Diffusion

In the absence of dependable data on rates of degradation or transformation, the

conservative assumption of no contaminant depletion is generally assumed.

In the subsequent sections, the movement of contaminants from the sediments

through the cap by both diffusion and advection are evaluated. The focus is on

the development of simple analytical models that can be expressed in algebraic

form. This generally limits the conditions evaluated to uniform sediment and

cap physical and chemical properties and an initial contaminant concentration

that is both uniform in the sediment and constant. Depletion of contaminant in

the sediment by either chemical degradation or mass depletion as a result of the

release of material through the cap is not considered. The models are thus

conservative indicators of contaminant release from the sediment (that is, they

overestimate the concentration in the sediment or the flux of contaminant to the

overlying water column).

Diffusion

Diffusion is a process that occurs at significant rates only within the pores of

the sediment and is driven by the difference in pore water concentration between

the sediment and the cap. The initial concentration of the contaminant in the cap

pore water is generally zero, while the concentration in the sediment is given by

Equation B15. Even without degradation, however, migration of contaminants

into the cap will deplete the underlying sediments as a result of the loss of mass

by diffusion through the cap.

Thoma et al. (1993) developed a model of diffusion through a cap that expli-

citly accounts for depletion in the underlying sediment. A simpler model of

diffusion through the cap, however, assumes that the contaminant concentration

in the underlying sediment is essentially constant. This would be most appro-

priate if the contaminant concentration in the sediment far exceeds the critical

concentration defined by Equation B7. Because the assumption of no depletion

in the underlying sediment overpredicts the driving force for diffusion, and

therefore the flux through the cap, it represents a conservative assumption of the

effectiveness of the cap. It will therefore be employed in the description that

follows.

One should first estimate the steady long-term flux of contaminants through

the cap via diffusion. This is the maximum flux that can occur through the cap

by the diffusive mechanism.

Maximum flux estimation (steady state)

If diffusion is the only operative transport process through the cap, the

pseudo steady-state flux through the cap (assuming constant contaminated sedi-

ment pore water concentration and no sorption effects in the cap layer) is given

Appendix B Model for Chemical Containment by a Cap