The scope of groundwater contamination on the Fort Peck Reservation caused by oil exploration that began in the 1950s has been estimated at 15 to 37 billion gallons spread across 17.9 square miles, according to a recent study.

“It ended up being larger than we imagined,” said Joanna Thamke, a U.S. Geological Survey hydrologist based in Helena, who was the lead author of the study.

Thamke has been studying the aquifer since 1989 and the USGS has been working with the tribes since the 1970s.

It was back then that an area farmer discovered water on his cropland, even though it hadn’t rained. The water stunted the crop’s growth.

The USGS got involved, drilling test wells and analyzing the groundwater, and discovered that the aquifer was being contaminated by brine.

“Brine is extremely saline water,” Thamke said. “So this is almost 10 times saltier than the ocean. It’s hyper saline.”

Salty water

A 1997 USGS study tied the groundwater’s contamination as well as salt water entering the Poplar River to development of the East Poplar Oil Field in the southeastern quadrant of the reservation. The still-active field contains more than 130 wells that produce 66,000 barrels of oil a year.

The 1997 USGS study also identified benzene, a common component of crude oil, in the groundwater. Long-term exposure to benzene can cause cancer and other health problems.

The groundwater is also contaminated by nitrates, which a 1990 study found could be from a combination of sources, including surface runoff into wells, from livestock near wells and even decomposition of wheat stubble fields.

Brine is a common byproduct of oil and gas exploration. Today, the only approved method of brine disposal is to inject it deep into the ground. But in the 1950s and until more stringent standards were adopted, oil producers often dumped the salty water into unlined settling ponds to evaporate where some also penetrated into the groundwater. The brine could also be entering the shallow aquifer from a perforated oil well casing, storage tanks, or brine-injection wells, Thamke said.

“These waters are from the same formation as where they are removing the oil,” Thamke said, although it’s difficult to pinpoint the exact source.

Moving brine

The brine plume has seeped southwest to the Fort Peck Reservation town of Poplar, the Poplar River and the Missouri River, along the way contaminating 100 wells that serve about 3,000 people.

Fortunately, by the time the water mixes with the much larger Missouri River the salinity drops to a level that is not harmful to the aquatic environment, Thamke said.

The groundwater contamination prompted the community of Poplar to seek a new source of water for its 2,900 residents, as well as the rest of the reservation. The portion of the Fort Peck-Dry Prairie Regional Water System that serves Poplar was finished in 2012. The projected cost for the entire project, which is meant to also serve drinking water to residents, livestock and businesses in Valley, Daniels, Sheridan and Roosevelt counties outside the reservation, was estimated at $193 million in 1998. The system will require 3,200 miles of pipeline and 20 pump stations. Water for the treatment plant, located southeast of Wolf Point, is pulled from the Missouri River.

Whether the brine plume will ever disappear from the aquifer is unknown.

“All of the sources of contamination are no longer contributing, but there’s no way to prove that,” Thamke said.

One source of contamination, the Biere well, was identified in 1999 and is being voluntarily cleaned up by Pioneer Natural Resources, a Texas oil company that inherited the problem when it purchased Mesa Petroleum.

Pioneer has plugged a leaking well casing from an abandoned well and pumped surrounding groundwater out of the aquifer for reinjection more than 7,000 feet underground to resolve the problem.

Sizing up the plume

As the USGS attempted to identify the extent of the brine plume, it became clear that it would take more than just drilling test wells. In the 1990s, the USGS decided that an aerial survey with electromagnetic equipment dangled from a helicopter would help outline the extent of the contamination in the shallow aquifers. The most recent USGS report does just that, Thamke said.

“There are many brine sources in the study area, resulting in multiple plumes,” she said. “Some plumes remain in the groundwater for decades and have merged together, making it difficult to identify original sources.”

Out of the area mapped, the study found 1.7 square miles of the aquifer were “considerably contaminated” based on water wells and monitoring wells. The aerial survey identified an additional 5.8 square miles of possible contamination and 2.1 square miles that could also be considerably contaminated.

Surveys like the one completed by the USGS could help oil producers avoid groundwater brine when drilling future wells. For example, in northeastern Montana’s Sheridan County, which contains prairie potholes that are known as “duck factories” for their importance to waterfowl reproduction, the USGS has identified saline formations to encourage producers to avoid them and protect surrounding wetland habitat.

The East Poplar Oil Field is on the western flank of the Bakken formation that is the source of the huge oil boom mostly in western North Dakota.

Drillers there must minimize the effects of salty water that is a byproduct of the oil production.

“It’s just really important to know how saline the water is and to treat it appropriately,” Thamke said. “That contamination could occur in other areas of the Williston Basin.

“Fortunately, I think most land and resource managers are aware of this, and people are making the best choices they can,” she added.

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