See more from this Division: Topical Sessions
See more from this Session: Advances in Surface Water–Groundwater Interactions: Investigations of Rivers, Lakes, and Wetlands
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.