Effective Thickness of a Cap (Cont.)

on a time scale that is rapid compared with the design lifetime of a cap. Con-

solidation of the cap directly reduces the thickness of a cap and the separation

between contaminants and the overlying water or benthic organisms while con-

solidation of the underlying sediment results in the expression of potentially

contaminated pore water. Using Lsed, A to represent the thickness of a cap com-

promised by a contaminant A during consolidation of the underlying sediment,

the effective cap thickness remaining for chemical containment is given by

(B9)

Lbio

Lcap

Lsed, A

Leff

where L0 is the initial thickness of the cap immediately after placement.

The depth of bioturbation can be assessed through an evaluation of the

capping material and recognition of the type, size, and density of organisms

expected to populate this material. Because of the uncertainty in this evaluation,

the bioturbed zone is generally chosen conservatively, that is, considered to be as

large as the deepest penetrating organism likely to be present. Due to the action

of bioturbating organisms, this layer is also generally assumed to pose no

resistance to mass transfer between the contaminated sediment layer and the

overlying water.

The consolidation of a cap can be estimated through use of standard consoli-

dation models; for example, the Corps of Engineers' Primary Consolidation and

Dessication of Dredged Fill (PCDDF) model (Stark 1991). Note, however, that

in addition to reducing the thickness of a cap, consolidation serves to reduce

both the porosity and permeability of a cap causing reductions in chemical

migration rates by both advection and diffusion.

The consolidation of the underlying contaminated sediment can also be esti-

mated through consolidation models. These models do not predict the resulting

movement of the chemical, however, and a model is described below. The

effective cap thickness estimated by Equation B9 is subject to chemical migra-

tion by advection and diffusion processes. The long-term chemical flux to the

water via these processes can be modeled.

The complete model of chemical movement through the cap must be com-

posed of two components:

An advective component considering the short-term consolidation of the

contaminated sediment underlying the cap.

A diffusive or advective-dispersive component considering contaminant

movement as a result of pore water movement after the cap has fully

consolidated.

The first component is operative for all caps but only for a short period of

time. The first component allows determination of the effective cap thickness

through Equation B9. The resulting effective cap thickness can then be used to

assess long-term losses through the cap by advective and/or diffusive processes.

Appendix B Model for Chemical Containment by a Cap