SEMIDIURNAL TIDES STRONGLY INFLUENCE GROUNDWATER FLOW AT A FRESHWATER TIDAL WETLAND IN NORTHERN VIRGINIA

Julie J. Metz Wetlands Mitigation Bank, the first mitigation wetland bank in northern Virginia approved by the Army Corps of Engineers, was constructed in two phases - 1995 and 1997 - by Wetland Studies and Solutions Inc. (WSSI). Covering approximately 227 acres, this mitigation wetland contains seven pods bordering the tidally-influenced Neabsco Creek. The creek valley, deeply incised into Cretaceous sands, contains a substantial alluvial apron along its valley-side toeslope. WSSI constructed the wetland by removing the toe of an alluvial fan and covering the underlying coarse gravel bed with a low permeability cap. Strong groundwater flow through the fan supports the wetland hydrology in the upper reaches of the mitigation wetland. Water fluctuations in the lower portion of the wetland indicate a connection with the semidiurnal tidal changes in the creek. The well data display a tidal lag of approximately one hour between the Neabsco Creek well and a monitoring well 25 meters inside the mitigated wetland. The short lag time reflects the highly permeable gravel bed connecting the tidal creek and wetland. Tidal changes also control the groundwater outflow gradient from the wetland to the creek; water normally only drains from the wetland during lows in the tidal cycle. Using Wetbud modeling software to incorporate groundwater with other water budget components, the results from this study can provide guidance for constructing future wetlands in a highly permeable tidal wetland setting.