POOR TASTE IN TEXAS: TRACING NATURAL AND ANTHROPOGENIC INFLUENCES ON SOLUTE VARIATION WITHIN THE WICHITA RIVER WATERSHED

The Wichita River watershed, an east-draining sub-watershed of the greater Red River system, is historically underutilized due to high concentrations of dissolved solids (i.e., Cl^-, SO₄⁼). Natural pollutants derive largely from groundwater and overland dissolution of evaporites in western headwater reaches. Anthropogenic influences include reservoir construction, Federal chloride control efforts to divert and impound saline headwaters, and freshwater importation from the southerly Little Wichita River. Watershed geology consists largely of N-striking Permian units that to the east (downstream) increase in age and change from marine evaporites to coastal plain siliciclastics. These downstream paleoenvironmental contrasts may be amenable to tracer studies with the potential to discriminate natural and anthropogenic influences on the dissolved solid load. To test this hypothesis we integrate isotopic (Sr, S) and elemental analyses on waters, freshwater bivalves, and evaporites collected systematically throughout the Wichita and Little Wichita River watersheds. Sr isotopes prove to be effective solute tracers in this system because of the extreme contrast in ⁸⁷Sr/⁸⁶Sr composition between Late Permian seawater (<0.707) and continental aluminosilicates (>0.710). Western headwaters are close to isotopic equilibrium with outcropping gypsum, but less radiogenic than impounded water for chloride control. The marine evaporite ⁸⁷Sr/⁸⁶Sr signal wanes downstream with the progressive influence of siliciclastic watershed units. However, the evaporite signal is strong and riverine ⁸⁷Sr/⁸⁶Sr compositions less than modern seawater (0.709) persist above the Red River confluence. Sulfur isotopes confirm a persistent Permian evaporite influence. These signatures contrast markedly from the Little Wichita River, the headwaters of which do not intersect evaporites. Temporal variations among various tracers at given localities strongly correlate with flow. Bivalves tend to have intermediate Sr isotopic compositions between high- and low-flow waters collected from the same locality, suggesting that they are good long-term integrators of salinity variation. Composite results demonstrate the efficacy of tracer studies to discriminate natural and anthropogenic influences bearing on water quality.