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About petroleum spills in Washington State:

In 1997, the Washington State Department of Ecology published an historical analysis of oil spills, that assembled the available information on spill type, size and method of occurrence. The Department of Ecology paper summarized oil spills going back to the 1960's, however it was not until the early 1990's that systematic reporting of all spills, regardless of size, took place. Before 1990, only larger spills were systematically reported, which makes a statistical analysis of the older data very problematic.

The data include both marine and land-based spills (underground leaking storage tanks are excluded), and include a wide variety of petroleum byproducts such as crude oil, fuel oil, lubricating oils, diesel fuel, jet fuel and gasoline (but no vegetable or animal oils). While all of these petroleum products are serious contaminants, the heavier hydrocarbons (crude oil and fuel oil) are much less volatile and much more persistent, even in high energy coastal environments such as Prince William Sound, Alaska. Heavy hydrocarbons that settle into beach deposits can be excavated by storms, re-contaminating the nearshore environment for decades following the spill.

Because the Washington State petroleum spills in the data set range from 30 gallons to 600,000 gallons, over 4 orders of magnitude, the logarithm of the spill size is calculated, binned and plotted against its frequency. As expected, small spills are the most abundant, whereas large spills are rather infrequent. Many phenomena such as landslides, earthquakes, volcanic eruptions, nuclear accidents, auto wrecks, and wildfires show a similar distribution of sizes; many small events, some medium sized events, and few large events. Size versus frequency for these phenomena often exhibit a power law relationship. If petroleum spills in Washington State follow this same relationship, then the Department of Ecology's compilation under-represents the smallest spills, which should be 2-3 times more frequent. This under-representation is consistent with the historical lack of comprehensive reporting.

Since the period of record is known (5 years, from 1991-1996), the recurrence interval for a spill of a given size can be calculated. Extrapolation to larger sizes using the power law can yield recurrence intervals for these rare events. On New Year's day in 1972, the General M. C. Meigs, a US Navy supply vessel, spilled 2.3 million gallons of heavy fuel oil into the Straights of Juan de Fuca between Washington State and Canada. What is the recurrence interval for an event of this size?

Source: Oil Spills in Washington State: An Historical Analysis; Washington State Department of Ecology Report 97-252.

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 Selected petroleum spills in Washington State, 1991-1996 Washington State Department of Ecology Report 97-252 spills in gallons of product year spill size bin size (log) frequency 1991 600,000 1-2 38 1991 3,025 2-3 37 1991 210,000 3-4 17 1991 210 4-5 4 1991 100,000 5-6 4 1991 3,528 sum 100 1992 2,100 1992 850 1992 100 1992 70 1992 370 1992 30 1992 20,000 1992 500 1993 800 1993 495 1993 5,400 1993 900 1993 6,260 1993 60 1993 35 1993 3,000 1993 100 1993 2,995 1993 50 1993 80 1993 360 1993 30 1993 3,295 1993 50 1993 264,000 1993 25 1993 560 1993 308 1994 40 1994 2,771 1994 300 1994 40 1994 30 1994 483 1994 5,500 1994 85 1994 200 1994 50 1994 1,000 1994 700 1994 325 1994 200 1994 100 1994 25 1994 500 1994 3,700 1994 100 1994 80 1994 50 1994 26,900 1995 300 1995 200 1995 2,520 1995 50 1995 30 1995 1,000 1995 400 1995 37 1995 250 1995 50 1995 200 1995 100 1995 55 1995 100 1995 30 1995 25 1995 100 1995 50 1995 30 1995 40 1995 50 1995 75 1995 50 1995 50 1995 85 1995 120 1996 30 1996 241 1996 25 1996 200 1996 378 1996 50 1996 308 1996 1,561 1996 450 1996 37 1996 65,000 1996 700 1996 35 1996 4,000 1996 100 1996 70 1996 1,500 1996 49,000
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