The extent of seawater circulation in the aquifer and its role in elemental mass balances: A lesson from the Dead Sea

This paper shows for the first time a field-based estimation of the volume of dispersive density-driven long-term seawater circulation in coastal aquifers, which is crucial to the understanding of water-rock interaction and to the assessment of its potential impact on elemental mass balances in the sea. The Dead Sea is an ideal place for studying this type of circulation due to the absence of tides and the accessibility of the shallow fresh-saline transition zone. The unique antithetical behavior of ²²⁶Ra and ²²⁸Ra during seawater circulation in the Dead Sea aquifer, where ²²⁸Ra is added and ²²⁶Ra is removed, provides a robust new method for quantifying aquifer circulation. Here we estimate water circulation through the Dead Sea aquifer to be 400 million m³/yr (~2.5 million m³/yr per 1 km of shoreline), which is ~20% of the fresh water inflow prior to the 1960s. This large volume can affect trace element concentrations in the Dead Sea, e.g. it is a sink for ²²⁶Ra, Ba and U and a source for ²²⁸Ra and Fe. These results suggest that dispersive density-driven seawater circulation in aquifers may play an important role in mass balances in other lacustrine and oceanic settings.