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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