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Cite This: Environ. Sci. Technol. 2019, 53, 6601−6615

Environmental Justice in Unconventional Oil and Natural Gas Drilling and Production: A Critical Review and Research Agenda Adrianne C. Kroepsch,*,† Peter T. Maniloff,† John L. Adgate,‡ Lisa M. McKenzie,‡ and Katherine L. Dickinson‡ †

Colorado School of Mines, 1500 Illinois Street, Golden, Colorado 80104, United States Colorado School of Public Health, University of Colorado Denver, 13001 E. 17th Place, Campus Box B119, Aurora, Colorado 80045, United States

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ABSTRACT: The drilling phase of oil and natural gas development is a growing area of environmental justice (EJ) research, particularly in the United States. Its emergence complements the longstanding EJ scholarship on later phases of the oil and gas commodity chain, such as pipeline transport, refining, and consumption. The growing scholarly attention to the EJ implications of drilling has been prompted by the surge in development of unconventional oil and gas resources in recent decades. More specifically, the oil and gas industry’s adoption of horizontal drilling and hydraulic fracturing (a.k.a., “fracking” or “fracing”) as methods for extracting oil and gas from a wider range of geologic formations has simultaneously heightened oil and gas production, brought extractive activities closer to more people, intensified them, and made well pad siting more flexible. Here, we provide a critical review of the novel EJ research questions that are being prompted by these on-the-ground changes in extractive techniques and patterns, propose an interdisciplinary conceptual framework for guiding EJ inquiry in this context, discuss key methodological considerations, and propose a research agenda to motivate future inquiry.



INTRODUCTION Oil and natural gas drillingin energy sector parlance, the “upstream” phase of oil, and natural gas developmentis an emerging area of environmental justice (EJ) research. Later phases of the oil and gas commodity chaini.e., its “midstream” and “downstream” componentshave long been a focus of EJ scholarship through, for example, analyses of the disproportionate impacts of pipeline placement, oil spills, refineries, and gasoline combustion on minority and lowincome populations.1 The potential EJ implications of upstream oil and gas activity have received comparably less attention, until recently. Scholarly interest has picked up on this topic in the United States in particular, in response to the country’s ongoing surge in domestic oil and gas development. In the past decade, the oil and gas industry’s growing adoption of the paired extractive technologies of horizontal drilling and hydraulic fracturing (a.k.a., “fracking” or “fracing”) has enabled a boom in so-called “unconventional” oil and gas development, which targets low-permeability geologic formations, such as shale.2 The novel combination and scale of these extractive techniques have simultaneously brought extractive activities closer to more people, allowed the oil and gas industry to concentrate and enlarge sites of extraction, and made well pads more flexible to site.3−8 These changes, which will be explained in more detail below, raise novel EJ questions, such as who lives near these increasingly intensive extractive activities? Does their proximity correlate with indicators of social inequality, such as race and socioeconomic status? Lastly, how do EJ concerns enter into decision-making processes © 2019 American Chemical Society

about the siting of these more intensive methods of oil and gas drilling, if at all? A growing subfield of the EJ scholarship has begun to answer these questions. Thus far, research findings have been mixed and have been complicated by methodological challenges. Furthermore, EJ patterns in human proximity to extractive activities have not yet been connected with the underlying processes that work to shape EJ outcomes (positive or negative). In this critical review, we build on this crucial start in upstream oil and gas-related EJ analysis by synthesizing initial results and offering ideas for advancing this area of inquiry in a rigorous and meaningful manner. We first explain why drilling is becoming increasingly important as an area of EJ inquiry. We then review findings of the nascent EJ literature on drilling and articulate the need for an interdisciplinary conceptual framework that connects EJ patterns with the decision processes that shape them, which range from individuals’ housing decisions to regulators’ rulemaking decisions. After articulating the need for such a framework, we offer one, explain its components using Colorado as a case study, and highlight key knowledge gaps in the existing literature. We then discuss methodological considerations associated with this type of EJ research, from quantitative and qualitative standpoints. Finally, we offer questions to motivate Received: Revised: Accepted: Published: 6601

January 10, 2019 May 10, 2019 May 17, 2019 May 22, 2019 DOI: 10.1021/acs.est.9b00209 Environ. Sci. Technol. 2019, 53, 6601−6615

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Environmental Science & Technology

production in 2018 and again in 2019, due primarily to increased production from unconventional or “tight rock” plays.8 Because the emerging EJ literature on drilling responds to this unexpected trajectory, it is predominately U.S.-focused and emphasizes intracommunity (or intraregion) comparisons in areas of UOG production specifically. In reviewing this literature, we acknowledge as per O’Rourke and Connolly that these evolving community-level energy geographies are occurring within a global context,1 and also that extractive industry debates are increasingly engaging questions of international and intergenerational climate justice.10,12,22 While analyses of these global EJ questions are highly important, they are beyond the scope of this article; we return to them in our questions for future research in the Guiding Questions section. If commercial-scale oil and gas extraction has occurred in the U.S. for more than 150 years, why has attention to the EJ implications of oil and gas drilling only emerged recently? We suggest three broad reasons derived from our own research experience and reviews of the EJ field writ large.10−13,23−27 First, the EJ scholarship has often (though certainly not exclusively) focused on urban environments and populations. While rural contexts have always been crucial sites of EJ activism and analysisnotably, the pathbreaking EJ studies of the 1980s focused on hazardous waste siting in rural areas of the U.S. South,28 in addition to Houston, Texas,29 and analyses of agriculturally related injustice have long emphasized rural regions30much of the EJ scholarship of the 1990s and early 2000s emphasized urban settings, urban planning, and was published in urban studies journals.12,13,25−27,31 With the notable exception of the Los Angeles oilfields, however, the production of oil and gas has historically been more likely to occur in areas that are rural and remote.32,33 As a result, literature on the impacts of extraction has historically been characterized by “boomtown” studies, which emphasize the relationship between the industry and entire rural communities.34,35 In addition to being rurally focused, the boomtown impact model has also implicitly assumed that a spatially concentrated resource will be developed in a single “boom,” followed by a one-time “bust,” leaving little room for inquiries about social inequality and extractive industry impacts over the long-term.36,37 Second, EJ research has also often (though, again, not exclusively) focused on the siting of large individual point-source polluters, such as hazardous waste sites or industrial plants that report to the U.S. Toxics Release Inventory database.13,23,28,29 Because hazardous waste sites, landfills, refineries, and other such large-scale projects are usually proposed by a single operator and weighed in a public process, they are high-profile events with relatively clear points of research access, including public hearings, media coverage, and easily identifiable interview subjects.38 Oil and gas drilling, on the other hand, has historically been small-scale and dispersed about the landscape, pursued by a mix of many industry actors, and sited via less accessible processes, such as individual contracts on private lands and federal leasing on public lands.33,39 These factors have made drilling a relatively more diffuse and inaccessible target of study. Third, and relatedly, EJ research has often focused on environmental hazards that include some degree of siting choice.28,29,40,41 Until recently, however, oil and gas development has historically proceeded along a more limited siting path, constrained by geology and well spacing requirements that were distributed and largely uniform.

a research agenda for EJ scholarship on oil and gas extraction and concluding thoughts.



A CHANGING ENERGY LANDSCAPE AND ITS ENVIRONMENTAL JUSTICE UNKNOWNS EJ scholarship is concerned with assessing the distribution of environmental benefits and burdens among populations, as well as the fairness of relevant decision-making processes. In a 1994 executive order, President Clinton established EJ as a goal of United States policy (EO #12989). Today, the U.S. Environmental Protection Agency defines environmental justice as “the fair treatment and meaningful involvement of all people regardless of race, color, national origin, or income, with respect to the development, implementation, and enforcement of environmental laws, regulations, and policies,” which will be achieved when “everyone enjoys (1) the same degree of protection from environmental and health hazards and (2) equal access to the decision-making process to have a healthy environment in which to live, learn, and work.”9 Dimensions of social inequality examined in EJ scholarship include the traditional indicators of race and socioeconomic status, as well as age; ethnicity; education; gender; immigration status; language; sexual orientation; socio-spatial factors, such as rural and/or urban geographies; and intersections among these characteristics.10−13 Academics typically distinguish among three components of EJ, all of which translate to the oil and gas context: traditional distributive justice (that the environmental burdens of development are not disproportionately borne by vulnerable populations); benefit-sharing distributive justice (that those who take on the burdens of development also share in its benefits); and procedural justice (that decisions about development are made in an inclusive and representative manner).14,15 To procedural justice, some scholars also add recognition (that stakeholders be recognized as having a legitimate seat at the table and be shown respect by other stakeholders and decision-makers).16,17 We endeavor to address all of these aspects of EJ in our review. Increasingly, scholars are also pursuing participatory EJ approaches that directly involve members of affected communities so as to better incorporate local values and knowledge in the process of identifying, identifying, measuring, and addressing environmental injustices.10 We note opportunities for participatory EJ methods throughout this review. EJ research often takes one of two approaches, comparisons within a single country (typically conducted within a single community) and comparisons among countries (typically conducted between nations in the global north and south).18 Until recently, EJ research on oil and gas primarily focused on the latter set of comparisons, as well as latter phases of the oil and gas commodity chain. For example, the most widely cited EJ review on oilan ambitious article by O’Rourke and Connolly covering the full oil lifecyclededicates many pages to the EJ implications of oil transport, refining, and consumption but provides only a brief overview of oil drilling.1 Because it was published just prior to the recent boom in unconventional oil and gas (UOG) production, that review provides an important launch point for this one. In this review, we turn our attention specifically to UOG development, which has already significantly increased extractive activities in the U.S., Canada, China, and Argentina and which is prompting exploration and development efforts in Algeria, Australia, Colombia, Mexico, Russia, and other parts of the world.19−21 The U.S. is expected to set a new record in fossil fuel 6602

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Environmental Science & Technology Notably, all three of these aspects of extractionrurality, dispersion, and constrained sitinghave been evolving with the uptick in UOG development. The rise of the paired extractive techniques of horizontal drilling and hydraulic fracturing have enabled record production levels, while also reshaping patterns and practices of extraction on the ground in ways that may have EJ implications. In simple terms, horizontal drilling involves drilling down toward a targeted geologic formation and then laterally into it in order to increase the total area of contact between a wellbore and hydrocarbonproducing rock. The lateral lengths of horizontal wells vary across the many regions of UOG extraction in the U.S. and are growingpushing beyond two miles in Colorado and four miles in Pennsylvania, for example.42,43 Hydraulic fracturing is the practice of injecting a high-pressure fluid into a well in order to fracture the targeted geologic formation and mobilize its hydrocarbons. Because horizontal wells require more hydraulic fracturing than vertical wellsoften greater than 20 stages in Colorado44the process is increasingly referred to as “high-volume hydraulic fracturing” in an effort to convey the significance of the higher volumes of fracturing fluid involved, plus associated chemical and proppant inputs. Together, these extractive techniques have enabled UOG production from “unconventional” geologic formations previously considered unprofitable to develop. The U.S. industry has shifted so intently toward UOG development that hydraulically fractured horizontal wells overtook all other methods of development in total footage in 2011 and accounted for the majority of new wells drilled in the U.S. by 2014.45 Presently, the U.S. EIA estimates that these wells make up about 670 000 of the 977 000 actively producing wells in the country. Changes in proximity to people, intensity, and siting flexibility have accompanied these changes in extractive techniques. Recent UOG development in the U.S. has occurred largely (90%) on private lands and also across wider geographies than before, given the broad spatial extent of UOG-rich areas.37,46 In addition to increasing production in remote regions, UOG development has also expanded extractive activities near more populated places, such as Colorado’s Front Range corridor, which we discuss here, as well as parts of Texas, Ohio, California, Oklahoma, and Pennsylvania, which rank as the top five states in terms of total population within 1600 m of an active well.4 Even ex-urban or rural areas of UOG may also have substantial populations arranged in towns and boroughs.37 As of 2014, more than 17.6 million people were living within a mile of an active oil or gas well in the coterminous U.S., and about half of those wells had been drilled since 2000.4 Notably, in places with growing extractive footprints and populations, some peoples’ proximity to well pads is a function of new housing development around existing well pads.7,47 Horizontal drilling is also generating larger well pads by enabling operators to colocate many wells, plus associated tanks and equipment, on a single pad (Figure 1). Holding as many as 20−60 wells, these large multi-well pads are described colloquially as “superpads,” “mega pads,” and “mini-industrial complexes.”43,48,49 These “superpads” serve to condense the surface impacts of UOG development to fewer, but larger, sites. This consolidation benefits the broader population in a given area, but at the expense of the few who live near more intensive drilling operations. Large multi-well pads pose correspondingly larger impacts and hazards, ranging from longer drilling and hydraulic fracturing

Figure 1. Comparison of well pads with vertical versus horizontal drilling. (Top) Aerial imagery of example well pads in the DenverJulesburg Basin, Colorado. (Middle) Simplified subsurface crosssection. (Bottom) Simplified plan view, including lines representing subsurface well bores (invisible in the vertical drilling case because they point straight down).

times, to more truck traffic and emitting infrastructure.6,50−53 Finally, as well pads have grown larger and horizontal laterals have lengthened, the siting and spacing options for them have also expanded, giving operators a wider siting geography than in the days of vertical drilling.5,54 In summary, UOG drilling today poses more potential impacts to more people, paired with wider discretion regarding the siting of those impacts. Together, these changes have raised the importance of EJ analysis and scholars’ attention to drilling as an EJ question. The extent to which EJ goalstraditional distributive, benefit-sharing distributive, procedural, and recognition-orientedare being achieved in areas of UOG development has yet to be clearly established. Broadly speaking, it is apparent that the benefits and risks associated with drilling are not evenly distributed and are conditioned by peoples’ positioning within socio-economic structures.55−58 The economic benefits of UOG development tend to accrue at the regional level or to individuals with mineral rights, while proximate populations experience acute hazards and nuisances alongside decision-making participation opportunities that are constrained by mineral rights ownership status and procedural limitations.5,14,15,39 This raises important questions about the distribution of benefits and burdens in UOG host communities. It also highlights, either by connection or by contrast, several EJ drilling-related issues that have long merited more scholarly attention but are beyond the scope of this article, such as the dearth of drilling research on the lands of sovereign Native American nations;33 the need for closer examination of drilling-related EJ in rural contexts;31 the shortage of available data regarding legacy issues from previous eras of extraction, despite related accidents and studies indicating that aging and abandoned wells demonstrate higher emissions rates;4,59,60 the absence of consistent social and economic impacts assessment, monitoring, and mitigation as a component of development;61 the lack of cumulative impacts assessments across UOG 6603

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Environmental Science & Technology landscapes;37,46 and the need for more data on the occupational health and safety of UOG workers.62

poverty levels and a largely white population, show no evidence of race-, poverty-, or income-based injustice, nor do they find evidence of education-based injustice.14 In the Marcellus Shale regions of West Virginia and Ohio, other vulnerable populationsthe elderly and childrenare more likely to live near wells.98 In Colorado, an in-depth analysis of home prices and well pad locations revealed that home values tend to be lower near well pads on the Front Range and in other parts of the state, and that the population within one mile of active wells is growing at a faster rate than the general population.7 The Colorado study did not directly include demographic variables but inferred socio-economic status from home values and raised the importance of studying the growing residential population living near UOG activities and infrastructure.7 Benefit-Sharing Distributive Justice. Initial studies of benefit-sharing distributive justice have yielded clearer trends. These studies focus on whether the benefits of drilling are shared by the populations that experience its burdens. Analyses have found that employment, wages, and nonwage income increase when UOG development begins in a community, and that the impacts are larger in communities with more wells drilled.97,99−101 Moreover, there is evidence that incomes increase at both high and low income levels, and that the percentage of residents under the poverty line decreases.14,102 However, these data also come with cautionary tales. In particular, there is evidence that, after a bust, oil and gasdependent communities suffer higher unemployment than communities with no resource extraction.36 These studies have focused on various spatial scales, from a single state or geologic formation,14,101 to regions,36,100 to nationwide.97,99,102 Income and employment analyses have at least two important limitations, however: (1) employment markets are geographically larger than the likely scales of environmental health harms near wells, making it difficult to discern the extent to which gains are accruing to residents very near wells (who are also exposed to higher risks) versus others in the same community; and (2) it is difficult to assess the extent to which employment and income gains are accruing to local residents versus outsiders. Several studies document the importance of nonwork income, such as payments to mineral rights owners. Brown et al. show that payments to mineral rights owners totaled $68 billion in 2014, and that “the income effect of oil and gas development in a county with an average degree of local ownership is 2.5 times higher than in a county with complete absentee ownership.”103 In the Barnett Shale region, research by Fry et al. documented mineral rights ownership and wealth in Denton, Texas, finding that the primary beneficiaries of drilling in Denton are “non-local mineral owners who comprise at least 61.4% of all mineral owners, receive at least 68% of the value, and, because of their distance from drilling activities, experience none of the costs associated” with UOG extraction in the city (p106).15 Local mineral ownersnamely, the city and several family trustsreceived only 6.3% of the total value held by mineral owners.15 In a study of land ownership in Pennsylvania, which serves as a proxy for mineral rights ownership due to the prevalence of unified estates there (i.e., the surface owner also owns subsurface minerals), Kelsey et al. found that, in 11 counties with significant UOG development, more than 25% of land was owned by nonresidents.104 In a Colorado study, McKenzie et al. found that 36−57% of wells drilled between 2000 and 2012 were located on a split estate,



REVIEW OF CURRENT ENVIRONMENTAL JUSTICE FINDINGS In this section, we begin with a review of the state of research findings on hazards, exposures, and health effects associated with UOG drilling to establish the public health context for EJ scholarship. We then turn to a synthesis of current results on distributive and procedural EJ and argue on behalf of a conceptual framework that draws together these research areas. Public Health. Since the Adgate et al. critical review published in this journal in 2014, the UOG-related public health literature has grown. New studies document increased ambient concentrations of hazardous air pollutants,52,63−66 particulate matter,67,68 and ozone,69,70 as well as increased residential noise exposure in areas of active development.71−73 Of particular note are studies that have attributed emissions of hazardous air pollutants directly to well sites and observed diurnal trends in ground level concentrations of these pollutants.63,64,74,75 The body of evidence suggesting that living near UOG sites could adversely affect both physical and mental health also has grown substantially. Studies have found that residents living within approximately 1000 feet of well pads may experience increased health risk due to exposure to hazardous air pollutants and noise.71,73,76 Current state or local setbacks likely leave residents at risk for catastrophic events, such as fire or explosion at nearby UOG development or production sites.77 Studies have also indicated that nearby residents may experience increased psychosocial stress due to disruptions to sense of place, distress over socio-economic and environmental change, and feelings of powerlessness.31,78−82 These psychosocial impacts vary based on economic factors, with those who benefit financially from UOG (e.g., mineral owners) reporting higher quality of life perceptions than other groups.57 Epidemiological studies using administrative data sources indicate that the likelihood of adverse birth outcomes;83−89 inpatient hospital admissions;90 childhood leukemia;91 fatigue, migraines, and chronic rhinosinusitis;92 and depression93 increase with increasing density or proximity to UOG sites. Traditional Distributive Environmental Justice. Studies of traditional distributive justice focus on who lives near extractive activities and whether proximity correlates with indicators of social inequality. Historical research has revealed obvious injustices from earlier periods of oil and gas extraction that are worthy of note by contemporary EJ scholars. For example, an analysis of home mortgage lending from 1900 to 1939 in the Los Angeles oil fields found that racial segregation was engineered in direct relationship to the location of oil development, with minority communities’ housing options constrained to petroleum-producing areas.32 The historical consolidation of poor minorities in Los Angeles’ extractive areas has ramifications in the present day.94 Meanwhile, contemporary studies of distributive justice in relation to the present production surge have yielded more mixed results.95 In the Eagle Ford area of Texas, a rural region with high variation in poverty levels and an ethnically diverse population, disposal wells for UOG production-related wastewater appear to be disproportionately sited in areas with higher proportions of people of color and people living in poverty.96,97 On the other hand, analyses of UOG well pad locations in the Marcellus Shale region of Pennsylvania, which includes relatively high 6604

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patterns and processes. We offer this conceptual framework with full acknowledgment that socio-economic and extractive conditions vary among UOG producing zones. Even so, we maintain that it will be useful in an array of contexts and hope to see it further refined through application to different regions.

such that the surface owner and mineral rights owner were not the same person. Areas with longer extractive histories tended to have more split estate wells.7 The large magnitude of mineral rights payments combined with variation in the degree of local mineral rights ownership between producing regions implies substantial inter-regional variation in UOG-related economic benefits. Procedural Environmental Justice. Procedural justice goals align with normative theories of deliberative democracy, which hold that citizens have a right to influence decisions that impact them via inclusive, representative, deliberative, and transparent debate.105,106 From an instrumental standpoint, meaningful public participation in industrial siting decisions has been shown to generate a number of positive outcomes, from more accurate evaluations of impacts to improved trust and opportunities for shared learning among industry, government, and public participants.38,107 In the UOG context, procedural justice is complicated by the fact that there are two major types of decisions that relate to drilling, encapsulated well in Fry et al.: private decisions regarding the leasing of mineral rights and the development process, plus public decisions pertaining to federal, state, and local regulations, such as pollution mitigation or minimum setback distances between wells and buildings.15 This division of decision-making is complicated further in the U.S. by the fact that the law privileges subsurface rights (mineral rights) over rights to surface uses and also allows subsurface and surface rights to be split among different owners.108 As a result, people who own both surface rights and mineral rights have the most say in private UOG decisions. Still, they report differing levels of influence, with some noting satisfaction,39 others relaying experiences of powerlessness,31 and some arguing that their preferences have been effectively nullified by “forced pooling,” which allows drilling to go forward as long as a majority of mineral owners lease their rights.109 UOG host community members who do not own relevant property rights have far less voice in decision processes. Nonmineral owners are excluded from private decisions and rely largely on municipal governments and community groups to represent their voices in public decisions. A third set of decisions of particular relevance to this reviewwell pad siting decisionssit at the intersection of private and public decision-making categories. Well pad siting is primarily conducted in the private sphere, through contracts between operators and landowners, but must also go through permitting processes with state regulators and local governments. In locations where the state retains mineral ownership (e.g., in countries outside the U.S., as well as on U.S. federal public lands), procedural justice dynamics are characterized by more extensive state involvement and less private decision-making; see, for example, the U.S. Bureau of Land Management’s federal oil and gas leasing process.110,111 Value of a Conceptual Framework. In sum, changes in UOG extractive techniques are generating new EJ questions and concerns, as well as mixed results, related to the health and well-being of proximate populations, the distribution of drilling’s benefits and burdens, and the fairness of related decision-making processes. We argue that this evolving energy landscape merits rigorous examination, and also that future research would be made more insightful by bridging EJ domains in ways that attend to both EJ outcomes (proximity and demographic patterns) and EJ drivers (the processes that shape those patterns). In the next section, we propose an interdisciplinary conceptual framework that draws together EJ



CONCEPTUAL FRAMEWORK LINKING ENVIRONMENTAL JUSTICE PATTERNS AND PRACTICES The multidisciplinary research reviewed abovebased in economics, environmental health, geography, policy, and related disciplineshas demonstrated that answering even basic questions about EJ in evolving UOG landscapes requires engaging with a complex set of interacting decision processes that are typically studied in isolation. Building from this research and its emphasis on human proximity to UOG development as a key EJ indicator, we argue that patterns of proximity are the outcome of decisions made at multiple scales, as visualized in the conceptual framework presented in Figure 2 and described below: • Individuals’ decisions on where to live, based on financial resources, household preferences, perceived risks and benefits of UOG development, as well as sociopolitical dynamics that shape or limit housing choices (e.g., poverty, historic and contemporary mortgage lending practices, family or community connection to place, etc.). • UOG operators’ decisions about where and how to develop, which are driven by geology, mineral rights ownership and leasing, technical, economic, regulatory, and political factors. • Housing developers’ decisions about the types and quantity of homes to build in proximity to UOG activities, conditioned by economic, regulatory, land, and mineral ownership parameters. • Community groups’ decisions to act collectively to influence well pad siting, which depend on individual and community characteristics, including political power and access to decision-makers. • Policymaker and regulator decisions that structure the other actors’ decision processes, such as establishing minimum well-to-building setback distances and public participation processes. In addition to identifying each of these decision processes, the conceptual framework extends previous EJ research by drawing on the strengths of multiple disciplines to articulate how they interact to generate human proximity to drilling activities and related EJ outcomes. The framework launches from a traditional economic EJ approach, which focuses on homeowner and firm (operator) decisions as conditioned by market-oriented factors, such as financial resources and available information. The economic approach centers upon the individual decision-maker and posits that people choose where to live and operators choose where to drill based largely on prices and similar market attributes. In an economic framework, the question is not whether a distribution is unequal so much as whether it reflects free choices by households in a setting with fair and effective regulations and nondiscriminatory firms. Rather than stopping there, the conceptual framework then draws inspiration from the fields of geography and policy studies to query how decisions by 6605

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Figure 2. Conceptual framework for EJ analyses of oil and gas drilling.

Figure 3. Population density and oil and gas wells (black dots on map) in the DJ Basin northeastern Colorado. Counties are labeled for reference.

individual homeowners and operators are shaped and constrained by decision processes underway at other levels in the present moment and historicallyby housing developers,

community collective action, local governments, and state regulators. These considerations expand upon the economic framework by asking how market fundamentals are being 6606

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with the spatial overlay of low home values and close UOG proximity revealed in previous research.7 Operator Siting Decisions. Prior work on UOG-related employment provides suggestive evidence that firms target wells to economically worse-off regions.97 Research has also shown that firms do take action to reduce their environmental violations in response to regulatory actions, although firm experience is an important mediator.118−121 When it comes to specific well pad siting decisions, operators’ choices are conditioned by legal, operational, regulatory, and political factors, which combine to shape development costs and spatial patterns of extraction. Research conducted in Colorado has demonstrated that the innovations in horizontal drilling that are generating larger well pads also give operators more flexibility in siting, since it has become possible to extract the hydrocarbons under a given area from a variety of surface locations (Figure 1).5 In making siting decisions, operators’ legal considerations include their ability to lease mineral rights and negotiate surface use agreements or purchase land for well pad siting; operational considerations include accessing their entire leasehold and midstream infrastructure; regulatory considerations include guaranteeing enough space for statemandated setback distances; and political considerations include whether the site is controversial.5 The extent to which operators consider community factors, such as demographics of nearby populations, is unknown. Operators’ responses to localized drilling resistance (or lack thereof) also merit closer investigation. Housing Developer Building Decisions. Housing developers’ building decisions have received only preliminary attention in oil and gas landscapes. Research conducted in Colorado and Texas indicates that housing development is occurring around existing UOG production sites, and also that setback distances between existing UOG infrastructure and new homes (called “reverse setbacks”) tend to be shorter than those mandated between existing homes and new UOG infrastructure, if they exist at all.7,47 These reverse setbacks are heavily influenced by local politics and zoning.47 In Colorado, the Colorado Oil and Gas Conservation Commission (COGCC) enforces setback distances between homes and new wells (rule 604).122 In contrast, the proximity of new homes to existing wells is regulated at the local level. A review of local development codes in Colorado found that few jurisdictions address the siting of houses in relation to existing oil and gas facilities.7 Local codes that do pertain are focused on including existing (and in some instances, proposed or permitted) well locations on housing subdivision plats. In the few local jurisdictions that have codes specifying a residential building exclusion zone around existing oil and gas wells, it is shorter than the COGCC regulated setbacks for new wells. For example, Adams County, Colorado currently specifies a building exclusion zone of 250 feet from existing wells, in contrast to COGCC setbacks of 500 feet from new wells (rule 604).122,123 Community Collective Action and Decisions. Previous research in Colorado and other U.S. states indicates that community collective action around UOG is often inspired by individual sites of extraction, especially well pads that are large in size and proposed near homes and schools.5,15,124−126 A recent review of the UOG social impacts literature termed these locations “focusing sites.”33,127 In Colorado, UOG focusing sites have emerged for reasons in addition to size and close proximity to homes and schools: because neighbors

producedor example, how political and social processes shape land value, what information is available to decisionmakers and why, how powerful or vulnerable various decisionmakers are, and whether decision processes are fair (in isolation and as they intersect). The combination of these disciplinary concerns and areas of inquiry builds toward a more rigorous assessment of EJ. Colorado’s Front Range Communities as a Case Study. We use research from Colorado to explain our conceptual framework because it is home to increasingly intense drilling intermixed with a large and growing population. In the western region of the U.S., the Denver− Julesburg (DJ) Basin, which encompasses densely populated areas along the mountain front corridor, known colloquially as Colorado’s Front Range (Figure 3), serves as a major example of a region in which concentrated UOG activity is occurring in and around communities. This intermixing of extractive and residential land uses has been called a “petro-suburban” impact geography.33 As of 2012, more than 378 000 Coloradoans were living within 1 mile of an active well, mostly in the DJ Basin.7 Between 2000 and 2012, the number of active wells in the DJ Basin grew from 10 922 to 21 044.7 In this part of Colorado, pads of 24 horizontal wells are increasingly common, and pads as large as 56 wells are being proposed.5,112 According to industry estimates, a 24-well site requires a total of 20 months of round-the-clock drilling plus hydraulic fracturing, including 55−108 daily round trips by heavy trucks.113 We focus on this “petro-suburban” impact geography as we elaborate upon our conceptual framework in the following sections, while noting the importance of others (e.g., remote and rural boomtowns, the extractive-agriculture overlay, oil and gas borderlands, and sovereign lands), and encouraging further exploration of these impact geographies in future research (the Guiding Questions section).33 Individual Housing Decisions. Economic analyses of EJ start from the assumption that households rationally choose their best location option, given budget constraints and available information, and test for deviations from the resultant equilibrium. If a poor household makes a fully informed decision to buy a cheap house near a polluting site, traditional economic models imply that this is the best choice available.114 Under this framework, some of the primary areas of economic inquiry have involved households’ preferences and access to information. There is some evidence that households indeed sort according to their preferences for local amenities.18 In addition, there is evidence from other contexts that homebuyers are not fully informed about locally undesirable land uses (LULUs), such as UOG development, and that variation in beliefs about local hazards influence housing location choices.115−117 In the case of UOG, previous research has raised questions about the extent and effectiveness of homeowner notification requirements.7,47 It is also unclear whether homeowners can accurately estimate their distance to the nearest well, or how such proximity estimates influence households’ location decisions. A key limitation of the traditional economic framework, however, is that it does not incorporate many underlying complexities of housing choices (such as the influence of social networks or histories of housing discrimination, among many additional factors) other than by assuming they are reflected in individual preferences.32 An additional knowledge gap is the question of if and how indicators of social inequity correlate 6607

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Environmental Science & Technology found them to be especially noisy,128 because they have been proposed or resited near vulnerable populations,129 or because of a lethal accident.130 An in-depth analysis of drilling history in Greeley, the city in Colorado with the most active wells (nearly 400), revealed complex collective action dynamics by local residents and UOG operators that led to the shifting of proposed well pads from some neighborhoods to others.5 Importantly, some well pad resiting efforts were focused on moving proposed wells away from vulnerable populations. Amidst these efforts, a subset of resiting cases raised distributive EJ concernsnamely, a proposed drilling location that an operator resite from a proposed location next to a charter school to a location next to a predominately lowincome and Spanish-speaking public school, which has prompted lawsuits by local residents and activists.5,129 Procedural fairness challenges have been part of the EJ dynamics. Operators have worked to plan their drilling in a comprehensive way at the scale of the city, while public participation is structured around individual sites. This scalar mismatch generates a fragmented decision process that does not allow residents or local governments to effectively deliberate the pros and cons of alternative drilling locations.5 A broader-scale examination is necessary to determine whether the EJ dynamics in this case study signal a more widespread problem. Regulatory Framework. In Colorado, the regulatory framework that governs well pad siting is multijurisdictional but gives primacy to state-level regulation by the COGCC. (This is beginning to change with the recent passage of SB19181 by the Colorado General Assembly, which grants more regulatory power to local goverments. Here we describe the status quo to the present, with the acknowledgement that oil and gas governance is rapidly changing in Colorado.) Regulations relating to well pad siting are more stringent for large well pads in (sub)urban areas than rural areas. For large facilities in (sub)urban areas, defined by COGCC as sites with at least 22 homes within 1000 feet, operators must consult with local governments about well pad placement; in permitting requests, operators must also explain why they did not select an alternative location (rule 305.A).122 These requirements do not apply to rural and exurban areas. New rules established in 2016 in response to intense local drilling conflicts now require that operators make a good faith effort to receive site approval from the local jurisdiction before proceeding to the COGCC for state-level approval (rule 305.A).122 Unless they have negotiated a waiver, operators must abide by minimum setback distances (500 feet from homes and 1000 feet from high occupancy buildings) (rule series 100, 305).122 A statewide ballot initiative that called for setbacks of 2500 feet failed in 2018 with 45% of Colorado voters in favor, 55% opposed, and campaign spending by energy companies that exceeded $41 million.131 Protracted conflicts over some “focusing sites” in Colorado are increasingly driving the use of policy tools that aim to bolster jurisdictional collaboration, such as Comprehensive Drilling Plans initiated by operators, Memoranda of Understanding negotiated between operators and local governments, and 5-year drilling plans initiated by operators at local governments’ request.48,132 All of these policy tools are in play in Colorado, but it is unknown who is using them and why, as well as how they affect UOG proximity and EJ outcomes. Summary. In brief, prior research has established that (a) UOG development and decision-making raise EJ issues

(distributive, benefit-sharing, and procedural), (b) UOG development has created patterns of proximity to extraction that signal EJ problems in some contexts but not in others, and (c) a complex combination of decision-making processes determine who ends up in proximity to UOG development. Our conceptual framework (Figure 2) synthesizes these research findings and trajectories. It also prompts a series of methodological challenges, to which we turn to next.



METHODOLOGICAL CONSIDERATIONS AND RESEARCH AGENDA A full examination of EJ dynamics requires understanding any patterns of differential exposure to environmental benefits or burdens, plus the processes that generate or prevent them. This requires a mixture of quantitative and qualitative methods. In this section, we discuss associated methodological considerations, adding to a growing body of literature focused on research approaches in UOG’s rapidly changing and politically heterogeneous landscapes.133,134 Patterns of Proximity and Distribution of Benefits and Burdens. How Close Is Too Close? The methodological considerations facing EJ analysts begin before proximity measurements, with the task of establishing what distances matter and why. It is clear that socio-environmental impacts of drilling have spatial dimensions, and that they are often a function of distance to well pads,135 but at this time “there is no single distance or set of distances from oil and gas wells that is accepted across the scientific community as conveying health consequences or lack thereof to adjacent human populations” (p7).4 In a recent effort to standardize a proximity methodology in the U.S., Czolowsi et al. analyzed buffers around wells at distances of 100, 400, 800, 1000, 1600, and 2000 m.4 Other peer-reviewed studies that incorporate distance measures utilize a variety of alternative distances.4,7,14,96,98 The variety of distances in the literature reflects differing scientific and policy approaches. From a policy perspective, proximity to well pads is largely governed by state-mandated setback distances, discussed above, which establish the minimum radius of space around a well pad (usually measured from the wellhead) within which other land uses are prohibited. What constitutes an appropriate setback distance is highly debated in areas of extractive activities.5,47,124,125 Comparative studies thus far suggest that setback distances “are not derived from peer-reviewed data, data driven analysis, or historical events,” but that they are “a compromise between governments, the regulated community, environmental and citizen interest groups, and landowners” (p1327).77 Setback distances vary significantly by location and over time, from as little as 150 feet from buildings in rural areas of Colorado prior to 2013, to a minimum of 1500 feet in some Texas communities today.47,124,125,136 Public health scholars have nevertheless been working to develop empirically grounded measures of distances from well pads associated with health consequences. A 2016 synthesis of literature on four exposure types associated with UOG developmentthermal exposure from an explosion, vapor dispersion of toxic gases, and air pollution from benzene concluded that minimum state setbacks in Texas, Pennsylvania, and Colorado were inadequate to protect public health and safety but did not offer a specific setback distance as a remedy.77 A panel of 18 public health experts reached a consensus that setbacks smaller than 1 mile (1320 feet) should not be recommended but did not reach a consensus on larger 6608

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Environmental Science & Technology setbacks.137 This lack of consensus arises from limited health and exposure studies that estimate risk to individual hazards at specific distances from single or multi-well pads. Some recent evidence indicates that, for example, exposure to hazardous air pollutants and noise near UOG sites increases the likelihood of adverse health effects from these stressors.71,76 At present, no individual study or synthesis of existing information provides scientists or decision-makers with the tools needed to systematically evaluate the combined risk from exposure to multiple hazards (chemical, physical, psychosocial) or evaluate trade-offs from different risk types associated with UOG drilling and production (e.g., short-term catastrophic versus long-term health risks). These complexities need to be addressed in the context of spatial and temporal realities of drilling, production, and maintenance of UOG infrastructure that is part of siting decisions by operators. Coupling this process with specific EJ values and goals will be important for developing setback policies that address the range of competing priorities among the various stakeholders described in our conceptual framework. Too Close to What? In addition to defining proximities of interest, EJ analysts must select infrastructure(s) on which to focus. First, they must decide whether the wellhead is the most important point from which to make proximity measurements, given the importance of infrastructure which may be found on the periphery of a well pad (e.g., oil tanks, produced water tanks, and compressors) or elsewhere (e.g., pipelines and processing plants).77 With the exception of the Johnston et al. study of wastewater injection wells, most oilfield EJ analyses have focused on individual oil and gas wellhead locations, which can be found via state regulatory or industry data sets.96 These data sets are large but often contain erroneous and/or missing data.4 Some EJ analyses differentiate among well types, though this has pros and cons. A growing number of studies focus on UOG wells only, in keeping with public and scholarly interest in hydraulic fracturing and horizontal drilling. Most studies also focus on active wells. These foci are sensible, but also problematic. Ignoring conventional oil and gas wells, which are prevalent in areas with longer extractive histories, leads to underestimating human populations at possible risk.4 This is especially true as conventional wells age. Overlooking the potential emissions of inactive wells also leads to underestimating risk. Inactive wells may be (1) temporarily inactive, (2) permanently inactive and properly plugged, or (3) permanently inactive and lacking a responsible operator (e.g., abandoned or orphaned).50 These wells may have high air pollutant emissions despite their inactive status, particularly if they are not plugged or were plugged long ago under less stringent standards.50,126,127 Unfortunately, inactive wells particularly abandoned or orphaned wellsare not adequately reported in state, industry, or federal databases. Emissions researchers have handled this by limiting their analyses to the inactive wells in state inventories and, in some cases, by looking for abandoned wells that are not recorded.50,126,127 Finally, it is also important to capture the cumulative spatial intensity of drilling and production activities, given the positive relationship between well density and elevated concentrations of air pollutants as well as the cumulative regional impacts of increased heavy vehicle traffic and midstream infrastructure associated with extractive activities.52,53,87,128−131 Measuring Proximity and Demographics. After proximities and infrastructure(s) of interest have been defined, the EJ

analyst’s next methodological tasks relate to measuring patterns of proximity and indicators of social inequity. This requires observing the locations of LULUs, such as UOG wells, and demographic data about nearby residents at appropriate geographic scales. EJ studies in urban settings have traditionally leveraged large data sets, such as demographic data from the Census Bureau at the block level13 or the tract level132 or real estate databases with individual home values as a proxy variable.6,133 In urban settings, Census data can provide adequate and spatially detailed data. Because urban tracts and block groups are relatively small and uniform, it is reasonable to assume that residents of a Census tract near a hazardous site are exposed to that site. Analysts typically examine whether members of disadvantaged groups are more likely to live near LULUs via a variety of statistical research designs, including regressing demographic measures on distance to LULUs9 and differencein-differences.132 Such approaches may be suitable in urban and suburban communities where UOG is becoming increasingly prevalent, but may pose problems in nonurban locales.23,27,132 Individual household level data available in real estate databases provides greater spatial resolution, but lacks demographic variables. Rural and ex-urban settings present more challenges for statistical analyses because they have lower population densities and because wells are not evenly distributed across landscapes. These areas have larger Census tracts, which likely extend beyond the distances of undesirable effects from drilling. One solution is to use Census blocks, the smallest unit generally available, but these can be large themselves. Using Census block level data may also pose logistical challenges as these data are only available via restricted access Census Data Centers. Analysts can assume a population distribution (e.g., uniform) throughout Census blocks or tracts, but this assumption may mask important variability in exposure.132 This can be combined with using a large spatial scope of study to gain a large number of observations, but that approach can mask important regional differences. Clough and Bell report that the largest Census block group in the Marcellus Shale region of Pennsylvania is more than 70 km2 and includes large areas with wells but no permanent residents.9 The authors sought to resolve this inconsistency with a technique that involved remapping Census data onto areas classified as “residential” in local land use data.9 Other studies have leveraged census point data.111 Scholars interested in studying EJ in settings where available Census geographies are too spatially expansive may need to make modifications such as these or turn to alternative data sets, such as school enrollment data or real estate databases. For EJ analysts interested in measuring demographic change over time, such as before and after a drilling boom, publicly available demographic data sets, such as the U.S. Census, may not consistently measure variables of interest. While data from decennial censuses are quite precise at fine spatial resolutions, survey samples in between decennial censuses are much smaller. This means that estimates of the demographics in a small area in a given year are relatively imprecise, which hinders analysis of change in demographics between decennial censuses. A related difficulty is that some publicly available data sets censor data when the number of respondents (such as firms in an industrial sector in a county) are small enough that anonymity would be compromised by public release. This 6609

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and semiurban settings, scholars focused on rural settings may confront the same problem laid out above in analyzing proximity; with a few households, standard spatially aggregated measures may be aggregated to too large a size to arrive at statistically defensible conclusions. Examining the processes that drive well pad proximity invokes the classic EJ problem of identifying which arrived first, people or well pads. In Colorado, for example, areas with longer extractive histories have more low-value homes being built next to existing well pads, while areas with shorter extractive histories tend to have wells built near more highvalue homes that predate oil and gas development.6 In some cases, energy development and home development occur in alternating waves (see McKenzie et al. for a relevant vignette);6 this dynamic is likely to continue as UOG development proceeds through cycles of “mini-booms and mini busts” given its sensitivity to commodity market dynamics.37 These data highlight the importance of understanding how proximity emerges in ever-changing energy and residential landscapes, and demonstrate the value of using qualitative approaches to understand the histories of specific sites in conjunction with larger-scale analyses. Drawing from Fernando et al.’s synthesis of mixed-method approaches in UOG impacts research, incorporating qualitative methods in EJ studies can be particularly useful for establishing context for land use changes, population changes, or siting conflicts to be explored using geospatial techniques or surveys, and to expand and enhance the reliability and validity of survey results or Census data sets by filling in important gaps, interpreting patterns, examining conflicting or contradictory findings, or pursuing deeper analysis of particular subgroups or cases.122 Qualitative approaches are also essential for disaggregating and understanding the differing experiences and perspectives of stakeholders within UOG host communities. Depending on the research question, relevant qualitative data may be gathered from in-depth interviews, focus groups, participant observation, public events or hearings, archives, media, and social media. Of course, qualitative approaches are accompanied by their own complexities, particularly related to representation and participation. In studies that characterize the views of a specific group, or compare and contrast the experiences of different stakeholder groups, care must be taken not to generalize findings more broadly.122 The contentious and ever-changing nature of UOG development can also make participation especially challenging. Lack of trust may prevent participation of some stakeholders, and boom/bust dynamics may prohibit interview saturation with a stakeholder group.122,123 Data gathering may also be limited by research fatigue among key informants, language barriers, cultural norms against collective action, time constraints (for parents, industry members, regulators, etc.), seasonal time constraints (for agricultural producers), digital accessibility (for the elderly), mobility (for temporary workers), and even legal barriers (for residents in legal proceedings with operators).133 Many of these barriers can be surmounted with enough resources, such as time dedicated to earning participants’ trust, the assistance of translators, incentives for research participants, and triangulating among multiple modes of qualitative data-gathering.

additionally hinders detailed demographic and economic data in low-population areas. Interpreting Proximity. To date, studies have focused on measuring air pollutants, ambient noise, and other stressors outside of people’s residences. Because people spend considerable time inside their residences and at other locations (i.e., work and school, indoors versus outdoors), it is also important to understand exposure to these stressors at these locations. Qualitative approaches can also be crucial for understanding time-activity patterns and identifying and understanding human proximity to well pads and related infrastructure. In particular, it is important to acknowledge that what counts as a “safe” distance between drilling activities and the public may always be, in part, a question of values. Understanding preferences about setback distances within communities and among immediate neighbors to drilling operations, and what shapes them, may be as important as measuring the distances themselves, making participatory research approaches key in this context. Moreover, geospatial well pad data describe only the final product of well pad siting processes. A combination of qualitative and quantitative research methods are essential for understanding how drilling sites are selected, contested, and finalized within complex UOG host community geographies and histories. Analyzing Benefit-Sharing. Assessing benefit-sharing justice requires measurements of the economic benefits of UOG drilling. Because mineral rights ownership data can be difficult to aggregate and royalty payments are private, researchers have turned to a number of proxy measures, such as tracking whether well permits are categorized as split estate,6 utilizing appraised mineral property values,10 analyzing income tax returns for changes to taxable income and reporting of royalty income,91 and incomplete data sets of mineral rights leases.93 Processes Driving Proximity. Which Processes Matter and Are They Fair? The decision-processes that drive proximity to drilling are many and intertwined. It is wellestablished that several aspects of the UOG development process disenfranchise those who do not own mineral rights or surface rights, such as neighbors to drilling activities and renters who lack relevant property rights but bear the risks and nuisances of extraction. That said, several decision-processes listed in our conceptual frameworksuch as how homeowners decide where to live, how operators and home builders decide to develop subsurface and surface resources, how community groups decide to mobilize, or how regulators decide to establish setback distances and permit wellsmay serve to ameliorate or further exacerbate the procedural justice challenges associated with these starting conditions. After these interacting decision processes have been identified, developing an understanding of how they shape EJ outcomes requires both unpacking them and evaluating whether they meet distributive and procedural justice criteria (and for whom).11,12 Participatory research approaches may be especially insightful here as well. Measuring Processes that Drive Proximity and Demographics. Economists have generally taken a statistical approach to analyzing household and firm siting decisions, as well as regulatory enforcement.17,134 These studies often combine real estate records, survey data, business records, and governmental administrative data. They then statistically test for patterns in household, firm, or regulator decisions that might explain observed patterns in environmental exposure. While this approach may transfer straightforwardly to urban



GUIDING QUESTIONS Here, we forward a research agenda for EJ scholarship focused on upstream UOG development (i.e., drilling and production). 6610

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Environmental Science & Technology Table 1. Future Areas of Inquiry for EJ Scholarship on UOG Development guiding questions

(1) Who is in close proximity to wells and related upstream UOG infrastructure? (a) Does proximity correlate with dimensions of social inequality, such as race and socioeconomic status, or additional and intersecting indicators including age, ethnicity, education, gender, immigration status, and/or sexual orientation? (b) Do the intensity of UOG development (i.e., well pad size and production volume) and/or the density of UOG development (i.e., number of wells in an area) correlate with indicators of social inequality or additional/intersecting indicators? (c) Are new UOG wells more likely to be built near existing homes, or are new homes more likely to be built near existing wells? Where and why? (d) Do relationships among proximity and indicators of social inequality vary within and/or among communities, states, regions, and/or countries? examining (2) How do individual, firm, community, and regulatory decision-making processes combine to produce the current distribution of UOG wells processes and people, and how do these decision-making processes vary? (a) How do differing community histories, geologies, industry norms, and regulatory structures shape patterns of proximity to UOG extraction? (b) How have these decision processes changed over time? (c) How do these processes vary by community, state, region, nation, and/or UOG impact geography? (d) How do EJ concerns enter into these decision processes, if at all? Through what formal or informal channels? informing oil and (3) How can the results of EJ analyses inform subsequent decisions about the proximity of wells and people? gas decisions (a) How can regulatory tools, such as setback distance regulations, incorporate community values, health equity considerations, and emissions exposure assessments? (b) What data do residents of UOG host communities need to make informed housing decisions, and how do these data needs vary among UOG impact geographies? (c) How can vulnerable populations be better and more proactively represented in UOG decision processes? bridging additional (4) How can an increased emphasis on UOG drilling contribute to bridging additional gaps in EJ research? gaps (a) How can intracommunity EJ analyses be expanded to engage the global context of UOG production? How and why do EJ outcomes vary and/or relate among UOG-producing countries? (b) How can the international and intergenerational justice questions raised by climate change be more directly connected with intracommunity UOG EJ analyses? (c) How can EJ research practices and possibilities be expanded to include chronically overlooked populations? (d) How can the legacies of previous eras of UOG development be better incorporated into EJ analyses? For example, how can abandoned and orphaned wells be consistently identified and remediated? (e) How can EJ analyses better account for the cumulative impacts of UOG development over time and/or across landscapes?

examining proximity

oil and gas commodity chain, increasing analytical attention to its upstream phase in addition to its midstream and downstream phases. Developing a clearer understanding of who lives near UOG sites, and why they do, would also inform policy decisions in the places where UOG resource development overlaps with the everyday lives of an increasing number of people.

In simple terms, the driving unknowns of this burgeoning area of inquiry are, first, whether the evolving UOG landscape is generating EJ problems; second, why or why not; and third, what should be done to either mitigate EJ problems if they are occurring or to continue avoiding them if they are not. In Table 1, we offer a series of research questions that engage these unknowns and draw from our conceptual framework. In addition, we propose a final set of questions that extend beyond our conceptual framework to motivate EJ inquiry and introspection even more broadly within the field.



AUTHOR INFORMATION

Corresponding Author



*Phone: 303-384-2565; Fax: 303-273-3751; E-mail: [email protected].

SUMMARY AND OUTLOOK Changes in the techniques of oil and gas development namely, proximity to people and the spatial consolidation, intensification, and siting flexibility enabled by horizontal drilling and hydraulic fracturingare raising new EJ questions and motivating a burgeoning subfield of EJ-related research focused on the upstream phase of UOG extraction. This critical review has aimed to advance this area of inquiry by synthesizing the results of existing EJ studies on drilling and production, offering an integrative conceptual framework that connects EJ patterns with the decision-making processes that generate them, discussing methodological considerations, and outlining an agenda for future inquiry. We have delineated this conceptual framework with examples from Colorado, where UOG and residential areas are increasingly intermixed, but the dynamics described by the framework are broadly transferable. We expect that the growing domestic and global reach of UOG will offer opportunities to refine scholarly understanding of the relationships articulated in the framework.14−16 Further research on the dynamics of extraction in the era of UOG would more evenly balance the weight of EJ inquiry across the

ORCID

Adrianne C. Kroepsch: 0000-0001-7731-8787 Lisa M. McKenzie: 0000-0003-4455-581X Notes

The authors declare no competing financial interest.



ACKNOWLEDGMENTS Drs. Adgate and McKenzie were supported, in part, by the AirWaterGas Sustainability Research Network funded by the National Science Foundation (NSF) under Grant CBET1240584. Any opinion, findings, and conclusions or recommendations expressed are those of the authors and do not necessarily reflect the views of the NSF. The authors thank the editors of ES&T for inviting this critical review and the anonymous peer reviewers for feedback which improved the article.



ABBREVIATIONS EJ environmental justice 6611

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UOG unconventional oil and gas



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