The Cuatro Cienegas basin in Coahuila, Mexico was declared a National Protected Area in 1994 by the Mexican government. Its principle uniqueness is in its aquatic fauna, which are dependent on the abundant springs in this desert valley. However, the basin's hydrogeology is not very well understood. The springs in the basin have extreme variability in temperature, salinity, water chemistry, and discharge over small spatial scales. By gathering physical and chemical data in the field and using standard computer methods, the extremely complex flow systems in this critically sensitive desert wetland have been delineated. Five distinct flow systems are defined in the basin by the chemical data. These include the Churince system in the southwest, the Garabatal-Becerra-Rio Mesquites system flowing from the southwest to northeast portion of the basin, the Tio Candido-Hundidos system in the center of the eastern side of the basin, the Santa Tecla system in the southeast, and the Anteojo system in the north. The physical characteristics of the basin, including spring and canal locations, have been examined and show variable mixing within each system and complex physical flow systems. The Churince, Rio Mesquites, and Tio Candidio systems have separate source springs and terminal playa lakes. The Tio Candido and Rio Mesquites systems are related through canal and possible karst flow. Chemical processes involved in the evolution of Cuatro Cienegas waters have been evaluated from the analyses of 35 samples collected in April 2004, June 2004, and January 2005. Calcium and sulfate dominate the water chemistry. However, high alkalinities are encountered in pools located closest to the mountains, especially in the Anteojo system. The Anteojo and Santa Tecla systems have lower conductivity/TDS than other systems. The water chemistry for the basin has been integrated into a Geographic Information System (GIS) to examine the spatial variations in chemistry and temperature throughout the basin. Highly saline waters (31.4 mg/L Na to 810.7 mg/L Na) are encountered throughout the basin, but the highest salinities occur toward the end of the flow systems. Furthermore, the temperature of the water at various pozas in the basin may reflect the source of the water via regional and/or karst flow. The Cuatro Cienegas water chemistry is mainly evaporation controlled, but there are other important processes involved, including the dissolution and precipitation of various mineral phases. In order to understand the variability within each flow system, a reaction pathway and mixing geochemical model has been developed for each system using PHREEQC. These baseline data provide a framework for necessary future hydrologic studies in the basinGeological Science
