CONCEPTUALIZATION OF GROUNDWATER FLOW SYSTEM USING HYDROCHEMISTRY AND ISOTOPIC COMPOSITIONS, PRESIDIO COUNTY, WEST TEXAS

The West Texas Bolsons aquifer forms the main water supply source in this arid part of Texas. In Presidio County, the West Texas Bolsons aquifer consists of the Presidio-Redford Bolson aquifer and the Ryan Flat Bolson aquifer. Igneous aquifer underlies most of the county. We determined geochemical evolution of the groundwater using elemental and isotopic compositions. We conceptualized the flow system for building a groundwater availability model. This understanding will help better manage groundwater resources in the area.

Groundwater in the Presidio-Redford Bolson is more saline (TDS up to 6,000 mg/l) than groundwater in the Ryan Flat Bolson and Igneous aquifers (TDS <500 mg/l). The ratios of Br/Cl in the groundwater do not notably vary in areas with soils of varying percolation rates and decrease downgradient. The ratios of Br/Cl decrease with an increase in Cl, Na, SO4, and F. This suggests that the waters were affected by dissolution of evaporites from below. Higher Br/Cl ratios in the Ryan Flat Bolson and the Igneous aquifers are indicative of an absence of evaporite dissolution. At higher salinity, Na decreases with respect to Cl suggesting its removal by cation exchange. Comparison of excess Na and Ca and Mg supports that Ca exchange for Na is a dominant process. Saturation indices suggest that the groundwater is saturated with respect to quartz, Ca-montmorillonite, kaolinite, and gypsum – minerals found as cements in the sediments.

Oxygen and deuterium suggest that the groundwaters were evaporated during recharge. Carbon-14 data suggests that the groundwaters in the Presidio-Redford Bolson aquifer are paleogroundwater with apparent ages of up to 18,000 years. Only a few samples in the downgradient areas have high tritium and carbon-14 indicating modern contributions. Groundwater from the Ryan Flat Bolson and Igneous aquifers are slightly younger with apparent ages of up to 9,000 years. Carbon-14 shows a negative linear correlation with carbon-13 isotopes indicating influences of soil CO2 in the younger waters while older waters show effects of dissolution from carbonate matrix. Spatial distribution of carbon-14 and tritium suggest that recharge is episodic and spatially biased, groundwater is compartmentalized locally due to lithologic heterogeneity, and groundwater moves slowly through the subsurface.