Capping

In situ (meaning "in place") capping is the placement of a covering or cap over an area of contaminated sediment. The cap may be constructed of clean sediment, sand, or gravel or may involve a more complex design using geotextiles, liners, and multiple layers.

In situ capping has been applied in rivers, in near-shore areas, and in estuaries. Conventional dredging and construction equipment and techniques can be used for these projects, but these practices must be precisely controlled. Projects to date and available monitoring data at several sites indicate that capping may be an effective technique for long-term containment of contaminants.

A variation of In situ capping could involve the removal of contaminated sediments to some depth, followed by capping the remaining sediment in place. This method could work when capping alone is not feasible because of hydraulic or navigation restrictions on the water depth. It might also be used where it is desirable to leave the deeper, more contaminated sediments capped in place (vertical stratification of sediments contaminants is common in many Great Lakes tributaries like the Fox River).

The primary technical considerations that affect the feasibility of in situ capping are the physical and hydraulic characteristics and the existing and future uses of the waterway. The suitability of in situ capping in a given site is less affected by the type or level of contaminants present, because it physically isolates the sediments and their associated contaminants. The ideal area for in situ capping would be a stable, sheltered area not exposed to high erosive forces or upwelling from groundwater. Key characteristics to look for include:

level bottom slopes
mild water currents, especially along the bottom
deep water that is relatively isolated from storm-induced currents
shallower water with protection from erosive forces
no nearby obstructions or structures
a site where capping materials must only be hauled short distances little or no heavy boat traffic.

Depending on the erosive forces present at the site, an in situ cap may have to be armored with stone or other material to keep the cap intact. The potential for large commercial vessels to scour the river bottom would necessitate a very large armor stone, making in situ capping difficult in or near most active navigation channels. The active navigation channel in the Fox River extends from Green Bay upstream to the tuning basin at Fort James Paper Corp. The federally authorized navigation channel extends from Green Bay to the outlet of Lake Winnebago.

In situ caps may further reduce water depth to levels that are not safe for existing or planned recreational boating, or may eliminate shallow water aquatic habitat. Construction of an in situ cap represents a deliberate change to the shape of the bottom of a waterway. Future human and ecological uses of the waterway may be limited by this change. Sites that are capped require perpetual maintenance, and there is always the risk that the cap could erode from flooding, aquatic organisms, stream-bank erosion, navigation, and recreational forces.

Besides capping, other in situ approaches to managing contaminated sediment exist. These include mixing chemicals into the sediment to alter the contaminants, adding binding agents to the sediment to solidify it and make it harder for contaminants to escape into the environment, and seeding the sediment with micro-organisms that ingest and break down the contaminants. A final in situ approach called containment involves the complete isolation of a portion of the waterway with a physical barrier. None of these approaches would be effective with PCB contamination or in settings like the Fox River.

For more information visit the U.S. Environmental Protection Agency's Web site to read the Guidance for In-Situ Subaqueous Capping of Contaminated Sediments (exit DNR).

Last Revised: Tuesday November 21 2006