Bioturbation

cap thickness component for chemical isolation based on modeling

and/or testing.

A flowchart illustrating the sequence of cap thickness evaluations

and the interdependence of the components is shown in Figure 20. More

detailed discussions of these design steps are given in the following para-

graphs.

Bioturbation

A design objective of a dredged material cap is to physically isolate the

contaminated material from benthic organisms. In the context of capping,

bioturbation may be defined as the disturbance and mixing of sediments

by benthic organisms. The importance of bioturbation by burrowing

aquatic organisms to the mobility of contaminants cannot be overesti-

mated. In addition to the disruption (breaching) of a thin cap that can re-

sult when organisms actively rework the surface sediments, there is the

problem of direct exposure of infaunal organisms to the underlying con-

taminated sediment. The best available knowledge on local infauna must

supplement generic assumptions concerning the bioturbation process.

Aquatic organisms that live on or in bottom sediments can greatly in-

crease the movement of contaminants (solid and dissolved) through the di-

rect movement of sediment particles or irrigation of pore water, increasing

the surface area of sediments exposed to the water column, and as a food

for epibenthic or pelagic organisms grazing on the benthos. The specific

assemblage of benthic species that recolonizes the site, the bioturbation

depth profile, and the abundances of dominant organisms are key factors

in determining the degree to which bioturbation will influence cap per-

formance. The depth to which organisms will bioturbate is dependent on

behaviors of specific organisms and the characteristics of the substrate

(i.e., grain size, compaction, organic content, pore water geochemistry,

etc.). In general, the depth of recolonization by marine benthos is greater

than that of freshwater benthos. Recolonization by benthic infauna at ma-

rine dredged material caps is primarily by suspension feeders as opposed

to burrowing organisms (Morton 1989; Myers 1979). The intensity of bio-

turbation is greatest at the sediment surface and generally decreases with

depth. Three zones of bioturbation are of importance (see Figure 21). A

surficial layer thickness of sediment will be effectively overturned by shal-

low bioturbating organisms and can be assumed to be a continually and

completely mixed sediment layer for purposes of cap design. This layer is

generally a few centimeters in thickness. Depending on the site charac-

teristics, a number of middepth burrowing organisms over time recolonize

the site. The level of bioturbating activity for these organisms will de-

crease with depth as shown in Figure 21. The species and associated be-

haviors of organisms that occupy these surface, and middepth zones are

generally well known on a regional basis. There may also be potential for

colonization by deep-burrowing organisms (such as certain species of mud

shrimp), which may burrow to depths of 1 m or more. However, knowledge

of these organisms is very limited. These cap design criteria assume that

deep bioturbators are not present in significant numbers.

Chapter 7 Dredged Material Cap Design