WILLISTON, N.D. — Oil and gas practices that were common decades ago in the Williston Basin left behind a salty legacy that still poses problems for groundwater and is the focus of ongoing research by the U.S. Geological Survey.

A recent study from agency scientists estimates that it could take several hundred years before the salt concentration in groundwater near at least one longtime oil- and gas-producing area returns to normal levels. Geological Survey research also looks at the potential impact of brine contamination on amphibians.

The research is important, as the contamination can render water wells unusable, and global amphibian populations are in decline.

When oil and gas is extracted, brine also comes up to the earth’s surface and must be disposed of somewhere. Today, the saltwater is injected back underground for storage. But decades ago, it was dumped into temporary “reserve” pits at well sites or at central collection facilities known as “evaporation” pits.

“They would pump out the water at the end and leave behind a saline slurry and backfill it, burying a large amount of salt,” Geological Survey geologist Todd Preston said.

In the early days of oil production in North Dakota, pits were not lined, and salt sometimes migrated into the groundwater.

“We have distances up to 1 mile away from reserve pits where we see elevated concentrations of salt in the groundwater,” Preston said.

Building upon work started in the 1980s by the Montana Bureau of Mines and Geology, Preston and other researchers looked at data on the concentration of salt in the groundwater near Goose Lake in northeast Montana. Eleven reserve pits and one evaporation pit were installed nearby in the 1960s, leading to high levels of salt in the area’s groundwater. Over time, the concentrations have been diluted.

“They have dropped significantly but they are still highly elevated,” Preston said.

By first analyzing how the concentrations have changed over the past 30 years, the researchers then predicted how long it will take for the levels to drop to meet water quality targets, barring any remediation efforts.

Their research concludes that meeting the less-stringent “acute” toxicity benchmark set by the Environmental Protection Agency could be achieved as early as 2045. But for the groundwater to match the quality of a site that has not experienced brine contamination, it could take until the year 2275.

Preston cautions that those estimates do not apply to all places in the Williston Basin where groundwater has been contaminated by brine. He said differences in geology, precipitation and initial concentration levels will factor into the length of time it will take for contaminated groundwater at each site to reach a higher quality.

Other recently published Geological Survey research focuses on how brine from historic oil and gas development might affect amphibians.

“You have a contaminated plume of groundwater that is moving downstream in the aquifer through these wetlands,” research hydrologist Kelly Smalling said. “Because the wetlands are shallow, they have interaction with the groundwater.”

The research analyzed water quality, sediment and tadpoles at 32 wetland sites across Montana and North Dakota in the Williston Basin. The tadpoles include three native species: the leopard frog, the boreal chorus frog and the tiger salamander.

Past studies in the region show that metals including sodium, strontium and vanadium are present in brine. Smalling said such metals can pose risks to aquatic life.

“Surprisingly, we didn’t really find many direct links between the metals that we detected in the sediment and the amphibians and the brine contamination,” Smalling said. “Most of the metals that were detected in the sediment and tissue weren’t directly linked with energy development.”

She said it’s unclear where the other metals came from, but it’s possible some could be related to agriculture or found naturally in the environment.

More work is needed to get a better understanding of how metals and brine affect amphibians, she said. The animals face other stressors like precipitation and temperature changes. Their populations are dropping across the world amid threats such as habitat loss, disease and contamination, Smalling said.

“That’s really the key, to protect and preserve our amphibian population,” she said. "We know there’s multiple stressors out in the environment, and that relationship is what’s really important for future studies.”

Smalling said researchers also are looking into whether native amphibian species that have been exposed to brine contamination are adapting to their environment better than ones that are not native to the area.

“In some instances you’d be surprised,” Smalling said. “You wouldn’t expect to see egg pools and masses in these wetlands, but they are there, surviving and breeding.”