TIDAL INFLUENCE UPON GROUNDWATER FLUCTUATIONS IN A FRESH-WATER TIDAL MITIGATION WETLAND, SOUTHEASTERN VIRGINIA

Geologic and hydrologic data collected across a created fresh-water tidal wetland allow testing of analytical models by Jacob (1950) that describe tidal forcing of groundwater fluctuations in coastal aquifers. The mitigation wetland was constructed on an artificial spit built for wave protection across a tidal flat along the edge of a sand-and-gravel mine. Dredged sand placed along the inner edge of the berm forms an unconfined aquifer approximately 80 m wide. Interpretation of data from ground-penetrating radar surveys, vibracores, and shallow borings verify the reported extent of the margin of the clay berm and that the bottom of the sand sheet deposited on the tidal flat muds depressed the base of the aquifer uniformly approximately one meter below low tide levels. Tides at the site average 0.84 m above mean low water although wind effects can generate significant deviations from predicted astronomical tide levels. A shore-to-upland transect of monitoring wells constructed prior to the regrading of the wetland site revealed that recharge from rainfall formed a minor watertable mound in the artificial spit that crested 10-to-20 cm higher than the tidal fluctuation. Tidal influence extended at least 21 m laterally into the thin unconfined aquifer, but tidal fluctuations were superimposed over and dampened by groundwater decline curves that followed recharge events or sizable tidal flooding. Analyses of the pre-construction data suggest the tidal efficiency (Hx/Ho) and time lag factors in the Jacob's analytical models for confined aquifers can adequately predict transmission of the tidal pulse through this unconfined system. The newly constructed mitigation wetland lies on a gently sloping bench carved into the artificial spit. Piezometers and monitoring wells now document groundwater movement through the aquifer underlying the fresh-water tidal wetland.