VERTICAL HYDRAULIC GRADIENTS AND HYPORHEIC EXCHANGE IN A PRE-RESTORATION URBAN FOREST STREAM: REEDY CREEK, CHARLOTTE, NC

The hyporheic zone is a dynamic environment that contains gradients of physical, chemical and thermal conditions beneficial to aquatic life and water quality. Many urban streams have excess nutrient loading from sources such as development, agricultural runoff, leaky sewage etc. Downwelling in the hyporheic zone increases hydrologic retention and can enhance nutrient processing (e.g. denitrification) relative to in-stream processes alone. This research focuses on Reedy Creek, a highly-incised urban forested stream in Charlotte, NC (watershed area 2.5 mi²) that has been selected for restoration within Reedy Creek Park. The objective of this research is to characterize hydrologic exchange between the stream and subsurface (upwelling/downwelling) prior to restoration scheduled to occur in 2017. To date, groundwater-surface water interactions and hyporheic exchange have been characterized in two ways at 5 stream cross-sections along Reedy Creek and its tributaries. Nitrate concentrations range from a reference stream (C1) with average NO₃^- concentration of 0.1 ppm as N to an impacted stream (A1) with average NO₃^- concentration of 0.5ppm as N. First, water levels and temperatures have been measured in streams and shallow riparian wells at 5-minute resolution since 2013. Second, vertical hydraulic gradient (VHG) measurements were taken at 25-75 cm deep piezometer nests during fall 2016 to quantify hyporheic exchange at several different geomorphic features within different sub-watersheds of Reedy Creek. Preliminary results suggest that a few piezometer nests had median downward VHGs at all depths. However, downward VHGs did not systematically occur where hypothesized (e.g. above debris features or gravel bars). Other piezometers exhibited inconsistent VHGs with time and/or with depth. Possibly these piezometers recorded hydrologic variability combined with heterogeneity (differences in hydraulic conductivity that cause the response times of piezometers to vary). Comparison of stream and well hydrographs indicates that riparian wells respond to precipitation and/or high streamflow to some degree. From these preliminary results, it appears that downwelling may occur at specific times and/or locations within the studied streams, which are assumed to gain flow overall along their length.