Available online at www.sciencedirect.com ScienceDirect Aquatic Procedia 4 ( 2015 ) 986 – 992 INTERNATIONAL CONFERENCE ON WATER RESOURCES, COASTAL AND OCEAN ENGINEERING (ICWRCOE 2015) Geochemical Modeling of Groundwater in Chinnar River Basin: A Source Identification Perspective Suma CS*, Srinivasamoorthy K, Saravanan K, Faizalkhan A, Prakash R, Gopinath S Pondicherry University, Department of Earth sciences, Pondicherry, India -605014 Abstract Understanding the geochemical evolution of groundwater is an essential part of the performance assessment and safety analysis of the geological system. Geochemical modeling techniques using PHREEQCI will aid in demarcating the main factors and mechanisms controlling the chemistry of groundwater. An attempt has been made in hard rock terrain of Chinnar basin, Dharmapuri district of Tamilnadu, India to interpret the processes and factors controlling hydrogeochemistry of groundwater. The area is made up of rock units belonging to Charnockite and Granitic Gneiss. Groundwater chemistry has been attempted + + 2+ 2+ - - - - based on laboratory observations of major cations like Na , K , Ca and Mg and anions like Cl , HCO3 , NO3 , F , 2- 3- SO4 and PO4 . Piper diagram reveals higher bicarbonate, calcium and magnesium along the recharge zones and tend to decrease along the flow path and vice versa for ions like sodium, potassium and chloride. Two flow paths based on piper and saturation index were identified for two major lithounits. The granitic terrain shows precipitation of calcium and sulfate minerals and dissolution of silicate minerals. The charnockite terrain shows precipitation of silicates and dissolution of calcium and sulfate minerals. From initial to final solution slight variation in calcium and sulfate stability were identified but drastic change with reference to silicate minerals stability due to effective dissolution of silicate minerals from the litho units of the study area. © 2015 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (©http://creativecommons.org/licenses/by-nc-nd/4.0/ 2015 The Authors. Published by Elsevier B.V.). PeerPeer-review-review under under responsibility responsibility of organizing of organizing committee committee of ICWRCOE of ICWRCOE 2015 2015. Keywords: Geochemical modelling, evolution, Piper, Stability, Saturation index, PREEQCI. * Corresponding author.. E-mail address: [email protected] 1. Introduction 2214-241X © 2015 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of organizing committee of ICWRCOE 2015 doi: 10.1016/j.aqpro.2015.02.124 C.S. Suma et al. / Aquatic Procedia 4 ( 2015 ) 986 – 992 987 The chemical interactions between groundwater and the geological materials of soils and rocks provide a wide variety of constituents. Geochemical modelling is an important tool for predicting the geological system. In hydrogeology it is very important for examining the geochemical behavior of the aquifer system. 2. Study area The study area Chinnar sub basin, located in Dharmapuri district, Tamilnadu state, India between longitude E 77° 36’and 78° 73’ and latitude N 11° 47’and 12° 33’ with a total geographical area of 1581km². The major litho units are granitic gneiss, charnockite, syenite and hornblende representing Archaean formations. Chinnar River is a tributary of Cauvery and originates on the high lands around Thali in Hosur taluk. After traversing in Dharmapuri district, it finally joins Cauvery River near Hogenakkal. Fig. 1. Location, Geology and the two flow paths selected for modelling studies 3. Hydrogeology 988 C.S. Suma et al. / Aquatic Procedia 4 ( 2015 ) 986 – 992 Groundwater in the study area occurs under phreatic condition in the weathered mantle and semi confined to confined condition in the fracturand fissured zones of these rocks. Thickness of weathered material varied widely from less than 1m bgl to more than 20m bgl. Water table fluctuation ranges betweenb10 to 20 m3/day. The estimation of groundwater resources comes under over exploited category. Dug wells are the most common groundwater abstraction structures used for irrigation in the district. 4. Materials and method A Total of 63 groundwater samples were collected during the month of January representing Post Monsoon (POM) season. The samples were analysed for major cations and anions (pH, conductivity, Ca2+ ,Mg2+, + + - 2- - 3- - - Na , K and Cl , SO4 , NO3 , PO4 , F , H4SiO4 and HCO3 ) using standard procedures (APHA,1995). Saturation index and inverse modelling were performed using PREEQCI program (Parkhurst and Appelo, 1999). 5. Results and discussion The statistics of chemical constituents in groundwater are given in table1.Ground water is generally acidic in nature with pH range between 6.14 to 8.17with and average of 7.16. Conductivity (EC) and Total Dissolved Solids (TDS) ranges between 1000 to 9640 μS/cm and 620 to 5580 mg/l with averages of 2400μS/cm 1641mg/l respectively. 5.1. Major water chemistry The general abundance of the major ions in groundwater follows the trend as: + 2+ 2+ + 2- - - - - 3- Na >Ca >Mg >K =HCO3 >Cl >NO3 >F >SO4 >PO4 The ground water of the study area shows higher concentration of Na+ ranging from 4.7 to 370.3mg/l with average of 109.67 mg/l, Ca2+ ranges between 14.1 to 235.0 mg/l and with average of 92.02 mg/l, Mg2+ from 7.56 to 162.3mg/l with an average of 61.18mg/l. Higher concentrations of cations are due to weathering of silicate minerals; Potassium was lower with ranges between 0.1 to 115.0 mg/l with average of 6.1 mg/l, due to its greater resistance to weathering and its fixation in the formation of clay minerals (Mohan et al 2000). Bicarbonate is the dominant anion ranging between 34.2 to 400.1 mg/l with an average of 208.5 mg/l - - followed by Cl between 8.6 to 450.6 mg/l with an average of 100.5 mg/l, NO3 ranges between 0.1 to 17.1 mg/l - 2- with an average value of 5.5mg/l, F ranges between 0.11 to 2.9 mg/l with an average of 1.26 mg/l, SO4 ranging 3- from 0.063 to 14.8 mg/l with average of 0.4 mg/l and PO4 ranges between 0.02 to 0.04 mg/l with averages of 0.03 mg/l. Silicate varied between 2.0 to 207.0 with an average value of 56.2 mg/l. Table1. Statistics of ground water sample Parameter Min Max Average St. Dev. pH 6.14 8.17 7.16 0.33 Cond 1000 9640 3512.22 2843.5 TDS 620 5580 2184.44 1723.3 2+ Ca 14.13 235 92.02 39.91 2+ Mg 7.56 162.3 61.18 29.29 + Na 4.75 370.3 109.67 99.93 C.S. Suma et al. / Aquatic Procedia 4 ( 2015 ) 986 – 992 989 + K 0.15 115 6.17 19.95 - Cl 8.62 450.6 100.51 88.5 - HCO3 34.25 400.1 208.55 78.82 2- SO4 0.063 14.8 0.49 2.09 - NO3 0.13 17.13 5.5 4.72 - F 0.11 2.91 1.26 0.57 H4SiO4 2 207 96.47 56.22 5.2. Hydrochemical facies A hydrochemical facies is a discrete zone of groundwater, identified to contain certain cation and , anion concentrations within definite boundaries. Piper diagram shows the percentage composition of major 2+ 2+ - cations+ and anions. From the piper plot (Fig 2.), major hydrochemical facies identified were: Ca - Mg - HCO 3 , 2+ 2+ + + 2+ + - - 2+ 2+ - + - Ca - Mg -Na+, Na - K - Ca , Na - K Cl and with minor representation of Ca - Mg - Cl and Na -Cl . Higher concentration of calcium, magnesium and bicarbonate were noted in the recharge zone and tend to decrease along the flow path direction, the ions like sodium, potassium, chloride are high in discharge region. (K. Sriniv asamoorthy et al, 2011). Fig. 2. Piper diagram of cation and anion showing post monsoon 5.3. Saturation indices (SI) of minerals in ground water chemistry The saturation index (SI) approach is to predict the reactive mineralogy of the litho units from the ground water data without collecting the samples of the solid phases and analysing the mineralogy (William J. Deutsch. et al). The aqueous speciation model PREEQCI (Parkhurst and Appelo, 1999) is used to calculate SI of minerals. The index is the log10 of the ion activity product of component of a mineral divided by the equilibrium constant for a mineral. SI greater than zero indicate mineral precipitated in water (over saturated), if less than zero, indicates mineral dissolved in water (under saturated), zero indicates equilibrium state of mineral constituents. Two flow paths were identified based on facies diagram (Hill Piper) and saturation 990 C.S. Suma et al. / Aquatic Procedia 4 ( 2015 ) 986 – 992 index value, topography and geology of the study area. For Charnockite (Sample no: 2 and 50) and for Granitic Gneiss (Sample no: 31-62) were selected as flow path (Fig 3). Silicate, Carbonate, Sufate, Chloride and Magnesium minerals reveals precipitation and dissolution nature along and quartz and chloride minerals like halite, magnesium and talc shows precipitation along the flow path; Carbonate minerals like Calcite, aragonite, and dolomite and sulphate minerals like anhydrite, gypsum, and sulphur shows dissolution along the flow path. In the charnockitic terrain silicate minerals, chloride minerals and magnesium minerals shows precipitation trend along the flow path; carbonate and sulphate minerals reveals dissolution. (Table2). Geochemical weathering causes precipitation and dissolution tendency in granite gneiss and charnockite minerals (K. Srinivasamoorthy 2012). Table2. Saturation index of minerals in two flow paths Phases Sample No:31 Sample No:62 Sample No:2 Sample No:50 Chalcedony 0.67 0.19 0.25 0.76 Chrysotile -5.02 -4.82 -5.00 0.15 Quartz 1.10 0.62 0.68 1.18 Aragonite -0.38 -0.56 -0.02 -0.02 Calcite -0.24 -0.41 0.13 0.13 Dolamite -0.31 -0.45 0.06 0.57 Anhydrite -4.92 -3.10 -4.36 -4.76 Gypsum -4.70 -2.88 -4.14 -4.54 Talc 0.01 -0.74 -0.80 5.36 5.4.