Determining the feasibility and cost of an ecologically-based design for a mitigation wetland in central Pennsylvania, USA

Studies indicate that mitigation wetlands are structurally and functionally different, not only from the wetlands they are intended to replace, but also from wetlands they ought to resemble based upon their position in the landscape. The objective of this study was to revisit a single wetland mitigation project to determine if a different, ecologically-based, design could have been developed in place of the original, at comparable cost. We selected a wetland in Pennsylvania (one of several built as mitigation totaling more than 12 ha) built during mitigation efforts for a major highway based upon available background data, the construction document package, and itemized costs. We prepared an alternative design using the original pre-construction site survey as a base map and local reference wetlands to inform the design process. We anticipated that the alternative design would cost less than the original, but due to site conditions and the objective of mimicking a natural wetland, much more earth needed to be excavated in the alternative design than anticipated. The cost of excavation, coupled with the added expense of hauling surplus fill off site, proved to be expensive. Nevertheless, other design differences, such the absence of water control structures, lack of instant habitat improvement devices, omission of an impermeable soil layer, and application of a simpler planting strategy, allowed for considerable cost savings.