RADIUM ISOTOPES AS A PROXY FOR GROUNDWATER-FRESH WATER LAKE INTERACTION: A CASE STUDY FROM THE SEA OF GALILEE, ISRAEL

Since radium adsorption decreases with increasing salinity, most of Ra isotope studies have focused on fresh groundwater discharge into coastal and estuarine environments, assuming Ra removal only by radioactive decay. Here we examine an opposite scenario where saline groundwater discharges to a fresh water lake. We investigated the distribution of the radium quartet in the Sea of Galilee (Cl~230 mg/L) and different saline springs that discharge to the lake. The ²²⁶Ra/Cl ratio in the lake is significantly lower than in the saline springs, indicating that simple dilution is not the only process influencing the activity of Ra in the lake. Given that the water residence time is only a few years, radioactive decay is not a significant sink for ²²⁶Ra (T_1/2=1600 years). Since the lake is under saturated with respect to BaSO₄, we determined that at least 70% of the Ra is being adsorbed onto sediments in the lake. The deficit in the shorter lived Ra isotopes, ²²⁴Ra and ²²³Ra, was found to be stronger. We show that this is due to a combination of radioactive decay of the short-lived isotopes and apparent preferential adsorption that inversely correlates with the half-lives of Ra isotopes.

In spite of the non-conservative behavior of Ra in the fresh lake system, we used the ratio of ²²⁸Ra/²²⁶Ra to determine the residence time of Ra in the lake (3.8 years). Moreover, we use the residence time equation to delineate possible Ra sources to the lake. Our data shows that predominance discharge of saline groundwater with ²²⁸Ra/²²⁶Ra ratio identical to that of Fulyia Springs (0.041, Fuliya B) provides realistic evaluation on the residence time of Ra. In contrast, discharge of other saline springs with different ²²⁸Ra/²²⁶Ra ratios yields unrealistic residence time and thus cannot explain the ²²⁸Ra/²²⁶Ra ratio in the lake. Overall, the use of the Ra isotope quartet in fresh water lakes provides essential information on the sources of groundwater discharge, rate of Ra removal, and residence time of Ra in the lake water.