Capping & Sand Cover Application
Along with dredging, the Fox River Cleanup Project is using engineered capping to contain PCBs in the river. The capping process involves placing a combination of sand, gravel and rock on top of the contaminated sediment bed in order to isolate the PCBs and keep them from entering the water and food chain.
The capping operations take place from April to mid November each year, depending on river and weather conditions.
Capping is done in areas of the river where dredging cannot be done safely or effectively, including areas around bridge pilings and shoreline structures. The caps are armored with stone to protect the cap from damage by storm flows and propeller wash.
The Fox River Cleanup Project will cap specified areas of the river bottom with varying thickness of sand, gravel and larger stone, depending on the local conditions, to isolate PCBs. The thickness of the caps varies from a six-inch sand "cover" to a 33-inch cover of sand, gravel and quarry spall.
Finer-grain sand is spread using unique spreading methods, which allow for uniform application of the material over large areas, with minimal disturbance of underlying contaminated sediment. This application technique also minimizes the mixing of clean cap material with the underlying contaminated sediment, and thereby maximizes the ability of the placed sand to isolate the PCBs.
Sand covers are used to isolate low levels of residual concentrations that may remain after completion of the dredging. These concentrations are identified through verification sampling performed after dredging. The sand cover is applied only if low level residual material is identified. Sand cover application began in 2009 at the southermost area of the Fox River Cleanup Project.
The engineered caps will be monitored after completion to ensure that they continue to form an effective and permanent barrier to the PCBs.
Staging for the capping/covering operations took place at a location near the De Pere - Rockland border for the first three years. The Green Bay Processing Facility site and other sites may be utilized to support the capping and sand covering as the project moves downstream. The capping material (sand, gravel and quarry stone) is trucked to the staging areas from several area material suppliers.
Sand from the staging areas is piped to the spreading barge (see below) where it is poured into a hopper that conveys material to the spreader. The spreader is located on the bow of the spreader barge nd distributes the moist granular capping material in a uniform pattern. Individual particles hit the water surface and fall through the water column at a reduced velocity, compared to direct discharge of slurry material, minimizing the potential for displacement of the underlying contaminated sediment.
After the engineered caps are in place, samples are collected, and the elevation of the river bottom is surveyed, to confirm that the layers of the caps are placed as required.
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