Hydrogeology of the Cuatrocienegas Basin, Coahuila, Mexico: An integrative approach to arid karst aquifer delineation

The Cuatrocienegas Basin is located in the Chihuahuan Desert, Coahuila, Mexico. Over 500 springs and groundwater-dependent ecosystems with >70 endemic species flank the 2,600-meter Sierra San Marcos that bisects the 1,200 square kilometer valley. The west sub-basin contains fracture-controlled springs with elevated relative discharge (∼85 percent of total), temperature (∼31.0°--34.0°C), and total dissolved solids (∼90 milligrams/liter chloride) compared to the east sub-basin, that has stratigraphically-controlled springs with lower relative discharge (∼15 percent), temperature (∼28.0°--30.0°C), and total dissolved solids (∼30 milligrams/liter chloride). Canals convey spring discharge out of the formerly closed valley. Groundwater development since the 1980s lowered groundwater levels >10 meters in adjacent basins and caused some springs to cease flowing.;The author hypothesizes that (1) both local and regional recharge are significant, (2) an integrative data approach can delineate recharge zones, and (3) the low-elevation of the Cuatrocienegas Basin, fault-associated secondary carbonate aquifer permeability, and stratigraphic influences on permeability control spring locations. To test these hypotheses, the research (1) develops hydrogeologic conceptual models of recharge areas using remotely-sensed topography, hydrogeologic data (e.g., spring geochemistry, temperature, and discharge), and geologic mapping; (2) delineates recharge areas by sequentially including upgradient catchments to match observed spring discharge using geographic information system catchment delineation, chloride-balance recharge estimation, and analytical model interbasin flow evaluation; (3) explains spatial variations in spring chloride, discharge, and temperature using environmental tracers (e.g., delta18O, noble gases, 3H) and an elevation-dependent recharge rate; and (4) uses land gravimetry surveys to generate best-fit hydrogeologic cross sections in areas of high spring density.;The author concludes that local precipitation is insufficient to generate observed spring discharge. Waters with <0.1 tritium units indicate regional flow and aquifer residence times of >50 years. Water-budget based catchment delineation suggests west basin fractures tap a (>10,000-square kilometer) regional carbonate aquifer. Thus, groundwater management should be extended outside the Cuatrocienegas Basin. East basin canyons intersect the potentiometric surface of a stratigraphically controlled (local/regional) aquifer recharged in the 500-square kilometer Sierra San Marcos. Sierra La Madera recharge explains Ocampo Valley predevelopment flow and chloride concentration. This approach can be applied to delineate similar developing arid aquifers.