Chemical flux processes

if the sediment to be capped has exhibited toxicity to benthic organisms, a

detailed assessment of long-term effectiveness would be advisable.

The additional cap thickness component for chemical isolation may be

defined as Ti and should be determined based on modeling and/or testing

as described in this section. The basis of design of a contaminant flux

thickness component will be project specific. The flux rates (mass of con-

taminant per unit area per unit time) pore water concentrations in the cap

and long-term accumulation of contaminants in cap sediments may be

evaluated and used in the design. For example, flux and the resulting im-

pact on overlying water quality may be compared with a water quality

standard or criterion in much the same way as water column contaminant

releases during the placement process. Compliance of the flux concentra-

tions at the boundary of the site or edge of an established mixing zone

would be appropriate. In this way, the cap thickness component for isola-

tion required to meet the water quality standards can be determined.

Chemical flux processes

Properly placed capping material acts as a filter layer against any mi-

gration of contaminated sediment particulates. There is essentially no

driving force that would cause any long-term migration of sediment parti-

cles upward into a cap layer. Most contaminants of concern also tend to

remain tightly bound to sediment particles. However, the movement of

contaminants by advection (movement of pore water) upward into the cap

is possible. Molecular diffusion over extremely long time periods will al-

ways occur. Advection refers to the movement of pore water. Such move-

ment could occur as an essentially continuous process if there is upward

groundwater gradient acting below the capped deposit. Advection could

also occur as a result of compression or consolidation of the contaminated

sediment layer or other layers of underlying sediment. Movement of pore

water due to consolidation would be a finite, short-term phenomena, in

that the consolidation process slows as time progresses and the magnitude

of consolidation is a function of the loading placed on the compressible

layer. The weight of the cap will "squeeze" the sediments, and as the pore

water from the sediments moves upward, it displaces pore water in the

cap. The result is that contaminants can move part or all the way through

the cap in a short period of time. This advective movement can cause a

short-term loss, or it can reduce the breakthrough time for long-term

advective/diffusive loss.

Diffusion is a molecular process in which chemical movement occurs

from material with higher chemical concentration to material with lower

concentration. Diffusion results in extremely slow but steady movement of

contaminants. The effect of long-term diffusion on the design cap thickness

is normally negligible, because long-term diffusion of contaminants

through a cap is an extremely slow process and contaminants are likely to

adsorb to the clean cap material particles.

Properly designed caps act as both a filter and buffer during advection

and diffusion. As pore waters move up into the relatively uncontaminated

cap, the cap sediments can be expected to scavenge contaminants so that

Chapter 7 Dredged Material Cap Design