USING GEOCHEMISTRY, ENVIRONMENTAL AND SULFUR ISOTOPES TO IDENTIFY SALINITY SOURCES FOR GROUNDWATER IN A MESOZOIC KARSTIC AQUIFER, NE MEXICO

Natural groundwater discharge in the Mesozoic karstic aquifer unit of Cuatrociénegas Basin (CCB), has created particular habitats where endemic species are reliant on local ecosystems. Groundwater in the CCB actually evolves from HCO₃-Ca to SO₄-Ca-Na type over a distance of less than 40 km. Trends in major (Cl, SO₄, Ca, Mg, Na), minor (K, F, Sr), trace element (Li, B, Br, Rb, Sr, Mo, As, U), and stable isotopes (δ¹⁸O, δ²H, δ³⁴S) concentrations in groundwater, indicate that baseline quality is largely controlled by water-rock interaction including carbonate and sulphate dissolution, dedolomitization and cation exchange. Environmental stable isotope data indicates an altitude effect in natural recharge; it is associated with fast flow conditions short after precipitation events in the mountains, and immediate infiltration of indirect natural recharge through main preferential zones, such as open fractures or karstic structures, avoiding evaporation during recharge events. Non-equilibrium evaporation over the discharged water under the arid climate with low humidity (45%) atmospheric conditions produces a noticeable enrichment in isotope content increasing salinity concentration. In addition, the Cl-δ¹⁸O relationship suggests mixing with a saline end-member associated with fluid inclusions derived from mineralization pulses that generated extensive celestite, barite, fluorite, Zn-Pb sulphur ore deposits; δ³⁴S data support dissolution of celestite, barite, gypsum minerals including the high salinity fluid inclusions. Actual water management in the CCB transferring water to the neighbouring La Madrid basin, is decreasing the water evaporation rates inside the basin; however this is in expense of an incomplete geochemical evolution in the playa lakes. This geochemical evidence can be considered as a warning signal suggesting that actual water management, sooner or later would lead to the reverse of the geochemical sequence in the water bodies (lowering salinity) bringing to an end the desiccation of playa lakes, producing detrimental effects on endemic species.