Hydrogeochemical characterization and evolution of a regional karst aquifer in the Cuatrocienegas area, Mexico

The Cuatrocienegas area is useful for the investigation of the effect of groundwater extraction in the Chihuahuan freshwater xeric ecoregion. It has been investigated at this time using a selection of geochemical indicators (major, minor and trace elements) and delta S-34 data, to characterize the origin of groundwater, the main geochemical processes and the mineral/groundwater interactions controlling the baseline geochemistry. The area is composed of limestones of Mesozoic age, with a composite thickness of about 500m, overlaid by basin fill (poorly consolidated young sediments). Substantial water extraction and modification of natural discharges from the area along the last century have produced a detrimental impact on ecosystem structure and function. Water-rock interactions, mixing and evaporative processes dominate the baseline groundwater quality. Natural recharge is HCO3-Ca type in equilibrium with calcite, low salinity (TDS<500mg/L), Cl- lower than 11mg/L and average Li+ concentration of 0.005mg/L. Along the groundwater flow systems, delta S-34 evidence and mass transfer calculations indicate that Cretaceous gypsum dissolution and dedolomitization reactions adjust water composition to the SO4-Ca type. The increase of water-rock interaction is reflected by Cl- values increase (average 68mg/L), TDS up to about 1500mg/L and an average Li+ concentration of 0.063mg/L. Calculations with chemical geothermometers indicate that temperature at depth could be at maximum of 15-20 degrees C higher than field-measured temperature for pozas. After groundwater is discharged to the surface, chemical evolution continues; water evaporation, CO2 degassing and precipitation of minerals such as gypsum, calcite and kaolinite represent the final processes and reactions controlling water chemical composition.