Urban Land-Use Effects on Stream-Groundwater Interactions along Inner Coastal Plain Streams, NC

To evaluate urbanization effects on stream-groundwater interactions, five low order Inner Coastal Plain streams and adjacent riparian zones were studied across a gradient of total impervious area (4-37% TIA) in Greenville, NC. Each site was equipped with three groundwater wells screened into the floodplain surficial aquifer and instrumented with groundwater level and temperature recorders along with surface water level, temperature, and specific conductance loggers (30 minute logging interval). Precipitation, groundwater, and surface water samples were collected monthly along with select storm events (7 events sampled to date). Baseflow and storm event samples were analyzed for stable isotope composition (18O and D). Stable isotope data suggest that urban watersheds have greater peak stormwater discharge, in proportion to TIA. In the most urbanized watershed, stormwater made up more than half of event discharge while the most rural watershed's stormwater comprised less than a quarter of event discharge, indicating a shift from baseflow dependence to stormwater dependence for urban streams. Stormwater runoff in the urban watersheds caused stream channel incision and in some cases disconnected the stream from the surficial aquifer system by downcutting into an impermeable marine clay unit. Channel incision along with decreased infiltration and recharge has caused a greater than 1 m decline in groundwater level in the floodplains of the urbanized streams. This process has resulted in the thinning of the surficial aquifer particularly in the most urban catchment. Stable isotope, temperature, and water level responses indicate that mixing of groundwater and surface water in the near-stream aquifer occur for brief periods during storm events at the most urban site, related to the flashy discharge response. Expansion of catchment impervious area has altered stream-groundwater interactions along these Inner Coastal Plain streams by altering recharge, runoff, and channel erosion processes.