Dredging Demonstration
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Deposit
N
Dredging Demonstration |
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In 1998 and 1999, the DNR and EPA sponsored a taxpayer-funded demonstration at Deposit N, a PCB hotspot located in the river between Little Chute and Kimberly, Wisconsin. The site covered about 3 acres and contained about 11,000 cubic yards of sediment. PCB concentrations were as high as 186 ppm PCBs. (50 ppm PCBs is classified as hazardous waste.) Of the 11,000 cubic yards in Deposit N, about 65 percent of the volume was targeted for removal. Approximately 8,200 cubic yards were removed, generating 6,500 tons of dewatered sediment that contained 112 total pounds of PCBs. The total included about 1,000 cubic yards from Deposit O, another contaminated sediment site nearby. This demonstration used a "swinging ladder" hydraulic dredge. The cutterhead looks like an egg beater with a suction hose going down the middle, sheathed on one side. During operation, the "egg beater" is suspended from a crane arm, and swings slowly back and forth sideways. It cuts the sediment in front in an arc. The hydraulic hose extended from the dredger to a site on land where the water was pressed out of the sediment, and the water was treated and returned to the river. (Unfortunately, the sediment was only landfilled, not detoxified.) Most of the scientists believe the swinging ladder is a messier style of hydraulic dredge, but even so, observers did NOT see a large amount of turbidity or stirring. During dredging, the site was surrounded with anchored silt curtains, which helped to reduce currents and filter-out floating sediment particles headed downstream. The major mistake at this site was timing. The DNR started in late November and the river froze by the end of December. They didn't allow enough time, and were lucky that December that year was warmer than usual. They only finished a small portion, then had to lift the silt curtains for the winter and close up shop. Then they had to set up and start over again the following year. Nevertheless, they did finish the site the next year, dredged down to the bedrock and removed a large mass of PCBs. Traces remain on the bedrock surface, but the TOTAL amount was greatly reduced and this site will no longer be a major PCB source. Monitoring data showed that the river was protected during the dredging and that wastewater discharged back to the river complied with all permit conditions. The project met the design specifications for the removal, such as the volume of sediment removed, sediment tonnage, and allowed thickness of residual sediment. The DNR noted that "the project’s goals were to test and meet the design specifications and focus on PCB mass removal, not to achieve a concentration-based cleanup, i.e., removal of all PCB-contaminated sediment above a certain cleanup level." The project cost about $4 million. This is a high per-unit cost due to its experimental nature, added monitoring, winter conditions (which should have been avoided), and the small size of the project. In addition, the DNR’s mistake regarding winter freeze-up was very costly, because it required the added take-down and set-up of equipment over 2 seasons, for a one season project. The proposed whole river cleanup will be more cost efficient, better timed (hopefully), and will benefit from the economy of scale. Links to More Deposit N Information Sediment Remediation Project Updates – Wisconsin Dept.
of Natural Resources
Sediment Demonstration Project – Deposit N – Foth &
Van Dyke
Dredging Found Effective (Deposit N) --- Univ. of Wisconsin
Seagrant Institute
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CONTENT BY: Rebecca Leighton Katers WEB DESIGN BY: DataScouts WEB HOSTING BY: Doteasy |
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