PHOTO 13 Frequent but small releases that accompany the high volume of petroleum consumed in areas with high population densities, coupled with higher percentage of paved surface leads to higher petroleum loads in surface water runoff. (Photo courtesy of Larry Roesner.)

and industrialization experienced over the last two decades, an increase in land-based runoff is expected.

Categories of inputs included in this report but not directly accounted for in the 1985 NRC report are the atmospheric deposition of VOC emitted from platforms and tankers, pipeline spills, aircraft dumping, discharges from small vessels under 100 GT in size, and discharges from recreational vessels. In particular, the operational discharges from recreational marine vessels were found to be significant.

Diffuse sources (natural seeps and run-off from land-based sources) are responsible for the majority of petroleum hydrocarbon inputs into North American waters, with contributions of 60 percent and 20 percent, respectively. In contrast, discharges from extraction and marine transportation of petroleum are responsible for less than 3 percent of the hydrocarbon inputs. Natural seeps represent the largest single petroleum hydrocarbon input, but there is a large range in the uncertainty estimation. Federal agencies especially the USGS, MMS, and NOAA should work to develop more accurate techniques for estimating inputs from natural seeps, especially those adjacent to sensitive habitats. Likely techniques will include remote sensing and ground truthing. This will aid in distinguishing the effects from natural processes verses anthropogenic activities.

The inputs and long-term fate of land-based sources (both due to run-off and atmospheric deposition) are poorly understood. The range of uncertainty of land-based run-off of petroleum hydrocarbons is four orders of magnitude. The upper limit, if correct, would dwarf all other inputs. The loads from rivers and air inputs are not being consistently monitored, and the background inputs from rivers are virtually unknown. In order to assess impacts attributable to different sources including oil spills and non-point sources, federal agencies, especially the USGS and EPA should work with state and local authorities to undertake regular monitoring of TPH and PAH inputs from air and water (especially rivers and harbors) to determine background concentrations.

For the period from 1990 to 1999, spillage from vessels in U.S. waters was less than one-third of the spillage during the prior decade, and now represents less than 2 percent of the petroleum hydrocarbon inputs into North American waters. Significant reductions in spillage were also realized worldwide. Improvements in vessel operation and design and the introduction of related federal and international regulations contributed to this decline in oil spills. In U.S. marine waters, the largest spills come from vessels, followed by pipelines and facilities. Vessels produced 109 spills greater than 34 tonnes (10,000 gallons) in size since 1990, and these larger spills had an average size of about 400 tonnes. During the 1990s, tanker vessels were responsible for about 81 percent of the spillage from vessels. The comprehensive port state control regime administered by the USCG, cooperative programs with ship owners and the boating community, and active participation at the International Maritime Organization in developing effective international regulatory stan

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