42-13 Surface Water-Groundwater (Dis)Connectivity in a Biologically Unique Endorheic Desert Basin

Abstract:

The Cuatro Ciénegas Basin (CCB) in Coahuila, Mexico is a rare endorheic desert oasis where thermal groundwater upwells through faulted Cretaceous carbonate rocks to support over 200 pools, or pozas, home to many endemic species (Felstead et al., 2015). Extensive regional groundwater pumping for agricultural and municipal use has triggered water table declines exceeding 30 meters, threatening the survival of these spring systems (Pedrozo-Acuña et al., 2026). This study evaluates the physical and hydraulic characteristics of the shallow subsurface to determine the resilience of these aquatic habitats to continued groundwater overexploitation. We hypothesized that the shallow subsurface is relatively homogeneous and highly electrically conductive, and pozas are generally in losing conditions and lack significant natural buffering against external hydrologic shifts. We installed monitoring wells at Rio Mezquites and Poza La Becerra, using RTK-GPS and Total Station surveys for precise topographic mapping. Subsurface structures and saturation were assessed through Electrical Resistivity Tomography (ERT), in-situ Guelph permeametry, and falling-head core analysis. Pressure transducers and stream gauging with acoustic Doppler velocity meters provided high-resolution data on head fluctuations and reach-scale mass balances. ERT results showed a highly conductive and physically homogeneous shallow subsurface. Water table levels rose rapidly (8-16 cm) following meteoric events, indicating a highly sensitive system with short recharge lag times. While localized gradients typically indicate losing conditions, precipitation events can temporarily reverse flow direction. These findings suggest that the lack of subsurface heterogeneity limits hydraulic resistance, leaving these springs highly vulnerable to continued groundwater extraction.

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