3 
of particulates in suspension is about 1 g/m of estuarine water. When 
this is combined with the estimate of rate of input of particulates a 
mean residence time for particulates in the water column is found to be 
about 15 days. This is about 1/10 of the estimated residence time for 
water in Puget Sound. This means that particulates are more apt to settle 
out within the system than to be exported by the net circulation fluxes. 
Thus Puget Sound is a sediment trap. 
Because many toxicants have an affinity for particulates, absorbing on 
their surface, the Sound can also be a trap or sink for toxicants. Size 
and chemical nature of the particles controls in part the scavenging of 
toxicants out of the water colutm. This and the circulation within the 
estuary determines where they eventually settle out. 
In the main basin rivers, shoreline, and urban discharges are sources for 
particulates. The pattern of their deposition is that finer and lower 
density particles in the sediments are at the greater depths along the 
central axis of the basin furthest from sources with coarser sediments 
along the edges closest to sources. A discrete source of mixed particle 
sizes or fines at the the shore edge or a wave-protected area can locally 
disrupt the general pattern. Concentrations of toxicants in the sediment 
are generally higher at depth in the fine sediments than in the coarser 
sediments of the near shore zone. If a localized source of toxicants and 
mixed-size particulates exists an elevated concentration of toxicants may 
be found in the near field of the source. If a localized source of clean 
particulates exists then their near field accumulation can reduce concen¬ 
tration levels of toxicants in the sediments (Fig* 4). 
Figure 4. Concentrations of lead in surface 
sediments are highest around Harbor Island and 
parts of the Elliott Bay waterfront, at the four 
mile rock dredge disposal site and at the site of 
the old north trunk sewer outfall between West 
Point and Shilshole Bay. 
(After Metro TPPS Study) 
8 
