HYDROLOGIC VARIATIONS BETWEEN NATURAL AND CONSTRUCTED MINERAL SOIL FLAT WETLANDS, SOUTHEASTERN VIRGINIA

Mitigation wetlands constructed as sites to foster the generation of palustrine forests should mimic most aspects of the hydrology of natural wetlands that support these vegetation communities. Because of soil compaction that results from construction using heavy machinery, many mitigation wetlands develop epiaquic (“perched”) hydrologic conditions. On Coastal Plain terraces of southeastern Virginia, many natural wetlands also display epiaquic characteristics, especially on mineral soil flats formed on broad interfluves carpeted with fine-grained sediments or thick Bt horizons. On sandier or relatively young surfaces, natural wetlands have endoaquic profiles (one phreatic surface). Because of the stresses to plants caused by rooting limits and more frequent drought conditions on newly constructed wetland surfaces, epiaquic soil conditions may hinder the development of the target vegetation community. The Virginia Department of Transportation has constructed one mitigation wetland at the Sandy Bottom Nature Preserve (SBNP) and another in Charles City County (CC). These large complexes lie on different Quaternary formations - the Tabb (SBNP) and Shirley (CC). Textural and hydrologic data collected from wetands in and around these mitigation sites permit comparisons of the functions in soils on natural and constructed wetlands, in soils with different textures, and in soils resulting from different construction techniques. A combination of hydrologic data collected at hourly and biweekly intervals reveal several significant trends. In the SBNP, piezometer data indicate that the natural loamy soil profiles display endoaquic responses to precipitation events. Lateral differences between several piezometer nests at the nearly flat SBNP indicate that subtle changes in landscape position can control the recharge-discharge status of a site. In the constructed wetland, both texture and compaction of the fill sediments and their hydraulic connection to surface canals control wetland soil hydrology. At the CC site, peizometer data indicate that wetlands constructed on compacted clayey soils may display epiaquic conditions. However deep desiccation cracks found in Btg horizons truncated during the construction process significantly increased vertical conductivity in soil profiles.