Assessing Stream-Groundwater Exchange in a Headwater Montane Catchment in Colorado

Abstract:

The dynamic sources and exchanges of stream water in headwater mountain catchments are not widely understood. Certainly precipitation and groundwater contribute to stream flow, and the hydrogeologic setting (i.e.., substrate, orientation of geological features, etc.) controls these interactions. Here we quantify the temporal patterns in groundwater gains to a stream and the mixing of stream and groundwater in riparian zones during summer baseflow recession in Gordon Gulch, Colorado. We use specific conductance and dissolved oxygen as natural tracers of stream and groundwater end-members to identify stream and groundwater mixing along a 1000 m reach. Beginning in June 2015, we collected data from the stream and 20 wells on an approximately weekly basis. Net stream flow gains to the stream range from <1% to >9%. The riparian wells are either consistently dominated by stream water or groundwater, or show varying dominance, with a few demonstrating a shift from stream water to groundwater dominance over this period. These results demonstrate that even in gaining streams, significant exchange of stream water and groundwater through the riparian aquifer is common, potentially fueling microbial activity in these zones.