Drainage Canal, eventually flowing to the Gulf of Mexico (National Research Council, 1995b), and were therefore included in coastal zone H. (Contact NRC staff to obtain a listing of the urban areas and corresponding 1990 populations in each coastal zone.)

  1. The oil and grease loading calculated in Step 1 was allocated to each coastal zone according to the percentage of the Mexican urban population allocated to that coastal zone. Thus, 65 percent of the total oil and grease loading from Mexico was allocated to coastal zone H, and the rest was allocated to coastal zone I.

Estimates of Petroleum Hydrocarbons and Polycyclic Aromatic Hydrocarbons

The land-based loading calculations of oil and grease described thus far were based on available data from the STORET database that was measured using either the Soxhlet extraction method or liquid-liquid extraction method. These methods determine groups of substances with similar physical characteristics on the basis of their common solubility in a specified solvent (American Society for Testing and Materials, 1999). Thus, “oil and grease” as measured by these methods includes not only petroleum hydrocarbons but also other substances, such as lipid material (American Society for Testing and Materials, 1999; Hoffman and Quinn, 1987a). An investigation was done of published literature to determine if quantifications have been made of the amount of petroleum hydrocarbons or polycyclic aromatic hydrocarbons (PAH) in oil and grease. The literature search revealed a scattering of studies that were generally focused on oil and grease data or specific hydrocarbons, but seldom on total hydrocarbons in oil and grease (Table I-5).

Eganhouse and Kaplan’s (1982) study of effluents from wastewater treatment plants in southern California remains the principal study that estimated the proportion of total hydrocarbons in oil and grease. The factor of 0.38 that was applied to oil and grease estimates in the previous National Research Council (1985) report to estimate petroleum hydrocarbon contributions from municipal wastewaters was obtained from the Eganhouse and Kaplan (1982) study. However, wastewater effluent in southern California is not representative of the petroleum hydrocarbon fraction in oil and grease in river water because there are many sources of petroleum hydrocarbons and oil and grease besides municipal wastewaters, the composition of petroleum-derived hydrocarbons varies widely from place to place, and there could be other sources of hydrocarbons such as those produced naturally by aquatic organisms that could be included in oil and grease measurements (Laws, 1993).

New studies were not available that compared concentrations of PAH or total hydrocarbons to oil and grease in water, but Michel (2001) provided data of measured total PAH on the lower Mississippi River in December 2000. These measurements were taken as a result of a spill on the river, but the background measurements of total PAH at three river stations varied from 100 to 156 ng L−1, with an average of 128.3 ng L−1. Using the average oil and grease concentration for the Mississippi River of 0.84 mg L−1 from the STORET data (see Table I-2), the estimated percentage of PAH in oil and grease in the Mississippi River would be about 0.015% based on the average total PAH concentration.

PAH typically constitute 0.1-1% of total petroleum hydrocarbons in oil (Wang et al., 1999b). However, since PAH are fairly soluble in water, they likely constitute a larger portion of total petroleum hydrocarbons in oil in water, so the range was expanded to 0.1-10% of total petroleum hydrocarbons, which was verified with comparisons of relative amounts of measured PAH and total hydrocarbons in water in studies in the literature (Table I-6). Thus, estimates of total petroleum hydrocarbons in the Mississippi River based on the December 2000 average PAH data of Michel (2001) would be from 1280 to 128,000 ng L−1. These estimates, when compared to the measured average oil and grease concentrations in the Mississippi River, are 0.15% to 15% of oil and grease, with a best estimate of 1.5%. The best estimate of total hydrocarbon loading from land-based sources was therefore calculated as 1.5% of the best estimate of oil and grease loading.

RESULTS

The average annual loads of oil and grease discharged to the sea were calculated for those rivers with reported oil and grease data in STORET (Table I-7). These total loads were then normalized to unit loads per urban land area. The final estimates of land-based contributions of oil and grease to the sea via all major inland river basins in the United States and Canada were then determined using the 1990s oil and grease data for the Delaware and Mississippi Rivers (Table I-8) with urban land area data from U.S. Bureau of Census (1998) and Statistics Canada (2000). About two-fifths of the estimated loading in North America was determined from actual measured data in STORET, with the remainder determined using the unit load approach.

The estimates of land-based contributions of oil and grease to the sea from both major inland rivers and coastal areas in the United States and Canada were totaled by coastal basin (Table I-9). Table F-9 also shows calculated values for coastal zones in Mexico, but these loads were not included in the totals for North America (i.e., the United States and Canada). The total loading for North America (3.4 million tonne yr−1) was used to obtain a world estimate of land-based oil and grease loading (9.4 million tonne yr−1; Table I-10). The regional distribution of this loading shows that North America and Europe contribute the majority of land-based oil and grease to the sea.

A factor of 0.015 was applied to the total oil and grease loading to estimate the fraction of hydrocarbons in oil and grease. The estimated worldwide loading of hydrocarbons to



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