ANALYSES OF NATUAL WETLAND HYDROLOGY ENHANCE SUCCESS OF WETLAND MITIGATION SITES, CENTRAL PIEDMONT FLOODPLAINS, 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, deeply incised streams commonly encountered in the Piedmont can be hydraulically disconnected from their floodplains. The lack of input from overbank flow to wetlands at these sites highlights the importance of groundwater needed to maintain sufficient wetland hydrology. Mitigation wetland designers who want to justify use of groundwater as a source of hydrology for such settings need a sufficient history of meteorologic and well-head data and stratigraphic information gathered prior to construction. Effective water-budget tools now couple models that reconstruct historic hydrographs (e.g. Effective Monthly Recharge (W_em) model) with flow models that incorporate site stratigraphy (e.g. MODFLOW).

Two case studies of natural toe-slope wetlands located in the central Piedmont of Virginia used 4-layer models to simulate groundwater flow from saprolite hillsides through valley floors buried by post-settlement alluvium that drains to perennial streams. Model results demonstrate how head elevation histories generated by W_em analyses on a monthly basis can be used to establish reliable constant head boundaries for 3-D transient-state simulations using MODFLOW. Both simulations run with daily time steps over 12 months used precipitation measurements and Penman ET calculations from local weather stations. Both the W_em and MODFLOW models used observed water-table fluctuations at multiple well positions (hillslope, toe-slope, mid-floodplain, and top of streambank) for calibration. The analyses demonstrate that groundwater seepage rising from saprolite hillsides into alluvium forms the most important hydrologic source for the toe-slope wetlands. The success of these simulations in natural wetlands suggests accurate models can be developed for pre-designed modules for mitigation sites in similar settings using publicly available data sources to implement both simple and advanced water level simulations.