Ecological Risk Assessment: Puget Sound Exhaust Gas Cleaning System
Washington State Department of Ecology (WDOE) expressed interest in understanding the potential effects of the discharge of the water from the Exhaust Gas Cleaning System (EGCS) associated with large vessels to marine water of the Puget Sound. The goal of this study was to provide empirical and modeled data to better understand these potential ecological risks. This study originated from a cooperative agreement between the Port of Seattle, WDOE, and Cruise Lines International Association (CLIA). The focus of the agreement was to develop a program to perform a risk assessment following U.S. laws, WDOE standards and procedures, and recent IMO recommendations to better understand the potential impacts of EGCS discharges to the Puget Sound.
A combination of analytical determinations (PAHs and metals) in pre- and post-EGCS waters, modeling of overboard discharge toxicity, pre-discharge evaluation of environmental toxicity, and post-discharge evaluation of environmental concentrations were employed. Two engine loads were evaluated, low load (≤ 50%, represents “in port operations”) and high load (≥70% represents normal transit operations). A representative set of samples were collected from six ships operated under low and high engine loads (total of 12 sampling scenarios). Environmental samples were collected from three locations from the EGCS; seawater intake, post-EGCS waters, and overboard discharge for each scenario.
Whole effluent toxicity tests were conducted with the Inland Silverside (Menidia beryllina), mysid shrimp, (Americamysis bahia), and sand dollar (Dendraster excentricus), following Washington Department of Ecology and U.S. EPA methodologies. The Silverside, mysid shrimp, and sand dollar bioassay methods are used by WDOE for monitoring National Pollutant Discharge Elimination System (NPDES) permitted discharges to the Puget Sound which are detailed in Whole Effluent Toxicity Testing Guidance and Test Review Criteria. The marine diatom bioassay with, Skeletonema costatum, was added to the test design based on the advice of the external Advisory Panel to decrease the uncertainty of the risk the washwaters would pose to the marine community. This test method followed the bioassay method detailed by the American Society for Testing and Material (ASTM, 2021).
The Target Lipid Model (TLM) was used to predict the toxicity of petroleum components (PAHs) in EGCS overboard discharged waters while evaluation of the toxicity of metal components was evaluated by comparing the chemistry results to the Predicted No Effects Concentrations (PNECs) established for each metal. The MAMPEC model (Marine Antifoulant Model to Predict Environmental Concentrations) was used to assess the fate and distribution of EGCS associated waters when discharged. MAMPEC has the ability to estimate hydrodynamic exchanges in six generalized “typical” environments (i.e., open sea, shipping lane, estuary, commercial harbor, yachting marina, open harbor); two scenarios were evaluated in this study. The Cornell Mixing Zone Model (CORMIX) was used to refine the estimates from the MAMPEC model for the in-port scenario. CORMIX model estimates were simulated under two discharge scenarios, discharge during slack tide and one-hour post slack tide.