Identifying source and evaluation of hydrogeochemical processes in the hard rock aquifer system: geostatistical analysis and geochemical modeling techniques

In order to quantify spatio-temporal changes in the hydrogeochemistry of Swarnamukhi river basin, Andhra Pradesh, India, a total of 239 groundwater samples have been collected for pre- and post-monsoon seasons of 2014 and 2015. The geology of the study area is comprised of granite, granitic gneisses, shales, quartzites, laterites and recent alluvium along the river course. Based on the study of geochemical processes of different seasons, it is found that the occurrence of Na and HCO3 in 2014 shifted toward Ca and HCO3 in 2015 due to cation exchange process. Fe shows the higher concentration value than safe limit due to dissolution of ferruginous minerals and domestic sewage discharges. Bivariate plots of various ions and ratios depict the predominance of silicate weathering over carbonate and evaporite dissolution apart from anthropogenic activities in the enrichment of groundwater constituents. High nitrate concentration was observed in the urbanized regions of the basin followed by intense agricultural areas. Geochemical modeling was carried out using mineral saturation index and thermodynamic stability plots to deduce major weathered products. Most of the groundwater samples in the study area are saturated with carbonate minerals but are undersaturated with silicate minerals. Thermodynamic stability plots suggest the formation of kaolinite secondary mineral and presence of aluminosilicate minerals. Fuzzy calculation is applied to determine the water quality index, which categorizes a majority of the samples as excellent to good category for human use. Factor analysis points toward the mixed source identification of ionic constituents. Saline water mixing index model calculation suggests the effect of salinization in coastal areas due to heavy withdrawal of the fresh groundwater resources for various uses, whereas in the mainland it is due to agricultural runoff and domestic waste water.