Exxon Valdez plus 10 years

Summary Points: 10 Years of Intertidal Monitoring After the Exxon Valdez Spill

NOAA biologist with small cage full of mussels.


NOAA's Hazardous Materials Response Division has monitored Exxon Valdez spill impacts and recovery processes on intertidal shorelines in Prince William Sound since 1990. The principle objective of this study is to understand the effects of oiling and cleanup measures so that we can improve responses to future spills. Study components include intertidal biology, chemistry and geomorphology. This independent study is not associated with trustee damage assessments or the Exxon Corporation. We are funded with restitution monies through the year 2005. Our study and conclusions do not include birds, mammals, or fish.

A casual observer would not likely see signs of the oil spill today in Prince William Sound. However, Exxon Valdez oil persists in certain environments, especially in areas sheltered from weathering processes, such as in the subsurface under selected gravel shorelines, and in some soft substrates containing peat.

We continue to grapple with the concept of ecological recovery: how to define and measure it. A commonly used definition, the "return to conditions as they were before the spill" is neither practical nor ecologically realistic for changeable intertidal systems. Specific measurable components that denote recovery of particular aspects of the ecosystem are discussed below.

Parallelism, a statistical test that measures whether abundances of plants and animals at oiled sites change over time in a manner similar to those at unoiled sites, provides an analytical and ecologically realistic measure of recovery. Recovery based on parallelism between oiled and unoiled intertidal populations occurred for most study species by 1992-1993.

Continuing differences between oiled and unoiled sites which suggest incomplete recovery (as of 1998) include species differences in infaunal populations, different grain size structures and lower population abundances at oiled sites. Some or all of these differences could be a result of natural variability, but continued monitoring will help elucidate whether oiling and subsequent hot water washing played a role.

Current evidence implies that oiled and hot-water washed sites initially suffered more severe declines in population abundance than oiled and not-washed sites. However, organisms from both types of oiled sites showed parallelism with unoiled populations on a similar time frame (by 1992-93).

Chemical contamination by polyaromatic hydrocarbons in tissues of mussels and clams were significantly elevated over background levels (all study sites combined) through 1992 for mussels and 1996 for clams.

Environmental exposure, sediment size and initial oil concentrations all affected oil weathering processes. By 1997, residual oil found in patches in sediments at a few locations ranged from moderately to extremely weathered, with oil from deep subsurface reservoirs under gravel beaches the least weathered.