The amount of petroleum that reaches Puget Sound in runoff and stormwater — once compared with the size of an Exxon Valdez spill every two years — appears to be dozens of times lower than initially thought, new studies show.

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In September 2008, the head of the state Department of Ecology told a PBS Frontline team that so much oil washes into Puget Sound that it equals an Exxon Valdez spill every two years.

A few months later, the agency attempting to restore Puget Sound made a slightly different case. It declared that an Exxon Valdez-size spill of “toxic chemicals” poured into Puget Sound every two years.

Neither is correct, according to new calculations of polluted runoff and stormwater the state published Tuesday. In fact, the amount of petroleum that reaches the Sound appears to be dozens of times lower than former Ecology Director Jay Manning told Frontline.

The confusion over precisely how much toxic stuff gets into the Sound underscores the complexity of tracking pollution rushing across the disparate landscapes that feed this vast water body. The data underlying the state’s grasp of so-called toxic loading grew more sophisticated between 2008 and 2011.

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But the inaccurate claims also reflect an eagerness within Gov. Chris Gregoire’s administration to seize on easy-to-grasp anecdotes that highlight Puget Sound’s ecological troubles.

“These studies did exactly what they should have: They refined over time our understanding of the problem,” said Manning, who’s now Gregoire’s chief of staff. “As a result, we now know that the relative contribution of petroleum to pollution in Puget Sound is lower than anticipated — by a significant margin. Do I regret my previous statement? I do. But we have to follow the science.”

Toxic pollution remains a problem for the Sound, and petroleum remains the single biggest contaminant by mass — at least 710,000 pounds per year. But it almost certainly isn’t the most harmful. Other pollutants, such as copper, are far more damaging to marine life in much smaller doses.

There’s also no clear evidence that any state policy was developed based on an incorrect understanding of the science. But the exaggerated figures clearly influenced debate.

Early in 2009 House lawmakers pushed a bill that would have boosted fees paid by the oil industry to improve the state’s stormwater-management system.

The rationale, described in the bill, was that petroleum is 90 percent of the problem. While later versions of the bill dropped those claims and expanded the share other industries should pay, some lawmakers repeated the charge in speeches. The measure never became law.

“When you’re throwing around numbers that equal Exxon-Valdez-spills-per-year, that’s something that people can get behind and understand,” said Lincoln Loehr, an oceanographer and attorney who first spotted errors in Ecology’s data. “But it’s just not true.”

The inaccuracy is rooted in a lack of information.

Ecology set out during the last decade to better understand just what gets funneled into Puget Sound through creeks and rivers, and when rain rushes across polluted parking lots and roads. An early version of the study, in late 2007, relied on historical data gathered elsewhere in the country that was then manipulated based on Washington land-use patterns, said Rob Duff, a program manager with Ecology.

The result: Far and away the largest single category of pollutants hitting Puget Sound was “oil and grease.” It was tens of millions of pounds a year.

In 2007, a reporter at The Seattle Times compared those numbers to a well-known event — the Exxon Valdez’s 10.8 million-gallon oil spill in Prince William Sound in 1989. He found the amount of oil and grease estimated to be getting into Puget Sound, in dribs and drabs, every two years was the equivalent of that spill.

His math was right. The data was not.

Loehr, the oceanographer, was sure that much oil would not escape notice.

“These numbers really perplexed me,” he said. “That was a huge amount of oil. We should be seeing seabirds covered in oil. But where were they?”

A friend led Loehr to a National Research Council report published in the early 2000s that showed the amount of oil spilled annually into all the lakes and rivers in North America was roughly the same as what Ecology suggested was getting into Puget Sound.

As it became clearer that Loehr was onto something, the Western States Petroleum Industry began paying him to do his research — just as it was fighting fee increases in the Legislature to pay for stormwater work. Over the course of several years, Loehr brought his concerns to Ecology, which determined he was right.

“I feel bad for the people at Ecology — these were totally unintentional errors,” Loehr said.

Petroleum is a far smaller portion of what constitutes “oil and grease” than thought. Duff agrees that Loehr found issues the state had not been aware of — but says the plan all along had been to revise the data over time.

“We always predicted this information would change,” Duff said. That’s why the studies were designed to come out in phases. The version released Tuesday is based on actual Puget Sound-region water samples.

Ecology began correcting its information, but by then the image of petroleum as the chief problem had stuck.

Still, Duff said the revised numbers don’t mean oil is harmless. “My perception of the problem of petroleum and stormwater is still approximately the same,” he said.

And even Loehr agrees stormwater is still a problem.

“There’s a lot of focus on stormwater and toxic loading. And to whatever extent those numbers have been used to tell people how bad it is … it’s not as bad as that,” Loehr said. But “should there be steps to reduce loading from stormwater? Yes, there should be.”

Craig Welch: 206-464-2093 or cwelch@seattletimes.com