Environmental impact of hydraulic fracturing include the potential contamination of ground water, risks to air quality, the potential migration of gases and hydraulic fracturing chemicals to the surface, the potential mishandling of waste, and the health effects of these, like cancer.[1][2] Many cases of suspected groundwater contamination have been documented.[3][4] The EPA has noted that "Ground water contamination with constituents such as those found at Pavillion is typically infeasible or too expensive to remediate or restore (GAO 1989)."[5] A review published in February 2012 found no direct evidence that fracking's actual injection phase resulted in contamination of ground water, and suggests that reported problems occur due to leaks in its fluid or waste storage apparatus; the review says that gaps remain in understanding fracking.[6][7] Because hydraulic fracturing was invented in the United States[8] and therefore has a longer history there, most of the studies of the environmental impact have been conducted there.

Challenges to research

It has been reported that Industry and governmental pressure have made it difficult to conduct and report the results of comprehensive studies of hydraulic fracturing. EPA investigations into the oil and gas industry's environmental impact have been narrowed in scope and/or had negative findings removed due to industry and government pressure[9][10][11] A 2012 Cornell University report noted that it was difficult to assess health impact because of legislation, proprietary secrecy, and non-disclosure agreements that allow them to keep the proprietary chemicals used in the fluid secret. Cornell researcher Bamberger stated that if you don't know what chemicals are, you can't conduct pre-drilling tests and establish a baseline to prove that chemicals found postdrilling are from hydraulic fracturing.[12] Pre-drilling tests and other assessments have found, however, that water supplies -- especially private water wells -- often suffer from high levels of naturally occurring contaminants, as the U.S. Geological Survey concluded in August 2011.[13] Nonetheless, these particular Cornell researchers recommended requiring disclosure of all hydraulic fracturing fluids, that nondisclosure agreements not be allowed when public health is at risk, testing animals raised near hydraulic fracturing sites and animal products (milk, cheese, etc.) from animals raised near hydraulic fracturing sites prior to selling them to market, monitoring of water, soil and air more closely, and testing the air, water, soil and animals prior to drilling and at regular intervals thereafter. Despite the lack of conclusive data, however, the researchers also wrote that "a ban on shale gas drilling is essential for the protection of public health."[12] The co-chair of the Chemicals Technical Options Committee for the United Nations Environment Programme, Dr. Ian Rae, recently criticized the Cornell researchers' conclusions, saying: "It certainly does not qualify as a scientific paper but is, rather, an advocacy piece that does not involve deep analysis of the data gathered to support its case." Rae added that the Cornell researchers "cannot be regarded as experts" in this particular field. Others have pointed out that the study does still raise important questions, and it echoes similar concerns of some landowners and environmental groups.[14] In addition, after court cases concerning contamination from hydraulic fracturing are settled, the documents are sealed. While the American Petroleum Institute deny that this practice has hidden problems with gas drilling, others believe it has and could lead to unnecessary risks to public safety and health.[15]
File:Steve Katz and Bob Castelli call for hydrofracking ban.jpg
New York State Assembly members Robert Castelli and Steve Katz call for a moratorium on hydraulic fracturing in the Croton Watershed in October 2010.

The New York Times reported that the results of the 2004 EPA Study were censored due to strong industry influence and political pressure (regulatory capture),[9] however EPA agency officials stood by the results.[citation needed] An early draft of the study discussed the possibility of dangerous levels of fracking fluid contamination, and mentioned "possible evidence" of aquifer contamination. The final report concluded simply that fracking "poses little or no threat to drinking water".[9] The study's scope was narrowed so that it only focused on the injection of fracking fluids, ignoring other aspects of the process such as disposal of fluids, and environmental concerns such as water quality, fish kills and acid burns. The study was concluded before public complaints of contamination started emerging.[16]:780 The study's conclusion that the injection of fracking fluids into coalbed methane wells posed a minimal threat to underground drinking water sources[17] may have influenced the 2005 Congressional decision that hydraulic fracturing should continue to be regulated by the states and not under the federal Safe Drinking Water Act.

The 2012 EPA Hydraulic Fracturing Draft Plan was also narrowed. It does not include studying the effects of iodine-131 (found in Philadelphia's drinking water)[18][19][20] or other radioactive tracer isotopes used in hydraulic fracturing.[21][22][23][4] Nor does the draft plan include evaluating the impact of wastewater. Christopher Portier, director of the US CDC's National Center for Environmental Health and the Agency for Toxic Substances and Disease Registry, argued that, in addition to the EPA's plans to investigate the impact of fracking on drinking water, additional studies should be carried out to determine whether wastewater from the wells can harm people or animals and vegetables they eat.[24] A group of US doctors called for a moratorium on fracking in populated areas until such studies had been done.[25][26]

Proponents of hydraulic fracturing have claimed in the press and other media that the recent University of Texas Study ("Fact-Based Regulation for Environmental Protection in Shale Gas Development") found that hydraulic fracturing caused no environmental contamination,[27][28] although the study did note that other steps in the drilling process -- excepting the actual injection of the fluid -- have been sources of environmental contamination.[29] Conflicting interpretations of this study are based on disagreement between industry and the environmental community about what "hydraulic fracturing" actually is: Industry notes that hydraulic fracturing is a specific process, which takes place after the well has been drilled and the drilling equipment has left the pad; the environmental community, however, uses hydraulic fracturing, or "fracking," to describe the entire production phase. The radioactivity of the injected fluid itself was not assessed in the University of Texas study.[29] While the EPA recognizes the potential for contamination of water by hydraulic fracturing,[citation needed] EPA Administrator Lisa Jackson testified in a Senate Hearing Committee stating that she is not aware of any proof where the fracking process itself has contaminated water.[30] State regulators from across the country have similarly noted that they have seen no evidence of hydraulic fracturing contaminating water in their respective jurisdictions.[31]

Air emissions

The main hydraulic-fracturing-related air emissions are methane emissions from the wells during fracturing and emissions from hydraulic fracturing equipment, such as compressor stations. According to the study conducted by professor Robert W. Howarth et al. of Cornell University, "3.6% to 7.9% of the methane from shale-gas production escapes to the atmosphere in venting and leaks over the lifetime of a well." According to the study, this is at least 30% and perhaps even 100% more than from conventional gas production. The study explains these higher emissions with hydraulic fracturing and drill out following the fracturing.[32] Methane gradually breaks down in the atmosphere, forming carbon dioxide. It means its greenhouse-gas footprint is worse than coal or oil for timescales of less than fifty years.[32][33] However, several studies have argued that the paper was flawed and/or come to completely different conclusions, including assessments by experts at the US Department of Energy,[34] by Carnegie Mellon University[35] and the University of Maryland,[36] as well as by the Natural Resources Defense Council, which concluded that the Howarth et al. paper's use of a 20-year time horizon for global warming potential of methane is "too short a period to be appropriate for policy analysis."[37] In January 2012, Howarth's colleagues at Cornell University responded with their assessment, arguing that the Howarth paper was "seriously flawed" because it "significantly overestimate[s] the fugitive emissions associated with unconventional gas extraction, undervalue[s] the contribution of 'green technologies' to reducing those emissions to a level approaching that of conventional gas, base[s] their comparison between gas and coal on heat rather than electricity generation (almost the sole use of coal), and assume[s] a time interval over which to compute the relative climate impact of gas compared to coal that does not capture the contrast between the long residence time of CO2 and the short residence time of methane in the atmosphere."[38] The authors of that response conclude that "shale gas has a GHG footprint that is half and perhaps a third that of coal," based upon "more reasonable leakage rates and bases of comparison." Howarth et al. responded to this criticism: "We stand by our approach and findings. The latest EPA estimate for methane emissions from shale gas falls within the range of our estimates but not those of Cathles et al, which are substantially lower."[39][40]

In 2008, measured ambient concentrations near drilling sites in Sublette County, Wyoming were frequently above the National Ambient Air Quality Standards (NAAQS) of 75ppb and have been recorded as high as 125 ppb.[41] A 2011 study for the city of Fort Worth, Texas, examining air quality around natural gas sites "did not reveal any significant health threats."[42][43] In DISH, Texas, elevated levels of disulphides, benzene, xylenes and naphthalene have been detected in the air, emitted from the compressor stations.[44] People living near shale gas drilling sites often "complain of headaches, diarrhea, nosebleeds, dizziness, blackouts, muscle spasms, and other problems."[45] Cause-and-effect relationships have not been established.[45] In Garfield County, Colorado, another area with a high concentration of drilling rigs, volatile organic compound emissions increased 30% between 2004 and 2006; during the same period there was a rash of health complaints from local residents. Epidemiological studies that might confirm or rule out any connection between these complaints and fracking are virtually non-existent.[2] In 2012, researchers from the Colorado School of Public Health showed that air pollution caused by fracking may contribute to "acute and chronic health problems" for those living near drilling sites.[46]

Groundwater contamination

As early as 1987, an E.P.A. report was published that indicated fracture fluid invasion into James Parson's water well in Jackson County, West Virginia. The well, drilled by Kaiser Exploration and Mining Company, was found to have induced fractures that created a pathway to allow fracture fluid to contaminate the groundwater from which Mr. Parson's well was producing. The oil and gas industry and the E.P.A. disagreed regarding the accuracy and thoroughness of this report.[15] In 2006 over 7 million cubic feet of methane were released from a blown gas well in Clark, Wyoming and shallow groundwater was found to be contaminated.[47] Directed by Congress, the U.S. EPA announced in March 2010 that it will examine claims of water pollution related to hydraulic fracturing.[10]

In 2009 13 water wells Dimock, Pennsylvania were contaminated with methane (one blew up).Arsenic, barium, DEHP, glycol compounds, manganese, phenol, and sodium were also found in unacceptable levels in the wells.[48] As a result, Cabot Oil & Gas was required to financially compensate residents and provide alternative sources of water until mitigation systems were installed in affected wells.[48] The company continues to deny, however, that any "of the issues in Dimock have anything to do with hydraulic fracturing".[16][49][50][51] The devices needed to prevent such water contamination cost as little as $600.[52] On Dec. 2, 2011, EPA sent an email to several Dimock residents indicating that their well water presented no immediate health threat. On Jan. 19, 2012, the EPA reversed its position, and asked that the agency’s hazardous site cleanup division take immediate action to protect public health and safety.[53][48] EPA began follow up testing and sampling local water supplies in Dimock in early 2012.[54] In May 2012 EPA reported that their most recent "set of sampling did not show levels of contaminants that would give EPA reason to take further action." Methane was found only in one well.[53] Cabot has held that the methane was preexisting, but state regulators have cited chemical fingerprinting as proof that it was from Cabot's hydraulic fracturing activities.[55][54] Both Duke University and University of Rochester are conducting studies of the age of the well water to confirm the sources of the various contaminants.[54] EPA plans to re-sample four wells where previous data by the company and the state showed levels of contaminants.[53]

In 2010 the film Gasland premiered at the Sundance Film Festival. The filmmaker claims that chemicals including toxins, known carcinogens, and heavy metals polluted the ground water near well sites in Pennsylvania, Wyoming, and Colorado.[56] The film was criticized by oil and gas industry group[57] Energy in Depth as factually inaccurate;[58] in response, a detailed rebuttal of the claims of inaccuracy has been posted on Gasland's website.[59]

Complaints about water quality from residents near a gas field in Pavillion, Wyoming prompted an EPA groundwater investigation. The EPA reported detections of methane and other chemicals such as phthalates in private water wells.[5] An EPA draft report dated December 8, 2011 suggested that the ground water in the Pavillion, Wyoming, aquifer contains "compounds likely associated with gas production practices, including hydraulic fracturing".[60][61][62] The EPA discovered traces of methane and foaming agents in several water wells near a gas rig. During the investigation Luke Chavez (EPA investigator), commented that the contaminants could have come from cleaning products or oil and gas production, but said that in either case, their presence suggested problematic practices.[49] Samples of water taken from EPA's deep monitoring wells in the aquifer were found to contain gasoline, diesel fuel, BTEX (benzene, toluene, ethylbenzene, xylene), naphthalenes, isopropanol, and synthetic chemicals (e.g., glycols and alcohols) used in gas production and hydraulic fracturing fluid, and high methane levels. Benzene concentrations in the samples were well above Safe Drinking Water Act standards.[60] The EPA report stated concerns about the movement of contaminants within the aquifer and the future safety of drinking water in the context of the area's complex geology. EPA's sampling of Pavillion area drinking water wells found chemicals consistent with those reported in previous EPA reports, including but not limited to methane and other petroleum hydrocarbons, indicating migration of contaminants from areas of gas production.[60] The report also said that contaminants in wells near pits indicated that (frack) pits are a source of shallow ground water contamination. It also said, "Detections of organic chemicals are more numerous and exhibit higher concentrations in the deeper of the two monitoring wells … (which) along with trends in methane, potassium, chloride, and pH, suggest a deep source of contamination." Their observations of chemical reactions in the field led them to suggest that upward migration of chemicals from deep underground is the culprit. They also found that the reports companies filed detailing jobs listed chemicals as a class or as "proprietary," "rendering identification of constituents impossible."[63] The draft report also stated: "Alter­na­tive expla­na­tions were care­fully con­sid­ered to explain indi­vid­ual sets of data. How­ever, when con­sid­ered together with other lines of evi­dence, the data indi­cates likely impact to ground water that can be explained by hydraulic fracturing."[64] The EPA also said that the type of contamination found is "typically infeasible or too expensive to remediate or restore."[5] Industry figures rejected the EPA's findings.[65] In 2010 the U.S. Department of Health and Human Services' Agency for Toxic Substances and Disease Registry recommended that owners of tainted wells use alternate sources of water for drinking and cooking, and ventilation when showering. These recommendations were still in place as of December 2011. Encana is funding the alternate water supplies.[60]

It is important to note that not every instance of groundwater methane contamination is a result of hydraulic fracturing. Often, local water wells drill through many shale and coal layers that can naturally seep methane into the producing groundwater. This methane is often biogenic (created by organic material decomposition) in origin as opposed to thermogenic (created through "thermal decomposition of buried organic material").[66] Thermogenic methane is the methane most often sought after by oil & gas companies deep in the earth, whereas biogenic methane is found in shallower formations (where water wells are typically drilled). Through isotope analysis and other detection methods, it is often fairly easy to determine whether the methane is biogenic or thermogenic, and thus determine from where it is produced.[66] The presence of thermogenic methane does not confirm the source of gas. The gas composition and isotopic finger print must compared by experts with other known sources of gas to confirm a match.[67]

In 2011, investigators from the Colorado School of Public Health performed a study in Garfield regarding potential adverse health effects, and concluded that residents near gas wells might suffer chemical exposures, accidents from industry operations, and psychological impacts such as depression, anxiety and stress. This study (the only one of its kind to date) was never published, owing to disagreements between community members and the drilling company over the study's methods.[45]

In 2011 a Duke University study published in Proceedings of the National Academy of Sciences examined methane in groundwater in Pennsylvania and New York states overlying the Marcellus Shale and the Utica Shale. It determined that groundwater tended to contain much higher concentrations of methane near fracking wells, with potential explosion hazard; the methane's isotopic signatures and other geochemical indicators were consistent with it originating in the fracked deep shale formations, rather than any other source.[68]

The 2011 University of Texas study described the environmental impact of each of the separate parts of the overall hydraulic fracturing process, or "phases of the shale gas development life cycle."[29] These parts include of (1) drill pad construction and operation, (2) the construction, integrity, and performance of the wellbores, (3) the injection of the fluid once it is underground (which proponents consider the actual "fracking"), (4) the flowback of the fluid back towards the surface, (5) blowouts, often unreported, which spew hydraulic fracturing fluid and other byproducts across surrounding area, (5) integrity of other pipelines involved and (6) the disposal of the flowback, including waste water and other waste products.[27][28] Associated problems include (1) Groundwater Contamination, (2) Blowouts and House Explosions, (3) Water Consumption and Supply, (4) Spill Management and Surface Water Protection, (5) Atmospheric Emissions, (6) Health Effects[29] All but the injection stage were reported to be sources of contamination in the University of Texas study.[29] The study concluded that if hydraulic fracturing is to be conducted in an environmentally safe manner, these issues need to be addressed first.[29] Proponents have reported that groundwater contamination doesn't come directly from the "fracking" part of the process (the injection of hydraulic fracturing chemicals into Shale rock formations) but from other parts of the hydraulic fracturing process. Injection cannot be accomplished, however, without the accompanying stages. Poorly constructed or damaged wellbores and pipelines can allow the fluid to flow into aquifers.[29] Volatile chemicals held in waste water evaporation ponds can to evaporate into the atmosphere, or overflow. In one of the cases described by a 2012 Cornell University study (conducted in Colorado, Louisiana, New York, Ohio, Pennsylvania and Texas) impounded wastewater was released into a field and pond, killing at least 70 animals.[12] The runoff can also end up in groundwater systems. Groundwater may become contaminated by trucks carrying fracking chemicals and wastewater if they are involved in accidents on the way to fracking sites or disposal destinations. Disposal of fracking fluid by injection can cause earthquakes, and release of unprocessed or under-processed waste water into rivers can contaminate water supplies.[29] Critics have noted that it is "difficult for researchers to be objective if their university receives a lot of grants and funds from the industry."[69] An Energy Institute spokesperson said that the study was not funded by the industry. He said funds came from the university, which has a variety of funding sources.[69] There are extensive links between UT and the oil & gas industry, with the giving of Royal Dutch Shell to the university currently standing at more than $24.8 million, $4m alone having been handed over for 2012.[70][71] Since 2011, Shell has partnered Texas in a program called Shell-UT Unconventional Research, and the university has a similar research program in place with Exxon Mobil.[72] Halliburton, the largest supplier of fracking services in the United States, has also given millions of dollars to the university.[73] Statoil announced a $5m research agreement with UT's Bureau of Economic Geology in September 2011, whose program director, Ian Duncan, was the senior contributor for the parts of the Texas study to do with the environmental impacts of shale gas development.[29][74][75]

In DISH, Texas, elevated levels of disulphides, benzene, xylenes and naphthalene have been detected in the air.[44] According to an article in 'Environmental Health Perspectives,' people living near shale gas drilling sites often "complain of headaches, diarrhea, nosebleeds, dizziness, blackouts, muscle spasms, and other problems."[45] Cause-and-effect relationships have not been established.[45] Additionally, the Colorado Oil & Gas Conservation Commission has found some wells containing thermogenic methane due to oil and gas development upon investigating complaints from residents.[76]

A 2011 report by the Massachusetts Institute of Technology addressed groundwater contamination, noting "There has been concern that these fractures can also penetrate shallow freshwater zones and contaminate them with fracturing fluid, but there is no evidence that this is occurring. There is, however, evidence of natural gas migration into freshwater zones in some areas, most likely as a result of substandard well completion practices by a few operators. There are additional environmental challenges in the area of water management, particularly the effective disposal of fracture fluids". This study encourages the use of industry best practices to prevent such events from recurring.[77]

A review published in February 2012 found no direct evidence that fracking's actual injection phase resulted in contamination of ground water, and suggests that reported problems occur due to leaks in its fluid or waste storage apparatus; the review says that methane in water wells in some areas probably comes from natural resources.[6][7]

Radioactive contamination

The New York Times has reported radiation in hydraulic fracturing wastewater released into rivers in Pennsylvania.[78] It collected data from more than 200 natural gas wells in Pennsylvania and has posted a map entitled Toxic Contamination from Natural Gas Wells in Pennsylvania. Sand containing gamma-emitting tracer isotopes is used to trace and measure fractures.[21] Individuals exposed to high enough levels of radiation may experience symptoms of acute radiation syndrome, including fatigue, leukopenia, fever, diarrhea, vomiting, nose bleeds, dizziness, disorientation, low blood pressure, seizures, and tremors.[79] The Times stated "never-reported studies" by the United States Environmental Protection Agency and a "confidential study by the drilling industry" concluded that radioactivity in drilling waste cannot be fully diluted in rivers and other waterways.[80] Despite this, as of early 2011 federal and state regulators did not require sewage treatment plants that accept drilling waste (which is mostly water) to test for radioactivity. In Pennsylvania, where the drilling boom began in 2008, most drinking-water intake plants downstream from those sewage treatment plants have not tested for radioactivity since before 2006.[81] The New York Times reporting has predictably been criticized by aggrieved parties,[82] but one venerable science writer has taken issue with one instance of the newspaper's presentation and explanation of its calculations regarding dilution,[83] charging that a lack of context made the article's analysis uninformative.[84]

According to a Times report in February 2011, wastewater at 116 of 179 deep gas wells in Pennsylvania "contained high levels of radiation," but its effect on public drinking water supplies is unknown because water suppliers are required to conduct tests of radiation "only sporadically".[85] The New York Post stated that the Pennsylvania Department of Environmental Protection reported that all samples it took from seven rivers in November and December 2010 "showed levels at or below the normal naturally occurring background levels of radioactivity", and "below the federal drinking water standard for Radium 226 and 228."[86] However the samples taken by the state at at least one river, (the Monongahela, a source of drinking water for parts of Pittsburgh), were taken upstream from the sewage treatment plants accepting drilling waste water.[87]

In Pennsylvania, much of this wastewater from hydraulic fracturing operations is processed by public sewage treatment plants. However, many sewage plants say that they are incapable of removing the radioactive components of this waste, which is often released into major rivers. Industry officials, though, claim that these levels are diluted enough that public health is not compromised.[78] This is a major concern as it provides the possibility for radioactive waste to enter into public water supplies.

The New York Times has implicated the DEP in industry-friendly inactivity, requesting rather than requiring them to handle their own flowback waste rather than sending it to public water treatment facilities.[88] However, former Pennsylvania DEP Secretary John Hanger, who served under Gov. Ed Rendell (D), has affirmed that municipal drinking water throughout the state is safe, but added that the environmentalists were accurate in stating that Pennsylvania's water treatment plants were not equipped to treat hydraulic fracturing water.[89] Current Pennsylvania DEP Secretary Michael Krancer serving under Gov. Tom Corbett (R) has denied that untreated wastewater is being discharged into the state's waterways.[90] It has been observed that Corbett received over a million dollars in gas industry contributions,[91] more than all his competitors combined, during his election campaign.[92] The New York Times reported that regulations are lax in Pennsylvania.[78] The oil and gas industry is generally left to police itself in the case of accidents. Unannounced inspections are not made by regulators: the companies report their own spills, and create their own remediation plans.[78] A recent review of the state-approved plans found them to appear to be in violation of the law.[78] Treatment plants are still not equipped to remove radioactive material and are not required to test for it.[78] Despite this, in 2009 the Ridgway Borough's public sewage treatment plant, in Elk County, PA, facility was sent wastewater containing radium and other types of radiation at at 275-780 times the drinking-water standard. The water being released from the plant was not tested for radiation levels.[78] Part of the problem is that growth in waste produced by the industry has outpaced regulators and state resources.[78] It should be noted that "safe drinking water standards" have not yet been set for many of the substances known to be in hydrofracturing fluids or their radioactivity levels,[78] and their levels are not included in public drinking water quality reports.[93]

Earthquakes

A report in the UK concluded that fracking was the likely cause of some small earth tremors that happened during shale gas drilling.[94][95][96] In addition, the United States Geological Survey (USGS) reports that, "Earthquakes induced by human activity have been documented in a few locations" in the United States, Japan, and Canada, "the cause [of which] was injection of fluids into deep wells for waste disposal and secondary recovery of oil, and the use of reservoirs for water supplies."[97] The disposal and injection wells referenced are regulated under the Safe Drinking Water Act and UIC laws and are not wells where hydraulic fracturing is generally performed.[citation needed]

Several earthquakes—including a light, magnitude 4.0 one on New Year's Eve—that had hit Youngstown, Ohio, throughout 2011 are likely linked to a disposal well for injecting wastewater used in the hydraulic fracturing process, according to seismologists at Columbia University.[98] Consequently, Ohio has since tightened its rules regarding the wells,[99] increased fees, and is considering a moratorium on the practice.[100]

See also

References

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  2. 2.0 2.1 Brown, Valerie J. (February 2007). "Industry Issues: Putting the Heat on Gas". Environmental Health Perspectives (US National Institute of Environmental Health Sciences) (115(2)). http://ehp03.niehs.nih.gov/article/browseIssue.action?issue=info:doi/10.1289/issue.ehp.v115.i02.
  3. Mall, Amy (a senior policy analyst with the Natural Resources Defense Council) (December 2011). "Incidents where hydraulic fracturing is a suspected cause of drinking water contamination". U.S. Natural Resources Defense Council. http://switchboard.nrdc.org/blogs/amall/incidents_where_hydraulic_frac.html. Retrieved 23 February 2012.
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  6. 6.0 6.1 Script error
  7. 7.0 7.1 Script error
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  16. 16.0 16.1 Script error
  17. (PDF) Evaluation of Impacts to Underground Sources of Drinking Water by Hydraulic Fracturing of Coalbed Methane Reservoirs; National Study Final Report (Report). EPA. June 2004. http://www.epa.gov/ogwdw/uic/pdfs/cbmstudy_attach_uic_final_fact_sheet.pdf. Retrieved 23 February 2011.
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