HYDROLOGIC VARIATIONS BETWEEN 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 have epiaquic (“perched”) hydrologic profiles. On Coastal Plain terraces of southeastern Virginia, many natural wetlands also have epiaquic profiles, especially on the 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 with 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. Biweekly data collected during the first year of study reveal several significant trends. In the SBNP, piezometer data indicate that the natural loamy soil profiles display endoaquic responses to precipitation events. Differences between widely-spaced natural wetland sites at the nearly flat SBNP also indicate that subtle changes in landscape position can control the recharge-discharge status of a site. At the CC site, peizometer data indicate that wetlands constructed on compacted clayey soils display epiaquic profiles. Minor differences in the shallow stratigraphic profiles present in neighboring experimental plots at CC apparently control the degree of hydraulic connection between the upper and lower phreatic surfaces. On-going studies will analyze vertical variations in permeability and the response time of wetland soils to individual precipitation events.