Quantifying saline groundwater flow into a freshwater lake using the Ra isotope quartet: A case study from the Sea of Galilee (Lake Kinneret), Israel

Abstract

We investigated the Ra isotope quartet in order to quantify the discharge of saline groundwater into a freshwater lake. The activities of 226Ra, 228Ra, 223Ra, and 224Ra were measured in the Sea of Galilee, Israel, surrounding saline springs, and the Jordan River, coupled with U, Th, and 226Ra measurements in sediment cores and laboratory Ra adsorption experiments under different salinity conditions. The 226Ra activity (0.007–0.008 Bq L−1) and 226Ra : Cl ratio in the lake were significantly lower than those of the inflowing saline springs, indicating that 75-86% of the incoming 226Ra is removed from the lake water. This "missing" Ra is likely adsorbed onto suspended particles. Given the observed differential depletion of Ra isotopes, we present a model consisting of adsorption-desorption, recoil, and decay of the short-lived 224Ra and 223Ra isotopes. We predict a removal time of ~2 weeks for Ra, inferring the apparent residence time of suspended matter in the lake. A mass-balance calculation of the different 226Ra inventories reveals a saline groundwater flux of 44–61 × 109 L yr–1 for "Fuliyatype" water. The residence time of dissolved 226Ra in the lake is estimated as 3–4 yr. The 228Ra : 226Ra ratio of the lake water and a mass-balance calculation show that 228Ra arrives from regeneration though bottom sediments, in addition to groundwater flux. Mass-balance calculations of the expected saline fluxes before the diversion of saline inflows to the lake (early 1960s) provide 226Ra activity estimates in the lake (0.018-0.020 Bq L−1) that are similar to the value (0.018 ± 0.0001 Bq L−1) measured in the lake in 1962.