Degraded marsh along the Mispillion River. USFWS photo.
Wetland Replaces Resources Lost at Superfund Site
This restoration is a Natural Resource Damage Assessment and Restoration (NRDAR) project for the DuPont Newport Superfund Site located in Wilmington, Delaware. The Trustees, Delaware Department of Natural Resources and Environmental Control (DNREC), National Oceanic and Atmospheric Administration (NOAA), and the U.S. Fish and Wildlife Service (USFWS) were authorized to recover damages to trust resources associated with a release of a hazardous waste.
The Trustees worked cooperatively with DuPont to assess the damages and identify a project that would replace the resources lost at the Newport Superfund Site. Thirty-nine scenarios were considered before this restoration was selected. In this case, the damages will be compensated by the preservation and enhancement of estuarine habitat.
The 56 acre site, located in Kent County on the Mispillion River, consists of one of the river’s original meanders and associated emergent wetland with 2,135 feet of river frontage. The quality of the estuarine tidal marsh located between the oxbow channel and the main channel had been degraded by sediment accumulation and an infestation of phragmites (Phragmites australis). The western edge of the marsh, adjacent to the main channel was severely eroded due to boat wakes. This, coupled with the highly erodible soils, would eventually destroy the marsh without intervention.
The objectives of this project were to:
- stabilize the western edge of the marsh using large coarse woody debris;
- restore a diverse, native marsh wetland plant community;
- enhance fish habitat by increasing tidal exchange in the oxbow channel; and
- enhance bird habitat in the historic tidal ponds by removing cattail (Typha sp.).
Woody debris stabilizes edge of marsh. USFWS photo.
Along the river, large diameter coarse woody debris (LWD) was used to provide immediate protection from boat wakes and trap organic and mineral sediment. Long-term protection will be provided by the re-establishment of vegetation. In addition to bank protection, the woody debris will enhance fish and wildlife habitat by providing roosting, nesting, refuge, and foraging opportunities.
Creation of Fish Rearing Areas
The oxbow channel has filled in over 50 years because tidal exchange has been diminished. A portion of the oxbow channel was dredged to a depth below Mean Lower Low Water. Selected mudflats were dredged to create fish rearing areas. Based on photo records from 1938, these mudflats were once tidal pools. Spoil from the dredging was broadcast over the marsh in a thin layer (<1 inch).
Removal of Invasive Species
Phragmites and dense stands of cattailswere removed by aerial spraying with glyphosate for three consecutive years. Standing dead canes were removed by controlled burns. A more diverse plant community has recovered in 90% of the marsh. Native plants, such as pickerelweed (Pontederia cordata), arrow arum (Peltandra virginica) and salt marsh cordgrass (Spartina alterniflora), now dominate the site. Wild rice (Zizania aquatica) has begun to colonize areas where soil is thin layered. A combination of tree seedlings and shrubs were planted on the natural upland levees adjacent to the oxbow channel in order to enhance the song bird habitat.
Completed project at high tide. USFWS photo.
Restoration was completed in June 2008. The shoreline stabilization was completed as planned however several modifications were made to the dredging component. To increase the amount of water in the ponds, flashboards will be placed in the channel leading into the tidal pools. By creating deeper water, the life of the ponds will be extended by preventing phragmites and cattail from re-colonizing. If the water quality declines, the boards can be removed. Several remaining patches of phragmites were sprayed in the fall of 2008.
A draft monitoring plan was developed as part of the Damage Assessment and Restoration Plan. This document is being revised by the Trustees, DuPont, and its contractor, URS. The project is scheduled to be monitored for 5 years. The property will remain in private hands and a conservation easement will be held and enforced by DNREC. At the conclusion of the 5 year monitoring phase, the project will be the responsibility of DNREC.