POOR TASTE IN TEXAS: TRACING ORIGINS OF DISSOLVED SOLIDS IN THE WICHITA RIVER WATERSHED

Quality and quantity of North Texas water resources vary tremendously as a function of climate and watershed geology. The Wichita River, 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 dissolution of evaporites in western headwater reaches, but anthropogenic contributions also exist from relic oil field brine disposal practices. Watershed geology consists almost entirely of N-striking Permian and Pennsylvanian units that to the east (downstream) increase in age and change from marine evaporites to non-marine siliciclastics. This study evaluates the use of strontium isotopes as tracers of upstream contributions to the dissolved solid load in conjunction with a lithochemical geographic information system (GIS). The study exploits the extreme contrast in ⁸⁷Sr/⁸⁶Sr composition between Late Permian seawater (lowest Phanerozoic value; < 0.707) and continental aluminosilicates (more radiogenic than seawater; >0.710). Historical surface water monitoring data are surveyed to establish natural hydrochemical variability and to determine empirical relationships between major chemical species and other water properties. These data are compared to analyses of Wichita River waters collected systematically throughout the watershed within 72 hour "snapshot" collection cycles. Preliminary data indicate that the marine evaporite ⁸⁷Sr/⁸⁶Sr signal wanes downstream as non-marine siliciclastic units progressively dominate watershed geology. However, the evaporite signal can be persistent and riverine ⁸⁷Sr/⁸⁶Sr compositions less than modern seawater (0.709) have been measured in the eastern extent of the watershed above its confluence with the Red River. The utility of strontium isotopes in freshwater bivalves to monitor long-term salinity variations is currently being evaluated. Results suggest that Sr isotopes have significant potential to evaluate the efficacy of Chloride Control projects proposed to improve water resources in the next 50 years.