determined by 1) the kind of source producing the contaminant, 2) the 
magnitude of the source discharge over time, 3) chemical phase 
transformation or alteration near the point of discharge, 4) dispersal 
mechanisms, and 5) post-depositional changes. Chemical properties of the 
contaminants and the processes listed above establish a link between 
pollutant sources and the sediments where pollutants accumulate. 
Most contaminants of concern bind to particles that become sediments 
at the bottom of Puget Sound. This removal process is an important means 
of immobilizing and eventually burying contaminants under layers of newer 
deposits. The general process of sedimentation is illustrated in 
Figure 2. Contaminated material from a source discharge mixes with 
material from other sources in the water column and is either degraded, 
transported away, or accumulated in surface sediments. 
Two major factors that determine the length of time contaminants will 
remain in surface sediments include the sedimentation rate (how fast new 
sediment accumulates) and the thickness of the mixed layer of surface 
sediments. Contaminants are quickly buried when there is a fast 
sedimentation rate and a thin surface mixed layer. Alternatively, 
contaminants can persist for years in surface sediments when there is a 
slow sedimentation rate and a thick mixed layer. Mixing results from 
biological activities (e.g., burrowing of benthic organisms and bottom 
fish) and physical processes (e.g., resuspension of sediments by storms 
and currents). When freshly deposited material is mixed extensively with 
older deposits, surface sediment concentrations will not quickly reflect 
changes in the discharge of contaminants from sources. 
Most marine sediments accumulate slowly. Sediment accumulation in 
Puget Sound (excluding major river deltas and other sites off major 
discharges of solids) typically ranges from 0.1 to 3 cm/yr in surface 
sediments (Carpenter et al. 1985). The sedimentation rates are determined 
using radionuclide techniques and are also expressed as the total mass of 
sediment accumulation per unit area (0.26 to 1.2 gm*cm~2.yr“l). The 
average mixed layer depth in Puget Sound sediments based on these 
techniques is approximately 10 cm, but ranges from 0 to 40 cm (Carpenter 
et al. 1985; Lavelle et al. 1986). Residence time calculations based on 
radionuclide techniques indicate that contaminants may be retained in the 
biologically active surface sediments for years or decades. 
After a source has been controlled (Figure 2) several years may pass 
before contaminants from the source are buried below the surface mixed 
layer. Even so, maintenance or remedial dredging can expose contaminated 
sediments that have been deeply buried. 
Toxic Effects in Sediments 
Because sediments are a major sink of contaminants discharged to the 
environment, they are also a major source of toxic chemicals to organisms 
exposed to the sediments. Laboratory and field evidence from Puget Sound 
suggests that adverse biological effects may be associated with high 
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