Introduction. N. Rajmohan Æ L. Elango

Transcription

1 Identification and evolution of hydrogeochemical processes in the groundwater environment in an area of the Palar and Cheyyar River Basins, Southern India N. Rajmohan Æ L. Elango Abstract The Palar and Cheyyar River Basins in Tamil Nadu state of Southern India are characterised by different geological formations, and groundwater is the major source for domestic, agricultural and other water-related activities. Hydrogeochemical studies were carried out in this area with the objective of identifying the geochemical processes and their relation to groundwater quality. Groundwater samples were collected once a month from 43 groundwater wells in this area from January 1998 to July Sampling procedures and chemical analysis were carried out as per the standard methods. Chemical data are used for mathematical calculations and graphical plots to understand the chemical process and its relation to the groundwater quality. The chemical composition of groundwater in the central part of the study area mainly depends on the recharge from lakes and the river, which is explained by a mixing mechanism. In addition, weathering of silicate minerals controls the concentration of major ions such as sodium, calcium, magnesium and potassium in the groundwater of this area. Further, the activity ratios indicate that the groundwater is in equilibrium with kaolinite, smectite and montmorrillonite. The reverse ion exchange process controls the concentration of calcium, magnesium and sodium in hard rock formations, and dissolution of carbonate minerals and accessory minerals is the source of Ca and Mg, in addition to cation exchange in Received: 25 August 2003 / Accepted: 2 February 2004 Published online: 31 March 2004 ª Springer-Verlag 2004 N. Rajmohan Æ L. Elango (&) Department of Geology, Anna University, Chennai, India elango@annauniv.edu Tel.: Fax: the sedimentary formations. In general, the chemical composition of the groundwater in this area is influenced by rock water interaction, dissolution and deposition of carbonate and silicate minerals, ion exchange, and surface water interactions. Keywords Groundwater Æ Reverse ion exchange Æ Dissolution and deposition Æ Silicate weathering Æ Mineral equilibrium Æ Palar basin Æ Tamil Nadu Æ India Introduction Geochemical processes occurring within the groundwater and reactions with aquifer minerals have a profound effect on water quality. These geochemical processes are responsible for the seasonal and spatial variations in groundwater chemistry. The geochemical properties of groundwater depend on the chemistry of water in the recharge area as well as on different geochemical processes that are taking place in the subsurface. The quality of water along the course of its underground movement is therefore dependent on the chemical and physical properties of surrounding rocks, the quantitative and qualitative properties of through-flowing water bodies, and the products of human activity (Matthess 1982). Schuh and others (1997) indicated that increases in solute concentrations in the groundwater were caused by spatially variable recharge, governed by microtopographic controls in the Northern Great plains of the US. Ramesam (1982), Elango (1992), and Ballukraya and Ravi (1999) reported that the rising water table in the post-monsoon period dissolves more saline matter from the soils and increases the salinity of water after monsoon. When groundwater is used for irrigation, its recycling will rapidly increase the salinity of groundwater by repeated circulation in addition to evaporation and evapotranspiration (Nativ and Smith 1987; Chourasia and Tellem 1992; Elango and Ramachandran 1991; Hamilton and Helsel 1995; Fisher and Mulican 1997; Kraft and others 1999). Further, the weathering of primary and secondary minerals are DOI /s Environmental Geology (2004) 46:

2 Fig. 1 Location map of the study area brought about by the release of cations and silica (Jacks 1973; Rajmohan and others 2000; Mohan and others 2000). Weathering of pyroxene, amphiboles and calcic feldspar minerals, which are common in basic rocks and are easily weatherable, is an important process controlling the concentration of these ions in groundwater (Jacks 1973; Bartarya 1993). The previous studies explain the influences of geology, hydrogeology, climate and human activities on groundwater chemical composition. Hence, it has become imperative to evaluate their effects on groundwater quality. Such a detailed study has not been attempted before in an area of the Palar and Cheyyar River Basins, Southern India, although earlier studies in this region conducted by government agencies (PWD 2000) explained the current status with regard to groundwater condition, potential and quality of this area. Rajmohan and Elango (2001) carried out a preliminary study on the movement of chloride and nitrogen in the unsaturated zone of this area. Therefore, no systematic hydrogeochemical studies have been carried out here. Hence, in the present study, a detailed investigation was carried out with the objective of identifying the hydrogeochemical process and its relation to groundwater chemical composition in an area of the Palar and Cheyyar River Basins, Southern India. Materials and methods Description of the study area The study area (Fig. 1) is in the Kancheepuram District in Tamil Nadu State. It forms a section of the Palar and Cheyyar River Basins, and is 70 km west of Chennai City. It covers an area of 234 km 2 extending approximately 18 km north-south and about 13 km east-west. The maximum elevation is 94 m above mean sea level in the eastern side of the study area and the minimum elevation is about 50 m above the mean sea level. Generally, the area slopes towards the east. The Vegavathi River flows in the northwestern part of the study area. Climate and rainfall The study area has dry climatic conditions, with a maximum temperature of around 39 C during the months of April and May, and a minimum temperature of around 21 C during the months of November and December. It receives an average annual rainfall of 1,113 mm, of which 60% is contributed by the North East (NE) monsoon (bringing heavy rain from October to December); the rest is during the South West (SW) monsoon (bringing some rain from June to September). Heavy rains are often associated with depressions and storms, which generally occur in the Bay of Bengal during the NE monsoon. Highest rainfall was received during 1998 according to the 48 Environmental Geology (2004) 46:47 61

3 Fig. 2 Drainage map of the study area rainfall record for the last 20 years. Rainfall is the major source of groundwater. The drainage map of this area is given in Fig. 2, which also indicates the locations of ponds and lakes. The rivers in this region only flow for a few days each year after monsoon rains. Therefore, the ponds and lakes have water for a maximum of only two months after NE monsoon, which are also the source for groundwater recharge. Geology The study area is comprised of partly sedimentary and partly crystalline formations (Fig. 3). The crystalline formations are charnockite, granitic gneiss and ultra basic rock. The crystalline charnockite and granitic gneiss of Archean age have been intruded by amphibolites, dykes of dolerite and occasionally by veins of quartz and pegmatites. The gneisses of this area have quartz, feldspars (potash feldspars and albite), hornblende, biotite, and so on. The acidic charnockite of this area has quartz, k-feldspars, hypersthene, and biotite minerals of a coarsegrained nature. These crystalline formations are unconformably overlain by sandy and clayey soils of Recent to sub-recent age. The sedimentary rock types include sandstone and shale. Alluvial deposits constitute the youngest formation, essentially composed of sand with intercalated clay and occurring along river courses. The alluvium frequently contains gypsum and carbonate minerals as evaporates. Figure 4 shows the lithology in some of the boreholes. Hydrogeology Groundwater occurs under water table conditions in weathered, fractured, jointed and faulted portions of crystalline rocks. The pore spaces developed in the weathered portions act as a shallow granular aquifer and form potential water-bearing zones. The depth of wells in this region reaches 10 m. Most of the wells in the hard rock region are large diameter dug wells. Generally, the water level in the crystalline formations is 6 9 m below ground level. Sometimes, during summer, these wells become completely dry. Alluvium is the most important sedimentary formation, occurring on both sides of the river. In these formations, wells have been dug to a depth of 23 m. Most of the wells are bore and dug cum borewells (open wells with a borewell at the bottom). The water level in these wells fluctuates between 5 20 m below ground level. Most of the area is covered by agricultural lands. The main cropping season is from September to January, when paddy (rice) is grown. During this period, both surface and groundwaters are used for irrigation. The second cropping season is from February to May when paddy, vegetables, pulses and groundnut are grown. These crops are mainly dependent on surface water in lakes/ponds and groundwater. Surface water is used for irrigation from October to May. Paddy and other crops are sometimes grown in some parts of the area from May to September, depending on groundwater. Groundwater sampling and analysis Initially, to understand the general variation in groundwater chemistry over the study area, a well inventory survey was carried out during November Almost all Environmental Geology (2004) 46:

4 Fig. 3 Geological map of the study area of the wells in the area were monitored for pumping patterns, groundwater levels, electrical conductivity (EC) and ph. These data were used to select the representative wells for routine groundwater sampling once a month from January 1998 to June Sampling wells are selected in such a way that they represent different geological formations of this area. Groundwater samples were collected once a month from 43 sampling wells. In total, about 660 groundwater samples were collected. Water samples were collected in clean polyethylene bottles. All sampling bottles were soaked with 1:1 HNO 3 and washed using detergent. These bottles were then rinsed with double-distilled water. At the time of sampling, sampling bottles were thoroughly rinsed 2 3 times using the groundwater to be sampled. In the case of bore wells, the water samples were collected after pumping the water for 10 minutes. In the case of open wells, water samples were collected 30 cm below the water level using a depth sampler. EC and ph of groundwater samples were measured in the field immediately after sample collection using a portable field kit. Water levels in the wells were recorded using a water level recorder. Apart from groundwater samples, river waters (Palar and Cheyyar Rivers) and lake water were also collected for a few months for comparison with groundwater. Samples collected were transported to the laboratory on the same day and they were filtered using 0.45 lm Millipore filter paper and acidified with nitric acid (Ultrapure, Merck) for cation analyses. For anion analyses, these Fig. 4 Borehole lithology of the selected wells in the study area samples were stored below 4 C. The samples were analysed for major cations (Na +,Ca 2+,Mg 2+,K + ) and anions (Cl,SO 4 2), HCO 3,CO 3 2 ). The chemical analysis was carried out as per the procedure given in APHA (1995). The analytical precision for the measurements of ions was determined by calculating the ionic balance error, which is generally within 5%. Apart from the collection and analysis of water samples, other data such as rainfall and borehole logs from the study area were collected from the Public Works Department (PWD 2000). 50 Environmental Geology (2004) 46:47 61

5 Table 1 Comparison of groundwater and lake water (March 1999) S. No. ph EC Na K Ca Mg Cl HCO 3 CO 3 SO 4 NO 3 Lake water Groundwater (Well No. 16) All values in mg/l except ph and EC (ls/cm) Table 2 Comparison of groundwater and Palar river water (October 1998) S. No. ph EC Na K Ca Mg Cl HCO 3 CO 3 SO 4 NO 3 Palar river water Groundwater (Well No. 1) All values in mg/l except ph and EC (ls/cm) Fig. 5 Sodium and chloride plots explaining the mixing process Results and discussion The results from the chemical analyses were used to identify the geochemical processes and mechanisms in the aquifer region. All of the identified processes are explained in detail in the following sections. Mixing process Lakes and rivers are important features of the landscape in the study area and provide a flux of fresh water into the groundwater system, as discussed earlier. The presence of these lakes modifies the configuration of the water table and controls the groundwater chemistry, particularly in Environmental Geology (2004) 46:

6 the central region of the study area. To determine the interaction between surface water and groundwater, water samples were collected from rivers and lakes in the study area and analysed for chemical constituents. The chemical compositions of groundwater and surface water from lakes (Table 1) are similar, which indicates recharge of surface water from the lakes in the central part of the area. Groundwater along the Palar River has a similar chemical composition to that of the river water during monsoon (Table 2). However, the groundwater is discharged into the rivers during non-monsoon periods. Similar results were also observed in groundwater along the Cheyyar River. The results confirm the mixing process between surface water and groundwater. The plot of sodium and chloride concentration of groundwater along with mixing line between lowest and highest salinity water of this area is shown in Fig. 5. The line in the plot was obtained by mixing of low and high salinity water in different proportions (20%+80%, 40%+60%, 50%+50%, 60%+40%, 80%+20%). Groundwater samples of the study area plot as a single group along the mixing line. This shows that the mixing of existing water with fresh recharge water is the major mechanism that is taking place in this area. This also indicates that the groundwater flows between sedimentary and hard rock formations. Silicate weathering process Weathering of silicate rocks in the region is one of the important processes responsible for the higher concentration of Na in groundwater of this area. In general, it is expected that the evaporation process would cause an increase in concentrations of all species in water. If the evaporation process is dominant, assuming that no mineral species are precipitated, the Na/Cl ratio would be unchanged (Jankowski and Acworth 1997). Hence, the plot of Na/Cl versus EC would give a horizontal line, which would then be an effective indicator of concentration by evaporation or evapotranspiration. The Na/Cl versus EC Fig. 6 Plot of Na/Cl ratio versus EC 52 Environmental Geology (2004) 46:47 61

7 Fig. 7 Plot of sodium and chloride explaining the evaporation process plot (Fig. 6) of this area clearly indicates that the ratio of Na/Cl decreases with an increasing EC value. A high sodium chloride ratio is observed at low EC value (<1500). Similarly, the sodium versus chloride (Fig. 7) plot indicates that most of the samples plot above the fresh water evaporation line. This indicates that evaporation may not be the major process controlling groundwater chemistry. Hence, sodium in the groundwater might have been derived from some other processes. If halite dissolution is responsible for sodium, the Na/Cl molar ratio should be approximately equal to one, whereas a ratio greater than one is typically interpreted as Na released from a silicate weathering reaction (Mayback 1987). In the present study, the molar ratio of Na/Cl for groundwater samples of the study area generally ranges from (Fig. 6). Samples having a Na/Cl ratio greater than one (Fig. 6) indicate 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 anion (Rogers 1989). This is because of the reaction of the feldspar minerals with the carbonic acid in the presence of water, which releases HCO 3 (Elango and others 2003). HCO 3 is the dominant anion in groundwater of this area. Hence, silicate weathering is the reason for sodium in groundwater. A plot of Na+K/Na+K+Ca versus TDS (Fig. 8) also indicates the importance of rock water interaction in this area (Gibbs 1970). However, samples with a Na/Cl ratio around and less than one indicate the possibility of some other chemical processes, such as ion exchange. Geochemical equilibria and reaction The equilibrium state of groundwater with respect to the possible reactant and product minerals was evaluated by mathematical and graphical approaches. The mathematical approach is often used for calculation of saturation indices of groundwater with respect to the mineral phases, providing some indication of the equilibrium state between the groundwater and the surrounding mineral rock assemblages (Njitchoua and others 1997). The graphical approach describes the mineral stability fields of minerals in equilibrium with groundwater, in terms of activity ratios of ions in groundwater (Helgeson 1968; Kramer 1968; Helgeson and others 1969; Paces 1972; Fritz 1975). The stability fields of crystalline minerals adequately serve as reference points to predict what minerals will react with the groundwater and the direction and extent of the reactions (Rogers 1989). The investigation of the mineral equilibria state of groundwater of this area has revealed that it is in equilibrium with the weathering product of silicate minerals. Interaction between Environmental Geology (2004) 46:

8 Fig. 8 Mechanism controlling groundwater chemistry weathered rock and groundwater is the major process in the area, as seen in the Gibbs diagram (Fig. 8). The important minerals in equilibrium with the groundwater of this area are kaolinite, smectite and montmorrillonite, due to rock water interaction. Kaolinite equilibria Kaolinite is a common weathering product of feldspar and other silicates (Garrels 1967; Nesbitt and Young 1984; Njitchoua and others 1997). The study area is characterised by the presence of weathered silicate rocks. Hence, in general, the groundwater of the study area is in equilibrium with kaolinite, as can be identified from the stability diagram of the partial system MgO Na 2 O Al 2 O 3 SiO 2 H 2 O (Fig. 9). Similar results are also obtained from the stability diagrams of the partial systems CaO Al 2 O 3 SiO 2 H 2 O CO 2 (Fig. 10), Na 2 O Al 2 O 3 SiO 2 H 2 O (Fig. 11) and ph pna versus log(ah 4 SiO 4 ) (Fig. 12) (Stumm and Morgan 1981). In addition, a high percentage of water samples clustered near the intersection of the saturation boundary of calcite in the stability diagram (Fig. 10). Smectite equilibria Smectite is also a common weathering product in feldspathic rocks (Garrels 1967; Nesbitt and Young 1984). Groundwater in a few wells of the study area is in Fig. 9 Stability field diagrams of the partial system MgO Na 2 O Al 2 O 3 SiO 2 H 2 O showing the chemical composition of groundwater samples (after Helgeson and others 1969, Rogers 1989) equilibrium with Mg-smectite and Ca-smectite, as identified from the stability diagram of the partial systems MgO Na 2 O Al 2 O 3 SiO 2 H 2 O (Fig. 9) and CaO Al 2 O 3 SiO 2 H 2 O CO 2 (Fig. 10). Garrels (1967) and Paces (1972) found that groundwater from feldspathic rocks is in equilibrium with kaolinite and smectite. Rogers (1989) also found that groundwater in a few regions of the US is in equilibrium with kaolinite and smectite. 54 Environmental Geology (2004) 46:47 61

9 Fig. 10 Stability field diagrams of the partial system CaO Al 2 O 3 SiO 2 H 2 O CO 2 showing the chemical composition of groundwater samples (after Nesbitt and Young 1984, Rogers 1989) Fig. 11 Stability field diagrams of the partial system Na 2 O Al 2 O 3 SiO 2 H 2 O showing the chemical composition of groundwater samples (after Helgeson and others 1969) Montmorillonite equilibria Groundwater in a few wells of the study area is in equilibrium with montmorillonite, as identified from the stability diagram of the partial systems of Na 2 O Al 2 O 3 SiO 2 H 2 O (Fig. 11) and ph pna versus log(ah 4 SiO 4 ) (Fig. 12) (Stumm and Morgan 1981). Jacks (1973) studied the chemistry of groundwater in the Coimbatore district in Southern India, and suggested that generally the water found in igneous rocks is just on the phase-border between kaolinite and montmorillonite. Therefore, the study of all of these stability diagrams prepared using the activity of ions revealed that the following are the major geochemical reactions controlling the groundwater chemistry of this region: CaCO 3 $ Ca 2þ Calcite þ CO 2 3 ð1þ Fig. 12 ph-pna plotted as a function of log(ah 4 SiO 4 ) (after Stumm and Morgan 1981) 6CaAl 2 SiO 10 ðohþ 2 +2H H 2 O $ 7Al 2 Si 2 O 5 ðohþ 4 +Ca 2þ +8H 4 SiO 4 a 2+ +8H 4 SiO 4 Ca - smectite Kaolinite MgAl 2 SiO 10 ðohþ 2 +2H H 2 O $ 7Al 2 Si 2 O 5 ðohþ 4 +Mg 2þ +8H 4 SiO 4 Mg - smectite Kaolinite 2.33NaAlSi 2 O H 2 O+2O 2 $ Na 0:33 Al 2:33 Si 3:6 O 10 ðohþ 2 + 2Na + 2HCO H 4 SiO 4 Albite Na montmorillonite ð2þ ð3þ ð4þ or 2NaAlSi 3 O H 2 O + 2CO 2 $ Al 2 Si 2 O 5 ðohþ 4 + 2Na + 2HCO 3 H 4 SiO 4 ð5þ Albite Kaolinite Such results were also reported by Rogers (1989). Dissolution and deposition Mineral equilibrium calculations for groundwater are useful in predicting the presence of reactive minerals in the groundwater system and estimating mineral reactivity (Deutsch 1997). If certain minerals such as calcite and dolomite are commonly found in equilibrium with groundwater, it is then reasonable to assume that these minerals are reactive in typical groundwater environments and that they can control solution concentration. By using the saturation index approach, it is possible to predict the Environmental Geology (2004) 46:

10 Fig. 13 Monthly variation of groundwater calcite saturation index (SI) and groundwater level (WL) in the study area Fig. 14 Monthly variation of groundwater dolomite saturation index (SI) and groundwater level (WL) in the study area reactive mineralogy of the subsurface from groundwater data without collecting the samples of the solid phase and analysing the mineralogy (Deutsch 1997). In the present study, to determine the chemical equilibrium between minerals and water, saturation indices (SI) of calcite and dolomite were calculated. If the groundwater is saturated with respect to a mineral, it is prone to deposit (precipitation) some of the solute load. On the other hand, if it is undersaturated (SI<0) it will take more mineral into the solution (dissolution). Hence, the saturation index of a mineral is calculated based on the following equation (Lloyd and Heathcode 1985): SI ¼ log IAP K s ð6þ where IAP is the ion activity product and K s is the solubility product of the mineral. The calcite and dolomite SI are defined by Sl C ¼ log Ca 2þ þ log CO 2 log KC ð7þ Sl D ¼ log Ca 2þ 3 þ log Mg 2þ þ 2 log CO 2 3 log KD ð8þ where SI C is the saturation index of calcite, SI D is the saturation index of dolomite, K C is the solubility product of calcite, and K D is the solubility product of dolomite. The SI of calcite and dolomite of groundwater of this area vary with respect to the season. During rainfall recharge, groundwater is undersaturated with respect to calcite and dolomite (Figs. 13 and 14) due to dilution. During dry periods, groundwater is oversaturated with respect to these minerals due to evaporation and therefore they are deposited. Such precipitation was noticed on the walls of the wells in this area. Recharge of rainwater during the subsequent monsoon dissolves these minerals, and evaporation enriched water in the soil zone is flushed into the groundwater. This increases the SI of groundwater. As rainfall continues, the saturation levels go down due to dilution. Several researchers have identified similar processes (Elango and Ramachandran 1991). Garrels (1967) and Paces (1972) found that the more highly evolved groundwater from feldspathic igneous rocks reached saturation with calcite. The study of the Ca/Mg ratio of groundwater from this area also supports the dissolution of calcite and dolomite present in the alluvium (Fig. 15). That is, if the tio Ca/Mg=1, 56 Environmental Geology (2004) 46:47 61

11 Fig. 15 Plot of Ca/Mg molar ratio dissolution of dolomite should occur, whereas a higher ratio is indicative of greater calcite contribution (Maya and Loucks 1995). Higher Ca/Mg molar ratio (>2) indicates the dissolution of silicate minerals, which contribute calcium and magnesium to groundwater (Katz and others 1998). In Fig. 15, the points closer to the line (Ca/Mg=1) indicate the dissolution of dolomite. Most of the samples having a ratio between 1 and 2 indicate the dissolution of calcite. Those with values greater than 2 indicate the effect of silicate minerals (Fig. 15). Reverse ion exchange The plot of Ca+Mg versus SO 4 +HCO 3 will be close to the 1:1 line if the dissolutions of calcite, dolomite and gypsum are the dominant reactions in a system. Ion exchange tends to shift the points to the right due to an excess of SO 4 +HCO 3 (Cerling and others 1989; Fisher and Mulican 1997). If reverse ion exchange is the process, it will shift the points to the left due to a large excess of Ca+Mg over SO 4 +HCO 3, which can be explained by the following reaction: 2Na þ þ CaðMg Þclay $ Na - Clay þ Ca 2þ Mg 2þ ð9þ The plot of Ca+Mg versus SO 4 +HCO 3 (Fig. 16) shows that most of the groundwater samples from hard rock formations are clustered around and above the 1:1 line (Fig. 16). An excess of calcium and magnesium in the groundwater of hard rock formations may be due to the exchange of sodium in the water by calcium and magnesium in clay material. Groundwater samples in sedimentary formations are plotted on and below the 1:1 line, which is due to excess bicarbonate. In addition, a plot of Na Cl versus Ca+Mg HCO 3 SO 4 (Fig. 17) also supports the hypothesized reverse ion exchange process. If ion exchange is the dominant Fig. 16 Relation between Ca+Mg and SO 4 +HCO 3 process in the system, the waters should form a line with a slope of )1. Similar results were observed for the groundwater of hard rock formations of this area Environmental Geology (2004) 46:

12 Fig. 18 Relation between Ca+Mg and salinity Fig. 17 Relation between Ca+Mg-HCO 3 -SO 4 and Na-Cl (Fig. 17). The diagrams show that the points give a line with a slope of ) This confirms that Ca, Mg and Na concentrations are interrelated through reverse ion exchange. However, such a relation is not observed in the groundwater of the sedimentary formations. The plot of m(ca+mg) versus m(cl) (Fig. 18) indicates that Ca and Mg increase with increasing salinity. The plots of m(na/cl) versus m(cl) (Fig. 19) and m(ca+mg) versus m(cl) (Fig. 20) clearly indicate that salinity increases with the decrease in Na/Cl and increase in Ca+Mg, which may be due to reverse ion exchange in the clay/weathered layer. During this process, the aquifer matrix may adsorb dissolved sodium in exchange for bound Ca and Mg. The sources of Ca and Mg in groundwater can be deduced from the (mmg+mca)/mhco 3 ratio. As this ratio increases with salinity (Fig. 20), Mg and Ca are added to solution at a greater rate than HCO 3. If Mg and Ca originate solely from the dissolution of carbonates in the aquifer materials and from the weathering of accessory pyroxene or amphibole minerals, this ratio would be about 0.5 (Sami 1992), as the governing weathering equation would be for Calcite CaCO 3 +H 2 O þ CO 2 $ Ca þ 2HCO 3 ð10þ for Pyroxene CaMgðSi 2 O 6 Þ þ 4CO 2 þ 6H 2 O $ Ca þ Mg þ 4HCO 3 þ 2SiðOHÞ 4 ð11þ for Amphiboles Ca 2 Mg 5 Si 8 O 22 ðohþ 2 þ 14CO 2 þ 22H 2 O $ 2Ca þ 5Mg þ 14HCO 3 þ 8SiðOHÞ 4 ð12þ As seen in Fig. 20, the low (Ca+Mg)/HCO 3 ratios (<0.5) could be the result of either Ca+Mg depletion by cation exchange or HCO 3 enrichment. However, high ratios cannot be attributed to HCO 3 depletion; under the existing alkaline conditions, HCO 3 does not form carbonic acid (H 2 CO 3 ) (Spears 1986). High ratios, therefore, indicate other sources for Ca and Mg, such as reverse ion exchange, which is observed in hard rock formations with an increase in salinity (Fig. 20). A ratio less than 0.5 may be due to the exchange of calcium and magnesium in water by sodium bound in the clay. This ratio does not vary much with respect to salinity, as observed in sedimentary formation. In addition, all of the groundwater samples of sedimentary formations have a (Ca+Mg)/HCO 3 ratio less than one. Hence, dissolution of carbonate minerals and accessory minerals is the source for Ca and Mg, in addition to cation exchange in sedimentary formations. Further, ion exchange and reverse ion exchange occurring in this region are also confirmed by the two indices of base 58 Environmental Geology (2004) 46:47 61

13 Fig. 19 Relation between Na/Cl ratio and salinity Fig. 20 Relation between (Mg+Ca)/HCO 3 and salinity Environmental Geology (2004) 46:

14 or weathered material. On the other hand, during the subsequent dry season when the water table is low, groundwater is oversaturated with carbonate minerals with high Ca and Mg. Hence, ion exchange is dominant; that is, an exchange of calcium and magnesium in water with the sodium in aquifer material. Conclusions Fig. 21 Relation between water level (WL) fluctuation and chloro alkaline indices (CAI) exchange (IBE), namely the chloro alkaline indices (CAI 1 and CAI 2), CAI 1 ¼ Cl ðna þ KÞ ð13þ Cl CAI 2 ¼ Cl ðna þ KÞ þ HCO 3 þ CO 3 þ NO 3 ð14þ SO 4 (all values expressed in meq/l) suggested by Schoeller (1965). When there is an exchange between Na or K in groundwater with Mg or Ca in the aquifer material, both of the indices are positive, indicating reverse ion exchange. If the exchange takes place between the Ca or Mg in groundwater with Na or K in the aquifer material, the indices will be negative, indicating ion exchange. In this area, these indices of groundwater vary with respect to time (Fig. 21). Generally, they are positive when the water table is high, and they become negative with the lowering of the water table. During rainfall recharge and the rise in the water table, the groundwater becomes undersaturated with carbonate minerals, as stated earlier, and so Na is dominant in groundwater. Therefore, reverse ion exchange is predominant; that is, there is an exchange of sodium in groundwater with calcium and magnesium in the alluvium The chemical composition of groundwater in a section of the Palar and Cheyyar River Basins is strongly influenced by rock water interaction, dissolution and deposition of carbonate and silicate minerals, ion exchange and surface water interactions. Groundwater chemical composition in the central part of the study area is mainly controlled by lakes and the river, which is explained by the mixing mechanism. Weathering of silicate minerals control the major ions such as sodium, calcium, magnesium and potassium in groundwater in this area. The activity ratios indicate that the groundwater is in equilibrium with kaolinite, smectite and montmorrillonite, which are the major weathering products from feldspar and other silicates. The SI of calcite and dolomite were calculated, which vary with respect to season. Groundwater is undersaturated with respect to calcite and dolomite during rainfall recharge and oversaturated during dry periods. The ionic ratio of Ca/Mg explains the contribution of calcite and dolomite to the groundwater calcium, magnesium and bicarbonate content. In addition, the reverse ion exchange process controls the concentration of calcium, magnesium and sodium concentration in hard rock formations. In the case of sedimentary formations, dissolution of carbonate minerals and accessory minerals are the sources for Ca and Mg, in addition to cation exchange. During rainfall recharge followed by a rising water table, reverse ion exchange is predominant; and during the summer followed by the lowering of the water table, ion exchange is dominant. Therefore, in general, the groundwater chemistry of this area is principally controlled by the mixing of waters, evaporation, mineral equilibria, dissolution and deposition, and ion exchange processes. Acknowledgements Thanks are due to the University Grants Commission, New Dehli for financial support under the Centre with potential for Excellence in Environmental Sciences and DRS- SAP schemes. The authors would like to thank the reviewers of this paper for their useful comments. References APHA (1995) Standard methods for the examination of water and wastewater, 17th edn. APHA, Washington, DC Ballukraya PN, Ravi R (1999) Characterisation of groundwater in the unconfined aquifer of Chennai city, India. J Geol Soc India 54:1 11 Bartarya SK (1993) Hydrochemistry and rock weathering in a sub-tropical lesser himalayan river basin in Kumaun, India. J Hydrol 146: Environmental Geology (2004) 46:47 61

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