Field data on long-term effectiveness

Field data on long-term effectiveness

Some field studies have been conducted on long-term effectiveness of

caps. Sequences of cores have been taken at capped dredged material

sites in which contaminant concentrations were measured over time periods

of up to 15 years (Fredette et al. 1992; Brannon and Poindexter-Rollings

1990; Sumeri et al. 1994). Core samples taken from capped sites in Long

Island Sound, the New York Bight, and Puget Sound exhibit sharp concen-

tration shifts at the cap/contaminated layer interface. For the Puget

Sound sites, these results showed no change in vertical contaminant distri-

bution in 5 years of monitoring with 18-month and 5-year vibracore samples

taken in close proximity to each other. In the New York Bight and Long

Island Sound sites, respectively, cores were taken from capped disposal

mounds created approximately 3 and 11 years prior to sampling. Visual

observations of the transition from cap to contaminated sediment closely

correlated with the sharp changes in the sediment chemistry profiles. The

lack of diminishing concentration gradients away from the contaminated

sediments strongly suggests that there has been minimal long-term

transport of contaminants up into the caps. Additional sampling for

longer time intervals is planned.

These results confirm that no gross movement of contaminated sedi-

ments or contaminants occurs with a properly placed cap, that only pore

water advection and molecular diffusion would act to move contaminants

into a cap over the long term, that such processes move contaminants at

extremely slow rates, and therefore contaminants are effectively isolated

from the aquatic environment for extremely long periods (Brannon and

Poindexter-Rollings 1990).

Acceptability of flux component design

If the flux evaluation indicates the design objectives are not met, addi-

tional cap thickness can be added or cap materials with differing properties

(grain size and TOC) can be considered to further decrease the contami-

nant flux. The evaluation process could then be run in an iterative fashion

if necessary to determine the chemical isolation component needed to

meet the design objectives. Of course, if no reasonable combination of

cap thickness and cap material properties can meet the objectives, other

alternatives or control measures must be considered.

Required Design Cap Thickness and Area

and Volume of Capping Material

Calculation of design cap thickness

The total design cap thickness, as initially placed, is determined as

follows:

Chapter 7 Dredged Material Cap Design