WILLISTON, N.D. (AP) — 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 by the Geological Survey shows 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.
Brine makes its way up to the earth's surface when oil and gas is extracted. Today, that saltwater is injected back underground for storage. Decades ago, brine 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.
Brine contamination can make water wells unusable as global amphibian populations are in decline, the Bismarck Tribune reported.
The Geological Survey 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.
After researchers first analyzed how the concentrations have changed over the past 30 years, they predicted how long it will take for the levels to drop to meet water quality targets, barring any remediation efforts.
Their research concluded that meeting the less-stringent "acute" toxicity benchmark set by the Environmental Protection Agency could be achieved by 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 cautioned that those estimates don't 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.
Information from: Bismarck Tribune, http://www.bismarcktribune.com