This study estimated present-day contaminant loadings to the Duwamish Estuary/Elliott Bay and Lake Union/Ship Canal.  Existing water quality, flow, and other data were used to estimate the mean annual current (circa 2015) loadings of contaminants entering the study areas from various pathways. Loadings were quantified for 14 contaminants of interest from 10 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 10 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.  

Results showed that uncontrolled CSOs were the largest contributing pathway of fecal coliform bacteria to both study areas.  Upstream watersheds and stormwater runoff that directly enters the study areas were the largest contributing pathways for total nitrogen, total phosphorus, TSS, total lead, total mercury, total zinc, BBP, BEHP, PBDEs, and PCBs.  Creosote-treated wood pilings were the largest contributing pathway of PAHs.  Antifouling vessel paint was one of the largest contributing pathways of total copper.