Robert A. Michaels; PhD, CEP (left)
RAM TRAC Corporation; Schenectady, New York
The role of natural gas in the overall U. S. and global energy portfolio has grown dramatically because of the emergence of shale gas as an economical resource for large-scale production. Shale gas is natural gas that is trapped within shale formations, which are fine-grained sedimentary rocks that can contain large amounts of gas and oil. The existence of large deposits of shale gas has been known for decades, but extracting it was uneconomical until development of horizontal drilling and hydraulic fracturing technologies in the 1980s and 1990s. Indeed, shale gas accounted for only one percent of U.S. natural gas production in 2000.
Development of new technologies under government-sponsored programs and a successful demonstration project in the Barnett Shale in north Texas together fueled a boom in shale oil production in multiple shale formations around the world. In the U.S., the Marcellus Formation, spanning much of the Appalachian Basin, propelled a major expansion of the U.S. gas industry starting in 2008. By 2010, shale gas accounted for more than 20 percent of U.S. natural gas production and, by the beginning of 2013, it reached nearly 40 percent.
The actual magnitude of gas reserves in the Marcellus and other shale formations is controversial. Estimates made at the beginning of the shale boom recently have come down substantially, as data from shale gas extraction have yielded a more accurate picture. The long-term abundance of natural gas from shale might (or might not) prove to be less than originally thought. Nonetheless, the era of large-scale exploitation of this resource is well underway, and has brought the issues of health and environmental impacts of hydraulic fracturing (or ‘fracking’) to the forefront. Rapid expansion of fracking technology adds urgency to issues raised by its use.
The term ‘fracking’, short for ‘hydrofracturing’, denotes forceful injection of fluid at high temperature and pressure to break up (‘fracture’) deep bedrock, and thereby extract trapped natural gas. The term is a euphemism, because ‘hydro’ implies misleadingly that the injected fluid is just water. Actually it is a mixture of water (98-99.5 percent) and a proprietary cocktail of organic and inorganic additives including sand and fluids (0.5-2.0 percent): acids to improve gas flow, biocides to prevent clogging, corrosion and scale inhibitors to prevent leaks, gels or gums to add viscosity, and friction reducers to maintain pressure from surface pumps to the furthest reaches of the wells (Clark, et al., 2012). These fluids may include toxic chemicals that are linked to cancer and other adverse health and environmental effects. The gas drilling industry has guarded the composition of fracking fluids, but the proprietary status of such information conflicts with necessary public agency establishment and implementation of monitoring requirements, and with the legitimate public right to hold drillers accountable for any damage that they might cause, which cannot be known if contaminants detected cannot be matched to fracking substances injected into gas wells.
New technologies now enable horizontal drilling in all radial directions, like spokes of a bicycle wheel, from the base of a vertical shaft, producing a shaped array of horizontal shafts miles beneath groundwater aquifers to deliver fracking fluids explosively. The same shafts then collect natural gas from all directions in an area of approximately 640 acres (Kurkowski, 2012), delivering it to the central vertical shaft that conveys gas upward, through aquifers, to the surface. In a perfect world, natural gas is collected for sale, and toxic fracking fluids are recycled. Residual gas and fracking fluids remain in the fractured bedrock, but are safely sequestered miles beneath the surface, never to pollute air, surface soil, or water. The real world and real bedrock, however, are imperfect.
Natural gas and fracking fluids must be contained reliably within lined well shafts, from which leakage of gas and fracking fluids otherwise can occur if containment is inadequate. Leakage, especially near the surface, can contaminate groundwater, surface water, surface soils, and air. If recovered fracking fluids are stored in open lagoons, they can leach into soils and groundwater and vaporize into air. Contaminated groundwater or surface water used for residential or institutional drinking supplies also can pose risks to public health. In Pennsylvania some homes were reported to have gas emanating from water faucets, causing odors and posing risks of fire and even explosion, though the linkage to fracking has been difficult to verify. Such consequences, if caused by fracking, are unacceptable, and have constituted a source of great concern in New York State.
Fracking in New York State would enable drillers to extract gas from vast deposits in the Marcellus shale, which also extends into Pennsylvania, West Virginia, and Ohio. Its exploitation in Pennsylvania and beyond already has nurtured a profitable industry. The costs, however, have included adverse impacts on public and environmental health (described above), and on the social fabric and economic life of affected communities. Alarmists prejudge these adverse impacts as being intrinsic to fracking, and therefore inevitable, based upon past experience.
Almost anything can be done badly. If accidents or poor practices in past experience had been deemed an appropriate criterion for disallowing an industry rather than improving it, we would have no airline or automotive industry, nor many other industries. Without prejudging the issue of whether fracking can be undertaken safely, we take a balanced approach: explore the feasibility of fracking safely, but acknowledge that accidents are not really accidents, but inevitabilities, in a permissive regulatory climate. This is because industrial processes and chemicals must be treated differently from American citizens, who are assumed innocent until proven guilty. In our regulatory system, industrial processes and chemicals generally are required to be proven safe, not presumed safe, prior to permitting. Exceptions have occurred however, especially with respect to substances that were in use prior to promulgation of regulations. No ‘grandfathering’ exception should apply to substances present in fracking fluids, especially if they have been regulated previously in other contexts.
The Southern Tier and Western New York State include a high proportion of economically depressed areas that might benefit from fracking if done correctly (Kurkowski, 2012). As advocates have acknowledged, the potential benefits of fracking may not be worth pursuing if the risks are too high, and/or if the benefits will materialize for gas companies to the detriment of residents. This viewpoint is sensible, as far as it goes, but we take the analysis further by identifying key issues at the global, national, state, and local levels that must be reconciled to assure that fracking in New York, if undertaken, will avert adverse impacts such as those listed above, reliably and perpetually.
The main global issue is whether or not fracking will generate more planetary greenhouse warming than its energy alternatives. Greenhouse gas generation from nuclear power, for example, is virtually nonexistent, and the potential for harnessing clean energy from sustainable solar, geothermal, wind, tidal, and other renewable sources is enormous. The current political and economic environment, however, is hostile to nuclear power; and renewables, for a combination of technical and economic reasons, are not yet in a position to provide for the majority of our energy needs. For a substantial period of time, we must rely heavily upon fossil fuels.
Natural gas is cleaner to burn than coal, oil, or gasoline. Coal is used to generate 80 percent of the world’s electricity, and constitutes the largest man-made source of greenhouse gas emissions. Replacement of coal plants by natural gas plants would cut carbon dioxide (CO2) emissions by 30-50 percent, and virtually eliminate emissions of mercury and other toxic metals, nitrogen oxides, and acid gases such as sulfates that contribute to acid rain. Replacing gasoline or diesel fuel with natural gas in vehicles would yield a 10-30 percent reduction in CO2 emissions. Clearly, if natural gas can be extracted without negative consequences, its use as a replacement for other fossil fuels would benefit pubic health and the environment.
Greenhouse gases are released, however, not only from using natural gas, but also from leakage of natural gas during production. Most of this leaked gas is methane, the primary constituent of natural gas. Argonne National Laboratory investigators have estimated shale gas leakage at 2.0 percent over the life cycle, and 1.2 percent for drilling, completion, and production activities. U. S. Environmental Protection Agency (EPA) estimates of these parameters prior to large-scale adoption of fracking were 1.4 percent for lifecycle, and 0.4 percent for drilling, completion and production (Clark, et al. 2012). In contrast, researchers at the National Oceanic and Atmospheric Administration (NOAA) reported measurements of methane leakage rates of four percent at a shale gas field near Denver (Pétron, et al., 2012), and rates of nearly nine percent in the Uinta Basin in Utah (Tollefson, 2013). Investigators have reached consensus on neither the magnitude of the methane release problem, nor its variation among gas fields and among well operators. More recently, the Environmental Defense Fund, the University of Texas at Austin, and nine major gas producers measured methane emission rates at U.S. shale gas wells. This is the most comprehensive study to date, and the work apparently is complete. The investigators, however, announced that the results only would be released upon publication in a technical journal.
Methane release from fracking contributes to total release of methane and, more broadly, to total release of greenhouse gases. In 2009, with fracking on the rise but the U.S. economy in the dumps, methane constituted only a small fraction of total U. S. greenhouse gas emissions according to the U. S. Energy Information Agency (US EIA, 2011). Even when increased to equivalent CO2 to account for its greater greenhouse warming potency, methane constituted only 11.1 percent of the total. However, this value cannot and should not be accepted uncritically. Data on methane leakage from fracking is incomplete and variable. The US EIA value, therefore, may be based upon voluntary reporting by gas drillers, which presumably would report, albeit accurately, their best cases rather than their typical cases. Further, the EIA data quantified methane emissions only up to 2009, showing an increase of approximately 10 percent in methane release during the 2005-2009 period, though shale gas production increased more than 100 percent. Thus, the EIA methane leakage data track the overall gas production numbers without any apparent uptick due to fracking. So either shale gas production is cleaner than other types, or EIA failed to capture methane emissions from fracking. The EIA data, therefore, are puzzling. Finally, methane leakage that increased between 2005 and 2009 would have continued to increase after 2009, which was early in the natural gas boom. Even if the methane emission values supporting the EIA (2011) report were atypical, at least they might represent a target that might be achievable and enforceable industry-wide.
The preponderance of greenhouse gas effects is exerted by CO2, which accounted for 81.5 percent of U. S. greenhouse gas emissions in 2009, if EIA’s methane leakage value is accurate. Other components also were emitted in smaller fractional amounts, including non-energy related CO2 (1.3 percent). More broadly, EIA reported that “a drop in the carbon intensity of the U.S. energy supply may represent a new trend: from 1990 to 2005, carbon intensity increased on average by 1.0 percent per year, but from 2005 to 2009 it fell by an average of 1.9 percent per year, as natural gas was increasingly substituted for coal, and renewable electricity generation continued to grow.” In early 2012 CO2 emissions reached a 20-year minimum (Carey, 2012), constituting approximately 70 percent of CO2 emission reductions targeted by the Kyoto Protocol relative to future forecasts that were made in 1998. This reduction of carbon emissions relating to energy consumption was attributed to increased shale gas production, largely accomplished via fracking. The implication is that increased methane emission due to fracking may be the price of decreasing CO2 emissions, with net benefit regarding global climate change.
A different picture, however, emerges from extensive modeling of climate impacts exerted by additional methane introduced by large-scale replacement of coal with natural gas obtained by fracking. The models assume a variety of methane release rates. The result of this modeling is that most of the previously reported methane leakage rates would raise global temperatures more than if coal burning continued. Similarly, a study published in the Proceedings of the National Academy of Sciences concluded that methane leakage rates would have to be below 1.6 percent for natural-gas-powered cars to exert less effect on climate change than cars running on gasoline (Alvarez, et al., 2012).
Multiple sources emit methane to the atmosphere, both natural and man-made. Natural background sources include biomass decay and hydrates (‘clathrates’) in seabeds and Arctic permafrost. When background production processes predominate over industrial processes, the release rate and its associated global warming rate are substantially fixed at the background rates. That is, even if industrial production were eliminated entirely, the predominant background release rates would persist irreducibly. The salient question is whether substantial substitution of fracking-derived natural gas for coal and oil in power generation, and for oil in automotive transportation, will reduce the climate effects of greenhouse gas emissions. If methane leakage in fracking is excessive, it might not. Transitions from mature technologies to emerging technologies often are characterized by relatively high environmental and public health impacts of the emerging technology, which may abate as the technology matures. Given the urgent time constraints for controlling global climate change, this transition in the case of fracking must be controlled more effectively than has been achieved during previous energy source and energy technology transitions.
Toward this goal, the gas industry should be required to continue evaluating methane leakage from shale wells quantitatively, and to develop and implement a program to reduce it aggressively to a sustainable emission rate. Such a program might be modeled on the sequentially tightened standards requiring automobile companies to increase their fleets’ corporate average fuel economy (CAFE). An important difference, however, is that automotive emissions were tolerated for many decades before a reactive regulatory attempt was made to reduce them, whereas ‘grandfathering’ of methane emission from fracking and associated climate effects must be prevented via proactive control.
All methods of natural gas extraction result in some level of methane release into the atmosphere. This also is true of coal mining and other fossil fuel extraction activities. If shale gas is to continue to be a major source of energy, it must be extracted in a manner that substantially retains the environmental benefits of natural gas, and thereby mitigates the impacts of extracting and using coal and oil.
U. S. National Issues
The main national issue relating to fracking is the need to develop energy self-sufficiency and strategic independence from uncertain foreign energy sources. Fracking may add vast quantities of versatile natural gas to our existing energy portfolio of domestic resources, which is currently dominated by coal. The abundance of coal and its resultant low price together have provided a strong economic incentive for its continued use despite negative environmental and health consequences. Recently, however, fracking has reduced the price of natural gas, and strong demand for coal, especially in China, has increased its price domestically. As a result, gas recently has overtaken coal in New York State. Coal is less versatile than natural gas, as efforts to convert it to a vehicle fuel have been unsuccessful. That failure prolonged U. S. dependency on foreign oil, constituting the basis for a U. S. national interest in promoting the use of more versatile natural gas, even if obtained from environmentally costly fracking.
The costs of reliance upon foreign energy sources are well known, and will not be addressed in detail here. They include political, economic, and military costs, all of which translate inevitably to environmental and public health costs on a national and planetary scale that must be considered. These considerations had led to a hands-off national policy on fracking. As part of the 2005 Energy Policy Act, Congress created the so-called ‘Halliburton Exemption’ for hydraulic fracturing, essentially taking the Federal Government out of the equation for regulating the gas industry. As a result, oversight and regulation of fracking has been left to individual states (for example, New York; NYS DEC, 2011), which often have been ill equipped to perform it. The Obama Administration, however, has sought to restore Federal oversight, despite gas industry opposition. Indeed, the President recently nominated to the Cabinet position of Energy Secretary M.I.T. physics professor Ernest J. Moniz, an advocate of accelerating replacement of oil and coal with natural gas, much if not most of which would be obtained via fracking, and the U. S. EPA is conducting a major study of its potential impacts on drinking water supplies (US EPA, 2011).
New York State Issues
New York State is grappling with the question of whether, and under what conditions, fracking might be permitted (NYS DEC, 2011). At minimum, specific conditions should be placed on fracking. An escrow fracking ‘superfund’ should be established from payments to be made into it by drillers to cover damage, if it occurs. Such a fund should be configured to provide incentives for the industry to prevent accidents, as in the Federal Superfund, in which companies must remediate damage or pay ‘treble damages’ (three times the price) into the Superfund if EPA must withdraw funds from it for remediation.
More generally, fracking should be regulated like any technology that potentially poses significant public health and environmental risks. Drillers should be required to avoid sensitive locations such as drinking-water reservoirs, and avoid damaged bedrock, such as previously drilled areas. They should be required to disclose the composition of fracking fluids that will be used, thereby assuring that detected contamination patterns can be matched to their source(s), which then would be held accountable. Permits should specify use of Best Management Practices (BMPs) and Best Available Control Technology (BACT), both to be defined for fracking by EPA and/or the New York State Department of Environmental Conservation (DEC). BMPs and BACT for fracking should include monitoring equipment capable of detecting contaminant releases before they become unmanageable, which could include double-hull piping to detect leaks in the inner tube that would be contained by, and detected within, the outer tube pending repair and/or replacement. The regulatory package should prohibit open, unlined lagoons; instead requiring closed containers for recycling and transporting fracking fluids.
Fracking is a local issue because communities must achieve an appropriate balance between possibly competing collective vs. individual interests. Individuals, most notably, may have an interest in selling mineral rights and/or rights-of-way to gas drillers, with major consequences to their communities. Communities, in contrast, typically have an interest in controlling the pace and direction of development having quality-of-life significance. Communities act on behalf of citizens who may be impacted negatively by decisions made by their neighbors, for example, via zoning regulations.
Communities also must plan with neighboring communities to avoid uncontrolled piecemeal development of their region. Indeed, the environmental impact statement (EIS) approach instituted by the National Environmental Policy Act (NEPA) of 1970 represents an administrative mechanism by which sponsors of projects apply for required agency permits, which may be approved or disapproved based upon EISs. NEPA, however, has been criticized for failing to account for cumulative effects of multiple projects of a kind in a region. Cumulative impacts can be assessed via permit procedures in which a lead agency invites participation of ‘stakeholders’, which might include regional chambers of commerce, local planning boards, and individual community representatives, all advised by technical experts. Retrospective impact assessment can be incorporated into permits to verify the accuracy of EIS forecasts and, if necessary, adjust permits and/or the number of permits issued.
Mechanisms for coordination of communities within a region toward a particular goal have been developed, for example to select and permit a deep geological repository for long-term storage of lowlevel radioactive waste from nuclear power plants. Implementation of such procedures has been balky and sometimes ineffective. The proposed Yucca Mountain repository in a Nevada salt dome, for example, was considered and litigated for years, and disapproved. The problem of long-term storage of so-called ‘rad-wastes’ today remains a major obstacle to siting nuclear power plants, notwithstanding their negligible greenhouse gas emissions and global climate impact. A case study (Smith, et al., 2012) of cumulative impacts of fracking on brook trout in a Marcellus shale watershed illustrates possible trade-offs between energy development and the benefits provided by natural ecosystems. Local issues, therefore, might be thorniest, as they may be unsolvable via objective scientific and engineering equations. The financial stakes for communities and gas companies are high, and may conflict with the interests of individual residents and/or other stakeholders.
Evolution of Energy Sources
Exploitation of most if not all energy sources poses risks to public health and the environment. Like the proverbial lunch, ultimately it is not free. This raises the philosophical question of whether our society should exploit energy sources even though doing so poses risks. A practical answer is that we have no choice in our circumstances. Modern societies can increase their energy efficiencies, but ultimately they depend upon having access to substantial amounts of energy. The established methods for obtaining this energy already pose considerable ‘baseline’ risks. Evolutionary biologists and game theorists describe such a situation as an ‘existential game’, meaning that we are in it, and must remain in it, whether we like it or not. In the present context, facing the risks posed by extracting and using energy is unavoidable, unless we prefer to face the greater risks posed by failing to do so. The bottom line is that modern societies, because they must face risks intrinsic to meeting their needs, also must manage and mitigate those risks to keep them within acceptable societal limits.
Risk management includes viewing fracking as a choice among alternative energy strategies, the relative risks and benefits of which must be evaluated, weighed, and balanced. Fracking, therefore, should not be viewed simplistically as being either safe or unsafe. Expanded coal mining, drilling for undersea oil, and using the resultant energy sources to meet our growing needs instead of natural gas, all have serious and growing consequences. New energy sources tend to be the most difficult to reach, the most expensive to extract, and/or the most onerous for public health and the environment; that is a major factor explaining why their exploitation, and the enabling technologies, are relatively recent developments. The ‘ecological footprint’ of an average barrel of oil, for example, has increased since 1950, and will continue to increase (Clark, et al., 2012). Thus, greenhouse gas emissions released in extracting a barrel of oil from newly exploited oil sands in Alberta, Canada were found to be 23 times greater than emissions from extracting an Alberta barrel of oil years ago via conventional drilling. Fracking, therefore, looks increasingly attractive when considered in the context of dwindling easy-toreach sources, but our regulatory system requires that permitting of technologies be acceptably safe, not just comparatively safe. Our available and evolving energy choices will determine how our overall energy portfolio likewise will evolve. Given the available alternatives, we cannot afford a decision not to exploit our abundant shale gas reserves. On the other hand, we also cannot afford to utilize those reserves in a way that will further damage human health and the environment.
From a climate perspective, the ‘right’ strategy would be to eliminate fossil fuel use; including coal, oil, and natural gas; and go immediately to renewables, but this is impractical because of the relative immaturity of renewable technologies, as well as their intrinsic properties. The major renewable technologies, specifically, wind and solar, deliver power intermittently. They are therefore difficult to use to meet the base power needs of the grid, and will remain so pending availability of a viable energy storage technology on a sufficiently large scale (or alternatively, the development of a sufficiently interconnected smart grid that can effectively shuttle power to where it is needed on demand). In contrast, fossil fuels are abundant enough to last for a longer time than our planet’s ecosystems can survive their use.
Even the most optimistic studies suggest that complete transition to renewable energy will take 20 to 30 years. In that time, the damage to public health and the environment caused by, for example, the planned 1,000 or more additional coal burning plants (many in China) if they are like those that exist today might be overwhelming. The pace of transition to renewables, therefore, must be gauged to public health and environmental factors because they are more urgent than the supply factor. This flies in the face of the temptation, which some would call an imperative, to delay the transition to renewable energy while fossil fuel remains available. This temptation should be resisted. As we are stuck with fossil fuels as a means of assuring that people’s energy needs will be met reliably and economically, we must find the least damaging way to use them while we urgently develop renewable energy alternatives.
Human societies will continue to increase their use of energy. We must evolve, therefore, toward emphasizing efficiency, moderation, and adoption of a sustainable ‘portfolio’ of energy sources, which necessarily will vary geographically. We must invest heavily in renewable energy sources so they eventually will suffice for our energy needs. At best, therefore, natural gas displacement of other fossil fuels, most notably coal, represents a bridge between our near-term energy needs and long-term goal of meeting those needs sustainably for our planet and its inhabitants. If we fail to do this in a proactive and timely manner -- that is, immediately -- our options for addressing global climate change will close quickly and irrevocably (Stocker, 2013), and we will be forced to go down the perilous road of experimenting with global climate geo-engineering to undo damage already done. All of this means, most essentially, that continued development of depletable, non-renewable, non-sustainable energy sources such as natural gas via fracking requires adoption of the safest practices now, and an exit strategy that will assure for our communities a sustainable future as a reward for making wise choices today.
Robert Michaels ([email protected]) is an environmental toxicologist consulting in health risk assessment and management. He has served numerous corporations, the U.S. Congressional Office of Technology Assessment, and the Natural Resources Defense Council (NRDC). He chaired the State of Maine Scientific Advisory Panel and the Certification Review Board of the Academy of Board Certified Environmental Professionals. Dr. Michaels has been Secretary of the NFPA Committee on Classification and Properties of Hazardous Chemicals, Board Member of the National Association of Environmental Professionals, and Member of the Editorial Advisory Boards of Springer-Verlag and Cambridge University Press journals. He earned his doctorate at SUNY at Stony Brook in 1979.
Randy Simon ([email protected]) is a physicist with over 30 years of experience in renewable energy technology, materials research, superconductor applications, and a variety of other technical and management areas. He has been an officer of a publicly traded Silicon Valley company, worked in government laboratories, the aerospace industry, and at university research institutions. He holds a PhD in physics from UCLA. Dr. Simon has authored numerous technical papers, magazine articles, energy policy documents, online articles and blogs, radio scripts, and a book, and holds seven patents. He also composes, arranges and produces jazz music.
1. Alvarez, R. A.; et al. Greater focus needed on methane leakage from natural gas infrastructure. Proceedings of the National Academy of Sciences, 109(17):6435-40, April 2012;
2. Carey, Julie M. Surprise side effect of shale gas boom: a plunge in U.S. greenhouse gas emissions. Forbes.com, http://www.forbes.com/sites/energysource/2012/12/07/surprise-sideeffect-of-shale-gas-boom-a-plunge-in-u-s-greenhouse-gas-emissions/; 7 December 2012; If you have trouble with the link, see Resourses below for .pdf.
3. Clark, C. E.; et al. The technology and policy of hydraulic fracturing and potential environmental impacts of shale gas development. Environmental Practice, 14:249-61, December 2012;
4. Kurkoski, Scott R. The Marcellus shale: a game changer for the New York economy? Albany, New York; New York State Bar Association, NYSBA Journal, 84(1):10-9, January 2012;
5. NYS DEC. Revised Draft Supplemental Generic Environmental Impact Statement On The Oil, Gas and Solution Mining: Well Permit Issuance for Horizontal Drilling and High-Volume Hydraulic Fracturing to Develop the Marcellus Shale and Other Low-Permeability Gas Reservoirs. Regulatory Program. Albany, New York; New York State Department of Environmental Conservation; 1,537 pages, http://www.dec.ny.gov/data/dmn/rdsgeisfull0911.pdf, 7 September 2011;
6. Pétron, G. et al. Hydrocarbon emissions characterization in the Colorado Front Range, a pilot study. Journal of Geophysical Research, 117, D04304, 2012;
7. Smith, David R.; et al. Shale gas development and brook trout: scaling best management practices to anticipate cumulative effects. Environmental Practice, 14:366-81, December 2012;
8. Stocker, Thomas F. The closing door of climate targets. Science, 339:280-2, 18 January 2013;
9. Tollefson, J. “Methane leaks erode green credentials of natural gas. Nature, 493, 12 January 2013;
10. US EIA. Emissions of Greenhouse Gases in the U.S. U. S. Energy Information Administration, http://www.eia.gov/environment/emissions/ghg_report/ghg_overview.cfm,, 31 March 2011;
11. US EPA. Study of the Potential Impacts of Hydraulic Fracturing on Drinking Water Resources: Progress Report. Washington, DC; U. S. Environmental Protection Agency, Office of Research and Development; EPA 601/R-12/012, http://www2.epa.gov/sites/production/files/documents/hf-report20121214.pdf , 278 pages, December 2012