Eleven-year retrospective of a wetland mitigation project in northeastern Pennsylvania
Wetland mitigation is an option of last resort for permitting encroachments into existing wetlands. Mitigation projects typically have low success rates, and relatively few are assessed beyond a five-year time period. In 2000, a developer in northeastern Pennsylvania received a permit from the Pennsylvania Department of Environmental Protection (DEP) to encroach into an existing wetland sized at 2.05 acres near Wilkes-Barre to build a retail facility. As part of the permit, the developer received permission to mitigate the wetland within a golf course 12 miles south of the encroachment. The mitigation design was based on an assessment of water levels in test pits within the area of interest, consisting of upland woods and disturbed meadow. The wetland was excavated in fall 2001 and final grading was completed in early spring 2002. The surface was covered by stockpiled topsoil and sparsely vegetated with clones of woolgrass (Scirpus cyperinus). Post-construction monitoring was conducted for two years, and then lapsed. At DEP’s request, the site was monitored again in fall 2013. The as-built wetland was redelineated and stratified into eleven communities based on vegetation type. Percent cover of each species, soil attributes, and hydrology were assessed in each community.
A delineation conducted in spring 2002, two months after final grading, found the as-built wetland occupied 1.95 acres, 95.1% of its original goal. The delineation conducted in 2013 sized it at 1.75 acres (84.9% of design). The shrinkage occurred along the wetland’s eastern fringe that was not sufficiently graded. Moreover, three narrow ungraded strips adjoining preexisting wetlands did not convert to wetland as anticipated. The areas that did successfully convert to wetland showed distinct wetland vegetation, soils, and hydrological indicators. The vegetation became dominated by 100% cover of native hydrophytic herbs and shrubs, including woolgrass, soft rush (Juncus effusus), pointed broom sedge (Carex scoparia), steeplebush (Spiraea tomentosa), and pussy willow (Salix discolor). The soils showed evidence of iron depletion and accumulation. Indicators of wetland hydrology included discolored leaf litter and oxidized root channels. Those parts of the mitigation zone were determined to be successfully functioning as wetland. Thus, this wetland appears to be a sustainable feature of the landscape, more than a decade after construction. Fully meeting the 2.05–acre target could have been achieved by overdesigning the system, though site-specific factors made doing so impossible in this case.