Aspects of Functional Analysis of Mitigated Wetlands Receiving Highway Runoff

The wetland mitigation and storm water management provisions in the 1987 Clean Water Act significantly affect transportation agencies. A common requirement of these federal storm water management provisions and state storm water regulations is the use of best-management practices (BMPs). The Virginia Department of Transportation has constructed more than 200 wetlands and many storm water BMPs, such as detention basins. A potentially cost-effective approach to satisfying wetland mitigation requirements and storm water regulations is to use mitigated wetlands as storm water BMPs. A multifunctional evaluation of two mitigated wetlands receiving highway runoff is presented to examine the feasibility of using mitigated wetlands as storm water BMPs. Influent and effluent water quality and quantity were monitored at the sites during storm events. Vegetation density and diversity and wetland wildlife were examined as functional indicators because they were believed to be the most likely to be impaired by highway runoff. Data collected were stored in a geographic information system, which was developed to serve as a database for current and future monitoring of mitigated wetland sites. Both sites had peak reductions in excess of 40 percent, with attenuation of greater than 90 percent for a system combining a detention basin and a mitigated wetland in series. Removal rates were as high as 90 percent for total suspended solids, 65 percent for chemical oxygen demand, 70 percent for total phosphorus and orthophosphate, and 50 percent for zinc. Despite having highway runoff as a primary water source, both sites support apparently healthy and diverse vegetative communities and provide habitat for a variety of wildlife.