USING GEOCHEMICAL METHODS TO TRACE GROUNDWATER/SURFACE WATER INTERACTION

The hyporheic zone, where groundwater and surface water interact in the streambed, is critical to the health of a stream as it provides benefits such as temperature regulation, biogeochemical cycling and contaminant attenuation. Restoration projects often rehabilitate the hyporheic zone within the stream, however there is a lack of consensus on which restoration techniques will be most beneficial. During the fall of 2019, a stream restoration project will take place at a small stream located on the University of Kentucky’s campus. Monitoring of the stream pre- and post-restoration will provide an opportunity to analyze how well the project achieved the stated goals of nutrient attenuation and increased hyporheic residence time.

To monitor mixing of water within the hyporheic zone, pore water samples will be taken from the hyporheic zone, along with surface water and groundwater samples. Stable isotope ratios d¹⁸O and dD will be analyzed in samples, since groundwater and surface water can have unique values. Stable isotope ratios in surface water show seasonal variations in precipitation and evaporation effects. In comparison, stable isotope ratios in groundwater are usually constant through the year and have values near the local meteoric water line. Additionally, a NaCl tracer will be utilized along with electrical resistivity to image the hyporheic zone. Electrical resistivity imaging data collected prior to the NaCl tracer injection is used to create a background resistivity model. This background model is then used to observe percent change in resistivity once the tracer travels through the monitored stream transect and quantify the extent of the hyporheic zone.

Analysis of the pre- and post-restoration state of the hyporheic zone make it possible to examine how well restoration efforts achieved stated goals. By quantifying physical and chemical changes in the hyporheic zone, future restoration efforts can better target proposed changes.