Validation of Wetland Mitigation in Abandoned Borrow Areas – Phase II

Abstract

Road construction in northeast Minnesota often causes wetland impacts that require compensatory mitigation. Borrow areas excavated for road construction material can be developed into wetland mitigation sites if hydric vegetation, hydric soils and adequate hydrology are provided. Fourteen wetland mitigation sites were constructed north of Virginia, Minnesota along the U.S. Trunk Highway 53 reconstruction project corridor. The sites were established with the goal of mitigating for project impacts to seasonally flooded basin, fresh meadow, shallow marsh, shrub swamp, wooded swamp, and bog wetlands. Monitoring results indicate that the 14 mitigation sites range in their potential to receive wetland mitigation credit. All but one of the sites consistently meet wetland hydrology criteria. The sites contain a variety of plant communities dominated by wet meadow, sedge meadow, and shallow marsh. Floristic Quality Assessment (FQA) condition categories for the sites range from "Poor" to "Exceptional." Invasive plant species, particularly reed canary grass and narrow leaf cattail, are present on a number of sites and should be controlled. Tamarack and black spruce plantings have been successful on some of the drier areas and should be expanded to increase the quality and potential mitigation credit for other sites. These sites have shown the potential for creating mitigation wetlands in abandoned borrow pits in conjunction with highway construction. Adaptive management, particularly water level regulation, early invasive species control, tree planting, and continued long-term annual monitoring can make mitigation sites like these successful options for wetland mitigation credit. Continued site monitoring to determine potential for mitigation credit is recommended.