Air Quality Trends in Texas and Colorado Associated with Unconventional Oil and Gas Development.

introductionThe shale oil and gas (O&G)

methodsAfter additional data quality assurance, we applied two types of trend analyses to the data: (1) a local, stepwise least-squares fitting method known as cubic splines, and (2) a fitting algorithm developed by the National Oceanic and Atmospheric Administration to analyze its greenhouse gas time series, which decomposes the data into a set of periodic and polynomial functions. Both routines allow user-defined settings and provide a long-term (multiyear) nonlinear trend function with uncertainty estimates as output. Permian Basin formaldehyde trends, which were temporally and spatially averaged, were also estimated from Ozone Monitoring Instrument Level 3 satellite data. The trends were compared with auxiliary data at both the regional (shale basin) and local (county) levels to assess whether changes in production could be driving emissions. Furthermore, derived trends were scrutinized in light of federal and state regulations passed to limit O&G development-related air pollutant emissions.

resultsObserved long-term trends varied significantly among the Texas sites. Barnett Shale AQM station data showed mostly flat to slightly decreasing long-term trends for the O&G emissions tracers ethane and propane, consistent with a gas production peak in 2011-2012, followed by an extended decline of 10%/yr. Air quality downwind of the Haynesville Shale was affected most during the initial boom years from 2006 to 2011 and then improved remarkably. Eagle Ford Shale AQM station data also correlated with O&G production data, showing that the highest ambient ethane and propane levels occurred during the peak production years of 2014-2015 and 2019, whereas the lowest levels were encountered during the lull in production in 2016-2017 and after 2020. Lastly, Permian-Midland Basin data showed a steady increase in ambient hydrocarbon levels throughout the production growth phase since 2016, also detectable via the satellite data-derived formaldehyde proxy, a trend that has been leveling in recent years. In both the Eagle Ford and Permian, casinghead gas production, related to gas flaring, correlated best with ambient ethane data. Similar to Texas, the Northern Colorado Front Range showed significantly higher ambient hydrocarbon levels, including methane, in air advected from the Denver-Julesburg Basin (DJ Basin) O&G region compared with the background air. However, Colorado ambient air quality data did not closely follow O&G production volume trends, but instead showed declining concentration trends. Atmospheric methane has continued to rise in step with global background levels, but at a slower rate. Over time, the difference between concentrations in polluted and clean air has narrowed, indicating that regional methane emissions are declining. For hydrocarbons, sites located closer to the shale basin showed higher concentrations overall, but the trend analyses reveal a steady decline of the petroleum hydrocarbon enhancement in outflow from the DJ Basin. The agreement between the methane and hydrocarbon trend behavior increases confidence in the finding of overall declining emissions from hydrocarbon production in the DJ Basin.

conclusionsLong-term trends in the concentrations of selected hydrocarbons associated with O&G production have differed markedly between the states of Texas and Colorado. In Texas, these trends generally mirrored changes in O&G production volumes, whereas in Colorado, they declined slowly but steadily. Unlike Texas, where only federal rules are in place to regulate emissions, Colorado passed several state-level rules targeting O&G emissions reductions that may have contributed to its air quality improvements.