Hydrochemical mapping of the Upper Thal Doab (Pakistan) using the geographic information system

The geographic information system (GIS) has become a widely accepted tool in all fields of life and particularly in groundwater exploration and hydrogeological investigations. Especially, the application of GIS in conjunction with hydrochemical analysis and environmental isotopic study provides an excellent tool for hydrochemical mapping and a better understanding of the quality of groundwater. A GIS-based hydrochemical study was conducted in the part of Upper Thal Doab (Pakistan). Groundwater samples were analyzed for major cations (Na⁺, Ca²⁺, Mg²⁺, K⁺), anions (HCO₃ ⁻, Cl⁻ and SO₄ ²⁻), stable isotopes (δ¹⁸O and δ²H) and radioactive isotopes (³H) to evaluate the groundwater quality, surface water/groundwater interaction and groundwater residence time. Based on these analyses, different irrigation indices such as SSP, SAR, MAR, KI and PI were calculated for further classification of the groundwater. Thematic maps using GIS were thus developed for depicting their spatial distribution and evaluating the overall trend and better understanding of the quality of groundwater. Based on these interpretative results, it is observed that different water types of groundwater exist in the Upper Thal Doab, most of which belong to either Na–Cl or K–HCO₃ water types. Other predominant hydrogeochemical facies are Na–SO₄, K–SO₄, Mg–Cl, K–Cl, Na–HCO₃, Mg–HCO₃ and Mg–SO₄. Most of the samples indicate medium to very high salinity hazard based on various indices such as TDS, EC, MH, SSP and KR at some places. However, groundwater from TH-27, TH-32, TH-36, TH-38, TH-43 and TH-47 are unsuitable for irrigation purposes. The environmentally stable isotopes (δ¹⁸O and δ²H) and radioactive isotopes (³H) from selected wells are used to assess the groundwater recharge mechanism and groundwater age, respectively. This study showed that the groundwater in the study area was recharged mainly by rainfall and the Indus Basin irrigation system. Furthermore, younger waters are available along the rivers Indus and Chenab up to greater depths and large areal extents.