Abstract

ABSTRACTDERICK RUSSELL HAYDIN. Vertical hydraulic gradients and groundwater surface water interaction under a pre-restoration Piedmont urban stream, Charlotte, NC. (Under the direction of Dr. DAVID VINSON)The hyporheic zone is a dynamic environment of physical and chemical gradients. These environmental gradients during groundwater-surface water interaction can be beneficial to aquatic life and water quality. Many urban streams have excess nutrient loading from sources such as development, agricultural runoff, and sewage. The hyporheic zone can be an area of mixing of groundwater and surface water, which can have distinct temperature and chemistry. Downwelling in the hyporheic zone increases hydrologic retention and can enhance biogeochemical processing (e.g. denitrification), with benefits for overall water quality. This research focuses on five short study reaches within Reedy Creek, a highly-incised urban forested stream in Charlotte, NC (watershed area 6.5 km2). The objective of this research is to characterize hydrologic exchange between the stream and subsurface prior to restoration scheduled to occur in winter 2017. From August 11, 2016 to February 3, 2017 groundwater-surface water interaction was examined in three ways at five stream cross-sections, 10 monitoring wells, and 87 piezometers in the Reedy Creek watershed. First, monitoring well water levels, stream water levels, and temperature have been measured since 2013. Second, vertical hydraulic gradient (VHG) measurements were taken at 25-75 cm deep piezometer nests (fall 2016-winter 2017) to quantify hyporheic exchange above and below geomorphic features in the streambed. Third, water quality samples were collected, and field measurements of pH, dissolved oxygen, and temperature were made in January-February 2017. To characterize groundwater that may discharge into the streams, slug testing was done on all piezometers and monitoring wells. Water samples were also collected from the monitoring wells located 5-60 meters from the stream channels. Slug testing showed that ~half of the piezometers installed were in impermeable sediments. The majority of piezometers that were in impermeable sediments were in the deeper sediments. Only two piezometer nests were hydrologically connected for all three piezometers. Slug testing data showed that the hyporheic zone in Reedy Creek is shallow within the sandy sediments that lay over impermeable clay and bedrock. The stream and well hydrographs indicate that riparian wells respond to precipitation and/or streamflow. VHG measurements suggest that a few piezometer nests had median downward VHGs at all depths. While downward VHGs (apparent downwelling) were detected at multiple locations, downward VHGs did not systematically occur where hypothesized (e.g. above debris features or above gravel bars). Other piezometer nests exhibited inconsistent VHGs with time and/or with depth (e.g. apparent downwelling at 25 cm and apparent upwelling at 50-75 cm). The data suggests these piezometers may have recorded stream flow variability interacting with subsurface heterogeneity. There are a few possibilities indicating from the VHG data. The stream could be perched on top of the impermeable sediments and bedrock, restricting any vertical exchange at these spots. Another possibility is that the groundwater at these spots is equilibrated with the streambed further downstream. The five study reaches experienced similar VHGs even though each study reach had their own unique number of geomorphic features. The median reach-wide downwelling VHGs for the reaches were from -0.01 to -0.06 for all five study reaches. Two study reaches, C1 and P1, were the only reaches that had a date of median reach-wide apparent upwelling VHG. The geomorphic features varied greatly in the extent of their VHGs. Both above debris jam and below a debris jam features had the largest hyporheic flux out of all features. Two of the five reaches had apparent perpendicular horizontal groundwater flow toward the stream while the three reaches apparently flowed away from the stream. Surface water and groundwater samples did not yield a great enough distinction between the two to be able to quantify the groundwater surface water interactions. Due to the high amounts of impermeable sediments and therefore low sub-streambed hydraulic conductivity, Reedy Creek has limited hyporheic exchange potential.