IN SITU CAPPING OF CONTAMINATED SEDI- MENTS: COMPARING THE RELATIVE EF- FECTIVENESS OF SAND VERSUS CLAY MIN- ERAL-BASED SEDIMENT CAPS

Ecological problems caused by sediment contamination occurring in deep water or wetland environments may be addressed through natural recovery, in-place containment or treatment, dredging and removal, or in some cases by in situ capping – which is defined as the placement of a subaqueous covering or cap of clean isolating material over an in-place deposit of contaminated sediment. While dredging and removal of contaminated sediments may be the most practical remedial method in many situations and sometimes necessary for navigational purposes, this remedial approach may not be the most environmentally protective and/or costeffective approach. In situ capping approaches are often considered to be more protective of faunal and floral communities inhabiting impacted ecosystems than dredging alternatives, or when converting an impacted area to a closed cell. According to current regulatory philosophy and recommendations, the three primary functions of an in situ sediment cap include (1) physical isolation of the contaminated sediment from the benthic environment; (2) stabilization of contaminated sediments, preventing re-suspension and transport to other areas or sites; and (3) reduction of the flux (transport) of dissolved contaminants into the overlying water column. To date, most in situ capping projects appear to involve the use of primarily granular (i.e., sandy) capping materials. Although such capping materials may adequately serve to meet stated cap functions at many sites, their relatively high permeability and low organic matter and clay content may limit their ability to reduce contaminant transport into the overlying water column. Furthermore, non-cohesive, granular materials can also be prone to erosional losses and redistribution, thus minimizing their effectiveness in isolating and stabilizing contaminated sediments. Finally, the thickness required to meet performance goals, many times on the order of several feet, can have a deleterious effect on channel hydraulics and waterway uses. As an alternative to granular sediment caps, a new in situ capping technology, AquaBlok™, has been developed for use in either deep water or wetland ecosystems. AquaBlok™ is a clay mineral-based capping material that offers several functional advantages over granular capping materials including lower permeability, higher resistance to erosive forces, and considerably higher attenuation capacity for many types of contaminants. The likely need for thinner AquaBlok™ caps at many sites would also minimize navigational constraints. In this paper, we compare the potential relative effectiveness of a typical granular sediment cap to that of a typical AquaBlok™ sediment cap, as each would be installed into “typical” impacted deep water or wetland ecosystems.