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

[EN]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 (δ18OH2O, δDH2O), while the solutes are provided by ascendant deep groundwater flows in the lakes bottom and in the surrounding area. Sulphate stable isotopes (δ18OSO4=; δ34SSO4=) 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

Hydrogeology of the Lagunas de Villafáfila area (Zamora)

Las Lagunas de Villafáfila constituyen un enclave natural en el valle del río Salado caracterizado por aguas con conductividades eléctricas entre 4000 y 5000 µS/cm). La piezometría elaborada en la zona revela el aporte de la recarga de las laderas que rodean las lagunas y el bajo gradiente hidráulico que existe en las zonas llanas perilagunares. El análisis físico-químico del agua de las lagunas y de las aguas subterráneas del entorno ha permitido caracterizar tres hidrofacies químicas: 1) bicarbonatada cálcico-magnésica; 2) clorurado-sódica; y 3) bicarbonatada clorurado-sódica. Estas representan respectivamente aguas recargadas en las vertientes de las lagunas, flujos profundos de largo recorrido que descargan en las lagunas, y mezcla de ambas. Las lagunas resultan de la mezcla de aguas con diferentes orígenes: aguas de lluvia + aguas de escorrentía + aguas de flujos locales + flujos regionales profundos + reciclado de salmueras. Las aguas cloruradas sódicas podrían procecer de la disolución de evaporitas de los relieves septentrionales de la Cordillera Cantabrica y/o de evaporitas situadas en el subsuelo desde esta Cordillera hasta el entorno de las lagunasVillafáfila lakes are natural lakes with electric conductivities between 4000-5000 µS/cm located in the valley of the Salado river. The piezometry of the area suggest that the hills surrounding the lakes are local recharge areas while the lakes and their surroundings around the lakes are discharge zones with low hydraulic gradient. Physico-chemical analyses of lake and groundwaters have allowed characterizing three hydrochemical facies: 1) calcium magnesium bicarbonate; 2) sodium chloride ; and 3) sodium bicarbonate-chloride waters. These hydrofacies record respectively, the local recharges of the surrounding hills, long and deep flow-paths discharging into the lakes, and a mix of the above. Villafálila lake waters are the result of a mixing of rain + runoff + local flows + regional deep flows + brine recycling. Sodium chloride waters could come from the dissolution of evaporites in the Cantabrian Mountains and/or from evaporites located in somewhere between these Mountains and the Villafáfila lake