Role of Geochemistry of Rocks and Soils for Groundwater Quality in the Chithar River Basin, South India—An Approach Using Enrichment Factor (EF) of Trace Elements and Chemical Index of Alteration (CIA)

The geology and hydrogeology of a study area primarily exert control over groundwater recharge and reserves, groundwater chemistry, suitability for various purposes, weathering processes in rocks and soils, as well as geochemical processes in groundwater. Establishing connections among these factors is crucial for gaining a comprehensive understanding of the groundwater regime. As such, the Chithar River basin, situated between latitudes 08° 48′ N–09° 14′ N and longitudes 77° 11′ N–77° 46′ E in Tirunelveli District, Tamil Nadu, India, has been selected for an integrated analysis of the aforementioned parameters. Examinations of the rock and soil composition reveal that the charnockites in this area are notably rich in quartz, plagioclase feldspars, and orthopyroxenes. Meanwhile, gneissic rocks in the region exhibit an abundance of potash feldspars and albite. Additionally, the limestone found here contains a higher concentration of mafic minerals, giving it a dolomitic characteristic. The weathering processes affecting soda feldspar (albite) and potash feldspars (orthoclase and microcline), which are prevalent in the biotite gneiss within the study area, significantly contribute Na+ and K+ ions to the groundwater. Similarly, calcic-plagioclase feldspars, pyroxenes, and amphiboles, commonly found in the silicate rocks of the study area, play a substantial role in introducing Ca2+ and Mg2+ ions into the groundwater through weathering processes.