Regional groundwater salinization and hydrostratigraphic heterogeneity in the Indus Delta, Pakistan: An integrated hydrochemical, isotopic, and geophysical assessment

Study Region The arid Indus Delta coastal aquifer in Sindh, Pakistan, faces reduced freshwater inflows, intensive groundwater abstraction, and seawater intrusion (SWI). Study Focus We assessed salinization through hydrochemical, isotopic, geophysical, and quantitative mixing analyses. Groundwater from multilevel wells (6–18 m) was analyzed for major ions, δ¹ ⁸O, δ²H, δ¹ ³C–DIC, δ³ ⁴S–SO₄²⁻, δ¹ ⁸O–SO₄²⁻, and tritium (³H), complemented by electrical resistivity sounding (ERS). The Seawater Mixing Index (SMI) and chloride-based seawater fractions assessed marine influence, providing the first integrated regional dataset. New Hydrological Insights for the Region Groundwater showed isotopic ranges (δ¹⁸O: −11.65 to +1.00‰; δ²H: −86.29 to +58.59‰). Correlations linked δ¹ ⁸O and EC (r = 0.79), Cl⁻/HCO₃⁻ and EC (r = 0.98), SMI and EC (r² = 0.999), and chloride-based seawater fraction and SMI (r² = 0.999). Coastal–inland patterns indicate marine mixing modified by evaporation, ion exchange, and water–rock interaction. δ¹ ³C–DIC (−18.88 to −1.30‰) indicates heterogeneous carbon evolution without uniquely identifying sources. Sulfate isotopes, supported locally by low DO, negative ORP, and detectable sulfide, are consistent with localized sulfate reduction. Tritium (0.76–4.55 TU) indicates modern recharge but cannot constrain groundwater age or intrusion timing. ERS reveals heterogeneous multilayered aquifers with conductive formations to approximately 150–250 m, consistent with shallow salinity patterns. SWI is the primary regional salinization influence, with secondary geochemical modification, supporting depth-specific abstraction and long-term monitoring.