ognizing the strong controls that exist for ports in North American waters, 98 percent compliance with MARPOL regulations is assumed for vessels greater than 400 GT. For vessels between 100 GT and 400 GT, which are not required to have oil-water separators and must transfer contaminated bilge water ashore, 90 percent compliance is assumed. This results in a calculated operational discharge of 81 tonnes per year. Intentional discharge of sludge in United States and Canadian waters are assumed to be small due to the rigorous enforcement efforts, and they are accounted for in the spill data.

Factoring in Canada and Mexico, the best estimate for bilge oil discharge in North American waters is 99 tonnes per year. Similar to the worldwide estimates, the minimum and maximum values were set at one-third and three times the best estimate, respectively.17

Operational Discharges (Vessels <100 GT)

According to U.S. Coast Guard figures for year 2000, there were 41,313 registered vessels under 100 GT other than tankers, cargo ships, and recreational vessels. The majority of these vessels were fishing boats and small passenger vessels. The average power of these vessels was about 350 HP. It was assumed that these vessels generated 0.09 gallons of bilge oil per day, operated 50 days per year, and that 60 percent of the vessels operated in marine (non inland) waters. These estimates were the best judgment of the committee, as data were not available. Based on the above, the total bilge-oil generation in the vessels less than 100 GT was calculated to be 380 tonnes per year. Assuming a 70 percent compliance level, the best estimate of operational discharge is 110 tonnes per year. Due to the very high level of uncertainty in these calculations, a range from 23 tonnes per year (one-fifth percent of the best estimate) to 570 tonnes per year (five times the best estimate) was selected. Worldwide estimates for vessels under 100 GT were not developed due to the lack of data.

Atmospheric Deposition

In this analysis, atmospheric deposition includes wet deposition (the scavenging of hydrocarbons from the atmosphere by precipitation), dry aerosol deposition (transport of aerosol particles and their associated hydrocarbons to the sea surface), and gas exchange. In the previous NRC 1985 report, each of these three processes were recognized, but only wet and dry aerosol deposition estimates were made, due to the uncertainty in the gas exchange calculation. Because gas-exchange is likely the dominant atmospheric deposition process, estimates of its magnitude are included in this

PHOTO 12 SeaWiFS (or Sea-viewing Wide Field-of-view Sensor) satellite imagery of the Mississippi River delta. As a result of high volume of tanker traffic, large number of oil and gas platforms, heavy input from the Mississippi River, and occurrence of natural oil seeps, the northwestern Gulf of Mexico experiences some of the largest average, annual inputs of petroleum to North American marine waters. (Image courtesy SeaWiFS Project, NASA/Goddard Space Flight Center and ORBIMAGE.)

17  

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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