Long and short-term cation exchange linked to a negative hydraulic barrier in a coastal aquifer

Abstract

Saltwater extraction in coastal aquifers generates a negative hydraulic barrier that prevents marine intrusion and produces a general freshening landward from this barrier. In the Andarax delta aquifer, SE Spain, two instances of saltwater extraction were performed and their effect on the aquifer hydrochemistry was studied. ¹⁴C groundwater dating, together with chemical analysis, reflects the presence of waters with different infiltration ages. Old marine groundwater (~10 ky) must be the remains of marine intrusion generated during the Holocene transgression at the same time the delta was formed. The freshening induced by the saltwater extraction triggers cation exchange between the aquifer substratum and groundwater. Unlike what is described in other examples of cation exchange in coastal aquifers, in the Andarax delta the freshening causes an exchange between Mg, which is released into the groundwater, and Na, which is held in the clay mineral structural unit. This process is reverted the moment the hydraulic barrier stops acting. Short saltwater pumping-stopping cycles generate fast inversions in this exchange chemical reaction. At the same time, a clear excess of Ca ion can be seen in all the groundwater samples. This excess is attributed to the release of this ion resulting from the overall marine intrusion in this area during the Holocene transgression. Contrasting what occurs with the Na-Mg exchange, the Na-Ca exchange process is more long-lasting in time.

Keywords: Groundwater freshening; Ion-exchange reaction; Marine intrusion; Paleoseawater; delta deposits.