It has been a little more than a year now since one of the state’s largest oil spills in active water was discovered in the Ash Coulee Creek, a tributary of the Little Missouri River.

Cleanup at the spill site north of Medora, where regulators say that 12,615 barrels of oil leaked from a Belle Fourche pipeline into the creek, is still underway, although it has now progressed from the creek to the hillside, where the leak began in December 2016.

“There is still some oil in that hillside,” North Dakota Spill Investigations Director Bill Suess said. “They did some hydrogen peroxide injections for the hillside, but we are not sure yet how well it did.”

Wendy Owen, a spokesperson for True Companies which owns the Belle Fourche pipeline company, said the company believes it is in the monitoring stages for the Belle Fourche cleanup, and that they are nearing completion of that effort.

“We have one individual who goes out to observe every week to see if anything is going on,” Owen said. “And if we find things that are in the traps, then we will take care of removing it, but we are talking about measurements in the gallons now, not barrels.”

True Companies also owns the recently fined Black Hills Trucking and the Bridger Pipeline Company. Bridger was responsible for a 758-barrel crude oil leak into the Yellowstone River in Montana in 2015, threatening Glendive’s water supply for a time and necessitating bottled water for residents of the area.

The particular portion of Belle Fourche’s Bicentennial system that leaked will not be put back in service, Owen said, though the company is continuing to comply with PHMSA directives related to the Dec. 5, 2016, leak.

The leak was caused when the hillside slumped, breaking the line, regulators have said, and was ultimately discovered by a landowner in the area after leaking unabated for at least two, and possibly more, days, according to PHMSA records.

The system was a gathering line, which means quite a bit of variability in flows, and that can make detection of a leak more difficult, even with electronic monitoring. An imbalance was identified by the monitoring system, company officials testified to PHMSA, but the data were misinterpreted.

Suess said he believes the hillside likely had more oil than the company expected based on the samples it took to map where and how much oil was released into the slumping hillside.

This is not an uncommon problem, given all the variables involved. An oil spill, like water, travels the paths of least resistance, filling up pockets here and there. Samples taken as little as a foot apart in a grid can vary dramatically.

The tests being the best information available at the time, however, the company decided to go with a hydrogen peroxide treatment. This can break down oil in place, without as much soil disturbance, and would be ideal if conditions are right.

“Hydrogen peroxide works best when there’s not a lot of free product,” Suess said. “It chemically breaks down oil, but if there’s too much, it just overwhelms it.”

To get hydrogen peroxide into a spill area, a series of wells are drilled, some of which will direct the oxidation chemical into the spill site and some of which will direct air in and out, since that chemical reaction generally requires oxygen to work.

A series of containment measures are in place throughout this process to ensure that no spilled oil can be pushed back into the water system, Suess added.

“We’ve got three different methods of protection to keep water from moving past that contaminated area into the stream,” he said.

There was first an existing layer of clay through which oil cannot pass. A containment wall was pushed down into that clay layer. Secondly, in the creek, there are booms set up to ensure that when the spring thaw comes, nothing can flow past them.

Thirdly, there are underflow weirs that stop the surface water, Suess said.

“The deeper water has to flow underneath and then over another wall, and that keeps oil or any free product from moving,” he explained. “It can then be vacuumed out.”

Once the company drilled the necessary vertical wells, or traps, hydrogen peroxide was directed into some of them and air into others. However, there was enough oil during the process that it was being pushed up into the trap wells, Suess said.

“They were able to pump that out,” Suess said. “The remaining hydrogen peroxide should react to clean up some of the stuff, but we won’t know how well it worked until spring when we can do another round of sampling.”

If there was too much oil, as Suess suspects, then what the company accomplished was more of an expensive flushing — an accepted cleanup method, but usually done with a less expensive material, such as hot water.

Hydrogen peroxide is most effective for trace amounts of oil that would remain after the bulk of a spill has been removed.

With flushing, hot water is injected into a spill area to push all the spilled crude out to a spot where it can be readily vacuumed up and removed.

“Hot water is a lot cheaper than hydrogen peroxide for that, though,” Suess said.

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