Modeling the in-water concentrations of ammonia if spilled into coastal and marine waters

Interest in ammonia as an alternative to fuel oils has risen steadily as governments and the shipping industry try to reduce carbon emissions. The risks of bulk ammonia spills into waterways are less understood than the well-studied consequences of fuel oil spills during transport or refueling. A previously validated trajectory and fate model, CHEMMAP, was applied to scope potential exposure volumes resulting from liquified ammonia spills. In-water exposure concentrations for potential worst-case discharges and smaller discharges were estimated to inform response planning, and to contrast long versus short-duration surface, as well as a shallow subsurface releases. An estimated 1 to 10 million m 3 of water would exceed a potential acute effects threshold of 1000 mg/m 3 total ammonia nitrogen (TAN) for up to 45 h following a hypothetical release of 44 metric tons of ammonia. Approximately 6 to 40 million m 3 of water could exceed 100 mg/m 3 TAN for several days, a threshold above which algal blooms and lowered dissolved oxygen are of concern. Larger (e.g., worst-case discharge) spills highly unlikely due to safety designs and practices in the industry. Modeling results indicate that the hydrodynamics and dimensions of the receiving water body influence fate and exposure. Releases into confined waterways where vertical mixing is limited by water depth resulted in more volatilization to the atmosphere than spills in open waters. Estimated volumes exceeding toxicity thresholds were of the same order of magnitude for ammonia as for completely dispersed crude oils and soluble chemicals of similar released mass. • Evaluated potential hazards of ammonia spills to aquatic biota using modeling • Mechanistic modeling accounts for transport and fate processes determining exposure • Large scale spills are highly unlikely due to specific safety features in shipping • Smaller spill estimates are <10 7 m 3 of water above the hazard thresholds for <48 h • Water body dimensions and currents largely determine exposure duration