Effects of Surface and Groundwater Interactions on the Solution Chemistry of a Subtropical Karst Stream

Abstract

Freshwater ecosystems in the Edwards Plateau region o f Texas face threats from a variety o f anthropogenic disturbances, including groundwater pumping, impoundment, siltation, agricultural practices, and the introduction o f exotic species (Bowles and Arsuffi 1993). Local streams are characterized by extreme hydrologic variability, a karst geology that fosters high connectivity between ground and surface water flow paths, and populations o f endemic species. These potentially fragile ecosystem s offer significant economic, scientific, and aesthetic value to the region, and a better understanding o f their physical, chemical, and biological processes is critical for future protection and remediation efforts. The Blanco River is a little-studied karst stream that traverses the Texas H ill Country and Balcones Fault Zone regions associated with the eastern Edwards Plateau. It is intimately connected with the Trinity Aquifer, the Balcones Fault Zone Edwards Aquifer, and the San Marcos River, all being extremely valuable regional resources. This study characterizes water quality in the Blanco River from the headwaters to mouth, with particular attention given to the effects o f spring and tributary inputs on concentrations o f dominant ions calcium, magnesium, and bicarbonate. Surface waters were almost always saturated with respect to calcite and dolomite, with the highest degrees of saturation found in the headwaters. Spring water was consistently undersaturated and exhibited very low variability in temperature. Historical correlations between discharge and concentrations o f specific ions are considered as evidence of active diagenesis, specifically dedolomitization, in rock units o f the Blanco River. A greater concentration of dolomitic weathering products in the headwaters region also supports this conclusion. Two 24-hour monitoring events examining trends in major dissolved ions and organic matter revealed high daytime levels o f calcite and dolomite saturation and two potential mechanisms for retention o f energy in the Blanco River ecosystem.