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To frack or not to frack?

Posted on 27 March 2013 by gws

In a recent op-ed in the New York Times, Geosciences Professor Susan Brantley and public policy expert Anna Meyendorff argued that “the facts on fracking” are mostly in favor of the practice, and that “… if natural gas displaces coal, then fracking is good not only for the economy but also for the global environment.” While we agree with several statements made in that article, a critical environmental impact, namely leaks, was treated inadequately by the authors. So it shall be considered here.

Existing, but yet limited research on atmospheric composition near shale-gas wells shows that fugitive methane emissions from the recent, rapid development of this unconventional natural gas source is possibly higher than presumed by EPA and the industry. If so, a large fraction of US CO2 emissions reductions attributed to shale-gas displacing coal as energy source may be undone by increased emissions of the much stronger greenhouse gas methane. This post shall summarize the specific knowledge development in the last two years.

The shale gas development controversy

Use of “fracking”, shorthand for hydraulic fracturing, has led to a large increase in natural gas production in the US, and is likely going to be practiced in other countries, such as the UK, in the near future. Fracking of rock, namely shale, at different depths releases initially large amounts of (“shale“-) gas, natural gas composed mostly of methane, which cannot be mined from the shale by conventional means. Fracking and its implications have been in the news for several years now, particularly due to (peer-reviewed) findings of methane-contaminated water-wells in areas where fracking is prevalent [1], but also due to findings of increased ambient air pollutant concentrations, such as benzene. There is a very critical documentary, there are blogs, and there even is a Hollywood movie now. All along, the industry has been aggressively defending its fracking endeavors, including with a documentary-style video and TV commercials. Suffice it to say, fracking is highly controversial in the US.

I was approached in summer 2011 at a gathering in San Antonio, TX, on what I thought about fracking. I replied with what I thought true: That there is no doubt that burning natural gas instead of coal drastically reduces combustion-related CO2 emissions per unit electric or heat-energy produced and that the current fracking boom, despite what I then considered to be localized issues with water and air contamination, was on the whole beneficial with respect to reducing greenhouse gas emissions. At that time, I had not yet read one of the very few peer-reviewed studies on the topic by Howarth et al. (2011)[2], which showed that fugitive methane emissions from the investigated shale-gas development may be significantly higher than from conventional natural gas development. Table 2 from that publication is reproduced below.

 Table 2 from Howarth 2011 paper

Total methane emissions may even be as high as to negate the natural gas’ greenhouse savings (compared to burining coal) if leak rates are applicable to the industry as a whole. The paper was followed by an interesting discussion in Nature [3, 4]. It did not take long before critical comments of the study appeared, including a peer-reviewed comment [5] and its rebuttal [6]. Another study, this time by NOAA researchers [7], appeared in 2012, again calculating higher than presumed methane (and associated other hydrocarbon) leak rates. That was contrasted by another publication arguing that actual emissions are much lower than estimated from bottom-up calculations [8].

Why is the leak rate so important?

Basically all studies comparing the life cycle greenhouse gas emissions of various fossil fuels find significant benefits of using natural gas as compared to coal, e.g. [9-12]; and find minor differences between conventional gas and shale gas [13]. However, these studies generally calculate from available inventory data over the lifetime of the well that the leak rate is around one percent of production. Critical for such estimates is which potentially leaking processes are included, and what emissions are assumed for each.

As the dominant component of natural gas is methane, leaks should be avoided as much as possible and the remainder abated (e.g. via flaring). This is because methane, molecule by molecule, is a powerful greenhouse gas, 25-72 times as effective compared to CO2. However, general practice in the industry includes for instance (natural gas) venting (to the atmosphere).

Through climate modeling, and assuming a range of different leak rates as gas replaces coal, Wigley [14](and references therein) showed that only at low leak rates, such as assumed by the industry [3], a transition from coal to gas confers the benefits attributed to it:

The most important result, however, […], is that, unless leakage rates for new methane can be kept below 2%, substituting gas for coal is not an effective means for reducing the magnitude of future climate change.

Another study by Alvarez et al. in PNAS [15] suggests that a leak rate around 3% would still be tolerable, especially when viewed over longer time scales. Whether low leak rates have been and are accomplished in shale gas exploration is coming more into question via some new research carried out in 2012 [16]. While the previous NOAA pilot study (which covered a larger region of oil and gas development in Colorado) from 2008 has been disputed [17, 18], newer data presented during the AGU 2012 Fall Meeting does not appear to paint a better picture. Throughout three sessions entitled Atmospheric Impacts of Oil and Gas Development, several research groups showed highly elevated methane concentrations in areas of new gas development. An example from the abstract of Caulton et al. (presentation A21J-03. Quantifying Methane Emissions from Shale Gas Wells in Pennsylvania) is shown below:

 ambient methane in fracking area

Figure 1: CH4 distribution over southwestern Pennsylvania on 6/21/12 between 9:00 and 10:30 EDT plotted in Google Earth.

Similar results were obtained in Dish, Texas, by Khan et al. (Presentation A21J-04. Fugitive greenhouse gas emissions from shale gas activities – a case study of Dish, TX), and by Presto et al. (Poster A23B-0204. Atmospheric Impacts of Marcellus Shale Gas Activities in Southwestern Pennsylvania), while lower enhancements were reported by Ramos-Garcés et al. (Poster A23B-0203. Methane and its Stable Isotope Signature Across Pennsylvania: Assessing the Potential Impacts of Natural Gas Development and Agriculture). All abstracts can be accessed via the Fall Meeting’s webpage.

High ambient abundances of methane do not necessarily equate to high leak rates. However, with the help of meteorological data, elevated methane concentrations in plumes from the fracking sites can be converted into real-world emissions data. Several authors noted that higher methane abundances occurred in areas with more active development. Simply assuming that shale gas development has similar leak rates than conventional natural gas development over the lifetime of the well is not accurate. This is because shale gas well production declines rapidly [4] within the first two to three years. Thus, the industry has to keep up with well decline through constantly drilling new wells. But as fugitive methane emissions occur dominantly during well development, the shorter lifetime of a shale gas well means larger methane emissions per marketed natural gas. A recent study by the Joint Institute for Strategic Energy Analysis [19] highlights the variability of calculated lifetime fugitive methane emissions based on well production variability in the Texas Barnett shale area. The average emissions were estimated to be 1.3%, but ranged from 0.8 to 2.8% for the Barnett shale area depending on well productivity. The report acknowledged accounting differences, such as compared to the Howarth et al. [2] estimates. It further highlighted that many of these fugitive emissions are preventable and that

… better and more recent measurements of fugitive emissions from well and processing equipment, as well as pipelines at all stages—gathering, transmission, and distribution lines—are warranted because the existing data are sparse and old.

Unfortunately, as the gas price is currently so low, there is also little incentive yet to invest in equipment to minimize fugitive emissions.

Many results presented at AGU are likely to enter the peer-reviewed literature in 2012. They may not conclusively answer the critical question of what a regionally representative leak rate of the current rapid shale gas development is. But a larger study asking this question is already underway. Meanwhile, the development of standards for shale gas exploration, such as this one, suggest that similar problems can be prevented in Europe and elsewhere.

An old problem

The shale gas controversy reminds us the other greenhouse gas problem our fossil fuel dominated energy system contributes to: (fugitive) methane and other hydrocarbon emissions. A recent review [20] suggests that 24% of all man-made methane emissions are due to fossil fuel infrastructure (18% from oil and gas plus 6% from coal mining). The authors’ Figure 1, reproduced below, shows that man-made emissions may still be growing.

 global anthropogenic methane emissions

Figure 2. (a) Methane concentration in the atmosphere. (b) Anthropogenic methane emissions by source in 2010. (c) Anthropogenic methane emission by sectors in 2010. (d) Methane emission trends by sectors from 1990–2010.

On the other hand, a recent study by Simpson et al. [21] showed that the past decline in methane atmospheric growth rate between the mid-1980s and 2010 was likely in large part due to “reduced fugitive fossil fuel emissions”. Another study presented during the AGU Fall Meeting, Schwietzke et al. (Poster A23B-0206. Reducing Uncertainty in Life Cycle CH4 Emissions from Natural Gas using Atmospheric Inversions), is aiming at constraining the average global leak rate from natural gas use. The preliminary results suggest that leak rates as high as 6% are unlikely. Unfortunately, both of these views are global and cannot directly inform about fugitive emissions from shale gas development.

Locally, old and ageing infrastructure is the other recognized source of fugitive emissions. As recently shown by Phillips et al. [22], storing, transporting and using natural gas has its own leak issues. As they showed (Figure 3), there are numerous gas leaks throughout Boston.

elevated methane mixing ratios in Boston

Figure 3.  A total of 3,356 methane leaks (yellow spikes) above background levels of 2.5 ppm mapped across Boston’s 785 road miles (red). source

What conclusions can we draw?

The brouhaha about shale gas development is hardly an invention of some leftwing greenies. There are some clear scientific objections and uncertainties, not only on the water side, but also with respect to atmospheric emissions. Critical questions asked now may prevent future regrets. Even moderate voices are calling for a tighter regulation of the industry, peer-reviewed science provides evidence that such regulation is justified, and a major international risk management company suggests tight planning, careful extraction, and environmental impact monitoring.

So instead of business as usual, critical questions ought to be asked, including whether touting shale gas as a "bridge fuel" is actually justified [23].


1.    Osborn, S.G., A. Vengosh, N.R. Warner, and R.B. Jackson, Methane contamination of drinking water accompanying gas-well drilling and hydraulic fracturing. Proceedings of the National Academy of Sciences of the United States of America, 2011. 108(20): p. 8172-8176.

2.    Howarth, R.W., R. Santoro, and A. Ingraffea, Methane and the greenhouse-gas footprint of natural gas from shale formations. Climatic Change, 2011. 106(4): p. 679-690.

3.    Tollefson, J., Air sampling reveals high emissions from gas field. Nature, 2012. 482(7384): p. 139-140.

4.    Howarth, R.W., A. Ingraffea, and T. Engelder, Natural gas: Should fracking stop? Nature, 2011. 477(7364): p. 271-275.

5.    Cathles, L.M., L. Brown, M. Taam, and A. Hunter, A commentary on "The greenhouse-gas footprint of natural gas in shale formations" by RW Howarth, R. Santoro, and Anthony Ingraffea. Climatic Change, 2012. 113(2): p. 525-535.

6.    Howarth, R.W., R. Santoro, and A. Ingraffea, Venting and leaking of methane from shale gas development: response to Cathles et al. Climatic Change, 2012. 113(2): p. 537-549.

7.   Petron, G., G. Frost, B.R. Miller, A.I. Hirsch, S.A. Montzka, A. Karion, M. Trainer, C. Sweeney, A.E. Andrews, L. Miller, J. Kofler, A. Bar-Ilan, E.J. Dlugokencky, L. Patrick, C.T. Moore, T.B. Ryerson, C. Siso, W. Kolodzey, P.M. Lang, T. Conway, P. Novelli, K. Masarie, B. Hall, D. Guenther, D. Kitzis, J. Miller, D. Welsh, D. Wolfe, W. Neff, and P. Tans, Hydrocarbon emissions characterization in the Colorado Front Range: A pilot study. Journal of Geophysical Research-Atmospheres, 2012. 117.

8.   O'Sullivan, F. and S. Paltsev, Shale gas production: potential versus actual greenhouse gas emissions. Environmental Research Letters, 2012. 7(4).

9.    Stephenson, T., J.E. Valle, and X. Riera-Palou, Modeling the Relative GHG Emissions of Conventional and Shale Gas Production. Environmental Science & Technology, 2011. 45(24): p. 10757-10764.

10.  Hultman, N., D. Rebois, M. Scholten, and C. Ramig, The greenhouse impact of unconventional gas for electricity generation. Environmental Research Letters, 2011. 6(4).

11.  Burnham, A., J. Han, C.E. Clark, M. Wang, J.B. Dunn, and I. Palou-Rivera, Life-Cycle Greenhouse Gas Emissions of Shale Gas, Natural Gas, Coal, and Petroleum. Environmental Science & Technology, 2012. 46(2): p. 619-627.

12.  Burnham, A., Life-Cycle Greenhouse Gas Emissions of Shale Gas, Natural Gas, Coal, and Petroleum (vol 46, pg 619, 2012). Environmental Science & Technology, 2012. 46(4): p. 2482-2482.

13.  Weber, C.L. and C. Clavin, Life Cycle Carbon Footprint of Shale Gas: Review of Evidence and Implications. Environmental Science & Technology, 2012. 46(11): p. 5688-5695.

14.  Wigley, T.M.L., Coal to gas: the influence of methane leakage. Climatic Change, 2011. 108(3): p. 601-608.

15.  Alvarez, R.A., S.W. Pacala, J.J. Winebrake, W.L. Chameides, and S.P. Hamburg, Greater focus needed on methane leakage from natural gas infrastructure. Proceedings of the National Academy of Sciences of the United States of America, 2012. 109(17): p. 6435-6440.

16.  Tollefson, J., Methane leaks erode green credentials of natural gas. Nature, 2013. 493(7430): p. 12-12.

17.  Levi, M.A., Comment on "Hydrocarbon emissions characterization in the Colorado Front Range: A pilot study" by Gabrielle Petron et al. Journal of Geophysical Research-Atmospheres, 2012. 117.

18.  Pétron, G., G.J. Frost, M.K. Trainer, B.R. Miller, E.J. Dlugokencky, and P. Tans, Reply to comment on “Hydrocarbon emissions characterization in the Colorado Front Range—A pilot study” by Michael A. Levi. Journal of Geophysical Research: Atmospheres, 2013. 118(1): p. 236-242.

19.  Logan, J., G. Heath, J. Macknick, E. Paranhos, W. Boyd, and K. Carlson, Natural Gas and the Transformation of the U.S. Energy Sector: Electricity, 2012. p. Medium: ED; Size: 255 pp.

20.  Yusuf, R.O., Z.Z. Noor, A.H. Abba, M.A. Abu Hassan, and M.F.M. Din, Methane emission by sectors: A comprehensive review of emission sources and mitigation methods. Renewable & Sustainable Energy Reviews, 2012. 16(7): p. 5059-5070.

21.  Simpson, I.J., M.P. Sulbaek Andersen, S. Meinardi, L. Bruhwiler, N.J. Blake, D. Helmig, F.S. Rowland, and D.R. Blake, Long-term decline of global atmospheric ethane concentrations and implications for methane. Nature, 2012. 488(7412): p. 490-494.

22.  Phillips, N.G., R. Ackley, E.R. Crosson, A. Down, L.R. Hutyra, M. Brondfield, J.D. Karr, K.G. Zhao, and R.B. Jackson, Mapping urban pipeline leaks: Methane leaks across Boston. Environmental Pollution, 2013. 173: p. 1-4.

23.  Stephenson, E., A. Doukas, and K. Shaw, Greenwashing gas: Might a 'transition fuel' label legitimize carbon-intensive natural gas development? Energy Policy, 2012. 46: p. 452-459.

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Comments 1 to 19:

  1. Another issue to consider is what I would call the "Keystone effect", where opponents of the pipeline argue that approval of it will ramp up and solidify tar sands extraction. Likewise, every new natural gas pipeline, and evey new well drilled, further entrenches the fossil fuel regime. These companies are making investments designed to pay off over decades. Natural gas may very well make sense as a bridge fuel, but that's certainly not how industry looks at it, nor do the politicians they support.  If they build the infrastructure, they're not just going to give it up if scientists say "sorry folks, burning any more natural gas isn't safe".  Bridges are designed to be crossed, not parked on.  We have a carbon budget we need to stay under, not just get to it in 2040 instead of 2030.

    I would say we need to stop all new drilling right now while exploting all the currently drilled wells to truly use as a bridge.  I've seen new infrastructure put in place in a matter of weeks to months; we can resume drilling at a later point if/when we have better data as to its safety

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  2. I've been looking carefully at the drilling/fracking thing and while I reckon that the fracking CAN be done safely under sufficient regulatory scrutiny, the effect is to supply a seive.  The Boston leak study is a deal killer.   The lack of effective regulation is a political problem and is solvable. 

    The fracked gas (ANY gas actually) can only be used as a bridge if the leaks are plugged.  That needs a financial incentive, and further regulation.  The price of gas can't be the only thing keeping people vigilant about the leaks, there has to be a price on the leaks.  The CO2 tax idea, on CO2 equivalents - from fossil sources (because a cow absorbs the CO2 it emits as part of the normal cycle, just like tree, fire, CO2, tree)... that CO2 tax has to be in the price of the gas, and the penalty for leaks has to be far far higher... so the economics of sealing them becomes   "persuasive".  

    Otherwise we are at the mercy of those attempting to reshape our our planetary environment to support the return of giant lizards.

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  3. Did they measure the d14C in this elevated level of methane around fracking wells? The lower than background d14C would confirm that the source is fosil fuel rather than active biosphere, therefore settle the question of leaks that FF industries are obviously denying despite rumours.

    Or are the expected isotopic differences too small to measure?

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  4. Chriskoz @3:

    The problem is that there is 'natural' fossil methane 50 meters down in some places, so the expected isotopic fingerprint would be that of fossil carbon. Also,  shallow methane is more likely to be present in areas where fracking occurs, because both may have the same source. Therefore, only a correlation cannot prove causation. The only way to know is to do a measurement before drilling, and one after.

    Also, even when you can prove a contamination is from a nearby well, the cause may not be the fracking process, but a leak near the top of the well. The latter can be mitigated by better regulation, the former cannot, so it is important to know what actually is the case. 

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  5. Distinguishing biogenic (from say coal beds) from thermogenic is not straightforward when biogenic source is too old for C14 to be of use. If you only have CH4 then thermogenic is lighter (d13 depleted) but it can be a difficult call. A better indicator of thermogenic gas is other hydrocarbons (eg ethane, butane) which would be unlikely biogenic gases. According to this report which looked at contamination discussed in the Gasland movie and did both geochemical and isotope studies, water contamination was from biogenic sources.

    And then you have the problem that fracking operations could theoretically at least mobilize biogenic methane.

    I havent seen Gaslands but I am suspicious of many anti-fracking claims. While needing a good regulatory environment (like all mining), it seems strange to suddenly have a hue and cry about something thats been done safely for decades.


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  6. "... it seems strange to suddenly have a hue and cry about something thats been done safely for decades."

    What's been done for decades was simple water and sand fracking; what is recently being done is the use of many toxic chemicals in the frack fluid, some of which the oil corps *will not* iallow nformation on. To me that is a huge big red flag. "Gasland" is a good movie, and you ought to view it; the 'flaming faucet' isn't far from where I live, and I live smack dab in the middle of one of the biggest fracking frenzies in the country. given the lies we were told, vis--avis 'safety' in the Gulf (and in many other despoiled environments), I have little reason to believe fracking is as safe as the oil corps claim it is.

    I am *not* willing to bet my source of fresh water--in a  region where it is already scarce--to it possibly being ruined. And when  you ruin an aquifer--think the Imperial Valley--it's ruined. No more water. An average fracked well uses between 4-6 million gallons of fresh water (brackish/salt water won't work) and the vast majority of that is ruined for animal use, human and non-human. If ever there was a call for the precautionary principle, writ larger, I'm not sure of where. There is no substitute for water.

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  7. 14C is not (yet) used in these studies. It would be challenging on methane anyway.

    @scaddenp: please follow the link in the post re water contamination. Thanks for the comment re 13C isotopes. OTOH, you are recycling a myth stating "that's been done safely for decades". Todays techniques are different and they do pose different challenges, particularly re fugitive emissions. Safety re water contamination is a different question.

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  8. gws - 14C would be no use in distinquishing biogenic coal gas from thermogenic methane - in both sources, 14C would be undetectably low.

    I followed the link but still scratching my head somewhat.The link I posted refers to analysis of aqifer that vroomie is worrying about and come up biogenic. I note that COGCC article does identify a well contaminated by thermogenic gas but tracing it to a leaking cap. There is not a lot of transparency around this data.

    I would be first to admit that I have next to no information on the fracking practise in the States. That extra chemicals (surfacants) are added is known to me, but that is common here too in geothermal. I would say that they pose managable risks involved with storage, transport and particularly in seepage from circulation pits. Is the risk from these any higher than that involved from other chemicals in common use in various industries? I dont see how the new chemicals  increase the risk of contamination of aquifers from the fracking process itself however. The key factors a casing seal and eventual well seal.

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  9. I do note from those reports that in the areas where fracking seems to cause concern (and the names mean nothing to me so could be corrected here), the thermogenic gas is "wet". (contains other higher chain hydrocarbons, -butane, ethane, propane). Their presense is more strongly diagnostic and easier to measure than isotope studies which can get muddied by mixing of biogenic and thermogenic methane.

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  10. Why ignore the science on fracking and go with anecdotes when you don't do that with climate change?  Do any of you know what chemicals go into fracking?  And do you know how many years and how many wells were drilled before any claims of water contamination occurred?  (If you want to protect water, you have to put safety seals on all wells so the owner cannot contaminate the well, regulate all water well drilling, etc.  Should we go there?) I'm not seeing the science here.

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  11. @10: Who is your comment addressed to?

    The post does not address the fracking&water issue and was not intended to. Please take discussions of the water issue to other venues. I know the issues cannot be completely seperated, that this discussion should be not morphing into one not related to the post topic.

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  12. "Do any of you know what chemicals go into fracking?"

    thanks for asking: here's the ones the industry *admits* to.


    Methanol? Formic acid? Potassium hydroxide? Trust your well water to thousands of oil and gas wells going through your aquifer? Not me, not yet.

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  13. gws:

    With all due repect, the title of your OP is all-encompassing with repsect to the upsides and downsides of fracking. Futhermore, there is no other article posted on SkS that addresses fracking and water impacts.

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  14. Just for a look at some at myths (pro and anti) in fracking, I found this popular mechanics article useful. It annoyingly doesnt include links but I found it easy enough to use Mr Google to follow up on interesting stuff raised there. US context only. Vroomie, I think pipes with chemicals going through your aquifer are pretty minor risk but it would depend on your regulatory environemnt. Ie what are the rules about penetrating an aquifer (for any reason from fracking to putting in a bridge pile) and do regulators check the seal tests for each and every well.

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  15. People may not know, or have forgotten, that vroomie is from Wyoming. Wyoming has a peculiar set of circumstances that make his concerns valid, chief among them that the aquifers there are deep and the shale beds are shallow ... typically 3000 feet below the surface, not the 6000 feet or more in the Marcellus, Natchez, or Texas shale beds. In his case, the chance of the fracking process itself, not just holes drilled through the aquifer, causing contamination is real. The EPA issued preliminary findings a couple of years ago to that effect around Pavilion a couple of years ago, although those findings were predictably contested by the usual suspects including James Inhofe.

    He also makes the point that it takes several million gallons of fresh water to frack a well, but doesn't talk about what happens to the contaminated water afterwards. Ideally, it is injected into older, retired wells, but I am not sure there are that many in Wyoming, or at least enough of them to cope with the demand caused by the fracking boom. What more typically happens there is the bad water is pumped into temporary holding ponds of earth with plastic liners, and left to evaporate. If they fail, they can do a great deal of local damage.

    On a side note, is that where the discrepancy in fugitive emissions between conventional and fracked gas wells comes from? Is there that much methane dissolved in the water/frack fluid mixture, or is it some other factor I haven't considered? While I have no cause to doubt the study findings, I am at a loss to explain so large a discrepancy.

    If he wants to know what is in the fracking fluids, all he has to do is convince the Wyoming state legislature to mandate disclosure, like Texas did. That may be a hard sell in the home of Dick Cheney, but there are enough pissed off ranchers to get it done if they got themselves organized.

    P.S. The Imperial Valley doesn't have an aquifer to speak of ... they get all of their water out of a big ditch from the Imperial Dam on the Colorado River. You might be thinking of the Westlands Water District in the northwest San Joaquin Valley, which did trash their ground water so badly that they have had to take major acreage out of production.


    Best wishes, 


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  16. some answers and analyses appeared on Climate Central here.

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  17. New study on natural gas migration into water wells here.

    From the abstract:

    "Methane was detected in 82% of drinking water samples, with average concentrations six times higher for homes <1 km from natural gas wells (P = 0.0006). Ethane was 23 times higher in homes <1 km from gas wells (P = 0.0013); propane was detected in 10 water wells, all within approximately 1 km distance (P = 0.01). Of three factors previously proposed to influence gas concentrations in shallow groundwater (distances to gas wells, valley bottoms, and the Appalachian Structural Front, a proxy for tectonic deformation), distance to gas wells was highly significant for methane concentrations (P = 0.007; multiple regression), whereas distances to valley bottoms and the Appalachian Structural Front were not significant (P = 0.27 and P = 0.11, respectively)."

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  18. Note that the UT/EDF study was just published in PNAS (a useful press release is here). It shows that extensive mitigation of emissions as advocated does work as expected, pushing total "leaked" emissions quite low (likely <1%). Unfortunately, mostly sites where active mitigation measures are in place were investigated.

    While we study the results and wait for more studies to be published, stay tuned for an update to this post some time in the fall.

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  19. The Purdue study listed above (Caulton et al.) has now been published:

    Toward a better understanding and quantification of methane emissions from shale gas development, Dana Caulton et al, PNAS, April 14, 2014, doi: 10.1073/pnas.1316546111

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