dissolved phase hydrocarbon concentration and the air-sea mass transfer coefficient described above. Because the analysis is calculating loads from the atmosphere to the ocean, volatilization fluxes are negative by convention.

The best estimate of atmospheric deposition (wet deposition + dry aerosol deposition + gross gas absorption) of petroleum hydrocarbons into the marine waters of North America is 21,000 tonnes per year, with a minimum of 9,100 tonnes per year and a maximum of 81,000 tonnes per year. Worldwide estimates are much larger, with the best estimate being 52,000 tonnes per year. Minimum and maximum values of 23,000 and 200,000 tonnes per year were estimated by varying the concentrations and equilibrium and mass transfer coefficients across their ranges of uncertainties. Given these loadings, there may be a regional impact on air quality.

Aircraft Dumping

There are inputs to the sea from deliberate and continual releases of fuel from aircraft. There are two sources: deliberate discharge due to emergency conditions aboard the aircraft, and normal operation releases including the release of partially burned fuel in inefficient engines or inefficient operating modes and emptying of fuel injection bypass canisters. Because of fears of dumping over residential areas, most dumping is conducted over pre-assigned areas of little habitation and in the case of airports located in coastal areas, the designated dumping region is in marine waters. Evaporation reduces the amount that directly deposits to between 5 percent and 70 percent of dump volume, depending on fuel type and weather conditions. Appendix E describes the methodology and computations of the inputs.

The computed volume of petroleum hydrocarbons dumped into North American waters is 1,000 tonnes per year (minimum). The best estimate is 1,500 tonnes per year (minimum + 50 percent), while the maximum is 4,400 tonnes per year (minimum + 340 percent).18 Worldwide estimates range from 5,000 to 22,000, with a best estimate of 7,500.

Summary and Recommendations

The estimated range of total input of petroleum hydrocarbons from all sources into North American waters is 110,000 tonnes per year to 2,300,000 tonnes per year. The best estimate is 260,000 tonnes per year (see Table 3-2). The source with the highest contribution into the best estimate is natural seeps, being 160,000 tonnes per year or roughly 57 percent of the total. Discharge of petroleum hydrocarbons by consumers (industrial and public) is the second largest category, accounting for 90,000 tonnes per year or roughly 32 percent of the total. Spills and discharges during transportation of petroleum hydrocarbon products account for 12,100 tonnes per year (~4 percent of the total). The smallest discharge occurs during accidental spills associated with the extraction of petroleum, accounting for 3,000 tonnes per year or roughly 1 percent of the total.

The estimated range of worldwide input of petroleum hydrocarbons into the oceans from all sources is 470,000 tonnes per year to 8,400,000 tonnes per year. The best estimate from all sources is 1,300,000 tonnes per year. Natural seeps remain the largest, estimated at 600,000 tonnes per year or 46 percent of the total discharge. Activities associated with consumption of petroleum discharges is 480,000 tonnes/year or 37 percent of the total. Accidental spills and operational discharges of cargo oil occurring during transportation of petroleum products account for 160,000 tonnes per year (12 percent), while extraction processes account for 38,000 tonnes per year (3 percent).

Estimates for three significant inputs (natural seeps, land-based runoff, and operational discharges from vessels) have especially high levels of uncertainty. In particular, the maximum estimate for worldwide input from land-based runoff is more than 35 times the best estimate. For those categories with such wide ranges in uncertainty, care should be taken when applying the best estimates.

In the 1985 NRC report, the total input of petroleum hydrocarbons into the sea by all sources was estimated to be 3,200,000 tonnes per year. The inputs reported in this report are significantly lower by nearly 2 million tonnes. This has probably resulted because the databases and computational methods have improved significantly since the 1985 NRC report, and there have been worldwide efforts to stem pollution of the world’s oceans. One major change between the 1985 report and the present report is the global discharge from natural seeps, 200,000 tonnes per year in 1985 and 600,000 tonnes per year in the present report. Inputs from the marine transportation of petroleum showed particular improvement. Accidental spills from tankers declined from the 1985 NRC report figure of 400,000 tonnes per year to 100,000 tonnes per year, and operational discharges of cargo oil from tankers declined from the NRC 1985 best estimate of 700,000 tonnes per year to 36,000 tonnes per year. Worldwide inputs from extraction processes are estimated to be 38,000 tonnes per year as compared to 50,000 tonnes per year in the 1985 report.

The 1985 NRC report did not report discharge by a consumption category; therefore, it is difficult to compare the figures. However, land-based runoff figures are present in both reports. In the 1985 report, the estimate was 1,180,000 tonnes per year while in the present report, the best estimate is 140,000 tonnes per year. These differences are likely related more to differences in the methodologies used to estimate the runoff than actual reductions in urban runoff of petroleum hydrocarbons. With the increases in population


The factors used to develop maximum and minimum estimates are somewhat subjective and reflect the committee’s confidence in the data available and the methods and assumptions used to complete the calculation.

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