ably over the past two decades and now account for only 5,300 tonnes per year, although they are still the dominant source from petroleum transportation activities (Figure 2-7). Tank vessel spills, globally, still discharge some 100,000 tonnes per year of petroleum hydrocarbons into marine waters, and they are the largest input from petroleum transportation worldwide (Table 2-2, Figure 2-7).
Tank vessels are permitted discharges related to both cargo and propulsion machinery, whereas nontankers are permitted only machinery-related discharges. Operational discharges from cargo washing are illegal in North American waters. Because of rigorous enforcement and the likelihood that intentional discharges will be detected as spills, no petroleum inputs are estimated for North American waters. Worldwide, operational discharges resulting from cargo washings represent 36,000 tonnes per year (Table 2-2). Increased compliance with international regulations has reduced this amount significantly from past estimates.
Discharges of oil in ballast and tank washing from oil tankers are prohibited within 50 nautical miles of the coast, thus most of the oil inputs from operational tanker discharges occur at sea. These discharges can cause impacts where heavily trafficked shipping lanes pass close to sensitive resources.
Spills from coastal facilities are primarily composed of refined products and account for an estimated 1,900 tonnes per year of petroleum hydrocarbons released to North American waters and 4,900 tonnes worldwide. Coastal pipelines that carry refined products and marine terminals account for 33 percent each of the total discharge. Because of their coastal location, these spills can have significant impacts from both episodic spills and chronic releases.
Loss of VOC during loading, washing, and transport on tankers contributes the smallest amount of petroleum hydrocarbons to marine waters from transportation activities, both in North America and globally (Figure 2-7). Most of the VOC is methane that enters in the atmosphere and is not counted in the volume entering the sea.
Based on the PAH content of oils spilled and released during shipping, an estimated 170 metric tonnes of PAH are released by transportation. This value does not include combustion-derived PAH produced by ships’ power plants that are released to the atmosphere (these contribute to the atmospheric PAH inventory and are included in the atmospheric deposition loadings). PAH release from transportation is approximately 7 percent of the total PAH loading from anthropogenic sources to the North American coastal ocean (Table 2-4). Tank vessel spills account for 70 percent of transportation-related PAH discharges.
The distribution of transportation-related inputs reflects the regions where refinery production and coastal tanker traffic are highest. All of the petroleum inputs into North American waters are point-source spills from pipelines, tank vessels, and coastal facilities, with the exception of a small amount of atmospheric deposition (note that operational discharges from vessels are illegal; thus they are estimated to