6.29 Schoeller Classification of Groundwater

37 SULPHATE REDUCTION PROCES The concentrations of sulphate in groundwater are very low through out the study area. They might have come from the dissolution of gypsum as in equation 6.8 because there is no acid rain or pyrite source in this area, which can supply sulphate to the groundwater. H 2 O + CaSO 4. 2H 2 O => Ca 2+ + SO H 2 O (6.8) Very low SO 4 /Cl ratios (low concentration of SO 4 ) (Figure 6.30) suggest that sulphate is being depleted, possibly by sulphate reduction (Lavitt et al. 1997). Earlier, Datta and Tyagi (1996) had observed that groundwater with high Cl and low SO 4 probably indicates reduction. Thus, the low sulphate concentration in the groundwater of this area may be due to sulphate reduction and perhaps lack of natural sources in the area. Figure 6.30 Plot of SO 4 Vs Cl indicates sulphate reduction

38 GROUNDWATER REDOX POTENTIAL Oxidation/reduction (Redox) reaction potential of groundwater (Eh) plays an important role in the geochemical processes that occur in groundwater. Redox is defined as the transfer of the electrons. Redox reactions are enormously important in aqueous environmental geochemistry Eh measurements are useful in identifying the redox zones as its value decreases with increases in residence time (Champ et al 1979) and reported that Eh values above 300mV indicate that sulphate would be stable in this area and it is a recharge area. High Eh values indicate the regions of good recharge and low Eh values are the regions of less recharge or discharge. Figure 6.31 shows the relation between Eh and ph were used to determine the groundwater conditions of the study area. The groundwater samples of the study area have high Eh value more than 557mV in the well no.34 and the lowest Eh value of the groundwater measured is 65mV at the sampling borehole no.36, but this borehole will give during the monsoon period of September 2006 as -65mV and the colour of the groundwater is yellow in colour throughout the sampling period. Eh values <100mV suggest that the redox conditions are low enough for sulphate reduction to occur (Champ et al 1979).Oxidation and reduction of sulphate and iron is a common process in an aquifer system. The ph vs Eh diagram shows (Figure 6.31) that how Eh in groundwater is governed in the upper range by oxidation of water to O 2 and lower range by reduction of hydrogen ions to H 2.The groundwater samples of the study area fall under ferrous (Fe 2+ ) i.e reduction state and Fe(OH) 2 i.e oxidation states. Fe 3+ + e - = Fe 2+ reduction state (6.9) Fe(OH) 3 + e - + H + = Fe(OH) 2 + H 2 O Oxidation state (6.10) From Figure 6.31 it is concluded that both oxidation and reduction states are taking places in the study area.

39 107 Figure 6.31 ph-eh diagram (May 2006) 6.11 MIXING OF SURFACE AND GROUNDWATER River, tanks/lakes and ponds are the important sources of the surface water resource in this area. In order to determine the interaction between surface water and groundwater, water samples were collected from surface water bodies located in the study area. These samples were analysed for the chemical constituents. The chemical composition of the groundwater along the Tondiar River is similar to the river water during the monsoon period (Table 6.1). The results confirm the mixing between surface water and groundwater. Similar the comparison of groundwater quality of pond and well was made in (Table 6.2) is similar. This indicates the effect of recharge from the ponds located in this region results in groundwater with low total dissolved solids. The storage in pond will be there generally for four months from the onset of monsoon.

40 108 Table 6.1 Comparisons of the groundwater and River water quality (November 2005) S.No EC Na K Ca Mg Cl HCO 3 So 4 Well No River water All values in mg/l except EC (μs/cm) Table 6.2 Comparisons of the Pond and Tank water quality (January 2006) S.No EC Na K Ca Mg Cl HCO 3 CO 3 So 4 Pond water (Korrakottai) Well no All values in mg/l except EC (μs/cm) The mixing process of various water is responsible for the chemical composition of groundwater in this region. The plot of Na and Cl concentration of groundwater indicate that the points fall on or near the 1: 1 indicating the dominance of mixing processes (Figure 6.24). The groundwater samples of the study area falls as a single group along mixing line. This shows that mixing of freshwater recharge with the existing water is the major mechanism taking place in the study area. Some of the samples in this area have high chloride concentration, which may be restricted flow. Thus in general, the groundwater chemistry of this area is controlled by mixing water, evaporation, rock water interaction and ion exchange process.