Authorities were scrambling to decontaminate a water treatment plant serving 6,000 eastern Montana residents after a cancer-causing component of oil was found downstream of a Yellowstone River pipeline spill.
Up to 50,000 gallons of crude were released in Saturday's spill.
As residents of Glendive lined up at a distribution center to receive bottled water, officials took initial steps Tuesday to decontaminate the city's water treatment plant by adding more activated carbon - a type of charcoal.
If that approach does not work, officials planned to add equipment to the plant that would pre-treat water coming into the facility from an intake beneath the river.
Elevated levels of benzene were found Monday in water samples from the treatment plant serving the agricultural community near the North Dakota border.
Montana Department of Environmental Quality Director Tom Livers said officials hoped to flush out any remaining contamination from the system and restore it to operation by Thursday.
Scientists from the federal Centers for Disease Control and Prevention said the benzene levels were above those recommended for long-term consumption, but did not pose a short-term health hazard. Residents were warned not to drink or cook with water from their taps.
Some criticized the timing of Monday's advisory, which came more than two days after the spilled from the 12-inch Poplar pipeline owned by Wyoming-based Bridger Pipeline Co. The spill occurred about 5 miles upstream from the city.
Adding to the frustrations was uncertainty over how long the water warning would last. Also, company and government officials have struggled to come up with an effective way to recover the crude, most of which appears to be trapped beneath the ice-covered Yellowstone River.
A mechanical inspection of the damaged line on Tuesday revealed the breach occurred directly beneath the river, about 50 feet from the south shore, Bridger Pipeline spokesman Bill Salvin said.
The cause remained undetermined.
By Tuesday, oil sheens were reported as far away as Williston, North Dakota, below the Yellowstone's confluence with the Missouri River, officials said.
"It's scary," said 79-year-old Mickey Martini of Glendive. "I don't know how they're going to take care of this."
Martini said she first noticed a smell similar to diesel fuel coming from her tap water Monday night. Officials previously didn't know whether the spill happened beneath the iced-over river or somewhere on the riverbank.
Martini said she was unable to take her daily medicines for a thyroid condition and high cholesterol until she picked up water from a public distribution center later in the day.
Representatives from the state and the U.S. Environmental Protection Agency earlier said preliminary monitoring of the city's water showed no cause for concern. The water treatment plant operated until Sunday afternoon, more than 24 hours after pipeline operator Bridger Pipeline discovered the spill, officials said.
Additional tests were conducted early Monday after residents began complaining of the petroleum- or diesel-like smell from their tap water. That's when the high benzene levels were found.
Benzene in the range of 10 to 15 parts per billion was detected from the city's water, said Paul Peronard with the EPA. Anything above 5 parts per billion is considered a long-term risk, he said.
Peronard acknowledged problems in how officials addressed the city's water supply, including not having the right testing equipment on hand right away to pick up contamination. But Peronard and others involved in the spill response said officials acted based on the best information available.
"Emergencies don't work in a streamlined fashion," said Bob Habeck with the Montana Department of Environmental Quality. "It's a process of discovery and response."
Several residents interviewed by The Associated Press said they first heard about the water problems through friends and social media sites, not the official advisory.
"They could have been more on top of it," Whitney Schipman said as she picked up several cases of bottled water for her extended family from a water distribution center. "As soon as there was a spill, they should have told everybody."