Construction Monitoring

Construction Monitoring

Monitoring to ensure that placement occurs as designed may include

baseline, postcontaminated material-placement, interim, and postcap

material-placement surveys. Baseline surveys consist of determining the

existing bathymetry of the site in order to determine changes in depth

resulting from disposal. The postcontaminated material-placement

monitoring determines where the contaminated sediments have been

placed so that a final plan of cap-placement locations can be developed.

Postcontaminated material-placement sampling is also needed as a

baseline for cap-thickness determinations based on bathymetry. Interim

surveys may be employed in large projects to determine where sufficient

cap has been placed and where additional material should be placed.

Finally, postcap material-placement monitoring is used to confirm the

final cap thickness and to serve as a baseline for future monitoring efforts.

Monitoring for Long-Term Effectiveness

The principal long-term concerns for capped deposits are (a) whether

the cap is remaining in place or whether erosion is occurring, and

(b) whether the contaminants remaining within the contaminated layer are

being transported to the sediment surface layer or to the water column.

Erosion can occur either due to daily tidal currents, propeller wash, or as a

result of storm-related surges or waves. Potential mechanisms for

contaminant movement through the cap include pore water movement,

diffusion, and biological mixing of the sediment (bioturbation).

Monitoring approaches for these concerns include sequential

bathymetric surveys or diver-inspected settling plates to determine

changes in deposit height, surface-sediment chemistry samples, sediment

and pore water chemistry profiles from cores, sediment physical structure

from cores, benthic community structure, and contaminant tissue

concentrations of mound resident benthic species. These and other

monitoring techniques discussed below can all be considered within the

framework of a tiered monitoring plan and conducted on time intervals

ranging from months to years.

After a severe storm, one with a 10- to 20-year return period, a modest

monitoring program should be conducted to confirm the cap has not

suffered any significant damage. Monitoring required after a severe storm

should probably be limited to bathymetry, grab samples, and perhaps SPI

and subbottom profiles.

Monitoring Techniques and Equipment

Selection of the types of samples or observations to be made, the

equipment to be used, the number of samples or observations, etc., is

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Chapter 9 Monitoring Considerations for Capping