GROUNDWATER HYDROLOGY OF LOW-ORDER COASTAL PLAIN STREAMS ACROSS AN URBAN LAND-USE GRADIENT

Urban land-use is expanding across the south-eastern U.S. and has been shown to affect stream hydrology. Variable responses of groundwater discharge to streams have been recorded in many areas of urban land-use. To evaluate the effects of urban land-use on the groundwater hydrology of low-order Coastal Plain streams, we monitored riparian groundwater levels at six sites spanning an urban land-use gradient in Greenville, NC. Impervious area for the catchments ranged from 3.8-36.7% and the catchment area ranged from 1.19 – 3.46 km2. Each site was equipped with 3 wells screened in the floodplain surficial aquifer. Groundwater and stream levels, stream specific conductance, and water temperature were manually monitored bi-weekly from June 2006 to June 2007. The well closest to the stream channel at each site contained a pressure transducer that logged groundwater level and water temperature every half hour. The Bouwer and Rice method was used to estimate floodplain hydraulic conductivity using slug test data from all 18 wells. Groundwater levels were much deeper in urban areas because the streams were incised or channelized thus lowering the groundwater table. In the most urbanized location, the mean annual groundwater level was 2.05 meters deep while at the most rural location groundwater was 0.51 meters deep. Groundwater levels were highest in the winter months and showed the most variability in the more urbanized locations. The magnitude of ground water fluxes at the floodplain scale was related to floodplain sediment hydraulic conductivity, which ranged from 1.34*10-4 to 2.66*10-6 cm/s. On an annual basis, specific conductance of surface water was more variable and surface water temperature was warmer for the more urbanized streams. Urban land-use effects included: drawdown of the water table in the riparian zone by incised streams, increased variability of groundwater levels, warming of the surface water, and increased variability of surface water chemistry with increased imperviousness. Increases in stormwater inputs, related to increases in catchment impervious area, have altered riparian groundwater systems in this Coastal Plain setting. Improvements in stormwater management can help ameliorate the effects of urban land-use on Coastal Plain streams.