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Hydraulic fracturing, or fracking, has long been tied to environmental risks such as spills. The frequency of spills, however, has long been murky since states do not release standardized data.

Estimates from the U.S. Environment Protection Agency (EPA) vary wildly.

"The number of spills nationally could range from approximately 100 to 3,700 spills annually, assuming 25,000 to 30,000 new wells are fractured per year," the agency said in a June 2015 report. Also, the EPA reported only 457 spills related to fracking in 11 states between 2006 and 2012.

But now, a new study suggests that fracking-related spills occur at a much higher rate.

The analysis, published Feb. 21 in the journal Environmental Science & Technology, revealed 6,648 spills in four states alone—Colorado, New Mexico, North Dakota and Pennsylvania—in 10 years.

The researchers determined that up to 16 percent of fracked oil and gas wells spill hydrocarbons, chemically laden water, fracking fluids and other substances.

For the study, the researchers examined state-level spill data to characterize spills associated with unconventional oil and gas development at 31,481 fracked wells in the four states between 2005 and 2014.

"On average, that's equivalent to 55 spills per 1,000 wells in any given year," lead author Lauren Patterson, a policy associate at Duke University's Nicholas Institute for Environmental Policy Solutions, told ResearchGate.

North Dakota reported the highest spill rate, with 4,453 incidents. Pennsylvania reported 1,293, Colorado reported 476 and New Mexico reported 426. The researchers created an interactive map of spill sites in the four states.

Although North Dakota is rich in oil, the state's higher spill rate can be explained by varying state reporting requirements. North Dakota is required to report any spill larger than 42 gallons whereas requirement in Colorado and New Mexico is 210 gallons.

Patterson points out that the different reporting requirements are a problem.

"Our study concludes that making state spill data more uniform and accessible could provide stakeholders with important information on where to target efforts for locating and preventing future spills," she told ResearchGate. "States would benefit from setting reporting requirements that generate actionable information—that is, information regulators and industry can use to identify and respond to risk 'hot spots.' It would also be beneficial to standardize how spills are reported. This would improve accuracy and make the data usable to understand spill risks."

The reason why the researchers' numbers vastly exceeded the 457 spills estimated by the EPA is because the agency only accounted for spills during the hydraulic fracturing stage itself, rather than the entire process of unconventional oil and gas production.

"Understanding spills at all stages of well development is important because preparing for hydraulic fracturing requires the transport of more materials to and from well sites and storage of these materials on site," Patterson explained. "Investigating all stages helps to shed further light on the spills that can occur at all types of wells—not just unconventional ones."

For instance, the researchers found that 50 percent of spills were related to storage and moving fluids via pipelines.

"The causes are quite varied," Patterson told BBC. "Equipment failure was the greatest factor, the loading and unloading of trucks with material had a lot more human error than other places."

For the four states studied, most spills occurred in the the first three years of a well's life, when drilling and hydraulic fracturing occurred and production volumes were highest.

Additionally, a significant portion of spills (26 percent in Colorado, 53 percent in North Dakota) occurred at wells with more than one spill, suggesting that wells where spills have already occurred merit closer attention.

"Analyses like this one are so important, to define and mitigate risk to water supplies and human health," said Kate Konschnik, director of the Harvard Law School's Environmental Policy Initiative in a statement. "Writing state reporting rules with these factors in mind is critical, to ensure that the right data are available—and in an accessible format—for industry, states and the research community."

Photo credit: Mark Schmerling / FracTracker Alliance

Earthquakes in Pennsylvania are usually rare but fracking operations triggered a series of small temblors in Lawrence County last year, officials at the state's Department of Environmental Protection (DEP) announced in a Feb. 17 report.

Hilcorp Energy Co., a Texas-based oil and gas company, was fracking a pair of wells in the Utica Shale when seismic monitors detected five earthquakes measuring between 1.8 and 2.3 on the Richter scale between April 25-26, 2016.

"Our analysis after doing the review... is that these events are correlated with the activity of the operator," DEP Acting Secretary Patrick McDonnell told Penn Live.

While the tremors were too small to be felt by humans or cause any damage, they are the first quakes in the state to be blamed on fracking. Pennsylvania happens to be the second largest natural gas-producing state in the country.

"At least within Pennsylvania, this is the first time that we have seen that sort of spatial and temporal correlation with [oil and gas] operator activity," Seth Pelepko, chief of well-plugging and subsurface activities for DEP's oil and gas management program, told Allegheny Front, a western Pennsylvania public radio program.

"No faults identified along portions of the well bore where these seismic events were detected," Pelepko continued.

Hilcorp spokesman Justin Furnace said operations were immediately suspended after learning about the tremors. Fracking and stimulation operations have since been discontinued at the well pad indefinitely.

The DEP said that Hilcorp was using a technique known as "zipper fracturing" at the time, which involves the concurrent fracking of two horizontal wellbores that are parallel and adjacent to each other.

So how did the earthquakes happen? As Penn Live explains:

Four wells were drilled to depth of about 7,900 feet in that location.

Evidence indicates that induced earthquakes occur when the separation between Utica Shale and basement rocks is lessened during drilling operations. That means, when someone drills too close to basement rocks, there can be earthquakes.

Pelepko said that seems to have been the case in Lawrence County, where the basement rock is shallow compared to other areas in the state.

The distance between Utica Shale and basement rocks were between 2,500 to 3,000 feet at the fracking site.

The DEP has since given a number of recommendations to Hilcorp, including the discontinuation of zipper fracturing near gas wells in North Beaver, Union and Mahoning Townships where the earthquakes occurred. Additionally, the company must shut down operations and notify the DEP should any earthquake larger than 2.0 or three successive quakes between 1.5 and 1.9 in magnitude occur within a three-mile distance of a wellbore path.

Earthquakes caused by fracking a well are uncommon. However, the notorious spate of earthquakes in Oklahoma, which were caused by the disposal of large quantities of fracking wastewater into underground wells, are rampant. The disposal of wastewater produced from fracking, has led to the alarming increase of earthquakes with magnitude-3 or larger by nearly 300 times, or 30,000 percent in north-central Oklahoma alone. In 2014, more than 5,000 earthquakes were reported.

But a 4.8-magnitude frack-quake that struck Alberta, Canada in Jan. 2016 set a world record for the largest earthquake triggered by the controversial drilling process.

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