The origin of the saline waters in the Villafáfila lakes (NW Spain). A hydrogeological, hydrochemical, and geophysical approach.

Villafáfila lakes are a natural reserve included in the intergovernmental RAMSAR agreements for conservation of wetlands, with special interest for their brackish-saline waters. These lakes are located at the western margin of the Duero basin, whose aquifer system has no evaporitic rocks upstream. Understanding the origin of the lake's salinity, the groundwater circulation and the distribution of the brackish-saline waters in the area is important not only for the preservation and management of the natural reserve, but for human water consumption as well. Three types of waters have been identified according to their chemical composition. Type 1 are calcium-bicarbonate fresh waters identified in the local recharge areas (surrounding hills); Type 2 are mixed waters dominated by sodium and chloride-bicarbonate, identified at the toe of the hills; Type 3 are brackish to saline sodium-chloride waters from the lakes, springs and boreholes. Time domain electromagnetic (TDEM) profiles have revealed the existence of a basement elevation that forces brackish regional groundwater flow to rise. Radiocarbon age of regional groundwaters points to residence times of 20-30 Ky. Villafáfila lakes are through-flow lakes nourished by meteoric waters (direct precipitation and shallow groundwaters) as deduced by stable isotopes (δO, δD), while the solutes are provided by ascendant deep groundwater flows in the lakes bottom and in the surrounding area. Sulphate stable isotopes (δO; δS) suggest that deep groundwaters have been in contact with Triassic and Cenozoic evaporites. Below the lake's bottom there is a brine (TDS = 27 g/L) contained within the lake-sediment aquitard that is concentrated by evaporation in the vadose zone and by salt recycling. A salinity inversion has been observed below the brine. The lack of saline crusts on the lake's bottom is favored by the SW outflow of the brine.