Construction Monitoring (Cont.)

layer often creates "noise" on instruments measuring the bottom depths.

Therefore, in addition to monitoring the mound from above, periodically,

core samples should be extracted from different locations on the sediment

mound to determine the thickness of remaining cap material. These cores

should be extracted from those locations on the mound from which it is

determined (by experience, surface measurements, and models) that most

erosion occurs.

Bathymetric monitoring of deposits to determine sediment losses needs

to be coupled with an understanding of consolidation processes. Consoli-

dation that occurs in the cap, contaminated sediment, and the original

base material within 6 to 12 months of disposal can result in substantial

reductions in mound height (Silva et al. 1994; Poindexter-Rollings 1990)

that could mistakenly be considered as erosion. Therefore, settlement

plates are very useful.

The SPC is a tool that can be used to detect thin layering within

sediment profiles. The SPC is an instrument that is lowered to the bottom

and is activated to obtain an image of sediment layering and benthic

activity by penetrating to a depth of 15 to 20 cm. As with bathymetric

surveys, the SPC approach also has limits in its ability to detect the extent

and thickness of deposits. The limiting depth of penetration limits the

thickness that can be detected. However, SPC can be used in conjunction

with bathymetric surveys to define the full range and extent of deposit

thicknesses. The SPC is extremely effective for mapping the extent of the

flanks of contaminated sediment around the central portion of the mound.

Knowing their extent is critical to successful capping since these flanks

can account for an area several times larger than that of the central mound

and can include 20 to 40 percent of the sediment mass.

Sediment samples can be taken using grab samplers or coring devices

to determine both physical and chemical parameters. In general, a core is

required to sample the full thickness of a cap layer and the underlying

contaminated material. Conventional boring techniques, vibracore

samplers, and a variety of gravity coring devices may be suitable.

However, site-specific factors such as the layering of the deposit (e.g.,

sand cap over relatively soft material), the material properties, and the

capability of a coring technique to collect samples from such deposits

should be considered when selecting a coring technique.

A variety of other instruments and approaches may be considered to

gain needed information regarding the physical condition and processes

occurring at capping sites. These include settlement plates (which must

be monitored by divers), use of remotely operated instruments, or divers

with photography and video cameras to obtain data on site conditions.

Biological monitoring may include sampling of fish and benthic

organisms. Fish and many shellfish are mobile; therefore, data using

these organisms are more difficult to relate to cause and effect. Sampling

design using such mobile species needs to carefully consider effects of

scale and migration dynamics. Most often, disposal mounds or sites are

inconsequential with respect to the ranges of such species, and linking any

observed changes in a species to disposal activities may be exceedingly

difficult.

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Chapter 9 Monitoring Considerations for Capping