GROUNDWATER CONTRIBUTION TO TOE-SLOPE WETLAND WATER BUDGETS, CENTRAL PIEDMONT, VIRGINIA

The weathered crystalline Piedmont of Virginia normally supports wetlands only in valley bottoms. There the sites most appropriate for efficient conversion to wetlands for regulatory mitigation lie on low terraces and floodplains, where wetlands rely on rain, tributary streams, groundwater and overbank flow. However, both the geological history and historical land usage in the central Virginia Piedmont generated large quantities of anthropogenic sediment and steepened gradients across many valley bottoms, inducing streams to deeply incise and resulting in streams that are hydraulically disconnected from their floodplains. The lack of input from overbank flow to wetlands at these sites emphasizes the contribution of groundwater to locations that maintain sufficient wetland hydrology.

Two case studies in the central Piedmont of Virginia demonstrate the significance of groundwater discharge along toe-slope seeps contributing to these wetlands. Quantification of groundwater discharge using Darcy’s Law indicates that during certain months groundwater can contribute up to 45% of total inputs, depending on the frequency and magnitude of rainfall. Piezometer data show that zones where groundwater rises to the surface along valley edges can extend many meters out into the valley throughout the entire year. In addition, hydraulic head fluctuations in hillslope and toe-slope wells correlate strongly with estimations of recharge across hilly watersheds (r² as high as 0.95, p = 0.00), even where fine-grained saprolite and colluvium dominate. These correlations suggest that the Effective Monthly Recharge (W_em) model can be used in similar Piedmont settings to generate synthetic groundwater hydrographs from historical weather records. Data from this study also serve as justification for modelers to include the use of groundwater as a source of hydrology as they plan construction of mitigation wetlands in these types of valley bottoms.