Compatibility of Capping and Contaminated Material Placement Options (Cont.)

c. Estimates of time required for initial consolidation of the contami-

nated material due to self-weight.

d. Monitoring requirements prior to cap placement.

The process of recolonization by opportunistic species may begin as

soon as contaminated material placement operations are completed

(Rhoads and Boyer 1982; Rhoads and Germano 1982). However, recruit-

ment and colonization processes for many assemblages of coastal benthic

organisms show definite seasonal peaks, usually a primary peak in spring

and a secondary peak in fall. For example, Scott et al. (1987) determined

that recolonization at a Long Island Sound dredged material disposal site

showed peaks during October and December of separate years. Ideally, to

minimize exposure durations of benthic organisms, placement of contami-

nated material and initiation of cap construction should occur prior to the

onset of a seasonal recruitment pulse. During intervals between peaks,

rates of colonization should be sufficiently slow to assume minimal expo-

sure over a period of 3 to 4 weeks. Once cap construction has begun,

those early colonizers of the contaminated deposit will be buried and thus

physically isolated. Assuming that cap placement proceeded at a reason-

able rate, it would be unlikely that any bioaccumulation that had occurred

prior to cap placement would result in unacceptable effects.

Some delay between completion of contaminated material placement

and initiation of capping is desirable from an engineering standpoint. Con-

solidation of the contaminated material and a corresponding increase in

density and strength occur due to the weight of the material as it is placed

in the deposit. This process is called self-weight consolidation. The con-

taminated material should be allowed to undergo initial self-weight con-

solidation prior to capping to increase its stability and resistance to

displacement during cap placement. This is especially important for slur-

ried materials placed by pipeline or by hopper dredge. For slurried materi-

als, a large portion of the self-weight consolidation occurs within a few

weeks of placement. Mechanically dredged materials placed by barge re-

lease are initially deposited at essentially the same density at which they

were dredged, and the potential degree of self-weight consolidation is less

than for slurried materials.

Monitoring is required to determine the areal extent of the contaminated

deposit prior to capping. Surveys and other sampling and monitoring

activities may require several weeks to complete. An appropriate delay

between contaminated material placement and capping must balance envi-

ronmental exposure with the engineering requirements of stability and

scheduling constraints for monitoring and dredging required for capping.

If appropriate precautions are taken to schedule the lag time for consolida-

tion during periods of low benthic recruitment, a period of 3 to 4 weeks

between completion of contaminated sediment placement and initiation of

capping should have minimal environmental effect.

Chapter 5 Equipment and Placement Techniques