Liquid petroleum (crude oil and its refined products such as tar, lubricating oil, gasoline, and kerosene) can be released as catastrophic spills from point sources (e.g., from tankers and blowouts) or as chronic discharges typically from nonpoint sources (e.g., from urban runoff or fallout from the atmosphere). Releases of petroleum into the environment occur naturally from seeps as well as from human sources. Together natural and human sources contribute about 380 million gallons of petroleum to the oceans each year. Of this, about 45 percent comes from natural seeps, and the remainder may be attributed to the human activities of petroleum production, transportation, and consumption. Discharges during petroleum production tend to be restricted to areas of exploration and extraction and are mostly due to the release of "produced waters" (water extracted with petroleum from the reservoir); these discharges contribute about 5 percent of the petroleum...