Wetland Mitigation and Ecological Considerations for Stormwater Management WETLAND MITIGATION IN ABANDONED GRAVEL PITS: CREATING FRESH MEADOW AND SHRUB SWAMP

It is becoming increasingly difficult to provide on-site mitigation for wetland impacts due to road construction in northeastern Minnesota counties that retain greater than 80 percent of their pre-settlement wetlands. Abandoned gravel pits are one of the few remaining areas that can serve as wetland mitigation sites within the impacted watersheds. The overall goal of the project is to develop cost effective methods for creating functional mitigation wetlands on abandoned gravel pit sites to compensate for wetland impacts due to road construction in northeastern Minnesota. More specifically, the aim is to achieve ―in-kind‖ compensation by creating wetlands of the same type and function as those being disturbed, such as fresh meadow and shrub swamp. A 1.3 hectare wetland demonstration site was established in an abandoned gravel/borrow pit within the U.S. Trunk Highway 53 reconstruction corridor in July 2007 to evaluate techniques for creating fresh meadow and shrub swamp wetlands. Aggregate material was removed from the site to a level below the water table and the resulting basin was filled with organic soil displaced by the road construction. Construction activities were monitored for subsequent cost analysis. A total of 50 – 5 m x 5 m plots were established to determine the effect of donor wetland soil applications, wetland temporary and native sedge/wet meadow seed mixes, and native willow hardwood cuttings on wetland establishment. Additional plots were established to determine growth and survival of hardwood cuttings of five native willow species. Plant species and percent cover, and willow survival and height were recorded for each plot in June and September of each year following establishment. Water level monitoring was conducted at the site throughout the growing season. Preliminary results for the site overall indicate a steady increase in plant species richness over time (September 2007 – 64 species, June 2008 – 101 species, and September 2008 – 130 species). Percent plant cover is variable with native species dominance also increasing. Analyses of treatment effects and construction costs are ongoing. Reed canary grass is present, but effectively controlled by spot spraying with glyphosate herbicide. Mean hardwood willow cutting survival in treatment plots after the first year was approximately 65 percent. In the willow species trials, Salix planifolia and S. petiolaris had the top survival rates at 88 and 84 percent respectively. Ongoing monitoring and data analysis will help determine the efficacy of the various wetland establishment strategies resulting in more cost effective strategies for creating mitigation wetlands in abandoned gravel pits.