Environmental and Human Impacts of Unconventional Energy Development.

Development T rise of the “shale revolution” at the beginning of the 21st century triggered a hotly contested public debate concerning the balance between the economic benefits of extraction of natural gas and oil from unconventional resources (unconventional oil and gas; UO&G) and the associated environmental and health risks.. This extraction involves first drilling a vertical or horizontal well and then injecting under high pressure millions of gallons of water, sand, and chemicals into the well to induce the formation of fractures in lowpermeability geological formations, allowing for flow of natural gas and/or oil, and returned hydraulic fracturing fluids mixed with formation water to the surface. Benefits are purported to include an expanded energy portfolio and reduced climate forcing through greater availability of natural gas, whose combustion emits less carbon dioxide and other air pollutants than for coal. Risks are purported to include increased climate forcing as a result of fugitive methane emissions, localized air and water pollution, and ecological and community impacts. While the early scientific evidence to inform this debate was slim, the current exponential growth of research in this area is in concert with the expansion of extraction operations, mostly in the U.S. but also in Canada, South America and China. Of the 180 studies published on this topic between 2010 and 2016 in Environmental Science & Technology (ES&T) and Environmental Science & Technology Letters (ES&T Letters), 75% were published after 2013 (Figure 1). Four years ago, the U.S. National Research Council organized workshops in which academic, industry, and government scientists identified risks associated with UO&G to (1) water resources; (2) air quality and climate change; (3) public health; and (4) socioeconomic and community effects. An evaluation of these risk domains were published as five reviews in a 2014 ES&T Special Issue: Understanding the Risks of Unconventional Shale Gas Development. To highlight research advances in more than 100 papers published since that Special Issue, ES&T and ES&T Letters have now organized a Virtual Issue containing 28 recent (2015−2017) papers that have contributed to the scientific understanding of UO&G within the following topics: fugitive methane emission estimates; air quality; impacts of waste fluids (hydraulic fracturing fluids and formation waters) on water quality and use; and risk governance policies. A 2014 review of UO&G impacts on global climate change identified that the accurate quantification of fugitive methane emissions from UO&G is a major priority. “Top-down” studies featured in this Virtual Issue involving aircraft-based methane measurements indicate that methane emissions from Barnett Shale oil and gas operations have been underestimated by a factor of 3 and Marcellus Shale unconventional natural gas well sites have been underestimated by >10−40 times. In the Marcellus Shale, UO&G well sites were estimated to emit 23 times more methane than conventional sites and ethane to methane ratios demonstrated that ∼80% of methane emissions in the Barnett Shale region derived from fossil fuel sources. Methods to apportion methane sources in UO&G basis have been developed and refined: stable isotope methods, particularly deuterium isotopes, can be used to distinguish biogenic (e.g., landfills) from fossil sources of methane, whereas alkane ratios can be used to distinguish natural gas and oil production sources. A 2014 review of UO&G risks in the air quality domain identified tropospheric ozone precursors, priority air pollutants, hazardous air pollutants, and particulate emissions from UO&G as specific risks to air quality. Five of the studies in this Virtual Issue have advanced understanding of these risks. Source apportionment models applied to measurements of these pollutants in the Barnett and Marcellus Shales provide specific pollutant emission rates from UO&G. The results indicate that emission rates vary by region, and the rates can be used to model effects on regional health and air quality. Additionally, air pollutant emissions (including methane) from UO&G are highly skewed toward a few high-emitting sources referred to as superemitters. For example, tank vents, hatches, and underperforming emission controls contributed to >90% of hydrocarbon emissions in a study of 8000 oil and gas well pads in seven U.S. UO&G basins. However, the drawbacks of air pollutant emissions from UO&G production must be weighed against potential benefits of switching among fossil fuel sources for combustion. In particular, switching from coal to natural gas