ANALYSES OF NATUAL WETLAND HYDROLOGY ENHANCE SUCCESS OF WETLAND MITIGATION SITES, CENTRAL PIEDMONT FLOODPLAINS, VIRGINIA
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 Wem 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 Wem 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.