THE EFFECTS OF URBAN LAND USE ON WATER TABLE AND NITRATE LEVELS IN GROUNDWATER DISCHARGING TO LOW-ORDER COASTAL PLAIN STREAMS

The majority of nitrogen transported to higher-order river systems has been attributed to low-order streams. The objective was to characterize the hydrogeologic setting, determine the fate and transport of nitrate through urban riparian zones of low-order Coastal Plain streams, and determine if urban land-use has affected groundwater nitrate processing. Water-quality (NO3-N, NH4-N, dissolved organic carbon, dissolved oxygen, Chloride, specific conductance, and temperature) and hydrologic data (surface water and ground water level) were collected during baseflow conditions along ground water flow paths in the floodplain sediments of three study sites located at the base of watersheds impacted by a range of urban land-use intensity. The data have been collected once per month from Oct. 10th, 2007 to the present from nested piezometers installed 5, 20, and 40 m from the stream channel and in the stream channel. Preliminary results suggest urban development to have a significant effect on reducing water table elevations. In the most urbanized stream catchment, channel incision due to stormwater runoff has resulted in groundwater table decline in riparian zone and floodplain sediments. Riparian zone groundwater has lost contact with surficial deposits of peat and other concentrated organic matter and now discharges through underlying coarse-grained sand. Preliminary water quality data suggest that decomposition of overlying organic matter in surficial organic rich peat promotes sufficient conditions in underlying groundwater to motivate significant nitrate reduction. Groundwater nitrate levels were consistently less than 1 mg/L in groundwater discharging to the urbanized streams. These levels are generally lower than those observed along low-order Coastal Plain streams in agricultural settings. If permeable floodplain sediments are present and well connected to the channel in urban settings a fast pathway to the channel may exist. With these conditions and either a new or more concentrated source or a lack of concentrated peat supplies, nitrate in urban groundwater could have a more significant impact on nitrate levels in streams draining urbanized catchments.