Evaluation of hydrogeochemical characteristics and the impact of weathering in seepage water collected within the sedimentary formation

Abstract

A study was conducted by collecting eight seepage water samples that drain through the sedimentary rocks, mainly sandstone and shale, to evaluate the hydrogeochemical characteristics. The collected samples were analyzed for physico-chemical parameters using standard procedures. Three water types were identified in the Piper plot and the hydrogeochemical evolution starts from a Ca–Cl facies (type 1) via mixed Ca–Mg–Cl and Ca–Na–HCO3 facies (type 2) to Na–Cl facies (type 3). Increasing trend of electrical conductivity (EC) values were observed from type 1 water to type 3 water. Lower ionic concentrations with an average EC value of 35.7 µs/cm in Ca–Cl facies indicate the recharge water by monsoonal rainfall, and ion exchange/weathering process is reflected in the mixing zone. Higher ionic concentration with an average EC value of 399 µs/cm is noted in Na–Cl facies, which indicates the ion exchange during water–rock interaction. Higher log pCO2 values are also found in this facies, revealing the longer residence time of seepage water in the rock matrix, which release more ions into the water. The relative mobility of elements during weathering suggest that the order of mobility in both sandstone and shale is Na > Ca > Mg > K. It was observed that the hydrogeochemistry of seepage water is mainly controlled by the bedrock geology.