Groundwater and Tidal Controls on Wetland Hydrology, Julie J. Metz Wetland Mitigation Bank, Woodbridge, Virginia
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 basal Quaternary gravels 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. Pressure transducer data from five monitoring wells along a transect across Pod 3 were used to generate hydrographs, create groundwater flow maps, calculate hydraulic conductivity (Ksat) by performing slug tests, and calibrate wetland water budget models using Wetbud software. Wetbud-generated water budgets using both Basic and Advanced scenarios calculated estimated water levels at Pod 3 for a typical Dry, Normal, and Wet year, and custom periods. Results from the pressure transducer data and the Wetbud Advanced Scenario indicate groundwater is the overall principal water source controlling water variability at Pod 3 in Julie Metz. However, surface water flow and direct precipitation contribute to the variability in Pod 3B. In Pod 3A, closer to Neabsco Creek, the water variability is strongly influenced by tidal forcings and storm surge. The T_TIDE (MATLAB) calculations indicate lunar tides, predominantly semi-diurnal, as well as wind contribute to fluctuations in Neabsco Creek. Because creek levels control the groundwater gradient they influence how quickly the groundwater leaves Pod 3. Based upon analyses of storm surge and tidal data, and using the current sea level rise rate (4.14 mm/year), Pod 3A will be consistently inundated in approximately 22 years. Permanent inundation of Pod 3 is projected in 132 years. The rise in sea level will gradually decrease the gradient and groundwater outflow from the wetland over time and the Julie Metz wetland will transition from a forested shrub-scrub wetland to an emergent wetland.
Groundwater, Hydrology, Julie J. Metz, Mitigation bank, Tidal, Wetand, Fresh Water Studies, Geology, Oceanography
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