Hydrogeology of the Cuatrociénegas Basin, Coahuila, Mexico : an integrative approach to arid karst aquifer delineation

Wolaver, Brad David, 1972-
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The Cuatrociénegas 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 Cuatrociénegas 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., [delta]¹⁸O, 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 Cuatrociénegas 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.