Two Birds With One Stone – The Fairview Church Wetland Mitigation Case Study

Abstract

Stormwater treatment basins, properly designed, control runoff and reduce sedimentation. Whether they can also be used as wetland mitigation sites has been a topic for debate among natural resource agents, academics, and consultants. Natural resource agencies, when reviewing wetland mitigation plans, often reject designs that include dual use stormwater treatment facilities/wetland mitigation sites for several reasons. First, routine maintenance at stormwater treatment basins usually includes vegetation removal (mowing) and dredging of accumulated sediment, both serious impediments to wetland viability. Second, the basins are meant to capture stormwater runoff, which translates to unpredictable flows and duration of water, and a high potential for capturing nutrients, heavy metals, hydrocarbons, and other pollutants found in runoff. Pollution and an uncertain water table can prevent the establishment of quality habitat. Finally, stormwater treatment basins and wetland mitigation sites are each subject to regulations regarding water quality and quantity, and one set of regulations may conflict with the other. In spite of these obstacles, a dual use stormwater/wetland mitigation site may be the best option available for on-site, in-kind remediation in certain situations. This article presents a situation in which a compensatory wetland mitigation design was successfully incorporated into an active stormwater management facility in York, Pennsylvania. The following includes a discussion of the rationale for selecting the wetland creation site, the significant design constraints, construction issues, and the results of the fi rst post-construction monitoring visit. Wetland mitigation project design, construction, and post-construction monitoring efforts are primarily overseen by two regulatory agencies in Pennsylvania: the U.S. Army Corps of Engineers (USACE) and the Pennsylvania Department of Environmental Protection (PADEP). USACE and PADEP guidelines for mitigation design stipulate that wetland mitigation projects should incorporate function for function replacement in their design. In other words, if a project impacts a mature silver maple forested wetland, a cattail marsh would not be an appropriate replacement, because it would not provide the same ecological functions and values. In this project, the original wetland site was a palustrine emergent marsh associated with an unnamed perennial tributary to Tyler Run. The original wetland was dominated by an herbaceous community of redtop (Agrostis alba L.), soft rush (Juncus effusus L.), and New York Ironweed (Vernonia noveboracensis (L.) Michx). The wetland site offered minimal riparian stabilization, terrestrial/semi-aquatic habitat, and floodflow attenuation functions as it was subjected to periodic mowing by local township personnel to remove the “weeds” along the stream banks. Water was supplied to the wetland from shallow groundwater sources, flood flows from the unnamed tributary flowing through the center of the site, direct precipitation, and overland flows from the surrounding agricultural, commercial, and residential areas. The upper seventeen inches of the soil profile were characterized by distinct silty clay A and B soil horizons, with B horizon soils showing clear hydric soil coloration (10YR 4/1).