ANALYZING GROUNDWATER FLOWPATHS IN A KARSTIC BASIN USING CROSS-PLOTS OF MAJOR IONS: CUATRO CIENEGAS BASIN, COAHUILA, MEXICO

We use published and new geochemical analyses from wells and springs in the Cuatro Cienegas Basin (CCB) and adjacent up-gradient Ocampo and Hundido valleys to create cross-plots of ions, infer groundwater evolution along hypothesized flow paths, and test the hypothesis that spring flow is derived from both local and regional flow systems. In the Chihuahuan Desert of NE Mexico, CCB hosts a unique groundwater-dependent ecosystem with springs and wetlands that provide habitat for over 70 endemic species. Surrounded by Cretaceous uplands, dominated by marine carbonates with minor evaporites, the basin receives annual precipitation of approximately 200 mm. Analyses were divided into 13 groups based on geographic location. Plots of conservative ions (e.g., B, Br, Cl, and Li) were completed along with Na, Mg, SO4, HCO3, Sr, etc. Contrary to previous studies, evaporative concentration is shown to be the main control on Cl-Br variation. Waters are predominately Ca-SO4 indicating gypsum dissolution. Along hypothesized flow paths from upgradient wells to downgradient springs and playa lakes, there is a general increase in all major ions except for HCO3 with a concomitant increase in Cl and SO4, indicating evaporative concentration and common ion effect (simultaneous gypsum dissolution and calcite precipitation). Homogeneous spring water 87Sr/86Sr suggests the source of SO4 is interbedded gypsum layers in the Cretaceous Aurora and, perhaps, Cupido aquifers. Ocampo groundwaters have a distinctive signature that may be related to volcanic rocks that occur there. Geochemistry of the largest CCB springs is consistent with fault-controlled regional discharge from the Hundido Valley. Lower discharge springs have a composition that implies a mixture of local and regional flow. 18/16O data indicate recharge occurs in the uplands and 3H data imply a mixture of young and old waters. Some springs in eastern CCB have a chemical signature of a strictly local flow system.