State environmental regulators have asked Exxon Mobil to justify its estimate for how much oil spilled into the Yellowstone River, citing the company's changing timeline on how long it took to stop a leaking pipeline.
The Texas-based company has said between 31,500 and 42,000 gallons of crude flowed into the Yellowstone following the leak near Laurel on July 1. That oil has fouled shoreline and contaminated backwaters along dozens of miles of the scenic river.
Exxon Mobil Pipeline president Gary Pruessing initially said it took six minutes to shut down the pumps on the Silvertip pipeline. But information submitted by the company to federal pipeline safety regulators later revealed it took almost an hour to fully stop the flow.
In a letter to Exxon executives, Montana Department of Environmental Quality Director Richard Opper asked for an explanation of why the spill volume was not changed given the longer timeline.
"Since the event occurred, ExxonMobil has increased its estimate of the duration of the spill event 10-fold from its original assertion," Opper wrote. "Despite this revision as to the duration of the event, ExxonMobil has not revised its estimate as to the volume of the spill into the river."
Exxon Mobil representatives said last week the spill estimate was based on the correct timeline. Spokeswoman Cynthia Bergman said Tuesday the company believes its estimate is accurate.
The estimate included oil lost after the pumps were shut down but before a series of valves were closed to fully seal off the section of failed pipeline, she said.
Opper also requested information from the company concerning the pipeline's pressure and flow rate beginning 48 hours before the spill through Monday. Exxon Mobil was asked to provide an answer by next Monday.
It's still unknown why the pipeline failed. Speculation has centered on high waters that potentially scoured out the river bed and exposed the buried pipe to damaging rocks and debris.
When water conditions allow, Exxon Mobil plans to use a side-scanning sonar to investigate the damaged section of pipeline, said Steve Merritt, the Environmental Protection Agency's on-scene coordinator. He said one question that needs to be answered is whether any oil remains trapped in the line.
Teams of trained spotters have been cruising sections of the river in boats over the past few days after water levels finally began to recede. Merritt said those teams have found moderate to light oil "staining" on vegetation.
State DEQ Deputy Director Tom Livers said a new area with oil was found near Hysham, about 90 miles from the pipeline failure. Officials and company representatives have said most damage from the spill appears to be concentrated within the first 30 miles downstream of the line.
Because much of the oil was swept away by the river, only 1 to 5 percent of the total amount spilled is expected to be recovered.
As of Tuesday, 942 barrels of oily water had been collected by crews. If the water were removed, that would equal roughly 9 barrels of crude oil, or less than 400 gallons, Merritt said
In some cases, Merritt said small quantities of oil could be left in place if removal would result in more damage.
Contractors also began collecting soil and sediment samples to determine in part if any of the oil has gotten mixed into the riverbed. They will be trying to determine potential damage to low-lying croplands smeared with oil.
No health problems from the spill have been reported in recent days, EPA and local health officials said.
In the first few days after the pipeline failed, some downstream residents suffered from acute exposure to the dangerous fumes given off by the oil. Those symptoms were expected to ease as the oil degraded, lessening its hazardous potential.
At the state Capitol building in Helena, about 70 protesters occupied the governor's office Tuesday, angry over the spill and plans to build another pipeline in the state that would carry oil from the tar sands in Canada.
The group banged drums and chanted such slogans as "the Yellowstone was the first. If Exxon stays it will get worse."
Gov. Brian Schweitzer told the group he would not cede to their demands over Keystone XL. They then played a piano and danced on a table in the governor's reception room in apparent frustration with the direction of the meeting.
Schweitzer left the room, and security personnel watched the group as work in the governor's office continued.
Matt Gouras contributed to this story from Helena.