More than 30 states drill for oil and gas via hydraulic fracturing, or fracking.
By drilling horizontally and shooting huge amounts of water into wells, companies can fracture the rock deep underground to unlock gas reserves that were previously unreachable with conventional, vertical drilling.
There are about 1.1 million active oil and gas wells in the US, according to the FracTracker Alliance.
And a small subset of these wells pose the risk of contaminating the drinking water around them.
Rob Jackson, an environmental expert from Stanford University, presented some of his research on drinking water contamination at the American Association for the Advancement of Sciences conference on February 14.
"Does hydraulic fracturing allow gas to reach drinking water?" he said at the meeting. "Yes, but not usually."
Contamination, namely methane, can reach drinking water through natural deposits in the surrounding rock, through cracks in the rock, and from abandoned oil or gas wells, Jackson said.
But the most worrisome way it can reach the water is through poorly cemented or completely un-cemented fracking wells, he said, where gas can move up to the water table "like a chimney."
Fracking wells are typically one to two miles underground, and the top 500 to 1,000 feet is what companies really have to worry about encasing in order to protect the ground water, Jackson said.
When companies drill a well, they insert a steel casing once they drill past the groundwater, according to a video from Marathon Oil Corporation. Then, they fill concrete in between the casing and the hole.
Sometimes, depending on the geology of where they're drilling and how deep the well is, the video said, companies may add a second casing to protect the water source.
Marathon Oil says in the video that they line the entire well, from the surface to the bottom, with casing.
But this isn't the case for all companies, a report from the Environmental Protection Agency found.
The EPA analyzed a statistically representative sample of wells throughout the US, and found that while 29% were fully cemented from top to bottom, 66% had one or more un-cemented regions.
The EPA also found that 3% of wells had un-cemented regions within the depth where well operators reported there was ground water — putting them at high risk of contaminating drinking water.
"This is a recipe for trouble," Jackson said at the AAAS meeting. "This is not what all companies do, but it's what some companies do."
Jackson has studied drinking water contamination, particularly at the Marcellus Shale in northeastern Pennsylvania, and has developed ways to identify whether it came from fracking or not.
Drinking water monitoring has to occur before, during, and after fracking, he said, in order to tell whether it was because of drilling operations or not.
Both Jackson's research and another EPA report on fracking safety released this past June have found that some drinking water has been contaminated, but that this amount is slight when compared with the overall number of fracked wells.
Jackson said these troubling cases haven't made him an anti-fracker. If companies simply line their entire wells with casing and concrete, and test the surrounding water, it could greatly reduce the risk of contaminating drinking water.
"I think hydraulic fracturing can be done safely," he said at the AAAS meeting. "It's been done in the entire state of Arkansas, and we've never found contamination."
