Models for Uncapped Sediment

depletion referred to here is simply accounting for the mass of contaminant lost

to the overlying water. Degradation of the contaminant is not considered.

To overcome this limitation of the preceding models, it is necessary to turn to

a numerical simulation of Equation B27. The numerical simulation should apply

Equation B27 both within the cap and in the underlying sediment assuming that

the concentrations and fluxes are continuous at the sediment water interface.

Arbitrary initial and boundary conditions could be applied. For the particular

case of an initially clean sediment cap overlying a finite layer of contaminated

sediment, the author has developed such a numerical solution. This model is

coded in FORTRAN and employs IMSL subroutines to conduct the numerical

calculations. An illustrative example using the model is presented later as is a

contact address for acquisition of the model.

Models for Uncapped Sediment

Although the primary purpose is the evaluation of contaminant concentrations

and fluxes associated with capped sediment, it is often convenient to compare

these quantities with concentrations and fluxes that would be observed in the

absence of a cap. Models similar to those above are available for uncapped con-

ditions and are especially useful for comparison purposes.

Let us consider the solution to Equation B27 subject to the uncapped boun-

dary conditions

sediment water interface (z 0)

Cpw

C0 or

deep sediment (z

Cpw

)

(B35)

initial sediment concentration

Cpw

These are the same conditions, however, as those leading to Equation B32 if the

z coordinate is directed into the sediment rather than out through the cap and if

the roles of C0 and Cw are reversed. Thus Equation B32 can be used to evaluate

concentrations in the uncapped case as well. Both the sense of U and z must be

reversed, and z = 0 now represents the sediment-water interface. Similarly, the

flux from the sediment to the overlying water is given by

Cpw(0, t)

F (0,t)

U Cpw(0, t)

Deff

(B36)

Similarly, finite contaminated layer models could be adapted from Equa-

tion B24. This would not be a fair basis for comparison, however, in that the

uncapped model would explicitly account for depletion of the sediment contami-

nants as a result of the loss to water while the cap version of the solution

assumes that the sediment concentration remains constant.

B17

Appendix B Model for Chemical Containment by a Cap