(AP) -- Montana 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 estimates between 31,500 and 42,000 gallons of crude flowed into the Yellowstone near Laurel on July 1, fouling the shoreline and 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, 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 Mobil 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 Cynthis Bergman said Tuesday the company believes its prior estimate was correct.
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.
Because much of the oil was swept away by the river, only 1 to 5 percent of the oil is expected to be recovered.
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