Effective Thickness of a Cap

In addition, the water in the pores contains contaminant sorbed to organic

carbon (dissolved or particulate organic carbon present at concentration oc, e.g.,

in milligrams/liter). To a first approximation, the partitioning to this suspended

organic matter is also governed by the organic carbon based partition coefficient,

Koc, and thus the total pore water concentration for that compound is given by

Cdiss (1

Cpw

oc Koc)

(B5)

sed

oc Koc)

Koc foc

Note, however, that the truly dissolved concentration can never exceed the

solubility of the contaminant in water, Cw , and therefore the pore water concen-

tration is bounded by

Cpw

oc Koc)

Cw (1

(B6)

As a result of this limit, there exists a critical sediment loading, crit, above

which the contaminant concentration in the pore water is independent of the

sediment loading. The dissolved concentration is always given by the water

solubility under these conditions, and the total pore water concentration is given

by the equality in Equation B5.

Koc foc CW

crit

(B7)

Cw (1

For

Cpw

oc Koc)

sed

crit

Thus the observed sediment-water partition coefficient for a hydrophobic

organic compound is given by

obs

sed

if measurements are available

Koc foc

estimate if

(B8)

sed

crit

oc Koc)

sed

estimate if

crit

sed

Cw (1

oc Koc)

Effective Thickness of a Cap

The effective thickness, Leff, of a cap is reduced by consolidation of the cap,

Lcap, consolidation in the underlying sediment, Lsed, and by bioturbation over a

depth, Lbio. Bioturbation, the normal life-cycle activities of benthic organisms,

leads to mixing and redistribution of contaminants and sediments in the upper

layer. The chemical migration rate within the bioturbated zone is typically much

faster than in other portions of a cap. In addition, consolidation typically occurs

Appendix B Model for Chemical Containment by a Cap