TIME-SCALES AND GEOCHEMICAL PROCESSES OF SEAWATER CIRCULATION IN AQUIFERS: A LESSON FROM THE DEAD SEA

Seawater circulation in coastal aquifers is now recognized as a major process affecting trace element mass balances in coastal areas. Estimates of submarine groundwater discharge (SGD) vary over several orders of magnitude (2-10⁶ m³/yr per meter shoreline). These estimates are sensitive to fresh–saline SGD ratios and to the temporal and spatial scales of the circulation. The Dead Sea system is an excellent natural field lab for studying seawater–groundwater interaction and large-scale circulation due to the absence of tides and to the minor role played by waves.

During Dead Sea water circulation in the aquifer several geochemical reactions occur, ranging from short-term adsorption–desorption reactions and up to long-term precipitation and dissolution reactions. These processes affect the trace element distribution in the saline groundwater. Dead Sea water is supersaturated with barite and celestine. These minerals precipitate during circulation in the aquifer, reducing barium (from 5 to 1.5 mg/L), strontium (from 350 to 300 mg/L) and the long-lived ²²⁶Ra (from 145 to 60 dpm/L) in the saline groundwater. Redox-controlled reactions cause a decrease in uranium from 2.4 to 0.1 μg/L, and an increase in iron from 1 to 13 mg/L.

²²⁸Ra (t_1/2=5.75 yr) activity in the Dead Sea is ~1 dpm/L and increase gradually as the saline water flows further inland until reaching steady-state activities (~27 dpm/L) with the aquifer sediments. The rate of ²²⁸Ra accumulation in the saline groundwater was used for estimations of groundwater velocity, which was found to be of several meters per year (3-8 m/yr). Based on the calculated velocities, rate constants were calculated for certain elements showing that most reactions occur over several years.

The decrease in ²²⁶Ra and increase in ²²⁸Ra in the circulation process provide a robust method for calculating the amount of Dead Sea water circulating in the aquifer. This volume was estimated to be 340 million m³/yr, affecting trace element concentrations in the Dead Sea and emphasizing the potential of long-term seawater circulation in oceanic mass balances.