Impact of soil salinity on groundwater chemistry in semi-arid regions in Western India: Insights from major ion and stable isotopic δ2HH2o, δ18oH2o, and δ13CDIC characteristics.

Detailed geochemical and stable isotopic (delta DH2O, delta(OH2O)-O-18, and delta(CDIC)-C-13) study of groundwater samples (n = 74) from a semi-arid region in Western India was carried out to constrain their solute sources and sub-surface weathering processes. Na+, Cl-, and HCO3- dominate major ion chemistry. This type of chemistry points to a significant solute supply from the salt-affected soils and bedrock dissolution. The average delta O-18 and delta D in our groundwater samples are similar to their corresponding values in precipitation samples. Spatial variability of the delta O-18 and delta D data depicts the impact of continental effect and variable extent of evaporation in this region. The delta O-18-delta D cross-plot yields a lower slope (5.2 +/- 0.5) than that of the global meteoric water line (-8), confirming the significant impact of evaporation on groundwater hydrology. delta C-13(DIC) values vary between-7.3%o and-16.4%o (average =-9.7%o +/- 1.7%o), and first-order binary mixing calculation shows that about 75 (+/- 16) % of the DIC in these samples are supplied via carbonate dissolution (range: 35-100%). Inverse model calculations involving elemental ratios of major ions show that the solutes are mainly supplied through salt-affected soils (30 +/- 22%), with sub-ordinate contributions from rain (31 +/- 13%), silicate (23 +/- 12%) and carbonate (15 +/- 13%). These estimates are consistent with the formation of salt-affected soils via evaporative water loss in semi-arid regions and the faster dissolution kinetics of these soil salts. The outcomes of this study underscore distinctly different compositions of semi-arid groundwater reservoirs, which has implications for drinking and irrigation usage.