GEOLOGIC INFLUENCES ON GROUNDWATER FLOW TO INTER-DUNE WETLANDS, SOUTHEASTERN VIRGINIA

False Cape State Park in southeastern Virginia Beach, Virginia contains a transgressive barrier island complex. Non-tidal and fresh-water wind-tidal wetlands carpet broad overwash flats in the island center, and mantle the long inter-dune swales near the ocean shore. The appearance and subsequent disappearance of redoximorphic wetland soil features in the young, sandy soils of the inter-dune swales here may stem from changes in the patterns of groundwater recharge and discharge across the island. These soils are being monitored by members of the interagency Mid Atlantic Hydric Soils committee and features there may be proposed for formal recognition as Hydric Soil Indicators.

Hydraulic head data from monitoring wells indicate that a strongly asymmetric freshwater lens crests under the eastern half of the island. The asymmetric shape of the water table could be due to differences in either vegetation cover or permeability of stratigraphic packages. The densely-vegetated western half of the island experiences higher rates of evapotranspiration (ET) than the more open-canopied eastern half. Results of groundwater models that spatially vary ET rates, and thus recharge, generate an asymmetric freshwater lens across a hypothetical island with uniform permeability. However, sedimentation and stratigraphic differences may prove to be more important than ET. Transducer data show that aquifer responses to rapid recharge events vary across the island in ways which suggest different net permeabilities for sediment packages on either side of the island. Permeameter tests also indicate slight differences in the permeability between the soils of each respective island half. Stratigraphic analyses of vibracore and Ground Penetrating Radar (GPR) data suggest beds in this portion of the island have relatively minor permeability contrasts that will not significantly affect shallow groundwater flow patterns in models and in the field.