Delineating the source and mechanism of groundwater salinization in crucial declining aquifer using multi-chemo-isotopes approaches

Abstract

Hydrogeochemical and multi-isotopic (18O, 2H, 3H, 13C, and 14C) approaches were applied accompanied with hydrogeological investigations to determine the origin and mechanism of groundwater salinization in Kashan Plain Aquifer (KPA). Evolution of hydrogeochemical facies in KPA is Ca-HCO3 (19.04%), Mix Ca-Cl, and Ca-Cl (9.52%), Mix Na-Cl, and Na-Cl (72.26%) based on the HFE-M. The isotopic composition of δ2H and δ18O varies from −8.38 to 6.48‰ with average of −4.49‰ for δ18O and −57.09 to 22.68‰ with average of −40.28‰ for δ2H. The fresh and saline groundwater samples were plotted near the GMWL, with a minor shift towards more enriched values, indicating that the present meteoric water is the main origin of groundwater in the study area. Intrusion from salt lake and saltpan was rejected by ionic ratios and stable isotopes approaches. Further, geophysical investigations ruled out contributions from the Nasr Abad salt dome, as it is buried at about 480 m depth underneath the KPA. In addition, the radioisotopes results (3H, and 14C) showed that the KPA groundwaters comprise of modern meteoric waters mixed with deep paleo-brine waters (10000 to 21000 yr BP). Further, the Na/Cl, I/Cl, Cl/Br, and Li/Cl/Br/Cl ratios and 2H and 18O data showed that, most probable source of salinity for this deep paleo- brine water is halite dissolution in Miocene Marl and URFs formation in the bedrock of KPA. The mixing ratio calculated by chloride concentration showed about 60% mixing between deep brine and groundwater samples. Consequently, the source of salinity in KPA groundwater is dissolution of halite and gypsum in deep paleo-brine which is likely upconing and mixing with fresh groundwaters during heavy pumping.