Figure 5-2. Contaminant Pathways for Nearshore CDFs

Framework for Dredged Material Management

May 2004

Figure 5-2. Contaminant Pathways for Nearshore CDFs

During the drying process, organic complexes become oxidized and decompose.

Sulfide compounds also become oxidized to sulfate salts, and the pH may drop

drastically. These chemical transformations can release complex contaminants to surface

runoff, soil pore water, and leachate. In addition, plants and animals that colonize the

upland site may take up and bioaccumulate these released contaminants.

Volatilization of contaminants depends on the types of contaminants present in

the dredged material and the mass transfer rates of the contaminants from sediment to air,

water to air, and sediment to water. Release of the dredged material slurry above the

water level in the CDF surface will enhance volatilization as the slurry impacts the CDF

surface, creating turbulence and releasing dissolved gases. The transfer rate for organics

such as polychlorinated biphenyls (PCBs) from water to air is generally slower, but of

longer duration, than from sediment to air (Thibodeaux 1989).

CDFs constructed totally or partially in water will usually receive dredged

material until the final elevation is above the high-water elevation. Three distinct

physicochemical environments may eventually exist at such a site: upland (dry

unsaturated layer), intermediate (partially or intermittently saturated layer), and aquatic

(totally saturated layer) (Lee et al. 1991).

When material is initially placed in an in-water CDF, it will all be flooded or

saturated throughout the vertical profile. The saturated condition is anaerobic and

reduced, which favors immobility of contaminants, particularly heavy metals. After the

site is filled and dredging ceases, the dredged material above the water level begins to

dewater and consolidate through movement of water downward as leachate, upward and

out of the site as surface drainage or runoff, and laterally as seepage through the dike. As