Using radon-222 and radium-226 isotopes to deduce the functioning of a coastal aquifer adjacent to a hypersaline lake in NW Iran

Abstract This study aims to assess the hydrogeochemistry of coastal groundwater, the occurrence of 222 Rn and 226 Ra, and their isotopic response to salinity and associated chemical compositions of groundwater in the coastal Urmia Aquifer (UA) at the western side of Urmia Lake (UL). The results of the PCA show that 87.3% of groundwater chemistry changes are controlled by six principal components. The interaction between groundwater and coastal igneous and metamorphic rocks in eastern areas (next to the UL) results in complex hydrogeochemical conditions than western areas. Based on correlation of U and salinity, some coastal samples display conservative and the others non-conservative behaviors. Differed from most previous studies, 226 Ra and 222 Rn concentrations in coastal groundwater samples of UA do not show a good correlation with salinity. Given 10% of groundwater 222 Rn is originated from host rocks, the radon concentrations recorded in the coastal groundwater samples are relatively in range that can effectively be supplied by the local rocks (5–49 Bq/l). Results of different chemical and isotopic parameters in this area indicate that there is no direct connection between fresh groundwater and UL saltwater. This is because that the hard and thick salty layer in the lakebed acts as an impermeable barrier to prevent the underground hydraulic connection. Results show that removing the salty layer of UL as an option to progress in rehabilitation program of this lake may result in more hydraulic connection between the lake and groundwater resources in some areas.