UNDERSTANDING GROUNDWATER RECHARGE IN PERCHED FLUVIAL AQUIFERS IN MOUNTAINOUS REGIONS OF SAN LUIS POTOSI, MEXICO

Unconfined fluvial aquifers in mountainous regions of the semi-arid San Luis Potosi region in México cover a limited area. Precipitation in this region is about 400 mm/ year; seasonal runoff can provide a water source for local natural recharge to perched fluvial aquifers. In the San Luis Potosi state, the mountainous regions commonly host economic ore deposits with metallic mining operations. In Cerro de San Pedro area (CSP), a very old (since 1592) mining district in San Luis Potosi, historical mining activities have produced large deposits of abandoned mine wastes (with oxidized minerals and sulfide content), some of them lying along the ephemeral streams associated with the local fluvial aquifers. These aquifers can constitute a long-term source of water supply for domestic use, so it is mandatory to investigate natural recharge (quantity and quality). For this study, we used hydrological, hydrochemical analysis techniques, and isotopic data to understand the provenance of natural recharge in fluvial perched aquifers. The fluvial aquifer unit in the CSP has an irregular distribution: it contains local groundwater flows in Quaternary alluvium (sand and silt with variable amounts of clay) and has a maximum thickness of approximately 50 m. Long-term (about 6 years) water level measurements in monitoring wells tapping the fluvial aquifer and/or the bedrock indicated that natural recharge is not an annual basis event; for these conditions, the soil has water deficit throughout the year, natural recharge is associated with extreme precipitation events only. The hydrological balance indicated 3-5% infiltration of total precipitation for the analyzed period (about 30 years record). Stable isotope data were consistent with recharge controlled by preferential pathways; results do not demonstrate the evaporation effect. Hydrochemical information indicates the perched aquifer has Ca-SO4 water type, high dissolved sulfate concentrations are attributed to sulfide oxidation in the presence of oxygen and water; sulfur isotopes in dissolved sulfate validate this geochemical conceptual model. Considering similar sites in Mexico, this methodology can be helpful to understand meteoric recharge in fluvial waters of semi-arid environments associated with economic ore deposits.