Hydrochemical evolution of groundwater in northwestern part of the Indo-Gangetic Basin, India: A geochemical and isotopic approach

The present study aims to understand the hydrochemical evolution of groundwater in the Ghaggar River Basin, representing a zone of excessive abstraction of groundwater in the northwestern Indo-Gangetic Basin. The study comprises a regional scale and high-resolution sampling of groundwater during pre- and post-monsoon seasons of 2013 and their analyses for major ions, δ¹⁸O, δ²H, and ³H. Variation in hydrochemical and isotopic data is found both in spatial and vertical scales. The significant vertical variation of TDS, NO₃⁻ and K⁺ allowed the classification of the aquifer system into two major groups: shallow (depth < 80 m bgl) and deep (depth > 80 m bgl). The depthwise variations of δ¹⁸O and δ²H support this categorization of the aquifers. The Ca-HCO₃ and Ca-Mg-Na-HCO₃ water facies with higher values of ³H in the proximal part of the basin characterize recharge areas under humid conditions. The dominance of Mg-Na-HCO₃ and Na-HCO₃ facies in shallow and deep aquifers in central part of the basin, illustrate the intermediate to advanced stages of hydrochemical evolution in the system. Dominance of brackish Ca-Mg-Cl-SO₄ and Na-Cl-SO₄ water types in the discharge areas is due to the prevailing geological conditions and anthropogenic activities. Geochemical modelling supports the reverse cation exchange and mixing during lateral and vertical flows, weathering of silicate minerals, dissolution of crustal salts, and evaporative enrichment are the natural processes governing the evolution of groundwater chemistry along the flowpaths. The developed process-based conceptual model will aid in the formulation of a suitable plan for groundwater resource management in the region.