Assessing wetland compensatory mitigation sites to aid in establishing mitigation ratios

Abstract

Compensatory mitigation has been a keystone of state and federal programs for regulating wetland loss. This study reviewed mitigation performance in Indiana, USA to propose mitigation ratios (area to be mitigated/area permitted for fill) based on the rate of wetland establishment by type. Between 1986 and 1996, the Indiana Department of Environmental Management (IDEM) required 345 mitigation sites. Of these, applicants constructed 214 of the sites; another 70 were not completed. No attempt was made to construct the required mitigation on 49 of the sites. Measurements of both the total wetland area and the area of each vegetation community in the mitigation site were taken at 31 of the sites identified as “constructed.” IDEM required 34.33 ha to compensate for the 13.73 ha of state waters lost through the permit actions associated with these sites. The mapping effort found that a total of 15.21 ha of wetland and other waters had established, a net gain of 1.48 ha. Vegetation community mapping revealed that palustrine forested areas, which had a failure rate of 71% and wet meadow areas (87% failure) were harder to establish than shallow marsh areas (17% failure) and open water areas (4% failure). These results suggest that federal and state regulatory agencies would have to require minimum mitigation ratios of 3.5∶1 for palustrine forested, 7.6∶1 for wet meadow, 1.2∶1 for shallow marsh, and 1∶1 for open water to compensate for the risk of failure. Additional mitigation may be needed to offset the effects of temporal loss of wetland function.