This study estimates changes in contaminant loads between 2015 and 2030 based on identified projects, programs, and regulations. In a complementary study, current (2015) loadings were quantified for 14 contaminants of interest (COIs) from ten major pathways. The 14 contaminants were: fecal coliform, total nitrogen, total phosphorus, total suspended solids (TSS), total arsenic, total copper, total lead, total mercury, total zinc, benzyl butyl phthalate (BBP), bis(2-ethylhexyl)phthalate (BEHP), polycyclic aromatic hydrocarbons (PAHs), polybrominated diphenyl ethers (PBDEs), and polychlorinated biphenyls (PCBs). The ten major pathways were: uncontrolled and controlled CSOs, wet-weather treatment facility discharges, stormwater runoff that directly enters the study areas, upstream watersheds loading directly into the study areas, local tributaries, leaching from antifouling vessel paint, leaching from creosote-treated wood pilings, atmospheric deposition, and bridge runoff. To estimate changes to these loadings, planned projects, programs, and regulations were reviewed, and when possible, their expected impact on loadings was quantified.
The results show that CSO control in the study area is estimated to reduce bacteria concentrations by about 80 percent. However, remaining pathways of bacteria are likely to still cause fecal coliform water quality criteria exceedances. A state ban on copper in anti-fouling bottom paint for some boats is estimated to reduce half of the current copper loadings to Lake Union and a quarter of copper loadings to the Duwamish Estuary. The annual PAH load is expected to decrease substantially in Elliott Bay as the result of the planned removal of about 13,000 creosote-treated pilings. Loadings from stormwater were estimated to be reduced by planned stormwater treatment and control projects, but the reductions were small in comparison to the total stormwater discharged each year. This study was not able to estimate changes in loadings from atmospheric deposition or the upstream pathways.
