0016-7622/2008-72-3-405/$ 1.00 © GEOL. SOC. INDIA JOURNAL GEOLOGICAL SOCIETY OF INDIA Vol.72, September 2008, pp.405-414 Hydrochemical Evolution of Groundwater in the Phreatic Aquifers of Chhattisgarh R. K. RAY and RUMI MUKHERJEE Central Ground Water Board, Dhamtari Road, Raipur - 492 001 Email: ranjan.ray@lycos.com, rumimukherjee@yahoo.com Abstract: The phreatic aquifers form the most important repository of groundwater in Chhattisgarh state, India. Based on 323 representative water samples of the phreatic aquifers, the present study is conducted to trace the materials and processes controlling groundwater chemistry. Groundwater is mostly low in dissolved constituents. Bicarbonate type is the dominant type of water irrespective of the rock type. Sulphate and Chloride type waters are restricted to small patches. δD and δ 18 O characteristics of groundwater samples suggest that recharge to the phreatic aquifer system is rapid with little or no evaporative enrichment of the meteoric water. Electrical conductivities and chloride concentrations, when plotted against the distance from water divide, do not show any trend. Dissolution of bicarbonate by CO 2 charged meteoric water is the principal process controlling bicarbonate content in groundwater. Dissolution of gypsum from the gypsiferous formations (Maniyari shale and Raigarh Formation) produces sulphate type water in parts of Durg, Janjgir Champa, Bilaspur and Raigarh districts. Chloride concentration in groundwater is low. Ca/Mg carbonates in sedimentaries, Ca-feldspars and ferromagnesian minerals in the crystallines are the major sources of Ca and Mg in groundwater. Cation exchange process in the sedimentaries, and albite weathering in the crystallines were found to be important processes controlling concentration of Na in groundwater. Keywords: Hydrochemical evolution, Groundwater, Chhattisgarh. The phreatic aquifer is the most important and most exploited aquifer in the state. The objective of this study is to hydrochemically characterise groundwater in the phreatic aquifers and to analyse the processes involved in hydrochemical evolution of these waters. GEOHYDROLOGICAL SETTING The study area is underlain by diverse rock types of different geological ages ranging from Precambrian to Recent (Fig.1). Hydrogeological characteristics of the different geological formations have been discussed in detail by Mishra and Mohapatra (2002). Geological formations can be grouped into five broad hydrostratigraphic units: 1. Precambrian crystallines 2. Precambrian sedimentaries, 3. Semi consolidated sedimentaries (Gondwanas and Lametas), 4. Deccan Traps and 5. Unconsolidated sediments (Recent). Precambrian metasediments and crystallines cover nearly 55% of the entire state and are characterized by granitoids and metasedimentaries of Archaean and Proterozoic age. Groundwater samples analysed here are mostly representative of crystallines including granites and INTRODUCTION The study area covers the entire state of Chhattisgarh, and occupies nearly 1,35,000 sq. km area in the central part of India. The state is bounded by 17°47' and 24°07' N. latitudes and 81°14' and 84°24' E. longitudes (Fig.1). The state can be divided into three major physiographic divisions: northern hills, Chhattisgarh plains in the central part and Bastar plateau in the southern part. Topography is highly undulatory in most parts. Based on the drainage, the state can be divided into five basins/ sub basins, namely Mahanadi, Lower Ganga, Godavari and Narmada Basins and Brahmani sub-basin (Fig.1). Out of these five major river basins, Mahanadi Basin is the largest, covering 56% area of the state. The state has a sub-tropical humid climate and receives rainfall from the southwest monsoon, lasting from mid-June to mid-September. The normal annual rainfall (IMD) in the districts varies from 1112 mm in Rajnandgaon district (west central part) to 1487 mm in Jashpurnagar (northeastern part). Rainfall occurs mostly within 3 months (July, August and September). Total rainy days in a year vary from 70 to 100 days. Nearly 80% area of the state is covered by hard rocks.