Roughly 10,000 pits were excavated as part of
the shoreline survey of Prince William Sound
during summer 2001.
The 1989 Exxon Valdez oil spill (EVOS) in
Prince William Sound, Alaska, released a minimum of 11 million gallons
of Alaskan crude oil into one of the largest and most productive
estuaries in North America. During the summer of that year, the
Alaska Department of Environmental Conservation (ADEC) estimated that
149 km of shoreline in Prince William Sound were heavily oiled and 459
km were at least lightly oiled. A year later a survey showed
oiling had decreased 73 percent. Two years later in 1991 an interagency
survey estimated only 1.4 km of shoreline were estimated to be heavily
oiled. By 1992 the estimate of heavily oiled shoreline was only
0.2 km. After 3 years of unprecedented efforts to clean the
polluted beaches and subsequent surveys showing declining contamination,
it was expected that natural processes would disperse any remaining oil.
However, in 1993 the EVOS Trustee Council funded an additional survey
that estimated 7 km of shoreline were still contaminated with subsurface
oil. Smaller-scale studies dealing with continued clean-up efforts and
restoration of oiled mussel beds conducted between 1995 and 1999 showed
that oil was surprisingly persistent and often in a relatively
unweathered state, containing high concentrations of toxic and
biologically available polycyclic aromatic hydrocarbons (PAH). Long-term
monitoring in the oiled areas has also shown that fauna from higher
trophic levels such as sea otters and sea ducks still have not
recovered. It now appears the remaining oil deposits may have become a
chronic source of low-level oil pollution within the spill-affected
Because a significant survey of Prince William Sound had not been
conducted since 1993 and the cumulative extent of the remaining oil was
unknown, concerns were generated by the public and scientific
communities about the oil’s possible continuing effects on humans and
fauna potentially exposed to the oil directly or indirectly. Public
perception of the amount of oil that remains on beaches within Prince
William Sound has varied widely. Without an accurate assessment of
the extent of the remaining oil, subsistence food-gatherers, consumers
of commercial fish products from the area, and tourists have used mostly
anecdotal evidence as the basis for economic decisions regarding
resource utilization in the affected area. Scientists and resource
managers also have lacked an accurate assessment of the amount of
remaining oil in Prince William Sound necessary for determining further
appropriate scientific studies and management actions.
Consequently, the Auke Bay Laboratory (ABL) with funding from the EVOS
Trustee Council, took on the task of assessing the remaining oil along
the shorelines of Prince William Sound during the summer of 2001. The
primary objective of the project was to measure the amount of oil
remaining in the intertidal zone of Prince William Sound. Secondary
objectives include determining the rate of decline of oil on these
beaches, estimating the persistence of the remaining oil, and
correlating the remaining oil with geomorphological features.
Table 1. Target sampling effort for 2001
Prince William Sound shoreline survey.
||Km in Strata
Previous attempts to estimate the oil remaining on beaches affected by
the Exxon Valdez oil spill have relied mainly on Shoreline
Contamination Assessment Teams (SCAT), field teams that perform
comprehensive foot surveys of impacted beaches. The SCAT survey
crews estimated oiled areas based mostly on visual clues at the surface.
Although SCAT were useful for directing cleanup efforts
immediately after the spill, it was determined that the SCAT methods
would not be useful for producing a quantitative estimate of
subsurface oil contamination 12 years after the spill.
Sampling efforts for the 2001 survey focused on
beaches assessed as heavily and moderately oiled during the
1989-93 ADEC/EVOS Trustee Council surveys (Table 1 above). Limited
additional effort was allocated to areas that had not been surveyed
since 1989 and where persistence was uncertain. The survey design
examined about 20 percent of the areas heaviest hit by the spill, with
an intensity that permits extrapolation to other hard hit areas for all
of Prince William Sound. The survey design guarantees a credible minimum
estimate of remaining oil in the area and will provide a confidence
interval for the most likely amount remaining throughout the affected
region. This information is needed to predict oil persistence into
the future and to determine associated risks to vulnerable biota.
The 2001 survey covered roughly 8,000 m of shoreline. Ninety-six sites
were randomly selected from the total number of oiled beaches assessed
during the ADEC/EVOS Trustee Council surveys. A 72-ft vessel was
chartered for 90 days (three 20-day legs and one 30-day leg) between May
and September of 2001. In addition to ABL staff, three contract laborers
per cruise were provided by the native villages of Tititlek and Chenega.
The Alaska State Department of Natural Resources provided an
archeologist on site to protect any cultural artifacts uncovered in the
Pits were dug manually with shovels and pry bars, mostly on cobbled beaches.
The 2001 survey adopted a stratified random/adaptive
sampling (SRAS) design. Two random pits were excavated to a depth of 0.5
m (1.6 feet) in every stratified block (0.5-m verticle drop in tide
height) within a grid system established at each site. If subsurface oil
was discovered in any of the randomly stratified origin pits, then
additional adaptive pits were excavated above, below, to the right, and
to the left of the origin pit until the extent of the oil patch was
determined. Sampling methods utilized only manual labor; holes
were dug with shovels and pry bars. Most beaches were cobble
or armored with boulders, making digging physically challenging. The
sampling effort translated into 6,775 random origin pits and
several thousand more adaptive pits within 7.7 km of the spill-affected
area (Table 2).
Table 2. Summary of sampling effort.
|No. of Sites
|No. of Random
|No. of Sites
|No. of Sites
Two additional measurements were made from every
oiled, random origin pit to aid in volumetric estimates. First, the oil
was categorized into a standardized visual classification scheme
developed by the previous surveys (light, moderate, or heavy oil
residues). Then, the oiled zone within the pit was measured to the
nearest centimeter. Also, a subset of pits representing each
visual oiling classification was weighed, homogenized, and collected for
chemical analysis. A regression relationship based on the quantity of
oil extracted and weighted from these samples will be generated to
establish an estimate of the mean amount of oil in each oiling
Oil was categorized into a standardized visual classification scheme.
Buried or subsurface oil is of greater concern than surface oil.
Subsurface oil can remain dormant for many years before being dispersed
and is more liquid, still toxic, and may become biologically available.
A disturbance event such as burrowing animals or a severe storm reworks
the beach and can reintroduce unweathered oil into the water. Results of
the summer shoreline survey showed that the oil remaining on the surface
of beaches in Prince William Sound is weathered and mostly
hardened into an asphalt-like layer. The toxic components of this type
of surface oil are not as readily available to biota, although some
softer forms do cause sheens in tide pools.
|Table 3. Number of random
oiled pits, not including adaptive pits, stratified by 0.5-m
tide heights. Intertidal sampling zone ranges
from +4.6-m to +1.8-m tide height.
0.5-Meter Vertical Drops (Tide heights)
Total Number of Pits
Our survey results indicate a total area of approximately 20 acres
of shoreline in Prince William Sound are still contaminated with oil.
Oil was found at 58 percent of the 91 sites assessed and is estimated to
have the linear equivalent of 5.8 km of contaminated shoreline (Figure
1). The overall 20 acre estimate of oil-contaminated
beaches was more than twice the estimate from the EVOS Trustee Council
survey conducted in 1993. (The 1993 surveys covered more beaches,
but dug far fewer holes). Most of the oil found in 2001 was
classified as lightly oiled, but was still easily observed once it
was uncovered (sheening, strong odor, and sticky) and did not require
the aid of a mechanical sniffer or chemical analysis for positive
In addition to the estimated area of remaining oiled beach, several
other important points were evident.
1) Surface oil was determined to be not a good indicator of subsurface
2) Twenty subsurface pits were classified as heavily oiled. Oil
saturated all of the interstitial spaces and was extremely repugnant.
These “worst case” pits exhibited an oil mixture that resembled oil
encountered in 1989 a few weeks after the spill - highly odiferous,
lightly weathered, and very fluid.
3) Subsurface oil was also found at a lower tide height than expected
(between 0 and 6 feet), in contrast to the surface oil, which was found
mostly at the highest levels of the beach (Table 3). This is
significant, because the pits with the most oil were found low in the
intertidal zone, closest to the zone of biological production, and
indicate that our estimates are conservative at best.
A pit containing a heavy oil residue.
The possibility of continuing low level chronic effects of the Exxon
Valdez oil spill seem very real now, although measurable population
effects would be very difficult to detect in wild populations. If
there are continuing effects, it would be most likely restricted to
populations residing or feeding in the isolated oil pockets. Sea
otters and harlequin ducks fall into this category. Researchers have
been monitoring these populations’ poor recovery in heavily oiled
areas since the 1989 spill. The 2001 shoreline survey has provided
new insights for possible sources of continued oil contamination. This
has stimulated future studies that will focus on the bioavailability of
the oil and its impacts on species such as sea otters, harlequin ducks,
and their intertidal prey.
We anticipate the significance of these results will
be controversial and stimulate discussion. Is the oil significant
and to whom? Are pink salmon or herring injured because of
continued intertidal contamination? Are near shore predators, like
otters or sea ducks, at risk because they prey in this zone? Are
the area’s subsistence users avoiding appropriate beaches, or are they
avoiding all beaches? What can or should be done about the
remaining oil? If more cleaning is requested or required, will it
do more good than harm?
The last beach assessment was completed in September 2001. Supporting
chemical analyses will be completed in fall 2002, and a final report
with statistical analyses and conclusions will be completed by April
By Jeff Short, Stanley Rice, and Mandy Lindeberg.
quarterly Jul-Sept 2001 sidebar
Auke Bay Lab