Significance of pco 2 values in determining carbonate chemistry in groundwater of Pondicherry region, India

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1 Front. Earth Sci. DOI /s RESEARCH ARTICLE Significance of pco 2 values in determining carbonate chemistry in groundwater of Pondicherry region, India S Chidambaram 1, M.V Prasanna ( ) 2, U Karmegam 1, C Singaraja 1, S Pethaperumal, R Manivannan 1, P Anandhan 1, K Tirumalesh 4 1 Department of Earth Sciences, Annamalai University, Annamalai, Nagar , India 2 Department of Applied Geology, School of Engineering and Science, Curtin University, Sarawak, Miri-98009, Malaysia State Groundwater Unit and Soil Conservation, Department of Agriculture, Pondicherry , India 4 Scientist D, Isotope Hydrology Division, BARC, Mumbai , India Higher Education Press and Springer-Verlag Berlin Heidelberg 2011 Abstract The partial pressure of Carbon-Di-oxide plays a significant role in the water chemistry. It reflects the geochemical process and relates to the saturation index (SI) of the Carbonate minerals. A total number of 98 samples were collected from layered sequential aquifers like Alluvium, Upper Cuddalore sandstone, Lower Cuddalore Sandstone and Cretaceous formations, during Pre-Monsoon and Post-Monsoon seasons. Chemical parameters of groundwater such as ph, EC, TDS, Na +,K +,Ca 2+,Mg 2+, Cl, HCO,SO 2 4,PO 4 and H 4 SiO 4 were determined. The study shows that an increase in the log pco 2 values during water-rock interaction which influences the process of mineral dissolution. Saturation index of the carbonate minerals like Calcite, Aragonite, Dolomite and Magnesite were derived and compared with the log pco 2 values. In both the seasons the decreasing log pco 2 increases the saturation index of most of the carbonate minerals studied. The saturation index of almost all carbonate minerals during both the seasons showed negative correlation irrespective of the formation. Log pco 2 also develops a negative correlation with ph in groundwater of the study area. Keywords hydrogeochemistry, log pco 2, saturation index(si), geochemical modeling, coastal aquifers 1 Introduction The origin of carbon in groundwaters includes: 1) atmospheric CO 2, 2) dissolution of carbonate minerals, and ) microbial oxidation of organic carbon and/or carbon Received October 26, 2010; accepted February 21, geoprasanna@rediffmail.com dioxide from plant respiration. Relatively higher log pco 2 values (up to atm) for groundwaters possibly indicate the roles of sources 2) or ) in addition to atmospheric CO 2. Partial pressure of CO 2 (pco 2 ) in rivers are commonly out of equilibrium with atmosphere. Two possible explanations for this apparent paradox are: 1) River waters (particularly perennial rivers) contain a significant fraction of high CO 2 groundwater and 2) the rate of re-equilibrium with the atmosphere (by releasing the excess CO 2 ) is relatively slow (Stumm and Morgan, 1996; Holland, 1978). This factor is significantly used to understand the state of recharge and its relation to geochemical process (Chidambaram et al., 2006, 2007, 2009; Prasanna et al., 2010). As Calcite and Dolomite are soluble in acid solution, even rainwater will dissolve Carbonate rocks. Likewise, a change in ph can result in precipitation of CaCO from a solution that was at equilibrium prior to the ph shift. The equilibrium constants for the various reactions in Carbonate equilibrium are as follows: Carbonate equilibrium constants at 25 C CaCO Ca 2þ þ CO 2 K CaCO ¼ αþþ Ca α CO ¼ α CaCO ¼ 10 8:5 (1) HCO H þ þ CO 2 K H 2 CO ¼ α H þ α HCO α H2 CO ¼ 10 6:4 (2) HCO H þ þ CO 2

2 2 Front. Earth Sci. K HCO ¼ α H þ α CO α HCO ¼ 10 10: () H 2 O þ CO 2 H 2 CO K CO2 ¼ α H 2 CO pco 2 ¼ 10 1:5 (4) The log pco 2 also governs the rate of crystallization of carbonate minerals. The fact that, at in constant (rate of Crystallization) a greater Dissolved Organic Carbon concentration is required to block the active sites on the Calcite crystals with increasing log pco 2 which may be due to combination of at least three factors: 1) the increase in negative charge on the Calcite surface with increasing log pco 2 (Charlet et al., 1990), 2) the decrease of bonding capacity of the fulvic acid with decreasing ph of the solution (Engebretson et al., 1996) and ) the decrease in the specific volume of DOC due to increase in ionic strength of the solution (Benedetti et al., 1996). The present study aims to bring out the significance of the log pco 2 in the groundwater chemistry and the relationship to the saturation index of the carbonate minerals. 2 Study area The Pondicherry Region is located on the east coast of India forming enclaves within the Cuddalore District of Tamil Nadu (Fig. 1). It is bounded by north latitudes and 12 0 and east longitudes 79 7 and 79 5 and forms parts of Survey of India topographical maps Nos. 58 M/9, M/1 and 57 P/12 and P/16. The region is bounded on the east by Bay of Bengal and on the remaining sides by lands of Cuddalore district. The region is 29 km 2 in extent consisting of 179 villages. Gingee and Pennaiyar are the major rivers in Pondicherry region There is only one rain gauge station available (at Pondicherry) in entire Pondicherry region. The average annual rainfall in Pondicherry is 1254 mm. The groundwater level data collected from various monitoring wells in the study area during Pre-Monsoon shows that the water level ranges between 5.18 m below ground level (bgl) and m bgl and deeper groundwater levels were observed in the central, north-west and south-western parts of the study area. Similar trend was also noticed during the Post-Monsoon, except for the variation in groundwater level, which ranges between 4 m and 0 m bgl. Materials and methods A total of 98 groundwater samples were collected during Pre-Monsoon season (May 2007) and Post monsoon season (January 2008). A total of 54 samples were collected from the tube wells drilled in different coastal aquifers during Pre-Monsoon (Alluviual aquifer (16), Upper Cuddalore aquifer (UCS-9); Lower Cuddalore Sandstone aquifer (LCS-19) and Cretaceous aquifer (10)) of the study area. Similarly, 44 samples were collected from tube wells during Post-Monsoon season of (Alluvium (11), Upper Cuddalore Sandstone aquifer (9), Lower Cuddalore Sandstone aquifer (15) and Cretaceous aquifer (9)) (Fig. 2). Samples were analyzed for major and minor cations and anions using standard procedures (APHA, 1995). ph and EC were measured in situ by ph conductivity meter model CONSORT C425. Na + and K + were analyzed by ELICO flame photometer (CL 78), HCO was analyzed by titration, Cl,Ca 2+ and Mg 2+ were analyzed using Ion chromatograph (Metrohm 861). SO 2 PO 4, 4 was determined by digital double beam spectrophotometer (SL 164). Except for a meager part of the north-eastern corner of Pondicherry region, the entire area is covered by sedimentary formations ranging in age from Cretaceous to Recent (Fig. ). Outcrops of these formations are sparse and to supplement the surface geology and to establish the stratigraphy of the area, the Oil and Natural Gas Commission (ONGC) had carried out detailed geological 1), geophysical surveys and drilled nine shallow boreholes. 4 Results and discussion The results of the geochemical analysis for the groundwater samples (Tables 1 and 2) show that the cation dominance is in the following order Na + > Ca 2+ > Mg 2+ > K + and that of anions are HCO > Cl > SO 2 4, the trend remains the same in both the seasons. The nutrients show that H 4 SiO 4 > NO > PO 4 trend is prominent in both the seasons. 4.1 Partial pressure of carbon dioxide (log pco 2 ) In hydro-geological studies, the equilibrium of Calcium carbonate in contact with natural water, either surface or ground, is one of the most important geochemical reaction. Neutral water exposed to CO 2 in the atmosphere will dissolve CO 2 equal to the partial pressure. The CO 2 will react with H 2 O to form H 2 CO, a weak acid, and resulting solution will have a ph of about 5.7. Soil CO 2 from organic decomposition is another more important study in groundwater. Partial pressures of CO 2 (pco 2 ) in water were calculated from measured ph and alkalinity data using the program WATEQF (Plummer et al., 1976). Log pco 2 of the pre monsoon season (Fig. 4) were studied for different formations among that the alluvial formation has an 1) Nair K M, Rao V P (1971). Result of shallow drilling in the area north of Pondicherry. Unpublished ONGC field season report of

3 S Chidambaram et al. Carbonate chemistry in groundwater of coastal aquifers Fig. 1 Location map of the study area average of 1.86, the UCS formation has an average of 1.57 and that of LCS is 1.77 followed by cretaceous with But the distribution of the values (Fig. 4) shows that values of higher range are noted in UCS than other formations. The lowest value of 2.78 is noted in the alluvium and highest in the UCS as During the post monsoon (Fig. 4) the same trend was observed with average log pco 2 of alluvium are 2.2 that of UCS is 1.6, LCS is 2.5 and that of cretaceous is 2.2. In both season UCS shows high value of log pco 2 in most of the samples than other formations. The mean annual evapotranspiration in tropical regions has been reported to be about 600 mm ( mm) (Korea Water Resources Cooperation, 199). According to Brook (198), the mean log pco 2 value in soil under the annual evapotranspiration (AET) of 600 mm is about This log pco 2 value is equivalent to 0.17 mmole of aqueous CO 2 at 25 C. The dissolution of silicate minerals consumes large amounts of CO 2. If Albite and Anorthite were dissolved, 1 and 2 moles of CO 2 are consumed for unit mole of Na + and Ca 2+, respectively, through the following reactions. 2NaAlSi O 8 þ 11H 2 O þ 2CO 2 > 2Na þ þ Al 2 Si 2 O 5 ðohþ 4 þ 4H 4 SiO 4 þ 2HCO (5) CaAl 2 Si 2 O 8 þ H 2 O þ 2CO 2 > Ca 2þ þ Al 2 Si 2 O 2 ðohþ 2 þ 2HCO (6) As an example, the release of 0.67 mmole Na + (15.4 mg/l) and 0.52 mmole Ca 2+ (21.0 mg/l) for a water sample owing to the incongruent dissolution of albite and anorthite requires the consumption of CO 2 by about 1.71 mmole (Kim and Mueller, 1987). This calculation indicates that if dissolution of plagioclase feldspar is preceded under the CO 2 closed system, the amounts of CO 2 required for the dissolution is larger than the amounts of CO 2 generated from biologic process. The Upper Cuddalore sandstone is chiefly composed of alkali feldspar and quartz 1) and this 1) Nair K M, Rao V P (1971). Result of shallow drilling in the area north of Pondicherry. Unpublished ONGC field season report of

4 4 Front. Earth Sci. Fig. 2 Sample locations indicated on the geology map aquifer is also reported with saltwater intrusion. Therefore, we should consider the geochemical evolution of groundwater under partially open system in which CO 2 is supplied continuously. 4.2 Ionic strength The few samples of Alluvial and Lower Cretaceous aquifers representing Pre-monsoon (Fig. 5) exhibits ionic strength greater than Most of the samples range between 0.01 and The ionic strength of the Lower Cuddalore Sandstone aquifer samples range from 0.02 to 0.01 and that of the Upper Cuddalore Sandstone is still lesser than The higher ionic strength indicates longer residence time or greater interaction with the host rock (Chidambaram et al., 2007). The samples of Post-monsoon (Fig. 5) ranges from to The log pco 2 (Raymahashay, 1986) for each samples are determined to study its relation to recharge (Prasanna et al., 2010; Chidambaram et al., 2008). Log pco 2 value, during postmonsoon ranges from 2.75 to Lower values were observed in the samples of Alluvial and Cretaceous aquifers along with few samples of Lower Cuddalore Sandstone aquifer, indicating recharge. The ionic strength linearly increases with log pco 2 values except for few samples in Lower Cuddalore Sandstone aquifer. This may be due to higher ph or lesser neutralizing ions like Ca 2+. The ionic strength for few Post-monsoon samples is almost equivalent to the fresh water. In general the dilution effect during Post-monsoon samples is evident from the low ionic strength. There is no much change in the samples of the Upper Cuddalore Sandstone aquifer during Post- Monsoon season. 4. Saturation index(si) of carbonate minerals by geochemical modeling Given the chemical compositions of the water samples, the aqueous speciation, distribution, and saturation state with respect to various mineral phases were calculated using the computerized geochemical model WATEQF (Plummer et al., 1976). The saturation state of the water samples is defined by the SI, where Ion activity product SI ¼ log Equilibrium solubility constant : (7) Positive values of SI indicate that the water is supersaturated with respect to the mineral phase, negative values indicate undersaturation and zero indicates equilibrium. Having characterized the thermodynamic state of the observed groundwaters, calculations to indicate what processes led to that observed composition were performed. Mineral replacement is likely for a groundwater

5 S Chidambaram et al. Carbonate chemistry in groundwater of coastal aquifers 5 Fig. Geology map with cross section along AB line

6 6 Front. Earth Sci. Table 1 Chemical concentration of groundwater samples during pre-monsoon of the max, min, and average (all values are in mg/l except, EC in μs/cm and ph) Lithology ph EC Ca Mg Na K Cl HCO SO 4 PO 4 NO Br I TDS Min Alluvium Upper Cuddalore Sandstone Lower Cuddalore Sandstone Cretaceous Max Avg Min Max Avg Min Max Avg Min Max Avg Table 2 Chemical concentration of groundwater samples during post-monsoon of the max, min, and average (all values are in mg/l except, EC in μs/cm and ph) Lithology ph EC Ca Mg Na K Cl HCO SO 4 PO 4 H 4 SiO 4 NO Br I TDS Min Alluvium Upper Cuddalore Sandstone Lower Cuddalore Sandstone Cretaceous Max Avg Min Max Avg Min Max Avg Min Max Avg composition that is supersaturated with respect to one mineral while undersaturated with respect to another. All these reactions would tend to move an observed groundwater composition from the predicted mixing curve toward equilibrium at SI = 0. Saturation index of carbonate minerals is mainly based on the Bicarbonate concentration in water. Bicarbonate derived by the pressure of weathering or from other source remains in the aqueous medium trying to equilibrate with different ions like Ca 2+,Mg 2+, or both to attain saturation with different minerals like Calcite (SI c ), Aragonite(SI A ), Dolomite(SI D ) and Magnesite (SI m ) (Table ). The saturation index of Dolomite is calculated in two forms, as dissolved (SI Dd ) and colloidal (SI Dc ). SI of Carbonate minerals shows under saturation to saturation state in both the seasons. The saturation state of Carbonate minerals are in the following orders SI c > SI a > SI Dc > SI m > SI Dd. 4.4 Relationship of pco 2 to state of saturation Partial pressure of CO 2 plays a significant role in the state of saturation of Carbonate minerals. Considering the above fact and relationship between log pco 2 and SI C graph was plotted. During Pre-Monsoon (Fig. 6) Calcite shows saturation for alluvial aquifer samples; near saturation for Cretaceous aquifer samples and under saturation for Cuddalore Sandstone aquifer samples. It is also observed that log pco 2 ranges from 2to 1. Few samples of Alluvium and Cretaceous aquifers show saturation to near saturation with respect to Calcite at lesser log pco 2 condition. The Post-Monsoon samples exhibit comparatively (Fig. 7) higher log pco 2 condition than Pre- Monsoon. Increase of SI C is noticed in Lower Cuddalore Sandstone and Cretaceous aquifers, due to the dissolution of silicate minerals which increases the availability of

7 S Chidambaram et al. Carbonate chemistry in groundwater of coastal aquifers 7 Fig. 6 Saturation index of Carbonate minerals during premonsoon to pco 2 in different aquifers Fig. 4 Range of pco 2 in different for mation during pre and post monsoon. (a) Pre-monsoon; (b) post-mmsoon Fig. 7 Saturation index of Carbonate minerals during postmonsoon to pco 2 in different aquifers Fig. 5 Relationship of log pco 2 to ionic strength during postmonsoon (*) and pre-monsoon silicate minerals in the groundwaters of the formation resulting in the SI of calcite. The upper Cuddalore formation is dominated by alkali feldspars and hence Slc is lesser. This results in the saturation of Calcite for more Post-Monsoon samples in all the formations except UCS than the Pre-Monsoon. The Upper Cuddalore Sandstone aquifer shows under saturation, with respect to SI C in both the seasons with higher log pco 2 values. In general it is noted in both the seasons that with the decreasing log pco 2 saturation index are increases. Fresh water samples are even supersaturated with respect to aragonite, calculation of such supersaturated conditions for the relatively fresh groundwater samples may be related to the flux of CO 2 gas. CO 2 out gassing from the samples would lead to a calculated increase in the saturation state with respect to all carbonate minerals. Excess CO 2 content would also cause the solution to be less supersaturated with respect to all the carbonate minerals. This effect can only be sorted out through more complicated mass balance calculations Although dolomitization is possible for any composition of the calculated mixture, extensive dolomitization is only expected for the conditions where dolomite could replace an unstable mineral (Herman et al., 1985), i.e., where another carbonate mineral is undersaturated. An increase in the Carbon dioxide partial pressure during water-rock interaction influences the process of mineral dissolution. Carbon dioxide may be generated by several processes as discussed earlier. In the absence of Calcite in mineral assemblage, saturation in Calcite and high ph may be easily reached during dissolution of plagioclase in water

8 8 Front. Earth Sci. with dissolved CO 2 coming from atmosphere and soil, and under such condition, for instance, of Kaolinite saturation (Venturelli et al., 200). Although the concentration of CO 2 in the gas mixture is low, interaction of diffused emission with groundwater s could be sufficient to increase the Silicate dissolution. 4.5 Relationship of log pco 2 and hydrogen ion concentration The degree of dissociation of the organic matter and functional groups depends largely on the ph of the solution. At higher ph, the mutual repulsion of the negatively charged sites causes the molecule with multiple functional groups to adopt a stretched configuration. As the ph of the solution is lowered and some of the charged sites are neutralized, a reduction in intra-molecular repulsion is predicted, resulting in a contraction of the polymer chain. Engebretson et al. (1996) defined an association index for humic acids using fluorescence anisotropy. This association index is a quantitative parameter for estimating the polymer contraction. They found that the association index increases with decreasing ph, consistent with a structural contraction of the humic acid molecules. Another factor affecting the characteristics of DOC is the ionic strength. Murphy and Zachara (1995) and Benedetti et al. (1996) found that the specific volume of humic substances is strongly depending on the ionic strength. Several attempts have been made to model electrostatic interactions of the humic substances with organic ligands and cations (Marinsky and Ephraim, 1986; de Wit et al., 1990, 199; Bartschat et al., 1992; Barak and Chen, 1992; Tipping and Hurley, 1992; Milne et al., 1995). They also found that groundwater ph decreases when log pco 2 increases. Log pco 2 in the groundwater samples during Pre- Monsoon ranges from 0.7 to 0.. Lower values of ph and high log pco 2 are noted in UCS (Fig. 8), followed by LCS, the samples fall between ph ranging from 6 to 8.5. Fig. 9 Relationship of log pco 2 to Cl/HCO in groundwater samples during pre-monsoon Most of the Alluvium, Cretaceous and few LCS fall in a definite zone between ph 7 and 8 with log pco 2 values ranging from 2.7 to1.7. The UCS shows comparatively higher values of log pco 2 to compare that of other formations during Pre-monsoon this may be due to high Cl/HCO ratio (Fig. 9) noted in this formation suspecting mixture of saline water into this aquifer as the study area forms a part of coastal ecosystem. In general there is a decrease of ph values with increases of log pco2 values. During Post-monsoon the values of log pco 2 range from 2.78 to This shows there is an increase in pco 2 values during Post-monsoon, it may be due to the acquiring of more CO 2 during the time of infiltration by microbial oxidation of organic carbon or from plant respiration (Prasanna et al., 2006). It is also interesting to note that almost all the samples fall between ph 7 and 8 (Fig. 10). On average there is a increase of ph in this season and the samples group around ph 7 and log pco to 1.4, but the trend reveals the same as that of Pre- Monsoon. The lowering of Cl/HCO ratio (Fig. 11) in the Fig. 8 Relationship of log pco 2 to ph of groundwater samples during pre-monsoon Fig. 10 Relationship of log pco 2 to ph of groundwater samples during post-monsoon

9 S Chidambaram et al. Carbonate chemistry in groundwater of coastal aquifers 9 Fig. 11 Relationship of log pco 2 to Cl/HCO ratio in groundwater samples during post-monsoon UCS during this season is reflected with lesser values of log pco 2 in samples of this formation. Waters with log pco 2 less than bar, low salinity (Σions < 40 meql 1 ), variable but commonly high ph (up to 9.7), and moderate to very low Sulfate and Chloride concentrations; many waters of this group have extreme composition with very low Calcium, Magnesium, Potassium, Chloride and Sulfur contents (Toscani et al., 2001). Under the closed CO 2 conditions the ph rapidly rises far above the measured values, while log pco 2 and ion concentrations are much lower. Under the completely open log pco 2 condition, on the other hand, the simulated ph is too low and log pco 2 high. Therefore, we consider that the evolution of groundwater in the study area occurs under the partially open CO 2 conditions. 4.6 Correlation analysis The statistical methods have not attained a comparable discussion in case of groundwater studies to date (Aruga et al., 1995; Ashley and Lloyd, 1978; Giammanco et al., 1998). The multi dimensional data analysis methods are becoming very popular in environmental studies dealing with measurements and monitoring. The correlation of the log pco 2 values with the Saturation index of different carbonate minerals were studied for both the seasons (Table ). It is found that all formations had negative correlations, except of the alluvium with poor negative in Pre-Monsoon to near positive correlations during Post monsoon. The increasing order of negative correlation during Pre-Monsoon is as follows Alluvium < UCS < Cretaceous < LCS; but during the Post monsoon it is Alluvium < Cretaceous < LCS < UCS. It is interesting to note that there is significant variation in the order but for that of the UCS which has developed a strong negative correlation during post monsoon. The alluvium does not respond much to season as it is behaves as an open system and that of, Cretaceous and LCS as closed system. The Pre-Monsoon results in the depletion of water level and sea water intrusion into the aquifers in the UCS and make it a partially open system, which results in the increase of log pco 2 values. 5 Conclusions The above study indicates that Lower values of log pco 2 were noted in the Alluvial and Cretaceous aquifers and also in few Lower Cuddalore Sandstone samples, indicating recharge effect during Post-Monsoon. The saturation state of Carbonate minerals are in the following orders SI Calcite > SI Aragonite > SI Dolomite(c) > SI Magnesite > SI Dolomite(d). During Pre-Monsoon, Calcite exhibits saturation for Alluvial aquifer samples, near saturation for Cretaceous aquifer samples and under saturation for Cuddalore Sandstone aquifer samples. It is also noted that log pco 2 ranges from 2to 1. Few samples of Alluvium and Cretaceous aquifer exhibit saturation with respect to Calcite at low log pco 2 condition. Post-Monsoon samples show higher log pco 2 condition when compared with Pre- Monsoon. Three different conditions such as open, closed and partially closed system are witnessed in the study area. The open system is clearly witnessed in the alluvial formation, closed in the Lower Cuddalore and partially closed in the upper Cuddalore sandstones and cretaceous formations. Thus the geochemical modeling study on log pco 2 reveals that there is definite relation of this parameter with saturation index of minerals and geologic formations. Table Correlation of the log pco 2 values with the saturation index of different carbonate minerals Minerals Alluvium Cretaceous Lower Cuddalore Upper Cuddalore PRM Magnesite Dolomite Calcite Aragonite POM Magnesite Dolomite Calcite Aragonite

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