About 1,000 barrels of oil leaked July 1 into the Yellowstone River near Billings, Mont., from the 12-inch Silvertip pipeline, figures released by Exxon Mobil indicate.
The EPA said it was sampling soil and sediment near the site of the spill and responders were working to remove oil residue as floodwaters along the Yellowstone River recede.
"Soil and sediment samples are used to determine locations where oil was transported and what compounds are present in the environment," the EPA said in a statement. "This, in turn, will greatly assist with ongoing assessments and cleanup efforts."
Montana Gov. Brian Schweitzer, a former rancher who holds a master's degree in soil science, sent a letter to Exxon last week requesting information about the oil in the Silvertip pipeline.
The Montana Department of Environmental Quality in a letter to Exxon said it wanted additional information about pipeline flow and pressure. The department notes that Exxon revised the duration of the spill from 6 minutes to about an hour yet hasn't revised its estimated spill volume.
Exxon in its latest update reported only that it had started work for the replacement of the pipeline.
Exxon Mobil Pipeline Co. President Gary Pruessing was to appear before a House of Representatives subcommittee on railroads, pipelines and hazardous materials to address questions related to the Silvertip pipeline.