GROUNDWATER AND STRATIGRAPHY IN TWO VALLEY BOTTOM WETLANDS, CENTRAL PIEDMONT, VIRGINIA, USA

In landscapes surrounding urban zones, exploration continues for large sites that can be converted to mitigation wetlands. Piedmont terrains normally support wetlands only in valley bottoms, so sites most appropriate for efficient conversion lie on low terraces and floodplains. In order to evaluate the hydrogeologic settings typical to valley bottom wetlands in the central Virginia Piedmont, sites near Powhatan and Chester were chosen for their apparently non-distinctive geology, topography and hydrology. Both analyses illustrate the role of stratigraphy, geomorphic processes, and land-use history in controlling the role of groundwater and overbank flow in valley-bottom wetlands. The expected stratigraphic package in Piedmont valley bottoms is a fining-upwards (gravel-sand-mud) point-bar/flood basin sequence deposited by a stream channel migrating laterally. Floodplains underlain by this sedimentary sequence often contain numerous riparian wetlands fed by rain, groundwater and overbank flow. This stratigraphic scenario seems to be less common than expected because of valley damming during the 18^th and 19^th centuries. Extensive and unexpectedly thick mud beds deposited in historic, abandoned millponds carpet both of valley bottoms analyzed and produce markedly altered subsurface hydrologic conditions. Surface flow often changes, too, because many streams have incised steep-sided channels 1-3 m deep into those deposits. The more deeply-incised streams are hydraulically disconnected from their old floodplains.

At both sites studied, groundwater discharge contributes significantly to wetland water budgets at toe-slopes along floodplain edges. In the middle of the valley bottom at the Powhatan site, though, wetland water on floodplain surfaces near incised streams drains downward into soil pipes and other megapores developed in the silty millpond sediments. This complex history of deposition, incision and piping create terrace surfaces with water levels too low to support wetlands that are not in the proximity of toe-slope seeps. In such settings, successful hydrologic designs for mitigation wetlands might rely upon the groundwater that emerges at toe-slope seeps and can be spread across regraded sloping surfaces carved from the historic milldam sediments.