Ra Isotopes as a Tool for Quantifying Saline Groundwater Flow into a Fresh Water Lake. A Case Study from the Sea of Galilee (Lake Kinneret), Israel

In spite of the non-conservative behaviour of radium in fresh water systems, here we demonstrate that the Ra isotope quartet is a valuable tool for quantifying discharge of saline groundwater into a fresh water lake. ²²⁶Ra, ²²⁸Ra, ²²³Ra, and ²²⁴Ra activities in the Sea of Galilee, Israel, surrounding saline springs, and the Jordan River were investigated, coupled with U, Th and ²²⁶Ra measurements in sediment cores and laboratory Ra adsorption experiments under different salinity conditions. The ²²⁶Ra activity (0.4-0.5 dpm L^-1) and ²²⁶Ra:Cl ratio in the lake are significantly lower than those of the inflowing saline springs, indicating 75%-86% of the inflowing ²²⁶Ra is removed from the lake water. This “missing” Ra is likely scavenged through adsorption onto suspended particles and sedimentation. 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 ~2-3 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 “Fuliya type” water, one of the main groups of saline springs entering the lake. The residence time of dissolved ²²⁶Ra in the lake is estimated as 3-4 years. The ²²⁸Ra:²²⁶Ra ratio of the lake water and a mass-balance calculation infers ²²⁸Ra arrives from regeneration though bottom sediments, in addition to groundwater flux. Mass-balance calculation of the expected saline fluxes before the diversion of the saline inflows to the lake (early 1960s), estimate ²²⁶Ra activity in the lake (1.1-1.2 dpm L^-1) that is similar to the value (1.1±0.007 dpm L^-1) measured in the lake in 1962.