Exxon's Valdez studies ignite controversy

Science News, May 8, 1993 by Janet Raloff

Alaska's Prince William Sound "has almost fully recovered from the 1989 Exxon Valdez oil spill:' assert officials with the Houston-based Exxon Co. USA. That assessment, based on a spate of new papers by company-funded researchers, provoked an immediate flurry of heated charges and countercharges last week.

Exxon scientists say their data indicate that widespread oil contamination has plagued Prince William Sound for more than a century. They interpret these findings to suggest that area aquatic life can coexist with low levels of oil - and even recover from occasional heavy oiling.

Government claims of long-term Exxon Valdez damage usually can be traced to a "faulty interpretation" of data, Exxon argues in a statement it released April 26. As a result, the company says, government scientists have mistakenly assumed "that large numbers of biologic and sediment samples from Prince William Sound contained remnants of Exxon Valdez crude when, in fact, they did not."

Exxon issued its statement at the opening of a four-day environmental session at an American Society for Testing and Materials (ASTM) meeting in Atlanta. Researchers funded by Exxon presented 25 papers there on the Valdez spill.

However, some of those same papers lead chemist Jeffrey W.. Short of the National Marine Fisheries Service in Juneau, Alaska, to suspect that there is at least some possibility that Exxon is ascribing to other sources a portion of the oil that actually came from the Valdez spill.

His concerns involve studies that attempted to identify the source of an oil from the chemical fingerprints of its polycyclic aromatic hydrocarbons (PAHs). These PAHs offer a relatively stable oil signature - one that persists even after a sample has weathered, or begun to turn tarry.

In one study, chemist David S. Page of Bowdoin College in Brunswick, Maine, and his co-workers assayed PAHs in more than 2,350 seafloor sediments collected in Prince William Sound and the adjacent Gulf of Alaska between 1989 and 1991.

Coauthor A. Edward Bence, a geochemist with Exxon in Houston, recalls how surprised he was to find a consistent background signature of crude oil -- one quite different from the Valdez oil -- going back at least 160 years throughout the supposedly pristine sediments in Prince William Sound. The age of the signature argued for some natural, continuing source of this petroleum.

Realizing that crude-oil seeps had been charted in several places along the eastern Gulf of Alaska, the researchers compared fingerprints of oil from the seeps to those of oil in Prince William Sound sediments. They matched.

Other Exxon studies offered an explanation of how the seeps' oil might have entered Prince William Sound. Clays from glaciers to the east readily combine with oil -- especially weathered oil -- to form flocculated emulsions (see story, p. 302). These buoyant floc particles would ride west on the Alaska coastal current (see diagram on facing page) until they hit the sound's slow waters and settled.

The Exxon survey of sediment fingerprints revealed large amounts of petroleum PAHs- concentrations sometimes approaching 500 to 1,000 parts per billion. In deep areas, most of the oil appears to have come from seeps. In shallower zones, diesel fuel was often present. And because this diesel oil, perhaps spilled during refueling, bore a signature quite similar to that of the Exxon Valdez oil, government chemists often mistook the two, Bence contends.

That's definitely possible and reflects "the bias I entered with," concedes Short, who led some of those analyses. ff an oil fingerprint bore the distinctive PAH peaks representing phenanthrenes and dibenzothiophenes - characteristic of North Slope crude oil --"I assumed it was Exxon Valdez oil:' he says. In fact, North Slope diesel oil contains the same two PAH peaks. Exxon differentiated between the two oil types by looking for chrysenes: The diesel lacks these PAHs.

Short's own PAH fingerprinting supports Exxon's finding of background contamination with nonNorth Slope crude oil in deep Prince William Sound sediments those in 40 to 100 meters of water. But his studies indicate that shallow, intertidal sediments generally remained pristine - totally free of petroleum residues - unless or until oiled by the Exxon Valdez spill.

And that's why at least one paper at the ASTM meeting bothered him. It attributed most o! the PAHs in biological materials from one tidal area to a mix of seep oil and North Slope diesel. If one assumes such shallow areas contain a mix of diesel, seep, and ValdezoiI, he says, then one can attribute much of any phenanthrenes and chrysenes present to seep oil, and much of the dibenzothiophene signature to diesel. This would allow you "to cover most of what you see with non-Exxon-Valdez oil -- even if Valdez crude is the only [oil] present:' he says. That prospect, he adds, "makes me suspicious."

Many government scientists lauded Exxon for the science reported at the ASTM meeting. Like Short, however, many also questioned how data from that research were being interpreted.