SPRINGS UNDER THE INFLUENCE OF SURFACE WATER: USING EDNA AS A TRACER

Surface reservoirs are commonly used for water storage to provide flood protection during high stream flow conditions and water supply during low stream flow conditions. Reservoir impoundments artificially raise water levels that may cause seepage into adjacent aquifers, creating new springs or changing conditions in existing springs. Groundwater users and managers often have water quality concerns, while surface-water managers may question the reservoir dam integrity or storage efficiency. A study below Stillhouse Hollow Reservoir in central Texas, in cooperation with Clearwater Underground Water Conservation District used ionic chemistry, isotopes and environmental DNA (eDNA) probes specific to invasive zebra mussels to assess surface-water influences on groundwater and springs. A combination of springs and wells at increasing distances from the reservoir were sampled periodically. Results indicated surface-water influences on wells and springs under both normal reservoir storage levels as well as during flood storage levels. The chemistry results could be explained in part by dilution from rainfall (recharge) and reservoir water. The isotopic data produced a convincing argument for surface-water influence as heavier isotopes associated with reservoir evaporation were more prevalent in springs near the dam. Springs near the dam showed positive results for eDNA. Environmental DNA results were essentially irrefutable because zebra mussels do not inhabit the aquifer. The eDNA is carried by particulate material that protects the DNA and is most promising as an indicator or tracer in karst systems with large openings and rapid flow rates. Although additional research is needed to better understand and use eDNA as a tracer, the present study demonstrates the potential of this novel approach.