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December 10, 2020

A new USGS study reports that shale-oil and -gas production from a major production area in Montana, North Dakota, and South Dakota has not caused widespread hydrocarbon contamination to date in nearby aquifer zones used for drinking-water supply.

The study used chemical tracers to determine if unconventional oil and gas production from the Three Forks-Bakken Petroleum System, a series of deep geologic formations, is contaminating groundwater used for water supply. Samples of untreated groundwater were collected from 30 drinking-water wells that tap nearby freshwater aquifers and analyzed for a broad suite of chemical and isotopic tracers.

There was some evidence for minor groundwater contamination associated with oil and gas production, possibly from underground or from land-surface activities. One of the 30 groundwater samples contained trace amounts of two volatile organic compounds—butane and n-pentane—likely associated with a nearby oil well. Another sample contained elevated concentrations of chloride potentially from oil and gas activities at the land surface, such as unlined disposal ponds and ruptured pipelines. Although some other samples contained methane and elevated chloride, chemical analysis indicated that the Three Forks-Bakken Petroleum System was not the source.

The age of the groundwater in the samples revealed that in most cases the groundwater recharged the aquifer thousands of years ago. Such old ages reflect very slow groundwater movement, which in turn could slow the widespread dispersal of chemicals if they reach the groundwater.

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