Underground petroleum leaks can trigger "potentially significant and overlooked arsenic groundwater contamination," says new research from the U.S. Geological Survey, adding a layer of complexity to dealing with pipeline spills like the one that occurred just last month near the Yellowstone River in Montana.

Research highlights "the need for long–term monitoring of arsenic and other chemicals that pose a water quality concern in areas associated with petroleum leaks and spills."

The long-term field study conducted by USGS and Virginia Tech scientists found that changes in geochemistry from the natural breakdown of petroleum hydrocarbons underground can prompt the release of naturally occurring arsenic, a toxic heavy metal, into groundwater. The mobile arsenic can flow with the underground water to new locations away from the original spill site.

The supporting data was collected over 32 years by a group of researchers at the site of a 1979 petroleum spill in a shallow, glacial aquifer near Bemidji, Minnesota.

"While arsenic is naturally present in most soils and sediments at various concentrations, it is not commonly a health concern until it is mobilized by a chemical reaction and dissolves into groundwater," reads a USGS press release. "Elevated arsenic levels in groundwater used for drinking water is a significant public health concern since arsenic, a toxin and carcinogen, is linked to numerous forms of skin, bladder, and lung cancer."

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The research article, published in the National Ground Water Association's bimonthly journal, highlights "the need for long–term monitoring of arsenic and other chemicals that pose a water quality concern in areas associated with petroleum hydrocarbon leaks and spills."

In November 2014, the Center for Biological Diversity revealed that in the 16 months prior to their analysis, there had been 372 oil and gas pipeline leaks, spills, and other incidents in the U.S. The CBD study, based on decades of records from the federal Pipeline and Hazardous Materials Safety Administration, showed that on average one significant pipeline incident occurs in the country every 30 hours.

"There’s no way to get around the fact that oil and gas pipelines are dangerous and have exacted a devastating toll on people and wildlife," CBD senior counsel Bill Snape said at the time, citing those statistics as a reason to oppose the Keystone XL pipeline. "The Obama administration’s own analysis says Keystone XL will spill oil, so it’s really troubling to see politicians wanting to add to this dangerous legacy of failed pipelines."

The U.S. Senate approved the Keystone XL pipeline last week; President Barack Obama has threatened to veto the bill.