Groundwater Contributions to Intermittent Streamflow in a Headwater Catchment: How do Geoclimatic Controls Influence Downstream Water Quality?

Hydrologic connectivity of headwater catchments affects surface water yield and quality of downstream drinking water supplies. Lower Gordon Gulch, a 2.75 km2 catchment, is part of the Boulder Creek watershed - the primary drinking water supply for the city of Boulder, Colorado. We hypothesize that the geologic and climatic environment within the catchment controls the magnitude, timing, and duration of hydrologic connection between the landscape and the stream, and thus the distribution of major ions to the surface water. Specifically, bedrock patterns, vegetation type and density, and snowpack dynamics influence how precipitation inputs move from the hillslopes to the catchment outlet. Preliminary results suggest that north-facing hillslopes with steeper slopes, deeper weathering of bedrock, denser vegetation stands, and a seasonal snowpack, provide consistently greater groundwater inputs to the stream compared to the south-facing hillslopes. We believe that this is in part due to subsurface bedrock patterns forcing a dominate cross-valley gradient. Through an extensive observation network of hillslope wells, periodic stream water balance measurements, and synoptic chemistry samples, we plan to continue our assessment of the spatio-temporal connectivity dynamics throughout the seasonal dry down (late summer through winter), during which streamflow can be intermittent. Results will help to guide landuse practices of upland catchments with respect to their role in Boulder's drinking water supply.