Dangerously high concentrations of air pollutants are threatening an unexpected place—rural Utah. During wintertime periods when air in the atmosphere tends to stagnate, a research team has found that volatile organic compounds (VOCs) released from oil and gas wells in Utah’s Uintah Basin reach levels exceeding those in U.S. cities (Environ. Sci. Technol. 2014, DOI: 10.1021/es405046r). The pollutants include benzene, a carcinogen, and compounds that are precursors of ozone, which can cause respiratory problems.
Ozone is a component of smog that’s produced when VOCs, such as those in vehicle exhaust, react with nitrogen oxide compounds in the presence of light. As a result, high ozone levels are typically found in cities. But several years ago, researchers began observing high concentrations of ozone in the Uintah Basin. Wintertime ozone levels in ambient air in the basin often exceed Environmental Protection Agency air quality standards.
The Uintah Basin is one of the largest oil- and gas-producing regions in the U.S., with more than 10,000 wells in operation. Detlev Helmig, an atmospheric scientist at the University of Colorado, Boulder, and his colleagues, including researchers at the National Oceanic & Atmospheric Administration, suspected that VOCs leaking from these oil and gas wells might be causing the ozone pollution. So, over two winter months in 2012 and 2013, they used gas chromatography to measure VOC concentrations in ambient air at a site on the northern edge of the basin’s most extensive gas field. They also collected air samples at a range of altitudes in the atmosphere using tethered balloons and analyzed the samples in the lab.
They measured methane and a suite of hydrocarbon VOCs that are released by oil and gas production. These include light alkanes, such as ethane and propane, and the aromatic VOCs benzene and toluene, which are directly toxic to humans. The team also measured ozone using an ultraviolet absorption monitor.
During several, multiple-day periods with substantial snow on the ground, they found that most of the light alkane VOCs built up to levels that are 10 to 100 times as great as concentrations in major U.S. cities. Helmig notes that snow cover drives this buildup: It prevents the ground from heating up, which slows surface air from mixing with colder, clean air from higher in the atmosphere. As a result, a layer of air about 50 to 100 m deep stagnates at the surface, accumulating pollutants. These periods coincided with ozone levels that exceeded EPA air quality standards.
The researchers estimated that total annual VOC emissions at the site are equivalent to those of about 100 million cars.
Throughout the study period, the researchers found that benzene levels frequently exceeded 1.4 ppb by volume, a health-based benchmark for chronic benzene exposure. However, because benzene is a carcinogen, the EPA does not define any safe threshold for it, says Lisa M. McKenzie, a public health researcher at the University of Colorado, Denver.
“This is quite amazing,” says Bernhard Rappenglück, an atmospheric scientist at the University of Houston. He notes that despite the basin’s sparse population, these conditions raise health concerns for people working at the wells and living in the area.
Other oil- and gas-producing regions are experiencing similar problems, Helmig says. High ozone levels have also been observed in Wyoming, and recent studies by NOAA have linked oil and gas emissions to summertime ozone production in the Colorado Front Range. Last month, Colorado tightened oil and gas development regulations to require operators to fix leaks and capture 95% of their hydrocarbon emissions, including VOCs.