Modeling applications for cap effectiveness

any pore water that traveled completely through the cap theoretically

would carry a relatively small contaminant load to the water column. Fur-

thermore, through-cap transport can be minimized by using a cap that has

sufficient thickness to contain the entire volume of pore water that leaves

the contaminated deposit during consolidation. For example, Bokuniewicz

(1989) has estimated that the pore water front emanating from a consoli-

dating 2-m-thick mud layer would only advance 24 cm into an overlying

sand cap (Sumeri et al. 1991). Contaminant flux processes are very much

dependent upon the nature of the cap materials. For example, a cap com-

posed of pure sand would not be as effective in containing contaminants

as a naturally occuring sand with an associated fraction of fines and or-

ganic content.

Some components for cap thickness should not be considered in evalu-

ating long-term flux. For example, the depth of overturning due to biotur-

bation can be assumed a totally mixed layer and will offer no resistance to

long-term flux. The component for erosion may be assumed to be absent

for short periods of time (assuming the eroded layer would be replen-

ished). Components for operational considerations, such as an added

thickness to ensure uniform placement would provide long-term resistance

to flux. The void ratio or density of the cap layer after consolidation

should be used in the flux assessment.

Any detailed assessment of flux must be based on modeling since the

processes involved are potentially very long term. Laboratory testing to

more precisely determine parameters for the available models may also be

conducted.

Modeling applications for cap effectiveness

A model has been developed by EPA to predict long-term movement of

contaminants into or through caps due to advection and diffusion proc-

esses. This model has been developed based on accepted scientific princi-

ples and observed diffusion behavior in laboratory studies (Bosworth and

Thibodeaux 1990; Thoma et al. 1993; Myers et al. 1996). The model con-

siders both diffusive and advective fluxes, the thickness of sediment lay-

ers, physical properties of the sediments, concentrations of contaminants

in the sediments, and other parameters. This model is described along

with example calculations in Appendix B.

The results generated by the model include flux rates, breakthrough

times, and pore water concentrations at breakthrough. Such results can be

compared with applicable water quality criteria or interpreted in terms of

a mass loss of contaminants as a function of time, which could be com-

pared with similar calculations for other remediation alternatives. The

model in Appendix B is applicable to the case of a single contaminated

material layer and a single cap material layer, each with a homogenous dis-

tribution of material properties. The diffusion relationships used in the

model have been verified against laboratory data. However, no field verifica-

tion studies for the model have been conducted.

There is a need for a comprehensive and field-verified predictive tool

for capping effectiveness, and additional research on this topic is planned.

Chapter 7 Dredged Material Cap Design