SPRING WATER GEOCHEMISTRY AS ENVIRONMENTAL TRACERS IN PAIRED KARST WATERSHEDS IN CENTRAL TEXAS

Woody plant encroachment in semiarid landscapes is a developing issue in ecohydrology, which, in central Texas, has been hypothesized to limit groundwater recharge and streamflow. A multidisciplinary study of paired control and treatment watersheds is currently underway at the Honey Creek State Natural Area to evaluate the effects of shrub control on watershed hydrology by selective removal of juniper from the uplands of the treatment watershed. The treatement and control watersheds are adjacent and have similar geologic and geomorphologic features, including springs discharging into Honey Creek. Spatial and temporal variations in the geochemistry of spring waters provide insight into the sources of dissolved ions and flow paths in these watersheds. These data provide a framework in which to interpret potential impacts of ongoing juniper clearing in the treatment watershed.

Prior to juniper removal, Sr isotope values (Sr-87/Sr-86) of soils from the two watersheds exhibit systematic differences. Treatment watershed soils have a mean isotopic composition of 0.7088, whereas control watershed soils have a mean of 0.7083. The lower Cretaceous limestone bedrock value is 0.7076. Spring water Sr isotope values fall between values for the limestone bedrock and the soils, reflecting a balance between these two primary sources of Sr to groundwater. Values for springs within both the treatment and control watersheds reflect the differences of the respective watershed soils: spring waters from the control watershed have lower Sr isotope values than spring waters from the treatment watershed. These results suggest that local soil variability exhibits a fundamental control on spring water geochemistry.

The spring waters also exhibit temporal differences in geochemistry, which may be indicative of a climatic influence on variations in recharge, groundwater residence time and water-rock interaction. Preliminary results indicate a decrease in springwater Sr isotope values in both watersheds in the drier summer months. This trend is consistent with a greater proportion of Sr being derived from bedrock limestone relative to overlying soils as a result of longer residence time and increased water-rock interaction during drier periods.