Publication Summary

The sediment trap design used here successfully collected undisturbed samples of suspended particulate matter (SPM) in sufficient quantity to allow analysis of a variety of physical and chemical parameters. This suggests that the trap design would also perform well when deployed in similar environments: nearshore marine areas with current velocities approximately in the range of 2-28 cm/sec. In addition, under the conditions present in City Waterway, the chemical and physical data presented suggest that the amount of resuspended material collected by the traps is probably low. However, additional data will be required to quantify the amount of resuspension.

The sediment trap design used here successfully collected undisturbed samples of suspended particulate matter (SPM) in sufficient quantity to allow analysis of a variety of physical and chemical parameters. This suggests that the trap design would also perform well when deployed in similar environments: nearshore marine areas with current velocities approximately in the range of 2-28 cm/sec. In addition, under the conditions present in City Waterway, the chemical and physical data presented suggest that the amount of resuspended material collected by the traps is probably low. However, additional data will be required to quantify the amount of resuspension.

The results to date indicate the value of extending sediment trap monitoring in the City Waterway past the initial year. These data are expected to provide the only empirical measure of the effectiveness of source control efforts in reducing the inputs of problem chemicals to the waterway. This is especially true since the city of Tacoma currently has no plans to continue monitoring storm drain discharges to the waterway.

Average sediment accumulation rates in the traps and sedimentation rates on the bottom for City Waterway calculated from trap data were 1.0 g/cm2/year and 1.2 cm/year respectively. These values are in good agreement with rates determined by other investigators from Pb-210 dated cores.

Preliminary indications from Phase 1 of this study are that concentrations of many problem chemicals (metals and semivolatile organics) associated with SPM currently entering City Waterway are lower than those measured in existing deposits. Two exceptions are mercury and lead at the mouth of City Waterway. Levels of problem chemicals in bottom sediments during the present study are similar to those recorded in 1984 during the Commencement Bay Nearshore/Tideflats Remedial Investigation. Comparisons of contaminant levels in SPM with available Apparent Effects Threshold values indicates mercury, lead, zinc, bis(2-ethylhexyl)phthalate, benzyl alcohol, and benzoic acid at the head (head to 11th Street Bridge) of City Waterway are at levels which could adversely affect marine benthic communities.