Estimation of Long-Term Losses

accumulation in the cap. The quantity of contaminant that can be rapidly

adsorbed by the cap material, cap (milligrams/kilogram dry cap material), is

generally assumed to be proportional to the concentration in the pore water (Cpw,

milligrams/liter),

obs

Kd,cap Cpw

(B11)

cap

where the constant of proportionality is the observed sediment-water partition

coefficient in the cap. Note that the observed partition coefficient is measured

during sorption onto clean cap material since this is the conditions that occur

after placement of a clean cap onto contaminated sediment. The maximum

quantity that can be sorbed by the cap is given by the product of the observed

partition coefficient and the initial pore water concentration of the contaminant

in the underlying sediment, C0.

As a result of sorption onto the immobile sediment, the distance that the con-

taminant migrates in the cap during consolidation of the underlying sediment by

a distance Lsed is given by

Lsed

Lsed, A

(B12)

obs

Kd,cap

This distance must be subtracted from the actual cap thickness to estimate effec-

tive cap thickness. Note that this model suggests that the more sorbing a cap, the

less important is consolidation in the underlying sediment. Sorption for hydro-

phobic organics such as polyaromatic hydrocarbons and polychlorinated biphe-

nyls is strongly correlated with the organic carbon content of the sediments.

obs

Kd,cap is typically of the order of hundreds or thousands for these compounds; if

a cap contains 0.5-percent organic carbon or more, the loss of effective cap

thickness due to penetration of the contaminant is a small fraction of the sedi-

ment consolidation distance. Metals also tend to be strongly associated with the

solid fraction, again reducing the migration of contaminant out of the sediment

as a result of consolidation.

Estimation of Long-Term Losses

Mechanisms and driving force

The effective cap thickness defined by Equation B9 is subject to advection or

diffusion or a combination of both throughout the lifetime of the cap. The long-

term contaminant release or loss requires estimation of the contaminant flux by

these processes. Diffusion is always present, while advection only occurs if

there exists a significant hydraulic gradient in the underlying sediments. The

relative magnitude of diffusion to advection in the cap of effective thickness, Leff,

can be estimated by the Peclet number.

Appendix B Model for Chemical Containment by a Cap