A COMPARISON OF THE RESPONSE OF AN INJECTED ARTIFICIAL TRACER AND NATURAL TRACERS TO A PRECIPITATION EVENT IN CENTRAL KENTUCKY

Tracer studies in hydrogeology often assume introduced tracers mirror natural flow patterns resulting from precipitation events. Research has shown that vertical flow through the vadose zone is often controlled by antecedent conditions, resulting in vastly different hydrologic responses depending on these conditions. A rainfall event in July 2018 above the catchment for a small cave spring at The Homeplace Farm on the Green River, near Campbellsville, Ky., gave us a unique opportunity to assess the responses of injected tracer versus natural tracers and what these tracers can tell us about flowpaths in a shallow karst system. Dye was injected 20 hours prior to the start of a large precipitation event; first recovery of dye at the spring occurred just prior to the arrival of the piston flow associated with the infiltration of precipitation into the system. Discharge increase showed a 9-hour lag behind precipitation. This lag showed that the precipitation pattern and the discharge pattern correlated poorly, which suggests activation of multiple pathways with variable residence times in the vadose zone. Temperature, specific conductance, turbidity, and nitrate responses mirrored precipitation, with a delay of 5 hours relative to discharge and r² values of 0.76, –0.84, 0.81, and –0.65, respectively. Although precipitation response suggested multiple flowpaths with variable residence times, dye recovery suggested a simpler flow system, with 91 percent of dye recovered at the spring within three days of injection. However, the shape of the recovery curve also suggests that although the initial dye was pushed through the system rapidly, likely following main conduits, some of the dye followed slower pathways and was later pushed through via piston flow associated with recharge from the precipitation event. These data suggest that artificial tracers partially identify activated flow paths in a groundwater system and that continuous monitoring of natural tracers provides a broader documentation of how water and associated solutes are mobilized in these settings.