International spill data were obtained from the Environmental Research Consulting database and includes information gleaned from the International Maritime Organization, ITOPF, and other national and regional agencies. The minimum computed discharge is 2,400 tonnes per year, while the best estimate is 4,900 tonnes per year (minimum + 100 percent). The maximum (see Table 3-2) discharge was estimated to be 15,000 tonnes per year (minimum + 500 percent).11
It is difficult to compare directly these numbers to those in the NRC 1985 report because of differences in definitions. The category, marine terminals, in the 1985 report included bunkering operations, but did not specifically indicate the type of facilities included in their computation. The present report defines coastal facilities in a very broad framework (see Table G-1, Appendix G). In the NRC 1985 report, the volume discharged into marine waters by “Marine Terminals” is 20,000 tonnes per year (best estimate), with a range of 10,000 to 30,000 tonnes per year. The best estimate in this report is 4,900 tonnes per year, a significant difference existing between the two estimates.
This report, based on the methodology described in Appendix E, reports a best estimate for imports from transportation of 160,000 tonnes per year, with a minimum of 120,000 and a maximum of 260,000 tonnes per year. The categories in the NRC 1985 report included major differences from those in the present report, so no overall comparison is possible. However, operational discharges from tankers (best estimate of 36,000 tonnes per year in this report compared to 710,000 tonnes per year in the NRC 1985 report) and oil spills from tankers (best estimate of 100,000 tonnes per year in this report compared to 390,000 tonnes per year in the NRC 1985 report) indicate very significant reductions in the amounts of petroleum hydrocarbons entering the oceans from transportation-related services. In North American waters, the best estimate of petroleum hydrocarbons discharged during the marine transportation of petroleum is 9,100 tonnes per year, while the minimum is 8,500 tonnes per year and the maximum is 11,000 tonnes per year.
The world population increased from 4.5 billion in 1980 to 6.0 billion in 1999, an increase of 35 percent (U.S. Census Bureau, www.census.gov). The world’s merchant fleet of vessels greater than 100 gross tons (GT) increased by 18 percent, from 73,832 in 1980 to 86,817 in 1999 (Lloyds Register, 1999). There has also been a significant increase in the number and use of recreational vessels. From year 1985 to 2000, the global consumption of petroleum increased from 9.3 to 11.7 million tonnes per day, an increase of over 25 percent (www.eia.doe.gov, 2001).
Significant petroleum hydrocarbon inputs into the oceans related to consumption of petroleum include river and urban runoff, oil spills from cargo ships, operational discharges from commercial vessels and recreational craft, and atmospheric deposition of petroleum hydrocarbons. Details concerning data sources, methodology, and computations can be found in Appendix E.
Because of the scarcity of available data for estimating loads from individual sources (i.e., municipal wastewaters, non-refinery industrial discharge, refinery discharges, urban runoff, river discharges, and ocean dumping), loading estimates presented in this analysis are based on loading from all land-based sources per unit of urban land area (see Box 3-3). These calculations assumed that most of the contributions of petroleum hydrocarbons to the sea from land-based sources were from urban areas. This approach accounted for loading from all of the sources in the United States and Canada, with the exception of Gulf coast loadings from coastal refineries, which were calculated separately. The overall calculations of hydrocarbon loadings from all land-based sources for the United States and Canada were then extrapolated to other regions of the world to form a world estimate.
For the calculations in the United States and Canada, the land-based sources were divided into two categories: inland basins and coastal basins. It was assumed that inland basins discharged into major river basins that drain to the sea along the coast of the United States and Canada. Coastal basins were assumed to discharge directly to the sea. Appendix I details the methodology and calculations utilized in determining the sources of petroleum hydrocarbons to the sea from land-based sources.
In order to compute the annual average load of petroleum hydrocarbon to the sea from land-based sources, it was decided to use oil and grease (O&G) as a surrogate measure, and consider TPH and PAH to be some fraction of the O&G. The reason for using O&G data is that these data have been measured historically in urban runoff, wastewater discharges, and rivers. There is no such database available for petroleum hydrocarbon. While some PAH data are available, there is no consistency from investigator to investigator, or study to study with respect to the PAH species that were defined as comprising PAH. The method for arriving at the fractions of O&G that are petroleum hydrocarbon and PAH is described in detail in Appendixes I and J.
The 100 percent factor used to develop a minimum estimate and the 500 percent factor applied to develop maximum estimate are somewhat subjective and reflects the committee’s confidence in the data available (including the judgment that spills in U.S. waters are more rigorously reported) and the methods and assumptions used to complete the calculation.