Influence of the paleogeographic evolution on the groundwater salinity in a coastal aquifer. Cabo de Gata aquifer, SE Spain

The groundwater of the Cabo de Gata detritic aquifer, in southeastern Spain, exhibit salinities of between 70–726 mmol/L of Cl⁻ (brackish-salt to hypersaline waters). We have investigated the causes of the high salinity anomaly, which at certain points exceeds that of present-day seawater (600 mmol/L). Two hypotheses are considered as possible sources for the saline water: (1) The deeper, more saline groundwater date back to an old marine intrusion that occurred at the end of the last Ice Age (14–17 ka), when seawater salinity was higher than in the present day. This hypothesis is supported by the values of ¹⁴C measured in this water (∼6–10 pmc), which indicate old water of up to 17 ka. However, the values of ¹⁸O and ²H are lower than would be expected. (2) The water is the result of mixing between fresh groundwater and seawater. The latter explanation agrees well with the low values of ¹⁸O and ²H. This mixture is later subject to evaporation, explaining its high salinity. Hydrogeochemical modelling was carried out for the most saline samples, assuming such mixing between freshwater and seawater followed by evaporation, and the results show a very good agreement between the measured and simulated values. According to the model calculation, the original mixture contained approximately 60% seawater and its volume subsequently was reduced through evaporation by around 30%. This mixing and evaporation could occur during the Flandrian Transgression (6000–8000 y), when this area accommodated a coastal lagoon.