Designating Management Actions

Benthic organisms are usually sedentary and often are considered good

indicators of the effects of physical and chemical alterations of the

environment. Benthic sampling devices include trawls, drags, box corers,

and grab samplers. Trawls and drags are qualitative samplers that collect

samples at the bottom interface, and therefore are good for collecting

epifauna and shallow infauna (top few centimeters). Quantitative samples

are usually obtained with box corers and grab samplers. Generally these

samplers collect material representing 0.02 to 0.5 m2 of surface area and

sediment depths of 5 to 100 cm.

Detection of chemical gradients or changes in the distribution of

contaminants within the mound can be monitored, but requires an

understanding of the baseline heterogeneity of contaminants within both

the contaminated deposit and the cap. For example, the contaminant

concentrations within the contaminated deposit can be expected to range

from hot spots to values that are similar to or even below the

concentrations within the cap. This is reflective of typical heterogeneity

within the original deposit and cleaner underlying layers of the channel or

harbor. Thus, while it may be possible to detect large transitions,

gradients may be much more difficult to observe, particularly if surface

contamination existed within the channel prior to dredging.

Sampling of tissues of marine biota that colonize the mound also needs

to be carefully considered. Typically, the chemical analyses require about

15 to 30 g (wet weight) of tissue per replicate. Unless the particular

region has large-bodied resident species that are easily collected, it may

take a day or more of field collection per station to obtain the necessary

sample requirement. Tissue sampling is also complicated by the natural

variation of benthic populations in both space and time. In some years,

the target species may be very abundant, while in other years the species

can be rare. These factors can result in large monitoring costs or produce

data that are of limited value.

Designating Management Actions

When any acceptable threshold values are exceeded, some types of

management actions are required. The appropriate management actions

should be determined/defined early in the disposal planning process; they

should not be determined after the threshold values have been exceeded.

Management options in early tiers could include increasing the level of

monitoring to the next tier, the addition of more sediment to form a thicker

cap, or stopping use of the site. Management options in later tiers could

include stopping use of the site, changing the cap material, or the addition

of a less porous material in cases where contaminant transport due to

biological or physical processes is occurring. For caps that are

experiencing erosion, additional cap can also be added, although it may be

advisable to choose a coarser material (coarse sand or gravel) to provide

armoring. In cases where extreme problems are encountered, removal of

the contaminated material and placement at another site could be

considered.

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Chapter 9 Monitoring Considerations for Capping