Evaluation of ecological risks associated with open loop exhaust gas cleaning system (EGCS) discharges in the Puget Sound, WA

Combustion of high‑sulfur fuels results in emissions of sulfur dioxide (SO₂), a known precursor to the formation of secondary particulate matter, which has been associated with adverse human health and environmental effects. To reduce SO₂ emissions and associated particulate matter formation, the International Maritime Organization (IMO) implemented stricter limits on the sulfur content of marine fuel oil under MARPOL Annex VI, requiring vessels to either use compliant low-sulfur fuels or adopt an approved equivalent compliance method. Many cruise operators have installed exhaust gas cleaning systems (EGCS) as an equivalent compliance technology to reduce sulfur oxide emissions. The objective of this study was to evaluate the potential ecological risks associated with open-loop EGCS water discharges in Puget Sound. The assessment combined empirical measurements, whole effluent toxicity (WET) testing, and environmental modeling. Concentrations of key constituents of concern, including pH, selected trace metals, and polycyclic aromatic hydrocarbons (PAHs), were measured in discharge waters and used to develop predicted environmental concentrations (PECs) in receiving waters. The Marine Antifoulant Model to Predict Environmental Concentrations (MAMPEC) was used to simulate vessel operations both underway and in port, and the CORMIX mixing zone model was applied to refine near-field dilution estimates for in-port scenarios. WET testing with standard marine species provided a direct evaluation of discharge toxicity. Integration of these multiple lines of evidence indicated limited biological effects in WET testing, with observed responses generally occurring under test conditions involving relatively high concentrations of EGCS discharge water. Conservative chemical-specific screening also identified potential concerns for some constituents before accounting for the dilution and environmental processes that occur after discharge. Under modeled real-world exposure conditions in Puget Sound, however, receiving-water concentrations were generally below applicable protective thresholds. Overall, the weight-of-evidence indicates that EGCS overboard discharge waters operated in compliance with applicable regulatory requirements pose little risk of adverse ecological effects in Puget Sound under expected real-world exposure conditions.