Major ion composition and 87Sr/86Sr of groundwater in the coastal Gujarat alluvial plain, India

Major ions, Sr concentration, and ⁸⁷Sr/⁸⁶Sr have been analyzed in groundwater of the coastal Gujarat Alluvial Plain, collected during monsoon, post-monsoon, and pre-monsoon seasons of 2016–2017. The major objective of this study was to understand the regional groundwater salinization mechanism. In the study area, the groundwater is mostly characterized by Na-Cl facies, with few samples of Ca-Cl, Ca-Mg-Cl, Na-Ca-HCO₃, and Ca-Mg-HCO₃ types. Whereas, the Narmada and the Tapi river water samples are particularly of Ca-Mg-HCO₃ type. The hydrogeochemical facies evolution (HFE) diagram depicts the coastal groundwater freshening irrespective of the season ruling out the lateral seawater intrusion far inland. However, the ⁸⁷Sr/⁸⁶Sr and Br/Cl ratios strongly suggest the modern marine influence on the regional groundwater. In the plot of 1/Sr versus ⁸⁷Sr/⁸⁶Sr, most of the groundwater samples fall on the binary mixing line between the seepage groundwater and modern seawater endmembers. Therefore, we suspect that the up-coning of recently trapped seawater by groundwater over-extraction is the most plausible reason for the groundwater salinization, which indicate the vulnerability of the coastal Gujarat alluvial plain to the near future sea ingress under the global warming scenario. A few exceptional groundwater samples far north of the Narmada River show more radiogenic ⁸⁷Sr/⁸⁶Sr indicative of silicate weathering.