Reconnaissance isotopic and hydrochemical study of Cuatro Ciénegas groundwater, Coahuila, México

Abstract

The springs of the Cuatro Ciénegas bolson (Four Marshes basin), Coahuila, México, support more than 70 endemic species of biota. The specifics of the groundwater flow regime, however, remain a mystery. Water samples were collected from a series of springs and pools and one canal in the Cuatro Ciénegas bolson and analyzed for field parameters (temperature, pH, conductivity, alkalinity) and stable oxygen and hydrogen isotopes in an attempt to begin a systematic study of the hydrogeology of the region. Groundwaters discharging directly along a fault-controlled spring line in Cretaceous carbonate rocks of the Cupido-Aurora aquifer are the most dilute (lowest conductivities) and warmest of those sampled. Cuatro Ciénegas waters are characterized by circumneutral pH (6.9–7.7) and reasonably low alkalinity (160–215 mg/kg as HCO 3). The δ 18O values of Cuatro Ciénegas waters range from −8.2 to −5.7‰, with a mean of −6.5±0.82‰, whereas δD ranges from −52 to −43‰, with a mean of −46.6±3.2‰. The majority of the water samples plot subparallel and beneath the local meteoric water line; those samples collected farthest from the spring line exhibit the most enriched δ 18O and δD values. The stable isotope data indicate that isotopic enrichment of groundwaters by evaporation following discharge and subsequent surface flow is an important process within the Cuatro Ciénegas bolson. The isotope data also suggest that a fraction of Cuatro Ciénegas groundwater originates with local recharge in mountains surrounding the bolson. Those springs that issue from the western base of the Sierra de San Marcos mountain range are recharged in part in these mountains, whereas groundwaters discharging from Laguna Anteojo in the northern part of the bolson are more likely recharged in the higher San de la Madera mountain range. An estimate of the water balance suggests that interbasin flow also may contribute to the considerable groundwater discharge.