GROUNDWATER AND TIDAL FLUX CONTROL WETLAND HYDROLOGY AT A FRESHWATER TIDAL WETLAND IN NORTHERN VIRGINIA, U.S.A

Julie J. Metz Wetlands Mitigation Bank, a 92-hectare freshwater wetland located in Woodbridge, Virginia, borders the tidally-influenced Neabsco Creek. Seven pods separated by earthen berms were built by Wetland Studies and Solutions Inc. (WSSI) in 1995 and 1997 by removing the toe of small sandy alluvial fans and covering the underlying coarse gravel bed with a low permeability cap. The Quaternary gravels, predominantly quartzite cobbles with Ordovician slate fragments, were deposited in a creek valley incised into a thick sand-and-clay Cretaceous delta. The alluvial-fan apron developed at the base of steep slopes along the sides of the valley. In the largest pod (3.1 ha), three hydrologic zones exist for transitory periods. Along the uphill side of the wetland, surface runoff from valley-side watersheds and groundwater flow through the alluvial fan control the hydrology. Hydrology of the central wetland area relies upon groundwater flow and direct precipitation. Near the creek, a semidiurnal tidal signal and rare storm surges influence water levels. The well data exhibit a one-hour tidal lag between the creek and a well located 25 meters inside the wetland. Tidal changes control the groundwater outflow gradient from the wetland to the creek where water normally only drains from the wetland during lows in the tidal cycle. The groundwater-and-precipitation hydrology of the central area characterizes most of the studied wetland except for short periods following heavy rain events and storm surges. As sea levels rise, the tidal signal in the wetland should move landward and progressively alter the groundwater outflow gradient from the wetland. Results from wetland water budget analyses (Wetbud) can provide guidance for constructing future wetlands as well as illustrate the impact of sea level rise on freshwater tidal wetlands such as Julie Metz.