Publication Summary

To evaluate the effectiveness of efforts to control sources of problem chemicals to Hylebos Waterway, samples of settling particulate matter (SPM) and in-place bottom sediments were collected between July 1990 and November 1991. All SPM samples were collected with the use of moored sediment traps. Chemical analyses focused on problem metals and organics, which were identified for Hylebos Waterway during the Commencement Nearshore/Tideflats Remedial Investigation.

To evaluate the effectiveness of efforts to control sources of problem chemicals to Hylebos Waterway, samples of settling particulate matter (SPM) and in-place bottom sediments were collected between July 1990 and November 1991. All SPM samples were collected with the use of moored sediment traps. Chemical analyses focused on problem metals and organics, which were identified for Hylebos Waterway during the Commencement Nearshore/Tideflats Remedial Investigation (Tetra Tech, 1985).

Mean concentrations of eight individual problem chemicals in SPM were high enough to adversely affect marine benthic communities based on comparisons with the Commencement Bay Sediment Quality Objectives (SQOs), (EPA, 1989). Arsenic was the only metal that exceeded the SQOs. No volatile organics identified as problem chemicals were measured above the SQOs. PCBs exceeded the SQOs at all locations tested.

Overall, the spatial distribution of most problem chemicals associated with SPM was in relatively good agreement with previous data on sediment contamination in Hylebos Waterway. The highest concentrations of arsenic and copper were measured at Station H-1 in the upper turning basin, while mercury and zinc were highest at Station H 2 also in the upper portion of the waterway. Lead levels were highest at Station H-6. The lowest metals concentrations were typically measured at Station H-7 at the mouth of the waterway. Problem organics in SPM peaked at two locations in the waterway; in the upper portion at station H-3 (LPAH, HPAH, phenol, and total PCBs) and in the outer portion at Station H-6 (trichloroethene, tetrachloroethene, ethylbenzene, xylenes, hexachlorobenzene, and hexachlorobutadiene). One exception was bis(2-ethylhexyl) phthalate which was highest in the upper turning basin at Station H-1.

Sedimentation rates for Hylebos Waterway calculated from sediment trap data ranged from 0.7-3.8 g/cm2/yr with a mean of 2.1±0.8 g/cm2/yr. Average bottom sediment resuspension rates were estimated to be 1.1 and 1.0 g/cm2/yr, at the head and mouth of the waterway, respectively. These data suggest that somewhere in the range of 25 45% of the material collected by the sediment traps could be resuspended bottom sediments.

Current velocities are generally low in the waterway even during periods of high tidal exchange. However, ship traffic and associated tug-boat activities can have a substantial short-term impact on currents in a localized area. Velocities £2 cm/sec occur approximately 97% of the time at the head. At the mouth current velocities are more variable, being £10 cm/sec 43% of the time.