Groundwater ages and reaction rates during seawater circulation in the Dead Sea aquifer

The Dead Sea system provides a unique opportunity to study flow velocities and reaction rates during seawater circulation in the aquifer. We present here a novel application of calculating groundwater age and velocity along the flow path of the hypersaline water from the Dead Sea into the aquifer using the buildup rate of ²²⁸Ra in this water. The calculated circulation velocities are 1-10m/y, which is in agreement with estimates based on the Na/Cl ratios in this water (1.5-4m/y). The latter is unique to the Dead Sea-aquifer system, where the Na/Cl ratio has been decreasing during the past 50 years due to the precipitation of halite in the lake. The velocity estimates facilitated the calculation of the rates of water-rock interactions in the Dead Sea aquifer.SO₄ is removed relatively fast (k = 0.8y^-1) due to gypsum precipitation while barite or celestine precipitation removes ²²⁶Ra and Ba in a time scale of years (k = 0.22y^-1). Similar rates were found for redox-driven reactions, such as U removal (k = 0.4y^-1) and Fe and Mn contribution due to the dissolution of oxides (k = 0.15y^-1). In the fresh-saline groundwater transition zone, gypsum which precipitated from hypersaline water in higher lake stands, is now being dissolved and enrich the water with SO₄.