USE OF EFFECTIVE MONTHLY RECHARGE CALCULATIONS TO EVALUATE MITIGATION WETLAND SITES IN EASTERN VIRGINIA

Effective Monthly Recharge (Wem) calculations provide estimates of historic water levels in wetlands and ponds whose only sources of water are precipitation and groundwater seepage. This empirical water budget model uses monthly values of rainfall and average temperature to calculate the relative amount of recharge for each month. Time-weighted averaged values from previous months permit simulation of the effect of previous months’ recharge upon the month in question. Calibration of the model’s variables compares the trends of calculated values against those of water level measurements taken in the pond or a representative monitoring well. The best calculated results are scaled to match the amplitude of the well records. Analyses of Wem hydrographs reconstructed from historical weather records permit site-specific estimates of the duration and timing of past water level fluctuations in both natural and constructed landscape settings. In one case, a one-year-old mitigation wetland site built in an abandoned sand mine lies on a relict barrier island. Analyses of water table wells indicate groundwater passes through the ponds as it moves away from the ridge crest to small creeks nearby. Wem calculations suggest that the pond levels respond to major recharge events with a two-to-three month lag. Regraded slopes on the up-flow side of two ponds have intercepted sufficient water in both “dry” and “wet” seasons to sustain a range of hydric soil conditions and high percentages of plantings. Wem analyses based on 43 years of weather records indicate this new wetland surface would have been saturated for at least half of the growing season during more than 70% of the years of record. A similar analysis of a former mine site in relict dunes on a fluvial terrace suggest it also can be regraded to form a slope wetland. Because the target vegetation for this site includes drought-intolerant rare plants, groundwater seepage must be relatively stable and continuous. Regional groundwater discharge through local stratigraphic pinchouts should provide these conditions at this site.