Oil drilling in the East Poplar oil field in Montana is the cause of brine contamination of an 18-mile shallow aquifer, a new report released by the U.S. Geological Survey (USGS) has revealed.

Researchers said that disposal of brine water from oil operations for decades has affected the quality of both private drinking water wells and public water supply wells for the city of Poplar. According to USGS estimates, the amount of groundwater impacted by contamination ranges between 15 billion and 37 billion gallons, affecting supplies to about 3,000 people in the area.

The report claimed that possible sources of groundwater contamination included pipelines, storage tanks, production wells and brine disposal wells. USGS researchers noted that identifying a specific source of contamination was often not possible because of the presence of several features occurring at the same time and place.

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An earlier study by the USGS involving samples of water taken in 2009/2010 confirmed that water from the public water supply of the city of Poplar was contaminated with chemicals present in oil-field brine. This prompted the city to construct a pipeline in 2011 to carry treated water from the Missouri River to the city and communities nearby. The pipeline substituted the use of the shallow aquifers as a source of water, the USGS said.