Integration of Ground-Water and Vadose-Zone Geochemistry...: A Case Study in ..the Northern Chihuahuan Desert, Trans-Pecos Texa

The hydrochemical history of ground water in the arid southeastern Hueco Bolson and southwestern Diablo Plateau was investigated by collecting soil-moisture samples from unsaturated siliciclastic bolson-fill sediments and groundwater samples from the Diablo Plateau aquifer, the Hueco Bolson silt and sand aquifer, and the Rio Grande alluvial aquifer. Major, minor, and trace solutes, stable isotopic compositions, and activities of tritium and carbon-14 were measured in ground-water samples; major solute concentrations were determined in soil-moisture samples. Soil samples were collected to determine the type and amount of material that could be readily dissolved by recharge water. Core samples of Cretaceous carbonate and bolson-fill material were analyzed to determine the mineralogy of sediment and aquifer matrix. The Hueco Bolson and Diablo Plateau aquifers contain mainly sodium-sulfate ground water that derived solutes by calcite, dolomite, and gypsum dissolution, coupled with exchange of aqueous calcium and magnesium for sodium on clay minerals and other ion-exchange sites. Rio Grande ground water is dominated by sodium and chloride derived from dissolution of salts precipitated in irrigated fields during times of high evaporation. All ground-water compositions were probably determined early in the flow history, principally through reactions in the unsaturated zone. Ages estimated from tritium and carbon-14 activities show that Rio Grande ground waters are youngest, reflecting the short flow paths to sampled wells after irrigation and percolation. Young ground waters are also found in the Diablo Plateau aquifer at wells both on the plateau and near the toe of the plateau escarpment. These are inferred to be recharge waters that rapidly moved along fractures to the water table. Other ground-water samples from the Diablo Plateau and Hueco Bolson aquifers are as much as 28,000 yr old. Carbon-14 ages and tritium activities do not vary uniformly along a flow path. However, the oldest waters are found in wells near the center of the bolson pediment, and the distribution of carbon-14 ages generally conforms to the salinity distribution, suggesting a systematic relation between residence time, chemical and isotopic composition of ground water, and regional hydrologic properties of the aquifers.