Assessing Source Contributions to Air Quality and Noise in Unconventional Oil Shale Plays

Unconventional oil and gas development (UOGD³) has enabled the exploration of previously inaccessible or uneconomic oil and gas resources in shale rock, resulting in thousands of extraction sites across the landscape, many near people's homes. Human exposure to air pollution and noise related to these activities poses a health risk. This study focused on characterizing air pollutants, greenhouse gas emissions, airborne radioactivity, and noise associated with UOGD in two shale production basins.

During one year of stationary air monitoring in Loving, New Mexico, in the western part of the Permian Basin (PB), we characterized the magnitude, frequency, and duration of UOGD-related emissions at temporal scales from 1 minute to seasonal. Continuous monitoring was performed for meteorological variables, near-surface ozone, nitrogen oxides (NO_x), sulfur dioxide, hydrogen sulfide, 20 speciated volatile organic compounds (VOCs) in the ethane to octane volatility range, and noise. Airborne radioactivity was measured in the gas and particle phases. Source apportionment was performed using nonnegative matrix factorization (NMF). To disentangle sound frequencies, we developed spectrograms and conducted machine learning regression to analyze sound sources and relationships to air pollutants. A network of passive hydrocarbon samplers that collected weekly measurements of 15 hydrocarbons was established throughout populated regions of both the PB and Eagle Ford Shale (EFS) areas to measure regional pollutant concentrations and their spatial gradients around UOGD. Visible Infrared Imaging Radiometer Suite (VIIRS) Nightfire (VNF) data were acquired to quantify gas flaring activity throughout the region during our field measurement period. VNF flares and estimated flare gas volume were linked to the stationary air quality measurements to examine associations.

Most of the monitored primary air pollutants showed high variability, with frequent concentration spikes that exceeded background mole fractions by up to three orders of magnitude. The frequency of concentration spikes, their maximum mole fractions, and averaged metrics (hourly, 8-hour, 24-hour, and annual) were higher than those reported from mostly urban comparison sites. Near-surface ozone measurements confirmed that this area of the PB is in nonattainment of the ozone national ambient air quality standard (NAAQS), exceeding the 70 ppb threshold more than 30 days of the year, and that this nonattainment is driven by UOGD-related emissions of VOCs and NO_x. The highest ozone values occurred during conditions of high temperatures, dry air, and slow advection of air masses across the PB from the south-southeast. VOC monitoring showed dominant impacts from saturated hydrocarbons, and associated NMF analysis identified five emission sources: a UOGD-related hydrocarbon source, two gas-flaring sources, a general combustion source, and a road transportation/traffic source. These sources correspond well with identified spatial distributions of surface well pad activity, gas flaring during the study period, and road traffic in the area. Radioactivity measurements displayed diurnal and seasonal changes consistent with prior work; elevated levels were observed from the north-northwest sector. Airborne radioactivity correlated with aromatic VOCs, petroleum hydrocarbon alkanes, and CO₂. Our noise measurements were dominated by low frequencies (<100 Hz), which were most strongly associated with CO₂. The passive sampler network identified northwest-to-southeast increases in ambient hydrocarbon concentrations, in line with the identified UOGD and traffic density in the area. Benzene levels across the network at times exceeded health-based reference values and were significantly higher than benzene levels recorded during air monitoring in large Texas metropolitan areas. Average hydrocarbon levels were significantly correlated with the well density surrounding each site in both shale basins.

Our extensive air quality research in the Permian-Delaware Basin revealed, at times, extraordinarily high levels of air pollution, including air toxics such as benzene, and frequent high ozone days in violation of the ozone NAAQS. Our analyses show that the overwhelming amount of this pollution is due to UOGD activities, including emissions from production and storage, gas flaring, and truck traffic.