Regional groundwater flow paths in Trans-Pecos, Texas inferred from oxygen, hydrogen, and strontium isotopes
Stable isotopes, radiogenic isotopes, and major ion chemistry are used to constrain flow paths in a fracture-controlled regional groundwater flow system in far west Texas. The flow system occurs in Permian and Cretaceous sedimentary rocks and Cenozoic alluvial basin fill. Samples from springs and wells in the study area were analyzed for major ions and hydrogen, oxygen, and strontium isotopes. δD and δ18O values fall close to the global meteoric water line, indicating that these values have not been significantly affected by evaporation or mineral–water reactions. δ18O values from samples along the regional flow path are lower by as much as 3‰ relative to local recharge values; the distribution of stable isotope values throughout the study area suggests that lower δ18O values represent recharge during cooler climatic conditions rather than latitude or altitude effects. 87Sr/86Sr values for water samples from along the regional flow path range from 0.7093 to 0.7148, with the highest values at the upgradient end of the flow system. These values are significantly higher than expected for groundwaters dominated by dissolution of Permian and Cretaceous marine carbonates and evaporites. The high 87Sr/86Sr values reflect dissolution of minerals of Precambrian and early Paleozoic origin in the crystalline rocks of the Carrizo and Baylor mountains and alluvial basin fill at the upgradient end of the flow system. Contours of 87Sr/86Sr values indicate a zone of high 87Sr/86Sr water that corresponds to the regional groundwater flow paths. Mass-balance mixing models and mineral reactions models indicate a three end-member mixing system modified by dissolution of strontium-bearing minerals in the alluvial fill at the upgradient end of the flow system.