STRATIGRAPHY AND GROUNDWATER FLUCTUATION IN A FRESH-WATER TIDAL MITIGATION WETLAND, CHARLES CITY, VIRGINIA

Geologic and hydrologic data collected across a created fresh-water tidal wetland allow a reevaluation of analytical models that describe tidal forcing of groundwater fluctuations in coastal aquifers. The site is on an artificial clay-and-sand spit built onto a tidal flat along the edge of a sand-and-gravel mine. After mine operations ceased, the clay berm was breached and the pit was enlarged to become a protected bay and port. Dredged sand placed along the inner edge of the berm forms an unconfined aquifer that underlies the created wetland. The spit is approximately 80 m wide. Recharge from rainfall forms a very minor and temporary watertable mound across the spit that may crest 10 to 20 cm higher than the tidal fluctuation. Tides at the site average 0.84 m above mean low water although wind effects can generate significant deviations from predicted astronomical tide levels. Interpretation of data from ground-penetrating radar surveys 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. Initial data from a line of groundwater monitoring wells across the constructed wetland site indicate the transmission of tidal influence extends at least 21 m laterally into the thin surficial aquifer, but dampens out by the middle of the spit where precipitation events control most water level fluctuations. Tidal fluctuations are superimposed over groundwater decline curves that follow recharge events or sizable tidal flooding. Analyses based on the initial data suggest the tidal efficiency factor in the analytical models adequately predicts transmission of the tidal pulse, but that this unconfined aquifer passes the pulse more efficiently than predicted by the time lag factor. On-going analyses aim to quantify variations in time lag and tidal efficiency throughout the surficial aquifer and to explain them using analytical and numerical solutions.