Recommendations

m. The spread and mounding behavior of contaminated material during

placement must be evaluated to predict the geometry of the deposit

and resulting cap material requirements. The capping material be-

havior must be similarly evaluated to determine if the design of

the cap and volume of capping material available are adequate.

The smaller the "footprint" of the contaminated material as placed,

the less volume of capping material will be required to achieve a

given cap thickness.

n. An evaluation of the consolidation and long-term potential for ero-

sion of the mound or deposit must be conducted to ensure that the

required cap thickness can be maintained. The design-cap thick-

ness must be adjusted to account for potential erosion and consoli-

dation. The cap can also be armored with coarser material to

minimize erosion.

o. Monitoring of capped sites is required during and following place-

ment of the contaminated and capping material to ensure that an ef-

fective cap has been constructed and to ensure that the cap as

constructed is effective in isolating the contaminants and that long-

term integrity of the cap is maintained. Design of monitoring pro-

grams must be logically developed, prospective in nature, and

tiered with each tier having its own thresholds, null hypotheses,

sampling design, and management responses based on exceedance

of predetermined thresholds.

p. Capping of contaminated material in open-water sites began in the

late 1970s, and a number of capping operations under a variety of

disposal conditions have been accomplished. Field experience with

these projects has shown that the capping concept is technically

and operationally feasible.

q. The cost of capping is generally lower than alternatives involving

confined (diked) disposal facilities. The geochemical environment

for subaqueous capping favors long-term stability of contaminants

as compared with the upland environment where geochemical

changes may favor increased mobility of contaminants. Capping

is therefore an attractive alternative for disposal of contaminated

sediments from both economic and environmental standpoints.

Recommendations

As more designs are completed and additional field experience is

gained, the technical guidelines in this report should be refined and ex-

panded. Additional research is also recommended to develop improved

tools for capping evaluations. Specific recommendations for further re-

search are summarized as follows:

a. More clearly define impacts associated with capping at water

depths exceeding 100 ft. PSSDA monitoring has shown material

dispersion can be predicted in 300- to 400-ft water depth in Puget

Sound.

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Chapter 11 Summary and Recommendations