Chemo-isotopic tracing of the groundwater salinity in arid regions: An example of Shahrood aquifer (Iran)

Abstract

Understanding the origin of groundwater salinity is critical to efficiently manage the scarce water resources in arid regions. In the current research, the source and mechanism of groundwater salinity in the Shahrood aquifer, central Iran, have been assessed using combination of hydrochemical techniques and environmental isotopes. A total of 115 groundwater samples were collected and analyzed for the major and minor ions (HCO3, Cl, SO4, Ca, Mg, Na, K, Br and I) and isotopes (δ18O and δ2H). Based on the results of the chemical approach, halite and gypsum dissolution, Na-silicate weathering and calcite precipitation induced by common-ion effect are the main source of salinity (>7500 mg/L) in the saline water zone (SWZ) of the Shahrood aquifer. The SWZ groundwaters are more depleted in δ2H as compared with fresh water zone (FWZ) samples which plotted on the Shahrood local meteoric water line (SMWL). Development of a groundwater divide line at the central parts of the aquifer causes low velocity and then enhancing the residence time of the SWZ groundwaters mainly originated from isotopic-depleted sub-modern precipitations in surrounding high-elevated mountains. Mixing water zone (MWZ) is identified between FWZ and SWZ, where the relative fractions of the saline groundwaters vary in the range of 23–99%. Managing groundwater withdrawals is highly recommended; as the groundwater over-exploitations enhance invasion of the saline groundwaters from SWZ and then overall degradation of the water quality in the aquifer. This is the first model explaining the groundwater salinity source and mechanism of the Shahrood aquifer as a case of mountain-front recharged plain aquifer, providing guidelines for salinity assessments in other arid-land aquifers around the world.