Emissions of Polycyclic Aromatic Hydrocarbons from Natural Gas Extraction into Air

Abstract

Natural gas extraction, often referred to as "fracking", has increased rapidly in the United States in recent years. To address potential health impacts, passive air samplers were deployed in a rural community heavily affected by the natural gas boom. Samplers were analyzed for 62 polycyclic aromatic hydrocarbons (PAHs). Results were grouped based on distance from each sampler to the nearest active well. Levels of benzo[a]pyrene, phenanthrene, and carcinogenic potency of PAH mixtures were highest when samplers were closest to active wells. PAH levels closest to natural gas activity were comparable to levels previously reported in rural areas in winter. Sourcing ratios indicated that PAHs were predominantly petrogenic, suggesting that PAH levels were influenced by direct releases from the earth. Quantitative human health risk assessment estimated the excess lifetime cancer risks associated with exposure to the measured PAHs. At sites closest to active wells, the risk estimated for maximum residential exposure was 0.04 in a million, which is below the U.S. Environmental Protection Agency's acceptable risk level. Overall, risk estimates decreased 30% when comparing results from samplers closest to active wells to those farthest from them. This work suggests that natural gas extraction is contributing PAHs to the air, at levels that would not be expected to increase cancer risk.

Figure 1

Average PAH concentrations grouped by distance to the closest active natural gas well. a. Sum of 62 PAHs, b. benzo[a]pyrene, and c. phenanthrene. The three distance groups are close (n = 5), middle (n = 12), and far (n = 6), defined in the text. Error bars represent one SD. The asterisk indicates a significant difference between the close and far groups for phenanthrene, p < 0.05.

Figure 2

Average sum of 14 PAHs, grouped by distance to the closest active natural gas well, with comparisons to previous studies.^{, -} All data are vapor phase PAHs. The three distance groups in the present study are close (n = 5), middle (n = 12), and far (n = 6), defined in the text. To visualize all data on a log scale, a (concentration + 0.1) transformation was used on all data in this figure. Error bars represent one SD.

Figure 3

Petrogenic vs. pyrogenic sourcing ratios, grouped by distance to the closest active natural gas well. a. Fluoranthene/pyrene, b. phenanthrene/anthracene, c. fluoranthene/(fluoranthene+pyrene), and d. anthracene/(anthracene+phenanthrene) ratios. Pyrogenic and petrogenic thresholds are defined in text. The three distance groups are close (n = 5), middle (n = 12), and far (n = 6), defined in the text . Error bars represent one SD.

Figure 4

Average carcinogenic potency of measured PAHs, grouped by distance to the closest active natural gas well. The three distance groups are close (n = 5), middle (n = 12), and far (n = 6), defined in the text. Error bars represent one SD. The asterisk indicates a significant difference between the close and far groups, p < 0.05