Model for Short-Term Cap Losses--Advection During Cap Consolidation

For simplicity and conservatism, the sediment underlying a cap may be assumed

to remain uniformly contaminated at the concentration levels prior to cap place-

ment. In reality, migration of contaminants into the cap reduce the sediment

concentration and the long-term flux to the overlying water. The consideration

of this situation, however, complicates the analysis and the models used to

describe contaminant flux. Analytical models are presented for the case of

constant concentration in the underlying sediment. The results of a numerical

model that incorporates the depletion of the underlying sediment concentrations

are referenced for comparison.

Model for Short-Term Cap Losses--Advection

During Cap Consolidation

After placement of capping materials, consolidation of both the cap and the

underlying sediment occurs. Consolidation of the cap results in no contaminant

release since the cap is initially free of contamination. Furthermore, the con-

solidation of the cap serves to reduce the permeability and, to a lesser extent, the

porosity of a cap. Both serve to reduce contaminant migration through the cap

by both diffusive and advective processes.

Consolidation of the underlying sediment due to the weight of the capping

material, however, tends to result in expression of pore water and the contami-

nants associated with that water. The ultimate amount of consolidation may be

estimated using standard methods; for example, the previously referenced PCD

model. The consolidation of the underlying sediment is likely to occur over a

short period (e.g., months) compared with the lifetime of the cap. It is appro-

priate, therefore, to assume that the consolidation occurs essentially instantane-

ously and estimate the resulting contaminant migration solely on the basis of the

total depth of consolidation and the pore water expressed. For a nonsorbing

contaminant, the penetration depth of the chemical is identical to that of the

expressed pore water. For a sorbing contaminant, the penetration depth is less as

a result of the accumulation of chemical on the sediment.

Mathematically, if Lsed represents the ultimate depth of consolidation of the

underlying contaminated sediment due to cap placement, the depth of cap

affected by this pore water (or nonsorbing contaminant), Lsed,pw, is given by

Lsed

Lsed,pw

(B10)

where is the porosity of the cap materials. The division by the cap porosity

recognizes that the expressed pore water moves only through the void volume

formed by the spaces between the grains of the capping material. Equation B10

assumes that the capping material is spatially uniform and that pore water is not

preferentially forced through a small fraction of the total cap area.

Although the depth of cap affected by the expressed pore water is given by

Equation B10, the migration distance of a sorbing contaminant is less due to

Appendix B Model for Chemical Containment by a Cap