Underground petroleum leaks can trigger arsenic spikes in groundwater, which could be a problem as the United States confronts a rising trend in pipeline-related accidents, such as the 39,000-gallon discharge into the Yellowstone River two weeks ago.
The arsenic release is more of a long-term problem compared to the immediate hazard of benzene and other toxins that prevented 6,000 residents of Glendive from drinking tap water for five days.
But U.S. Geological Survey researcher Barbara Bekins said it illuminates the difficulty of dealing with oil spills.
“It showed up as an unintended consequence of the cleanup of other things,” Bekins said from her office in Menlo Park, California. “Where you’re trying to clean solvents, dry cleaning fluids or petroleum, what’s often done is you add organic carbon to the ground. That turns the groundwater anaerobic, which can mobilize arsenic. Regulators need to think about when should people worry about arsenic mobilization - how far does it travel?”
New USGS research released last week found that when bacteria break down petroleum underground, the chemical process can release naturally occurring arsenic.
That toxic heavy metal then can dissolve into underground aquifers.
A study in Bemidji, Minnesota, found such petroleum plumes produced arsenic concentrations 23 times higher than federal drinking water standards advise. The mobile arsenic can flow with the underground water to new locations away from the original spill site.
Arsenic mobilization requires anaerobic, or oxygen-free, conditions.
The Poplar leak on Jan. 17 affected Yellowstone River surface water, which should not support the bacteria that mobilize arsenic. However, state Department of Environmental Quality workers are now testing private underground wells around Glendive for petroleum infiltration.
The Missoula Valley depends on a large underground aquifer for its drinking water. While it has had frequent problems with high arsenic levels, those have stemmed more from old mine waste, such as the former Milltown Reservoir or industrial deposits like the defunct paper mill at Frenchtown.
“We would need a very large (petroleum) spill for that to be a problem here, and other things would be of concern first,” said Missoula City-County Environmental Health Supervisor Peter Nielsen.
An Associated Press review of U.S. Department of Transportation records found that pipeline accidents involving crude oil have grown 87 percent between 2009 and 2014.
At least 73 incidents occurred last year, including the Bridger Pipeline LLC failure at Poplar. Almost half the incidents in the past five years involved pipelines installed more than 40 years ago.
The Poplar line was last inspected in 2011, as part of a safety review triggered by the blowout of an Exxon Mobil pipeline beneath the Yellowstone near Billings. That same year, Missoula County officials became concerned about another pipeline getting exposed in the Clark Fork River near Turah. That line was re-buried without leakage.
The Bridger Pipeline delivered about 42,000 barrels of oil a day. It runs near where the proposed Keystone XL pipeline would carry more than 800,000 barrels a day – mostly heavier-than-water Canadian bitumen from the tar sand deposits in northern Alberta.
Congress approved the Keystone XL pipeline construction on Thursday, but President Barack Obama has threatened a veto of the bill.