Ground Water Hydrochemistry in the Southeastern Hueco Bolson,Trans-Pecos Texas

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Fisher, R. Stephen
Mullican, William F.  

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The hydrochemical history of groundwater in the arid southeastern Hueco Bolson 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 groundwater samples, while 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 Belson and Diablo Plateau aquifers predominantly contain Na-SO4 groundwater, derived from solutes through carbonate and gypsum dissolution, coupled with the exchange of aqueous calcium and magnesium for sodium on clay minerals and other ion exchange sites. Rio Grande groundwater is dominated by sodium and chloride, derived from the dissolution of salts precipitated in irrigated fields during periods of high evaporation. Major compositional characteristics of all groundwater types appear to be acquired early in the flow history, primarily through reactions in the unsaturated zone.

Ages estimated from tritium and carbon-14 activities indicate that Rio Grande groundwater is the youngest, reflecting short flow paths from land surface following irrigation, infiltration, and deep penetration from the river to sampled wells. Groundwater from the Diablo Plateau and Hueco Bolson aquifers ranges in age from a few hundred to nearly 29,000 years. Carbon-14 ages and tritium activities do not vary systematically along a flow path; however, the oldest waters are found in wells near the center of the bolson pediment. The irregular distribution of carbon-14 and tritium suggests that the Bolson and Diablo Plateau aquifers are internally complex, and flow velocities are not readily predictable solely based on the potentiometric gradient and estimates of regional porosity and permeability data.


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