Isotopic and hydrochemical evidence for the source and mechanism of groundwater salinization in Kashan Plain aquifer in Iran

Groundwater salinization and interaction between Playa Lake and regional groundwater were investigated using multi-chemo-isotopic evidences. Forty groundwater and 26 Kashan Playa Lake (KPL) water samples were collected and analyzed for their geochemical compositions. The evolution of hydrochemical facies in Kashan Plain Aquifer (KPA) to KPL is Ca-HCO₃ (19%), Mix Ca–Cl (9%), Ca–Cl (17%), and Mix Na-Cl and Na-Cl (55%). Also, the Hydrochemical Facies Evolution Diagram (HFE-D) proposed cation exchange as the main process of salinization in KPA. Based on the binary hydrogeochemical diagrams of (Na⁺/Cl⁻)/Cl⁻, (Ca²⁺ + Mg²⁺)/HCO₃⁻ + SO4²⁻, and Cl/Br, dissolution of halite and gypsum in the Miocene marlstone in the KPA is the main source of salinity. The δ¹⁸O of water in aquifer and playa water samples varies from –10.03 to 7.03‰ (VSMOW) with an average of –6.95‰ and –60.73 to 25.08‰ with average of –45.82‰ for δ²H. Based on the results, the relation between δ¹⁸O and δ²H, and δ¹⁸O and Br, approves discharge of saline water from KPA to KPL. Likewise, the isotopic composition of δ³⁴SO₄, varies from 5.95 to 22.55‰ CDT in KPA, and 5.95 to 9.99 ‰ CDT in KPL. Also, the relations between δ¹⁸O-δ³⁴SSO₄ and Cl-δ³⁴S were non-linear, indicating that sulfur concentration in KPA and KPL changed due to sulfide oxidation and sulfate reduction in the freshwater and deep brines in the aquifer and mixed during the over-pumping in the KPA. Oxidation of sulfide minerals in the mineralized region in the western part of the aquifer (recharge zone) may have been the source of sulfur leached by seasonal runoff. Water–rock interaction, ion exchange, and hydraulic gradient have been the dominating factors in changing the water chemistry between aquifer and playa leading to saline groundwater discharged to the playa.