A.24 Grubers Grove Bay; Prairie Du Sac, WI
A.24.1 Contacts
Regulatory Contact(s): Wisconsin, DNR
A.24.2 Summary
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Environment: |
Lake remnant |
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Scale: |
Full |
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Contaminants of Concern: |
Mercury, methylmercury, lead, copper, and zinc |
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Source Control Achieved Prior to Remedy Selection? |
No |
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Final Remedy: |
Dredging and dewatering of sediments using Geotubes followed by placement on site and cappingTechnology which covers contaminated sediment with material to isolate the contaminants from the surrounding environment. with a soil cover |
A.24.3 Site Description
Grubers Grove Bay is located in Lake Wisconsin, an impoundment of the Wisconsin River created by the Wisconsin Power & Light dam constructed in 1915 at Prairie du Sac. The primary source of contamination at the site is contaminated wastewater discharge from the Badger Army Ammunition Plant (BAAP) in Sauk County. Discharges took place between 1942 and 1976. Gruber’s Grove Bay, due to its elevated mercury levels, was added to USEPA's impaired waters (303d) list in 2000.
The Badger Army Ammunition Plant (BAAP) operated between 1942 and 1976. The BAAP was built and operated by the U.S. government to produce various formulations of nitrocellulose-based propellants including nitroglycerine. Nitric and sulfuric acid were also produced as intermediate materials used in manufacturing those propellants. At the time of its construction, it was one of the largest ammunition plants in the world. The operation of the BAAP caused extensive contamination of soils, surface water sediment, and groundwater. Both production and waste-disposal practices at BAAP account for contamination at this site.
A.24.4 Remedial Objectives
The primary remedial objective relevant to the sediment portion of the remedy was removal of contamination to promote recovery of the benthic community.
A.24.5 Remedial Approach
Final selected remedy: Hydraulic dredging, sediment dewatering, and disposal in an on-site containment facility (OCF).
Two sediment cleanups were performed. The initial cleanup was performed in 2001 and resulted in the removal and dewatering of approximately 90,000 yd3 of contaminated sediment. Subsequent sampling, however, found that mercury and other metals were still present over a wide area of the bay’s sediment at levels above the cleanup goal. A second cleanup was completed in 2006. Approximately 60,000 yd3of sediment harboring mercury, methyl mercury, lead, and copper were dredged over a 17 acre area. The sediment was pumped with water from the bay into 42 geomembraneA kind of geosynthetic material made up of an impermeable membranes. Their uses include solid waste containment (such as landfill liners), mining, and water containment applications. tubes placed on a plastic liner on BAAP land where it was covered with a final soil capA covering over material (contaminated sediment) used to isolate the contaminants from the surrounding environment.. More than 66 million gallons of the dredged water was collected as it drained to sediment collection tubes and was distributed as irrigation water to three areas on BAAP property.
During dredging operations, a silt curtain was placed across the mouth of the bay to prevent contaminant releases to Lake Wisconsin. Dredging depth was verified using GPS in conjunction with an echo sounder. Problems affecting the dredging operation included heavy precipitation1) The formation of a solid in a solution or inside another solid during a chemical reaction or by diffusion in a solid; or 2) rain, sleet, hail, snow and other forms of water falling from the sky., tears in Geotubes, and the presence of debris within the sediment bed.
Sediment slurry was pumped from the dredge to a series of Geotubes located on the BAAP plant site. A polymer was added to the dredge slurry to increase flocculent growth and retention of fine particles in the water. Water from the Geotubes was discharged to a storage lagoon for spray irrigation. Mercury levels in the effluent were generally non-detect or below the discharge permit level.
The cleanup target for mercury was 0.36 milligrams per kilogram (mg/kg), the background standard for mercury in the bay, in the top 6 inches of sediment. Also removed from the bottom were lead, copper, and zinc, metals bound together in the sediment particles.
It is estimated that the two dredging operations removed 500 pounds of mercury, 12,000 pounds of copper, 16,000 pounds of zinc, and 36,000 pounds of lead from the bay.
Approximately 2 acres of habitat restoration was completed following implementation of the dredging operation.
A.24.6 Monitoring
WDNR performed testing at the site and found eight of the ten sediment samples exceeded the cleanup goal of 0.36 ppm. WDNR test results for mercury ranged from 0.24 to more than 9 ppm.
RAOs/project objectives achieved? It is unclear if the project objectives were achieved.
A.24.7 References
Major Contaminated Sediment Sites Database; Sept. 2004 as updated 2008. http://www.smwg.org/MCSS_Database/MCSS_Database_Docs.html.
Publication Date: August 2014