An examination of reports from a variety of sources, including industry, government, and academic sources, indicate that although the sources of petroleum input to the sea are diverse, they can be categorized effectively into four major groups, natural seeps, petroleum extraction, petroleum transportation, and petroleum consumption. Natural seeps are purely natural phenomena that occur when crude oil seeps from the geologic strata beneath the seafloor to the overlying water column. Recognized by geologists for decades as indicating the existence of potentially economic reserves of petroleum, these seeps release vast amounts of crude oil annually. Yet these large volumes are released at a rate low enough that the surrounding ecosystem can adapt and even thrive in their presence. Petroleum extraction can result in releases of both crude oil and refined products as a result of human activities associated with efforts to explore for and produce petroleum. The nature and size of these releases is highly variable, but is restricted to areas where active oil and gas exploration and development are under way. Petroleum transportation can result in releases of dramatically varying sizes, from major spills associated with tanker accidents such as the Exxon Valdez, to relatively small operational releases that occur regularly. Petroleum consumption can result in releases as variable as the activities that consume petroleum. Yet, these typically small but frequent and widespread releases contribute the overwhelming majority of the petroleum that enters the sea due to human activity.

Based on analysis of data from a wide variety of sources, it appears that collectively these four categories of sources add, each year on average, about 260,000 metric tonnes (about 76,000,000 gallons) of petroleum to the waters off North America. Annual worldwide estimates of petroleum input to the sea exceed 1,300,000 metric tonnes (about 380,000,000 gallons). Although these are imposing figures, they are difficult to interpret in terms of their ecological significance, as they represent thousands or tens of thousands of individual releases whose combined effect on the environment is difficult to clearly establish. Regional or worldwide estimates of petroleum entering the environment are useful only as a first order approximation of need for concern. Sources of frequent, large releases are rightfully recognized as areas where greater effort to reduce petroleum pollution should be concentrated, despite the fact that not every spill of equal size leads to the same environmental impact. This study, as did the 1975 and 1985 NRC reports, attempts to develop a sense of what the major sources of petroleum entering the marine environment are, and whether these sources or the volume they introduce, have changed through time. Thus, this report not only attempts to quantify the amount released each year, but makes an effort to examine the geographic distribution and nature of releases of petroleum to the marine environment, as well as the processes that can mitigate or exacerbate the effect of these releases on the environment. Where appropriate, comparisons of estimates of petroleum pollution among studies over the last 25 years provide the basis needed to explore the performance for prevention efforts implemented during that time.

Natural Seeps

Natural seepage of crude oil from geologic formations below the seafloor to the marine environment off North America is estimated to exceed 160,000 tonnes (47,000,000 gallons), and 600,000 tonnes (180,000,000 gallons) globally, each year. Natural processes are therefore, responsible for over 60 percent of the petroleum entering North American waters, and over 45 percent of the petroleum entering the marine environment worldwide. Oil and gas extraction activities are often concentrated in regions where seeps form. Historically, slicks of oil from seeps have been attributed to releases from oil and gas platforms, and vice versa. In North America, the largest and best known natural seeps appear to be restricted to the Gulf of Mexico and the waters off of southern California, regions that also have extensive oil and gas production. As mentioned earlier, the seepage of crude oil to the environment tends to occur sporadically and at low rates. Federal agencies, especially USGS, MMS, and NOAA, should work to develop more accurate techniques for estimating inputs from natural seeps, especially those adjacent to sensitive habitats. This effort will aid in distinguishing the effects of petroleum released by natural processes versus anthropogenic activities. Furthermore, areas surrounding natural seeps are extremely important natural laboratories for understanding crude oil behavior in the marine environment, as well as how marine life responds to the introduction of petroleum. Federal agencies, especially USGS, MMS, NSF, and NOAA, should work with industry and the academic community to develop and implement a program to understand the fate of petroleum released from natural seeps and the ecological response to these natural releases.

Petroleum Extraction

Activities associated with oil and gas exploration or production introduce, on average, an estimated 3,000 tonnes (880,000 gallons) of petroleum to North American waters, and 38,000 tonnes (11,000,000 gallons) worldwide, each year. Releases due to these activities, therefore, make up roughly 3 percent of the total petroleum input by anthropogenic activities to North American waters and 5 percent of the total worldwide. Although dwarfed by some other sources of petroleum to the marine environment, these inputs are not trivial, as they can occur as large spills or as slow, chronic releases concentrated in production fields. Furthermore, those releases from petroleum extraction activities that take place near shore or even on shore can pose

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