The individual coastal basin metropolitan areas, along with data from Statistics Canada (2000), were then aggregated into the appropriate coastal zones. The annual loads for each coastal zone were calculated by multiplying the unit loads from the rivers for which data were available by the urban land area in each coastal zone.
Because almost one-fourth of the crude oil distillation capacity of the United States is located along the Gulf coast (Radler, 1999), the petroleum refining industry discharges a substantial amount of additional O&G to coastal waters in that area. To estimate this contribution, the operating capacities for coastal refineries in Louisiana and Texas (Radler, 1999) were multiplied by an assumed rate of O&G loss that corresponded to effluent guidelines for refinery discharges. These loadings were added to the coastal discharge for the coastal zone for the western Gulf of Mexico.
The total inland river and coastal basin annual loads of O&G for the United States and Canada were summed (with the exception of coastal zones for Mexico) to give an overall estimate for North America, based on urban land area. The North American O&G loading was then extrapolated to world estimates based on the number of motor vehicles in different regions of the world (World Resources Institute, 1998).
Data on motor vehicles from the World Resources Institute (1998) were used to calculate the O&G loadings for Mexico because of a lack of data regarding urban land area for metropolitan areas in Mexico. The total load for Mexico was divided by partitioning urban areas in Mexico with populations of more than 100,000 inhabitants as of 1990 (United Nations, 1998) depending on whether urban drainage from those areas drained to the Gulf of Mexico or the Pacific Ocean.
The total petroleum hydrocarbon load was estimated as a fraction of the oil and grease load to the sea. The calculated value of land-based inputs of petroleum hydrocarbons to North American marine waters, using the unit loadings per urban land area, is 2,600 tonnes per year (minimum), while the best estimate is 54,000 tonnes per year and the maximum estimate is 1,900,000 tonnes per year (Table 3-2).12 The worldwide best estimate, as determined using the methodol
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. As discussed in Appendix I, uncertainties regarding the percent of TPH in O&G measurements account for the vast majority of this uncertainty.