Mitigation Wetlands


        The Clean Water Act (1972) is often viewed as the most important single piece of legislation promoting federal wetlands protection in the United States. Implementation of the Act has helped to stop, and in some areas even reverse, the net decline of wetland habitat that had proceeded unchecked for more than a century. The Act gave the U.S. Environmental Protection Agency ultimate authority over wetland preservation, and designated the U.S. Army Corps of Engineers (USACOE) as the watchdog for enforcing the new federal legislation.
        Under current regulations in Pennsylvania, any destruction of existing wetland habitat exceeding 1 acre in size is only permissible following the successful submission of a “joint application” to the USACOE and PA Department of Environmental Protection (PADEP) by the developer/land owner. The application must demonstrate that the wetland destruction is “unavoidable” (the land development project cannot be sited elsewhere), “minimizes” wetland loss in the development plan, and will provide “mitigation” (compensation for the loss). Mitigation options vary, but most frequently involve new wetland creation as close as possible to the site of wetland loss.
        Because portions of the Brandywine watershed have experienced rapid growth and changing land use during the past several decades when these regulations have been in force, new “mitigation” wetlands have been built as a result of such projects as the Rte. 30 By-Pass (Fig. 14), and the construction of new shopping malls. Although these new wetlands are intended to replicate the environmental conditions destroyed by the development project, in many instances they are ecologically similar, in having shallow and permanent standing water, to constructed ponds.
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Fig. 14. Pond at Embreeville Wetland Preserve (Chester County Parks and Recreation), shortly after being built as mitigation for wetland habitat loss during construction of the Rte. 30 Bypass.

achieving these objectives, and considerable research has therefore been directed toward the use of wastewater treatment ponds as secondary polishers of sewage effluent.
        About a third of wastewater treatment plants in the United States currently use some form of pond system. Most are clearly not intended as habitat amenities, and often have a very “industrial appearance”, with rectangular, cement basins and high fences. It is nonetheless possible to design treatment ponds that add aesthetic value to their surroundings, as shown by the treatment facility at Okehocking Preserve (Fig. 13).
        The nutrient removal efficiency of wastewater treatment ponds is often quite high, but depends on size and shape (especially depth), aeration, and the composition of the algal and plant communities (Crites et al. 2005). Duckweeds are an effective means of nutrient removal from the water column, and are intentionally grown in some wastewater systems (Körner et al. 2003).