GEOCHEMICAL SIGNATURES AND AQUATIC LIFE USE MONITORING AT REBECCA SPRINGS DURING THE CRITICAL PERIOD OF FLOW IN THE HILL COUNTRY AREA OF SOUTH CENTRAL TEXAS

Located in northwestern Comal County in South Central Texas, Rebecca Springs issue from fissures and cavities in the Cow Creek Limestone Member of the Middle Trinity Aquifer and flow into the Cypress lined Rebecca Creek, a tributary to the Guadalupe River above Canyon Lake. The spring and surrounding land were acquired by Wimberley Valley Watershed Association (WVWA) to assure long-term protection. As part of the Texas Clean Rivers Program (CRP), WVWA collaborates with Guadalupe-Blanco River Authority (GBRA) staff by providing water quality data as well as access to properties like Rebecca Springs for water monitoring fieldwork. This work is key to establishing baselines and assessing the health of perennial karst springs in watersheds of the Texas Hill Country region. Through scientific investigation, preliminary biological sampling and habitat assessments were conducted at Rebecca Springs during the summer of 2018 to determine if the aquatic assemblages (fish, benthic macroinvertebrates) and physical habitat meet aquatic life use criteria as established in the Texas Surface Water Quality Standards. The current aquatic life use designation for Rebecca Creek is “Exceptional”. The single two-part aquatic life monitoring event at Rebecca Springs scored aquatic use ratings which were not supporting its designated uses. An additional sampling program was conducted to characterize the chemistry of water discharging at Rebecca Springs. Baseflow groundwater chemistry was interpreted as being of the calcium bicarbonate type using a Piper Diagram, which illustrates water quality resulting from cations and anions from water samples collected from the main outflow of Rebecca Creek Springs. Future work will include continuous monitoring of discharge, which will be provided by the Edwards Aquifer Authority (EAA). Combined with continuous flow monitoring, multi-parameter water quality instrumentation and automatic samplers will be used to collect hydrogeochemical and isotopic data, which will provide a better understanding of Rebeca Springs response to storm events and karst groundwater flow paths in both space and time. Other projects will involve Electrical Resistivity (ER) surveys 11 meters above the cave to locate karst conduits in the Cow Creek Limestone upstream of the springs.