Application of radon and radium isotopes to groundwater flow dynamics: An example from the Dead Sea

•Comprehensive Ra and Rn isotopic composition dataset of Dead Sea groundwaters•The processes dictating secular equilibrium of radium isotopic ratios•The definition of a new concept of groundwater system age•Implications of the isotopic composition of radium for Dead Sea groundwaters

This study presents the behavior of radon and radium isotopes and their application to groundwater age and flow dynamics. The research was conducted in the complex Dead Sea groundwater system, which includes a large variety of sediments, groundwater salinities, flow mechanisms and groundwater ages. Groundwater around the Dead Sea contains high activities of radon (up to tens of thousands dpm/L) and radium (up to hundreds dpm/L). Adsorption of radium, which is partially salinity controlled, is an important source of unsupported 222Rn, which is used for estimating the adsorption partition coefficient of radium. In addition to salinity, the concentration of Mn and Fe oxides and aquifer heterogeneity are important factors controlling the adsorption partition coefficient. The different nature of the rocks on both sides of the Dead Sea transform, with lower Th/U ratios in the carbonate rocks on the western catchment of the Dead Sea compared to higher ratios in the sandstone aquifer east of the Dead Sea, is reflected in a higher 228Ra/226Ra activity ratio in the eastern compared with the western groundwaters (averages of 0.76 and 0.15, respectively). The different groundwater groups around the Dead Sea contain secular or non-secular equilibrium ratios, which depend on the age of the groundwater (the time since the groundwater entered the aquifer) or whether the groundwater system is in a steady state (the age of the groundwater system). Young groundwater, such as the Dead Sea water that circulates in the aquifer or freshwater springs, is depleted in the long-lived radium isotopes compared to the short-lived isotopes, whereas old groundwater contains relatively high activity of 226Ra (∼ 500 dpm/L) and the radium activity ratios are close to secular equilibrium. The common secular equilibrium ratios between all four radium isotopes in the Dead Sea groundwaters suggest that many of the groundwater flow paths did not change significantly during the past 8000 years.