Figure 1. PAH concentrations with depth at Milwaukee CDF, Milwaukee, WI

Technical Note DOER-C5

September 1999

In the Great Lakes, vegetation quickly covers the exposed dredged material, and the resulting

evapotranspiration assists dewatering.

CDF closure studies conducted by the Detroit District have shown that organic contaminant

concentrations in CDFs tend to be reduced in the upper unsaturated zone, whereas contaminant

concentrations remain unchanged in the saturated zone. For example, PAH analysis of samples

from six borings in the Milwaukee CDF shows that the upper layers are less contaminated than the

lower layers (Figure 1). Although variations in contamination levels in the dredged material

disposed over two decades could account for the profiles shown in Figure 1, the fact that the most

recently dredged material has been placed in the open-water area of the CDF (Figure 2) suggests

that this is not the case. The profiles are more consistent with the top-down, self-cleaning hypothesis

(Myers 1996) involving intrinsic biodegradation and clearly show that the top meter of material is

substantially cleaner than the material lower down in the CDF. Similar contaminant levels with

depth profiles have been observed at the Renard Island CDF in Green Bay, WI.

Figure 1. PAH concentrations with depth at Milwaukee CDF, Milwaukee, WI

PAHs appear to be biodegradable compounds (Herbes and Schwall 1978; Cerniglia 1984; Bauer

and Capone 1985; Mihelcic and Luthy 1988; Lewis 1993; Jones, Araujo, and Rogers 1996), but

only under optimal conditions. Typically, the sediment or soil must have an abundant supply of

oxygen, nutrients, and moisture. Fortunately, polluted dredged material is typically high in macro