CONTROLS ON GROUNDWATER-SURFACE WATER INTERACTION IN A FORMERLY GLACIATED VALLEY, ROCKY MOUNTAINS, NORTHERN COLORADO

Groundwater discharge may supply a significant portion of streamflow in mountain watersheds. In unconfined mountain valleys above the Pleistocene glacial limit, large quantities of groundwater may be stored in glacio-fluvial sediments adjacent to modern streams. This study investigated sources of groundwater recharge and controls on groundwater-surface water interaction in a glacial valley traversed by a perennial mountain stream in northern Colorado. The ~3 square kilometer study area, spanning elevations ranging from 2740 to 2940 meters amsl, is instrumented with two 10-meter deep monitoring wells, several riparian zone wells, and two streamflow gaging stations equipped with pressure transducers for near-continuous recording of water levels. Hydrologic data collected over a 2-year study period (2020-2021) were used to constrain a numerical groundwater model with a streamflow routing component. Analyses reveal complex temporal and spatial variation of gaining and losing stream conditions within the study reach. Spatial variations are explained by channel planform and valley geometry, which are incorporated into the numerical model. An increase in valley cross-sectional area contributes to a downstream transition from gaining to losing conditions. Streamflow differencing and flow modeling indicate that the study reach changes between overall gaining and overall losing conditions multiple times between May and October. These temporal variations in groundwater-surface water exchange are driven by seasonal changes in surface water contributions to streamflow and evapotranspiration. Stable isotope (δ²H, δ¹⁸O) analyses, combined with the flow modeling and particle-tracking analysis, suggest that localized recharge from stream leakage and water-filled kettles within glacial moraines are important sources of aquifer recharge. The results of this study highlight how spatio-temporal heterogeneities in water sources and hydrogeologic characteristics strongly influence groundwater-surface water interaction in the mountain environment.