BILLINGS, Mont. (AP) -- Montana officials said Wednesday they are investigating several sites with suspected oil left over from an Exxon Mobil pipeline break last year, but recent tests showed at least one such site downstream of the spill is oil-free.
The July 1 accident spilled an estimated 1,500 barrels of crude oil, or 63,000 gallons, into the Yellowstone River near Laurel.
In recent weeks, Montana Fish, Wildlife and Parks workers have found sheens or other evidence of oil at several sites downstream of the spill, said agency spokesman Bob Gibson.
Department of Environmental Quality scientist Laura Alvey said that includes a sheen she saw last week on an island east of Laurel. She said there was "no question" the sheen came from oil. Officials are on the lookout for more crude to get stirred up in coming weeks as high waters from mountain snowmelt shift sandbars that could hold trapped oil.
However, recent tests on water and sediment samples at another site, a riverfront residence near Laurel, were negative for oil contamination. Earlier tests in the vicinity of a fish kill near Huntley about two months ago also came back negative.
In both cases, Alvey said, the sheens appeared to be naturally occurring — from algae, decomposing organisms or something else besides oil.
Homeowner Jim Swanson had contacted the DEQ after seeing sheens along the river behind his Laurel house. His property suffered extensive contamination last year, which Exxon Mobile Corp. workers attempted to remove as part of an estimated $135 million in cleanup and pipeline repair work.
The company recovered an estimated 1 percent of the oil spilled, which marred about 70 miles of shoreline and numerous islands in the river channel.
"I was looking at Mr. Swanson as a worst-case scenario and thinking if (the sample results) came back dirty, we would have to do more work or have Exxon go out and do more work," Alvey said.
That could still happen if the sites with suspected oil identified by Fish, Wildlife and Parks are confirmed for contamination. Gibson said it is uncertain when the test results will come back.
As part of a settlement over state water pollution law violations, Exxon is obliged to do any additional cleanup work that is necessary. However, government regulators and the company already have agreed that much of the remaining contamination should be left in place to naturally degrade.
Exxon is monitoring 45 such sites along the river to gauge whether that is working.
Swanson welcomed Wednesday's news that there were no toxic compounds in the sheens found on his property but said it "boggles my mind" that they could be from something other than oil.
He said he can still see rings of degraded oil around trees and rocks on his property and oil in brush that was left to break down naturally.
"The fact remains they only recovered 1 percent," Swanson said. "There's still an ugliness there but at least it's not toxic and at least it's going away."
Swanson is a plaintiff in a lawsuit against Exxon from riverfront landowners who allege they suffered diminished property values and other damages because of the spill. A trial in the case is set for October 2013 before state District Judge Gregory Todd in Billings.
Exxon spokeswoman Amber Flournoy said in an emailed statement that the company remains committed to a full cleanup and remediation.
"We continue to work cooperatively with the (DEQ)," Flournoy wrote. She added that the company "has agreed to monitor and document the degradation of visible oil over time."
Additional lab tests are pending on fish collected from the river by state biologists last month. Evidence of oil was found in some fish last year, but officials said it was not at high enough levels to pose a threat to people who might eat them.
The latest round of tests is meant to determine if there were any residual effects on fish that came into contact with oil.
Although the oil itself likely would have worked its way through a fish's system, Gibson said, a fish still "may have liver problems or gall bladder problems, some residual damage that would indicate petroleum hydrocarbons."
That information will be used in part to determine how much Exxon will be asked to pay to cover natural resource damages from the spill. The investigation into those damages could take years to complete and is separate from the DEQ cleanup-related work.