Unconventional oil and gas spills: Materials, volumes, and risks to surface waters in four states of the U.S.
Graphical abstract
Distribution of spills attributed to unconventional oil and gas wells by state. Light green polygons indicate shale basins (basin nomenclature and shapefile from USEIA (2011)).
Introduction
Development of oil and gas from unconventional shale sources (unconventional oil and gas, or UOG) has dramatically increased over the past ten years in large part due to the combination of horizontal drilling and hydraulic fracturing. Horizontal drilling refers to the process where a wellbore aligns horizontally with the target formation, thus increasing contact with the reservoir, and hydraulic fracturing refers to the process that stimulates oil and gas within the reservoir by expanding fractures in shale through injection of fracturing fluid (i.e., water, proppant and chemicals) (USDOE, 2009). The U.S. is currently the leader in developing UOG resources, where from 2000 to 2016 daily production of shale gas (dry) increased by 20-fold (2.2 to 44.0 billion cubic feet) and tight oil increased by > 10-fold (0.4 to > 4.5 million barrels) (USEIA, 2016). Other countries are beginning to commercially produce oil and gas from shale and low-permeability formations (USEIA, 2015), and by 2040, unconventional gas production is projected to triple to account for almost a third of global natural gas production (IEA, 2015). Given the rapid, recent development of UOG, data are scarce on its long-term environmental impacts, and there is a need to better quantify risk to people and nature (Finkel and Hays, 2013, Small et al., 2014, Souther et al., 2014, Werner et al., 2015).
UOG development can affect species, ecosystems, and the services they provide to people. In central North America, estimates suggest that oil and gas development (including coal bed methane) reduced net primary productivity, an important measure of a region's ability to provide ecosystem services, by ~ 4.5 Tg of carbon from 2000 to 2012 (Allred et al., 2015). Further, land application of hydraulic fracturing fluid resulted in leaf drop and 56% mortality of trees where the application occurred (Adams, 2011). Forest interior bird counts increased with distance from a well pad in Pennsylvania (Barton et al., 2016), abundances of sagebrush songbirds decreased with increased well density in Wyoming (Gilbert and Chalfoun, 2011), and mule deer have been documented to avoid well pads with active drilling by at least 800 m in Colorado (Northrup et al., 2015). In Kentucky, an accidental release of hydraulic fracturing fluid into a stream increased gill lesions and other indicators of stress in fish (Papoulias and Velasco, 2013), and in Pennsylvania, juvenile mussels below a brine treatment plant had lower survival rates than mussels located above the plant (Patnode et al., 2015). Streambed microbial diversity was lower below an oil and gas waste injection plant in West Virginia (Akob et al., 2016), and water downstream from this site had higher endocrine-disrupting activities than reference water (Kassotis et al., 2016). Despite the emerging evidence, studies establishing causal relationships between environmental changes and UOG activities are scarce; this is particularly true for spills and releases of materials used in and produced by UOG development.
Summary reports on UOG spills are starting to emerge; however, they are typically restricted to a single state, short on detail regarding materials spilled or reasons for spills, or are characterized by a small sample size. For example, the Colorado Oil and Gas Conservation Commission (COGCC, 2014) reported that equipment failure and human error were the two main causes of spills, most spills occurred during the production stage, process piping, pipelines and tanks were the main sources of spills, and the volume of 12% of the spills were > 100 barrels (15,900 L); however no detailed analysis on spilled material was presented. Brantley et al. (2014), using the Pennsylvania Notice of Violation (NOV) database, reported that one-fifth of wells were given a non-administrative violation from 2005 to 2013, and Rahm et al. (2015) reported that Pennsylvania NOVs (2007–2013) related to spills and erosion were the most common NOV issued. Neither study, however, conducted a detailed analysis on volumes or materials spilled or their potential impacts to surface waters in Pennsylvania. Finally, the U.S. Environmental Protection Agency (USEPA, 2015a) reviewed over 36,000 spill records from nine states but was able to confidently identify only 457 incidents associated with hydraulic fracturing (~ 12,000 contained insufficient information and ~ 24,000 were not related to hydraulic fracturing). The USEPA reported most spills were small (< 1000 gal, 3785 L), flowback and produced waters were the most commonly spilled material, human error was the most common cause of a spill, storage units were the common source of spills, and 300 of the spills reached an environmental receptor; however, the study did not include spills that occurred during the drilling stage.
The objectives of this study were to characterize the volumes and compositions of the materials spilled from horizontal, hydraulically fractured oil and gas wells, and evaluate the risk that spills posed to streams and surrounding watersheds important to human drinking water. Our first objective aimed to fill the knowledge gap on the materials and volumes spilled during UOG development. Our second objective focuses on streams because they provide habitat that supports a high level of biodiversity (Meyer et al., 2007), are particularly vulnerable to UOG development due to their tight coupling with upstream catchments (Hynes, 1975), and are sensitive to small changes in catchment conditions from anthropogenic activities (Maloney et al., 2012). Further, over 1/3 of the U.S. population uses public drinking water systems that rely, at least in part, on intermittent, ephemeral or headwater streams (USEPA, 2009). The spatial position of anthropogenic activities within the catchment often affects these relationships (King et al., 2005), which is especially important for UOG because wells are frequently located in close proximity to streams (Entrekin et al., 2011). We therefore evaluated the risk of spills to streams by quantifying the spatial position of spills to the nearest stream and how these distances related to current setback regulations. Because a large population relies on surface water for domestic use, our second objective also explored risks to drinking water using the U.S. Forest Service's Forest to Faucets data set. We provide a broad analysis of spill features to improve understanding of the potential environmental risks of spilled materials from UOG development and to inform management practices and policy formulation.
Section snippets
Study sites and setback regulations
We sampled state databases on spill records for four states (Colorado – CO, New Mexico – NM, North Dakota – ND, and Pennsylvania – PA) that have accessible oil and gas spill data and that are underlain by a number of shale basins (USEIA, 2011). Each of the states experienced an increase in horizontal hydraulic fracturing over the past decade (Fig. S1); however, they vary in production type from ND being primarily an oil producer to PA being primarily a gas producer. These states provide a
Well and spill temporal patterns
We identified 6622 UOG spills from 5958 unique reports in the four states' databases (Table 1). Six hundred and fifty eight reports (11.1% of total) had more than one material reported on the same incident report. North Dakota had the most horizontal UOG wells, followed by PA, NM and then CO (Table 1). While all states showed a sharp increase in the number of horizontal wells over time (Fig. S1), the number of spills increased sharply in the late-2000s only for ND and PA; for PA, the increase
Discussion
Understanding the characteristics of spills is key to effectively evaluate environmental risk due to UOG development. Risk is a function of the frequency of spills, the type of material spilled, the volume of material spilled, and the proximity of the spill to surface waters and other ecologically sensitive systems. In our study, wastewater and crude oil were two of the most frequently spilled materials across all states, which is consistent with previous reports (COGCC (Colorado Department of
Conclusions
Concerns over potential environmental issues resulting from UOG development have spurred a flurry of recent articles on its potential ecological effects to associated ecosystems (Brittingham et al., 2014, Evans and Kiesecker, 2014, Souther et al., 2014). An important gap in our understanding of potential effects of UOG on associated ecosystems is the surface release of chemicals, waste materials and oil and gas. We found that wastewater, crude oil, drilling waste and hydraulic fracturing
Competing financial interest declaration
The authors declare no competing financial interests.
Acknowledgements
We thank the various state agencies who have made their oil and gas spill data publically available and the staff at the National Center for Ecological Analysis and Synthesis who assisted with data scraping. We also thank three anonymous reviewers and Adam Benthem of the USGS whose comments greatly improved the manuscript. This work resulted from the SNAPP: Science for Nature and People Partnership Impacts of hydraulic fracturing on water quantity and quality Working Group at the National
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