Montana Gov. Brian Schweitzer last week said there were no surprises in the state's analysis of the crude oil spilled from the Silvertip pipeline in July. The crude oil, he said, didn't contain high levels of heavy metals or other toxic additives that persist in the environment.
The U.S. Environmental Protection Agency said its sampling results of air, water, soil and sediment found that residual oil was decomposing naturally.
"The data tell us that we have weathered crude in the environment that is readily biodegradable," EPA on-site coordinator Steve Merritt said. "The oil is weathering as we expected it would."
Pipeline owner Exxon Mobil sent more responders to the scene to clean oil-soaked debris left behind by raging floodwaters in the Yellowstone River. A report from The Wall Street Journal warned that the erosive power of rushing waters was leaving some pipelines under riverbeds exposed.
Silvertip had carried oil from tar sands in Canada since the 1980s but tar sands oil wasn't at the site of July spill.