A study led by researchers at the University of Colorado (UC) found that of the hundreds of compounds used in hydraulic fracturing (fracking) operations, 15 are of particular concern if they ever become groundwater contaminants due to several factors, including toxicity.
In a seven-page paper published online in Environmental Science & Technology Letters, which is published by the American Chemical Society, researchers tested 659 of the 996 compounds that the Environmental Protection Agency (EPA) and the website registry FracFocus have listed as compounds for use in fracking.
Under a fast groundwater transport scenario, the researchers found that 15 compounds listed on at least 50 FracFocus reports had an elevated exposure potential -- defined as 10% or more -- of their initial concentrations remaining after a transport distance of 94 meters (308.4 feet), a distance roughly the equivalent to the average setback for oil and gas wells in the United States.
The researchers tested more than 50,000 wells in four states -- Colorado, North Dakota, Pennsylvania and Texas -- since 2011.
"We wanted to evaluate which compounds we should be paying attention to based not just on toxicity but also on the extent a particular compound travels from one place to another through groundwater," said CU Professor Joseph Ryan, the lead researcher. "We found 41 compounds that were considered mobile and persistent, and a handful of those we would not want in our drinking water."
Of the 15 compounds, two -- naphthalene and 2-butoxyethanol -- were identified on more than 20% of FracFocus reports. "Both compounds are commonly identified as ingredients in surfactants, corrosion inhibitors, and non-emulsifiers, and naphthalene is a constituent of some petroleum-based additives," the study said.
The study also found three of the 15 compounds -- acrylamide, and the petroleum hydrocarbons ethylbenzene and xylenes -- have Maximum Contaminant Level (MCLs) standards set by the EPA.
"The occurrence of these compounds at concentrations above their MCLs would necessitate regulatory action," the study said. "Acrylamide was identified on 3.2% of FracFocus reports as a residual ingredient in non-hazardous acrylamide polymers commonly used in fracturing fluids as friction reducers.
"The petroleum hydrocarbons were rarely identified as individual additives on FracFocus...but they are minor constituents of some petroleum-based additives like hydrotreated light petroleum distillates...identified as an ingredient in friction reducers on 70% of reports. Benzene, also a minor constituent of petroleum distillates, was predicted to be sufficiently mobile and persistent by the screening framework but was individually identified on [less than] 50 reports."
Ryan said that although the study may soothe concern that hundreds of chemicals used in fracking are at risk of contaminating groundwater, "it also demonstrates a subset of these compounds could result in potentially hazardous exposures following spills or well failures."
According to Ryan, the researchers used a screening process that included degradation rates of particular organic compounds as they are horizontally transported from a point of release. They also considered the chemical structure and stability of each compound, its behavior in water, and employed a wastewater treatment process simulator called BIOWIN 4 to help estimate the biodegradation of the organic compounds over time.
Ryan said the researchers used two transport scenarios: a highly porous aquifer with relatively fast groundwater velocity, and a less porous aquifer with slower velocity. The compounds were evaluated for potential toxicity using the National Primary Drinking Water Regulations and Health Assessment Information.
"Just because a compound has been put into the ground doesn't mean it is going to migrate to a place where humans will be exposed to it," Ryan said. "On the other hand, problems with particular compounds in groundwater could take a decade or so to even show up. We don't know enough about some of these processes right now."
CU said the researchers may next evaluate chemical products produced by the breakdown of fracking fluids, or consider the return of known fracking compounds to the surface. The research is being funded through a $12 million grant by the National Science Foundation (NSF), and is supported by the NSF's Sustainability Research Network program and the EPA's STAR Fellowship program.