Acknowledgments - trdoer10199

In the next case a cap has been placed and the flux through the cap is esti-

mated subject to the previously discussed assumptions of constant concentration

in the underlying sediment. This system is described by Equation B32. The

result is presented by the broken line in Figure B2. The flux is initially zero

until cap breakthrough, and the flux then slowly increases with time. After

several thousand years in this example, the flux with and without the cap

approaches the constant value of 20 mg m-2 year-1. Again, both models approach

the same value because the contaminated region is assumed infinitely thick and

advection ultimately controls the flux.

In the final case, the conditions are identical to the capped case above, but

mass transfer is recognized to cause depletion of the contaminant beneath the

cap and the actual thickness (and therefore finite mass) of the contaminated

region is explicitly considered. The thickness of the contaminated region is

assumed identical to the effective thickness of the cap, 35 cm. No degradation is

assumed, consistent with the previous examples. The solution by the numerical

model is given as the dotted line on Figure B2. Of the three models, this is the

only one that satisfies the material balance in that the loss to the overlying water

is reflected in reductions in mass in the contaminants in the sediment.

The plot of flux with time for an uncapped system shows a high initial flux

owing to a large concentration gradient at the surface initially. With depletion in

the near-surface sediment, the flux asymptotically approaches a limit given by

the advective flux from the deep-sediment concentrations. With a cap, the con-

taminant takes some time to seep through the clean capped region. Hence there

is an initial time period when there is essentially no contaminant flux. Since

there is an assumption of constant contaminant concentration at the base of the

cap, the flux asymptotically approaches a maximum that would ultimately equal

the uncapped flux. The realistic model that accurately accounts for contaminant

depletion in the sediment shows a flux that never reaches as high as the flux

from either of the two preceding models, and it steadily decreases at long time.

Note that in either capped case, the total mass released to the water column is

significantly reduced for any period of time. The total mass released is the inte-

gral under the flux curves.

In this example it was assumed that the bioturbated region offers no resist-

ance to the transport of contaminants. A model explicitly accounting for the

bioturbated region could also be developed. Similarly, the effect of cap thick-

ness and contaminated layer thickness or inhomogeneity on the long-term flux

profile can be studied using the numerical model. This is not possible using the

conservative analytical model Equation B32.

Acknowledgments

This appendix was prepared with the partial support of the U.S. Environ-

mental Protection Agency through the Hazardous Substance Research

B30

Appendix B Model for Chemical Containment by a Cap