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A Critical Review of the Risks to Water Resources from Unconventional Shale Gas Development and Hydraulic Fracturing in the United States

07 Mar 2014-Environmental Science & Technology (American Chemical Society)-Vol. 48, Iss: 15, pp 8334-8348
TL;DR: Analysis of published data reveals evidence for stray gas contamination, surface water impacts in areas of intensive shale gas development, and the accumulation of radium isotopes in some disposal and spill sites.
Abstract: The rapid rise of shale gas development through horizontal drilling and high volume hydraulic fracturing has expanded the extraction of hydrocarbon resources in the U.S. The rise of shale gas development has triggered an intense public debate regarding the potential environmental and human health effects from hydraulic fracturing. This paper provides a critical review of the potential risks that shale gas operations pose to water resources, with an emphasis on case studies mostly from the U.S. Four potential risks for water resources are identified: (1) the contamination of shallow aquifers with fugitive hydrocarbon gases (i.e., stray gas contamination), which can also potentially lead to the salinization of shallow groundwater through leaking natural gas wells and subsurface flow; (2) the contamination of surface water and shallow groundwater from spills, leaks, and/or the disposal of inadequately treated shale gas wastewater; (3) the accumulation of toxic and radioactive elements in soil or stream sediments near disposal or spill sites; and (4) the overextraction of water resources for high-volume hydraulic fracturing that could induce water shortages or conflicts with other water users, particularly in water-scarce areas. Analysis of published data (through January 2014) reveals evidence for stray gas contamination, surface water impacts in areas of intensive shale gas development, and the accumulation of radium isotopes in some disposal and spill sites. The direct contamination of shallow groundwater from hydraulic fracturing fluids and deep formation waters by hydraulic fracturing itself, however, remains controversial.

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A Critical Review of the Risks to Water Resources from
Unconventional Shale Gas Development and Hydraulic Fracturing in
the United States
Avner Vengosh,*
,
Robert B. Jackson,
,
Nathaniel Warner,
§
Thomas H. Darrah,
and Andrew Kondash
Division of Earth and Ocean Sciences, Nicholas School of the Environment, Duke University, Durham, North Carolina 27708,
United States
School of Earth Sciences, Woods Institute for the Environment, and Precourt Institute for Energy, Stanford University, Stanford,
California 94305, United States
§
Department of Earth Sciences, Dartmouth College, Hanover, New Hampshire 03755, United States
School of Earth Sciences, The Ohio State University, Columbus, Ohio 43210, United States
*
S
Supporting Information
ABSTRACT: The rapid rise of shale gas development through horizontal drilling
and high volume hydraulic fracturing has expanded the extraction of hydrocarbon
resources in the U.S. The rise of shale gas development has triggered an intense
public debate regarding the potential environmental and human health eects
from hydraulic fracturing. This paper provides a critical review of the potential
risks that shale gas operations pose to water resources, with an emphasis on case
studies mostly from the U.S. Four potential risks for water resources are
identied: (1) the contamination of shallow aquifers with fugitive hydrocarbon
gases (i.e., stray gas contamination), which can also potentially lead to the
salinization of shallow groundwater through leaking natural gas wells and
subsurface ow; (2) the contamination of surface water and shallow groundwater
from spills , leaks, and/or the disposal of inadequately treated shale gas
wastewater; (3) the accumulation of toxic and radioactive elements in soil or
stream sediments near disposal or spill sites; and (4) the overextraction of water
resources for high-volume hydraulic fracturing that could induce water shortages or con icts with other water users, particularly
in water-scarce areas. Analysis of published data (through January 2014) reveals evidence for stray gas contamination, surface
water impacts in areas of intensive shale gas development, and the accumulation of radium isotopes in some disposal and spill
sites. The direct contamination of shallow groundwater from hydraulic fracturing uids and deep formation waters by hydraulic
fracturing itself, however, remains controversial.
1. INTRODUCTION
Production from unconventiognal natural gas reservoirs has
substantially expanded through the advent of horizontal drilling
and high-volume hydr aulic fracturing (Figure 1). These
technological advances have opened vast new energy sources,
such as low-permeability organic-rich shale formations and
tight-sand reservoirs, altering the domestic energy landscape
in the United States.
13
The total production of natural gas has
increased by more than 30% during the past decade. In 2012,
unconventional shale gas and tight sand productions were
respectively accounting for 34% and 24% of the total natural
gas production in the U.S. (0.68 trillion m
3
).
4
The increase in energy production has been broadly
distributed across the United States (Figure 2) and densely
distributed within speci c shale plays (Figure 3). Unconven-
tional hydrocarbon extraction from organic-rich shale for-
mations is now active in more than 15 plays in the U.S. In PA
alone, 7234 shale gas wells were drilled into the Marcellus
Formation
5
in addition to the 34 376 actively producing
conventional oil and gas wells in that state (2012 data; Figure
3).
6
At the end of 2012, the Marcellus Shale (29%), Haynesville
Shale (23%), and Barnett Shale (17%) dominated production
of natural gas (primarily methane, ethane, and propane) from
shales in the U.S., with the remaining 31% of total shale gas
production contributed by more than a dozen basins (Figure
1). Oil and hydrocarbon condensates are also targeted in
numerous basins, including the Barnett, Eagle Ford, Utica-Point
Pleasant, and Bakken.
4
Future energy forecasts suggest that U.S. unconventional
natural gas production from shale formations will double by
Special Issue: Understanding the Risks of Unconventional Shale Gas
Development
Received: November 18, 2013
Revised: February 9, 2014
Accepted: February 18, 2014
Critical Review
pubs.acs.org/est
© XXXX American Chemical Society A dx.doi.org/10.1021/es405118y | Environ. Sci. Technol. XXXX, XXX, XXXXXX

2035 and generate 50% of the total domestic natural gas
production.
4
Similarly, U.S. domestic oil production from
unconventional shale formations is projected to increase by as
much as 15% over the next several decades.
7
Unconventional
extraction (hor izontal drilling and high volume hydraulic
fracturing) for shale gas has already expanded to Canada
8
and will soon be launched on a global scale, with signi cant
reservoirs in South America, northern and southern Africa,
Europe, China,
9,10
and Au stralia.
11,12
The current global
estimate of natural gas reserves in unconventional shale is
approximately 716 trillion m
3
(2.53 × 10
13
Mcf).
11,12
Despite the large resource potentials and economic benets,
the rapid expansion of shale gas development in the U.S. has
triggered an intense public debate over the possible environ-
mental and human health implications of the unconventional
energy development. Some primary concerns include air
pollution, greenhouse gas emissions, radiation, and ground-
water and surface water contamination.
1,3,1336
These concerns
have been heightened because the 2005 Energy Policy Act
exempts hydraulic fracturing operations from the Safe Drinking
Water Act (SDWA). The only exception to the exemption is
the injection of diesel fuel. Additionally, because environmental
oversight for most oil and gas operations is conducted by state
rather federal agencies, the r egulation, monitoring, and
enforcement of various environmental contamination issues
Figure 1. Evolution of the volume of natural gas production from
dierent unconventional shale plays in the U.S. Data from the U.S.
Energy Information Administration.
4
Figure 2. Map of unconventional shale plays in the U.S. and Canada, based on data from the U.S. Energy Information Administration.
4
Environmental Science & Technology Critical Review
dx.doi.org/10.1021/es405118y | Environ. Sci. Technol. XXXX, XXX, XXXXXXB

related to unconventional shale gas development are highly
variable throughout the U.S.
3739
This paper provides an overview and synopsis of recent
investigations (updated to January 2014) into one set of
possible environmental impacts from unconventional shale gas
development: the potential risks to water resources. We identify
four potential modes of water resource degradation that are
illustrated schematically in Figure 4 and include (1) shallow
aquifers contaminated by fugitive natural gas (i.e., stray gas
contamination) from leaking shale gas and conventional oil and
gas wells, potentially followed by water contamination from
hydraulic fracturing uids and/or formation waters from the
deep formations; (2) surface water contamination from spills,
leaks, and the disposal of inadequately treated wastewater or
hydraulic fracturing uids; (3) accumulation of toxic and
radioactive elements in soil and the sediments of rivers and
lakes exposed to wastewater or uids used in hydraulic
fracturing; and (4) the overuse of water resources, which can
compete with other water uses such as agriculture in water-
limited environments.
2. GROUNDWATER CONTAMINATION
2.1. Stray Gas Contamination. Elevated levels of methane
and other aliphatic hydrocarbons such as ethane and propane in
shallow drinking water wells pose a potential ammability or
explosion hazard to homes with private domestic wells. The
saturation level of methane in near-surface groundwater is
about 28 mg/L ( 40 cc/L) and thus the U.S. Department of
the Interior recommends monitoring if water contains more
than 10 mg/L (14 cc/L) of methane and immediate action if
concentrations rise above 28 mg/L. Several states have dened
a lower threshold (e.g., 7 mg/L in PA), from which household
utilization of methane-rich groundwater is not recommended.
Stray gas migration in shallow aquifers can potentially occur
by the release of gas-phase hydrocarbons through leaking
casings or along the well annulus, from abandoned oil and gas
wells, or potentially along existing or incipient faults or
fractures
40
with target or adjacent stratigraphic formations
following hydraulic fracturing and drilling (Figure 4).
27
The
latter mechanism poses a long-term risk to shallow ground-
water aquifers. Microseismic data suggest that the deformation
and fractures developed following hydraulic fracturing typically
extend less than 600 m above well perforations, suggesting that
fracture propagation is insu cient to reach drinking-water
aquifers in most situations.
41
This assertion is supported by
noble gas data from northeastern PA,
42
yet stray gas migration
through fractures and faults is considered a potential
mechanism for groundwater contamination.
40
Across the northeastern Appalachian Basin in PA, the
majority of shallow groundwater had detectable, naturally
occurring methane with thermogenic stable-isotope ngerprints
(e.g., δ
13
CCH
4
and δ
2
HCH
4
).
2729,42,43
These ndings
imply that the high methane in shallow aquifers from this
region is predominantly thermogenic in origin.
28,29,42,43
In
northeastern PA, however, a subset of shallow drinking water
wells consistently showed elevated meth ane, ethane, and
Figure 3. Map of active unconventional (yellow) and conventional (purple) oil and gas wells in Pennsylvania and West Virginia. Note areas of
coexisting conventional and unconventional development (e.g., southwestern PA and WV) relative to areas of exclusively unconventional
development (e.g., northeastern PA). Well locations were obtained from the West Virginia Geological Survey (http://www.wvgs.wvnet.edu/) and
the Pennsylvania Department of Environmental Protections oil and gas reporting Web site (https://www.paoilandgasreporting.state.pa.us/
publicreports/Modules/Welcome/Welcome.aspx). The background topographic map, Marcellus Formation outline, and state boundaries were
downloaded from http://www.pasda.psu.edu/ and the Carnegie Museum of Natural History.
148
Environmental Science & Technology Critical Review
dx.doi.org/10.1021/es405118y | Environ. Sci. Technol. XXXX, XXX, XXXXXXC

propane concentrations (i.e., relatively low hydrocarbon ratios
(C
1
/C
2
)) and relatively enriched thermogenic carbon isotope
ngerprints in groundwater exclusively <1 km from shale gas
drilling sites. A subset of samples with evidence for stray gas
contamination display isotopic reversals (Δ
13
C=
δ
13
CH
4
δ
13
C
2
H
6
> 0) and proportions of methane, ethane
and propane that were consistent with Marcellus production
gases from the region, while some other wells had natural gas
compositions consistent with production gases in conventional
wells from the overlying Upper Devonian formations.
27,29
New
evaluations of the helium content
29
and noble gas geo-
chemistry
42
in these samples further supports a distinction
between naturally occurring background hydrocarbon gases
and groundwater with stray gas contamination in wells located
near (<1 km) shale gas drilling sites. Background gases
typically had lower proportions of ethane and propane and
elevated helium concentrations that reect the history of
natural gas migration from the Marcellus source rock to the
Upper Devonian reservoir rocks throughout geological
time.
29,42
Thus, the combination of gas geochemical nger-
printing suggests that stray gas groundwater contamination,
where it occurs, is sourced from the target shale formations
(i.e., the Marcellus Formation) in some cases, and from
intermediate layers (e.g., Upper Devonian Formations) in
others.
In cases where the composition of stray gas is consistent with
the target shale formation, it is likely that the occurrence of
fugitive gas in shallow aquifers is caused by leaky, failing, or
improperly installed casings in the natural gas wells. In other
cases, hydrocarbon and noble gas data also indicated that
fugitive gas from intermediate formations apparently owed up
through the outside of the well annulus and then leaked into
the overlying shallow aquifers.
27,29,42
In these cases, the isotopic
signatures and hydrocarbon ratios matched the gases in
intermediate formations rather than Marcellus shale production
gases. In sum, the combined evidence of hydrocarbon stable
isotopes, molecular hyd rocarbon ratios, and helium geo-
chemistry indicate that stray gas contamination occurs in a
subset of wells <1km from drilling in northeastern PA.
In contrast to these reports, other investigators
22,43,44
have
suggested that higher methane concentrations in shallow
groundwater were natural and could be explained by topo-
graphic factors associated with groundwater discharge zones.
Geochemical data do suggest that some natural gas migrated to
shallow aquifers in northeastern PA through geologic time.
However, these characteristics occur in areas with higher
hydraulic connectivity between the deep and shallow
formations.
34,42
A recent analysis showed that topography
was indeed a statistically signicant factor in some cases but did
not explain the variations in methane and ethane concen-
trations with respect to distance to gas wells.
29
Additional evidence for stray gas contamination because of
poor well construction is provided by the isotopic composition
of surface casing vent ow (SCV). Integrating the δ
13
C data of
methane (C1), ethane (C2), and propane (C3)
4547
showed
that stray gas contamination associated with conventional oil
wells in Alberta, Canada reected methane sourced from
intermediate formations leaking into shallow aquifers and not
from the production formations such as the Lower Cretaceous
Mannville Group.
48
Jackson et al. (2013)
49
listed several other
case studi es that demonstrate evidence for stray gas
contamination. While such studies have shown evidence for
methane, ethane, and propane contamination associated with
conventional oil production
48,50
and coal bed methane,
45
Muehlenbachs (2013)
51
also showed direct evidence for SCV
leakage from intermediate zones in newly completed and
hydraulically stimulated horizontal shale gas wells in the
Montney and Horn Ri ver areas of northeastern British
Columbia, Canada.
51
Methane leaking from the annulus of
conventional oil and gas wells was also demonstrated in PA.
52,53
Combined, these studies suggest that stray gas contamination
can result from either natural gas leaking up through the well
annulus, typically from shallower (intermediate) formations, or
through poorly constructed or failing well casings from the
shale target formations.
The migration of natural gas to the surface through the
production casing and/or well annulus is acommon
occurrence in the petroleum industry
51
and can aect a large
fraction of conventional wells. Among the 15 000 production
oil wells tested from the Gulf of Mexico, 43% have reported
cement damage after setting that leads to sustained casing
pressure (SCP). These eects increased with time; whereas
30% reported damage during the rst 5 years after drilling, the
percentage increased to 50% after 20 years.
54
Likewise, the BP
Deepwater Horizon oil spill was partly attributed to the fact
that cement at the well bottom had failed to seal o
Figure 4. Schematic illustration (not to scale) of possible modes of
water impacts associated with shale gas development reviewed in this
paper: (1) overuse of water that could lead to depletion and water-
quality degradation particularly in water-scarce areas; (2) surface water
and shallow groundwater contamination from spills and leaks of
wastewater storage and open pits near drilling; (3) disposal of
inadequately treated wastewater to local streams and accumulation of
contaminant residues in disposal sites; (4) leaks of storage ponds that
are used for deep-well injection; (5) shallow aquifer contamination by
stray gas that originated from the target shale gas formation through
leaking well casing. The stray gas contamination can potentially be
followed by salt and chemical contamination from hydraulic fracturing
uids and/or formational waters; (6) shallow aquifer contamination by
stray gas through leaking of conventional oil and gas wells casing; (7)
shallow aquifer contamination by stray gas that originated from
intermediate geological formations through annulus leaking of either
shale gas or conventional oil and gas wells; (8) shallow aquifer
contamination through abandoned oil and gas wells; (9) ow of gas
and saline water directly from deep formation waters to shallow
aquifers; and (10) shallow aquifer contamination through leaking of
injection wells.
Environmental Science & Technology Critical Review
dx.doi.org/10.1021/es405118y | Environ. Sci. Technol. XXXX, XXX, XXXXXXD

hydrocarbons.
55
In PA the overall reports of cementing, casing,
and well construction violations total 3% of all shale gas wells.
22
However a closer look at the distribution of violations shows
large variations in percentage with time (before and after 2009),
space, and type of wells.
5,56
In particular, the percentage of well
violations was much higher in northeastern and central counties
in PA (1050%).
5
Consequently, reports of stray gas
contamination in areas of unconventional shale gas develop-
ment in the northeastern Appalachian Ba sin (U.S.) and
Montney and Horn River Basins (Canada) may be associated
with leaking of oil and gas wells.
In contrast to the results from the Marcellus, Montney, and
Horn River Basins, the Fayetteville Shale in north-central
Arkansas showed no evidence of methane contamination in
groundwater. Stud ies in this area showed low meth ane
concentrations with a mostly biogenic isotopic ngerprint.
36,57
The authors hypothesized the potential for stray gas
contamination likely depends on both well integrity and local
geology, including the extent of local fracture systems that
provide ow paths for potential gas migration.
36
In addition to groundwater, surface waters could serve as an
indicator of regional migration from unconventional shale gas
development. To date, streams in areas of shale gas drilling
have not shown systematic evidence of methane contamination.
A new methodology for stream-gas sampling as a reconnais-
sance tool for evaluating natural and anthropogenic methane
leakage from natural gas reservoirs into surface waters was
recently demonstrated using inorganic and gas geochemical
tracers and could be applied more widely in areas of oil and gas
development.
59
2.2. Groundwater Contamination with Salts or Other
Dissolved Constituents. The presence of fugitive gas in
shallow drinking water wells could potentially lead to
salinization and other changes of water quality in three possible
ways. First, the leaking of natural gas can be associated with the
ow of hydraulic fracturing uids and saline formation waters to
overlying shallow aquifers. Given the buoyancy of gas, the ow
rate of denser saline water would be substantially slower than
the ow of natural gas, and would depend on both the pressure
gradients and hydraulic connectivity between the overpressur-
ized annulus or leaking sites on the wells and the overlying
aquifers.
53
An EPA study
60
near the town of Pavillion, Wyoming found
water contamination in two shallow monitoring wells. Although
this initia l stu dy was questioned for adequate s ampling
protocols,
22
a follow up study by the U.S. Geological Survey
conrmed elevated levels of specic conductance (1500 mS/
cm), pH (1011), methane (2527 mg/L), ethane, and
propane.
61
However, the mechanisms that caused the apparent
contamination of the shallow groundwater in this area are still
under investigation (i.e., contamination from surface ponds or
subsurface leaking cement from shale gas wells).
The ability to trace and identify contamination from shale gas
exploration is limited because of the relatively short time frame
since the beginning of large-scale shale gas exploration in early-
2000s c ompared to typical groundwater ow rates (i.e.,
decades). However, an evaluation of water contamination
associated with conventional oil and gas exploration provides a
much longer time frame for evaluating possible groundwater
contamination. Possible evidence of long-term (20002007)
increases in the sali nity of groundwater as sociated with
conventional oil and gas drilling was reported from Gar eld
County, CO. There, a rise of chloride concentrations in
drinking water wells was associated with an increase of methane
with a thermogenic isotopic ngerprint, both of which were
associated with an increase in the number of conventional oil
and gas wells.
62
The fraction of drinking water wells that had
chloride concentrations >250 mg/L (EPA threshold for
drinking water) in groundwater from Gareld County doubled
between 2002 (4%) and 2005 (8%), with chloride up to 3000
mg/L in drinking water wells.
62
The parallel rise in salinity and
methane with a thermogenic isotope signature in Gareld
County could reect either migration from leaking oil and gas
wells or contamination from unlined surface impoundments.
62
Overall, the geochemical composition of the salinized ground-
water in such scenarios would mimic the composition of either
the formation water in the production formations
34
or the
uids in the shallower or intermediate units (that typically have
a dierent water chemistry). While there might be evidence for
water contamination in some areas of conventional oil and gas
exploration, groundwater sites in areas aected by stray gas
contamination near shale gas sites in northeastern PA have not
to our knowledge shown signs of salinization induced directly
by leaking natural gas wells.
27,29,34
Unlike other areas in PA,
northeastern PA was developed recently and almost exclusively
for shale gas (Figure 3), with few legacy wells reported in the
area. Thus, any water contamination in this area attributable to
natural gas extraction would be related to current shale gas
operations rather than to older legacy wells. Therefore
conclusions regarding contamination from saline water and
hydraulic fracturing uids ow are restricted in both space and
time and further studies are needed to address this question.
A second mode of groundwater contamination that could
evolve from stray gas contamination is oxidation of fugitive
methane via bacteria l sulfate reduction.
50
Evidence for
dissimilatory bacterial sulfate reduction of fugitive methane
near conventional oil wells in Alberta, Canada, includes sulde
generation and
13
C-depleted bicarbonate, with lower residual
sulfate concentrations relative to the regional groundwater.
50
Bacterial sulfate reduction reactions due to the presence of
fugitive methane could trigger other processes such as reductive
dissolution of oxides in the aquifer that would mobilize redox-
sensitive elements such as manganese, iron, and arsenic from
the aquifer matrix and further reduce groundwater quality. Low
levels of arsenic and other contaminants, recorded in some
drinking water aquifers in TX, were suggested to be linked to
contamination from the underlying Barnett Shale,
63
although
evidence for a direct link to the Barnett remains uncertain.
Athirdhypotheticalmodeofshallowgroundwater
contamination associated with the presence of stray gas
contamination is the formation of toxic t rihalomethanes
(THMs), typically co-occurring with high concentrations of
halogens in the saline waters. THMs are compounds with
halogen atoms (e.g., Cl, Br, or I) substituted for hydrogens in
the methane molecule. The formation of THMs were
previously recorded in untreated groundwater in the U.S.,
unrelated to shale gas activities, but associated with agricultural
contamination of shallow aquifers.
64,65
Numerous studies have
demonstrated that the presence of halogens together with
organic matter in source waters can trigger the formation of
THMs, specically in chlorinated drinking water (see
references in Section 2.1). However, no data has to our
knowledge been reported for the presence of THMs in
groundwater associated with stray gas contamination from shale
gas wells.
Environmental Science & Technology Critical Review
dx.doi.org/10.1021/es405118y | Environ. Sci. Technol. XXXX, XXX, XXXXXXE

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    [...]

Journal ArticleDOI
TL;DR: Using noble gas and hydrocarbon tracers, this work identifies eight discrete clusters of fugitive gas contamination in eight clusters of domestic water wells overlying the Marcellus and Barnett Shales, including declining water quality through time over the Barnett.
Abstract: Horizontal drilling and hydraulic fracturing have enhanced energy production but raised concerns about drinking-water contamination and other environmental impacts. Identifying the sources and mechanisms of contamination can help improve the environmental and economic sustainability of shale-gas extraction. We analyzed 113 and 20 samples from drinking-water wells overlying the Marcellus and Barnett Shales, respectively, examining hydrocarbon abundance and isotopic compositions (e.g., C2H6/CH4, δ13C-CH4) and providing, to our knowledge, the first comprehensive analyses of noble gases and their isotopes (e.g., 4He, 20Ne, 36Ar) in groundwater near shale-gas wells. We addressed two questions. (i) Are elevated levels of hydrocarbon gases in drinking-water aquifers near gas wells natural or anthropogenic? (ii) If fugitive gas contamination exists, what mechanisms cause it? Against a backdrop of naturally occurring salt- and gas-rich groundwater, we identified eight discrete clusters of fugitive gas contamination, seven in Pennsylvania and one in Texas that showed increased contamination through time. Where fugitive gas contamination occurred, the relative proportions of thermogenic hydrocarbon gas (e.g., CH4, 4He) were significantly higher (P < 0.01) and the proportions of atmospheric gases (air-saturated water; e.g., N2, 36Ar) were significantly lower (P < 0.01) relative to background groundwater. Noble gas isotope and hydrocarbon data link four contamination clusters to gas leakage from intermediate-depth strata through failures of annulus cement, three to target production gases that seem to implicate faulty production casings, and one to an underground gas well failure. Noble gas data appear to rule out gas contamination by upward migration from depth through overlying geological strata triggered by horizontal drilling or hydraulic fracturing.

393 citations

Journal ArticleDOI
TL;DR: In this article, the authors argue that the switch from coal to natural gas for electricity generation will reduce sulfur, nitrogen, mercury, and particulate air pollution, but the question of whether natural gas will displace coal compared with renewables is open.
Abstract: Unconventional oil and natural gas extraction enabled by horizontal drilling and hydraulic fracturing (fracking) is driving an economic boom, with consequences described from “revolutionary” to “disastrous.” Reality lies somewhere in between. Unconventional energy generates income and, done well, can reduce air pollution and even water use compared with other fossil fuels. Alternatively, it could slow the adoption of renewables and, done poorly, release toxic chemicals into water and air. Primary threats to water resources include surface spills, wastewater disposal, and drinking-water contamination through poor well integrity. An increase in volatile organic compounds and air toxics locally are potential health threats, but the switch from coal to natural gas for electricity generation will reduce sulfur, nitrogen, mercury, and particulate air pollution. Data gaps are particularly evident for human health studies, for the question of whether natural gas will displace coal compared with renewables, and fo...

364 citations


Cites background from "A Critical Review of the Risks to W..."

  • ...These naturally occurring brines are often saline to hypersaline (35,000 to 200,000 mg/L TDS) (37) and contain potentially toxic levels of elements such as barium, arsenic, and radioactive radium (37, 94, 95)....

    [...]

  • ...For the first pathway, >100 violations associated with spills and leaks were reported for Pennsylvania since 2008 (37)....

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  • ..., 1 million of 4 million gallons) mixed with an increasing proportion of natural brines from the shale formations through time (37)....

    [...]

  • ...Ferrar et al. (99) documented discharge from water treatment facilities in Pennsylvania with TDS values ∼4 times the concentration of sea water (120,000 mg/L) and with elevated levels of barium, radium, and organics such as benzene....

    [...]

  • ...Across many plays (37), including the Barnett, Marcellus, and Fayetteville Shales, hydraulic fracturing typically requires 8,000 to 80,000 m3 (2 to 20 million gallons) of water for a single well (Table 1)....

    [...]

References
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Journal ArticleDOI
TL;DR: The brominated DBPs were the most genotoxic of all but have not been tested for carcinogenicity and highlighted the emerging importance of dermal/inhalation exposure to the THMs, or possibly other DBPs, and the role of genotype for risk for drinking-water-associated bladder cancer.
Abstract: Disinfection by-products (DBPs) are formed when disinfectants (chlorine, ozone, chlorine dioxide, or chloramines) react with naturally occurring organic matter, anthropogenic contaminants, bromide, and iodide during the production of drinking water. Here we review 30 years of research on the occurrence, genotoxicity, and carcinogenicity of 85 DBPs, 11 of which are currently regulated by the U.S., and 74 of which are considered emerging DBPs due to their moderate occurrence levels and/or toxicological properties. These 74 include halonitromethanes, iodo-acids and other unregulated halo-acids, iodo-trihalomethanes (THMs), and other unregulated halomethanes, halofuranones (MX [3-chloro-4-(dichloromethyl)-5-hydroxy-2(5H)-furanone] and brominated MX DBPs), haloamides, haloacetonitriles, tribromopyrrole, aldehydes, and N-nitrosodimethylamine (NDMA) and other nitrosamines. Alternative disinfection practices result in drinking water from which extracted organic material is less mutagenic than extracts of chlorinated water. However, the levels of many emerging DBPs are increased by alternative disinfectants (primarily ozone or chloramines) compared to chlorination, and many emerging DBPs are more genotoxic than some of the regulated DBPs. Our analysis identified three categories of DBPs of particular interest. Category 1 contains eight DBPs with some or all of the toxicologic characteristics of human carcinogens: four regulated (bromodichloromethane, dichloroacetic acid, dibromoacetic acid, and bromate) and four unregulated DBPs (formaldehyde, acetaldehyde, MX, and NDMA). Categories 2 and 3 contain 43 emerging DBPs that are present at moderate levels (sub- to low-mug/L): category 2 contains 29 of these that are genotoxic (including chloral hydrate and chloroacetaldehyde, which are also a rodent carcinogens); category 3 contains the remaining 14 for which little or no toxicological data are available. In general, the brominated DBPs are both more genotoxic and carcinogenic than are chlorinated compounds, and iodinated DBPs were the most genotoxic of all but have not been tested for carcinogenicity. There were toxicological data gaps for even some of the 11 regulated DBPs, as well as for most of the 74 emerging DBPs. A systematic assessment of DBPs for genotoxicity has been performed for approximately 60 DBPs for DNA damage in mammalian cells and 16 for mutagenicity in Salmonella. A recent epidemiologic study found that much of the risk for bladder cancer associated with drinking water was associated with three factors: THM levels, showering/bathing/swimming (i.e., dermal/inhalation exposure), and genotype (having the GSTT1-1 gene). This finding, along with mechanistic studies, highlights the emerging importance of dermal/inhalation exposure to the THMs, or possibly other DBPs, and the role of genotype for risk for drinking-water-associated bladder cancer. More than 50% of the total organic halogen (TOX) formed by chlorination and more than 50% of the assimilable organic carbon (AOC) formed by ozonation has not been identified chemically. The potential interactions among the 600 identified DBPs in the complex mixture of drinking water to which we are exposed by various routes is not reflected in any of the toxicology studies of individual DBPs. The categories of DBPs described here, the identified data gaps, and the emerging role of dermal/inhalation exposure provide guidance for drinking water and public health research.

2,668 citations

Journal ArticleDOI
12 Jul 2013-Science
TL;DR: The current understanding of the causes and mechanics of earthquakes caused by human activity, including injection of wastewater into deep formations and emerging technologies related to oil and gas recovery, is reviewed.
Abstract: Background Human-induced earthquakes have become an important topic of political and scientific discussion, owing to the concern that these events may be responsible for widespread damage and an overall increase in seismicity. It has long been known that impoundment of reservoirs, surface and underground mining, withdrawal of fluids and gas from the subsurface, and injection of fluids into underground formations are capable of inducing earthquakes. In particular, earthquakes caused by injection have become a focal point, as new drilling and well-completion technologies enable the extraction of oil and gas from previously unproductive formations. Earthquakes with magnitude (M) ≥ 3 in the U.S. midcontinent, 1967–2012. After decades of a steady earthquake rate (average of 21 events/year), activity increased starting in 2001 and peaked at 188 earthquakes in 2011. Human-induced earthquakes are suspected to be partially responsible for the increase. Advances Microearthquakes (that is, those with magnitudes below 2) are routinely produced as part of the hydraulic fracturing (or “fracking”) process used to stimulate the production of oil, but the process as currently practiced appears to pose a low risk of inducing destructive earthquakes. More than 100,000 wells have been subjected to fracking in recent years, and the largest induced earthquake was magnitude 3.6, which is too small to pose a serious risk. Yet, wastewater disposal by injection into deep wells poses a higher risk, because this practice can induce larger earthquakes. For example, several of the largest earthquakes in the U.S. midcontinent in 2011 and 2012 may have been triggered by nearby disposal wells. The largest of these was a magnitude 5.6 event in central Oklahoma that destroyed 14 homes and injured two people. The mechanism responsible for inducing these events appears to be the well-understood process of weakening a preexisting fault by elevating the fluid pressure. However, only a small fraction of the more than 30,000 wastewater disposal wells appears to be problematic—typically those that dispose of very large volumes of water and/or communicate pressure perturbations directly into basement faults. Outlook Injection-induced earthquakes, such as those that struck in 2011, clearly contribute to the seismic hazard. Quantifying their contribution presents difficult challenges that will require new research into the physics of induced earthquakes and the potential for inducing large-magnitude events. The petroleum industry needs clear requirements for operation, regulators must have a solid scientific basis for those requirements, and the public needs assurance that the regulations are sufficient and are being followed. The current regulatory frameworks for wastewater disposal wells were designed to protect potable water sources from contamination and do not address seismic safety. One consequence is that both the quantity and timeliness of information on injection volumes and pressures reported to regulatory agencies are far from ideal for managing earthquake risk from injection activities. In addition, seismic monitoring capabilities in many of the areas in which wastewater injection activities have increased are not capable of detecting small earthquake activity that may presage larger seismic events.

1,823 citations

Journal ArticleDOI
TL;DR: It is concluded that greater stewardship, data, and—possibly—regulation are needed to ensure the sustainable future of shale-gas extraction and to improve public confidence in its use.
Abstract: are consistent with deeper thermogenic methane sources such as the Marcellus and Utica shales at the active sites and matched gas geochemistry from gas wells nearby. In contrast, lower-concentra- tion samples from shallow groundwater at nonactive sites had isotopic signatures reflecting a more biogenic or mixed biogenic/ thermogenic methane source. We found no evidence for contam- ination of drinking-water samples with deep saline brines or frac- turing fluids. We conclude that greater stewardship, data, and— possibly—regulation are needed to ensure the sustainable future of shale-gas extraction and to improve public confidence in its use.

1,285 citations

Journal ArticleDOI
17 May 2013-Science
TL;DR: Improved understanding of the fate and transport of contaminants of concern and increased long-term monitoring and data dissemination will help effectively manage water-quality risks associated with unconventional gas industry today and in the future.
Abstract: Unconventional natural gas resources offer an opportunity to access a relatively clean fossil fuel that could potentially lead to energy independence for some countries. Horizontal drilling and hydraulic fracturing make the extraction of tightly bound natural gas from shale formations economically feasible. These technologies are not free from environmental risks, however, especially those related to regional water quality, such as gas migration, contaminant transport through induced and natural fractures, wastewater discharge, and accidental spills. We review the current understanding of environmental issues associated with unconventional gas extraction. Improved understanding of the fate and transport of contaminants of concern and increased long-term monitoring and data dissemination will help manage these water-quality risks today and in the future.

1,263 citations

Journal ArticleDOI
TL;DR: In this article, the authors evaluate the greenhouse gas footprint of natural gas obtained by high-volume hydraulic fracturing from shale formations, focusing on methane emissions, and find that 3.6% to 7.9% of the methane from shale-gas production escapes to the atmosphere in venting and leaks over the life time of a well.
Abstract: We evaluate the greenhouse gas footprint of natural gas obtained by high- volume hydraulic fracturing from shale formations, focusing on methane emissions. Natural gas is composed largely of methane, and 3.6% to 7.9% of the methane from shale-gas production escapes to the atmosphere in venting and leaks over the life- time of a well. These methane emissions are at least 30% more than and perhaps more than twice as great as those from conventional gas. The higher emissions from shale gas occur at the time wells are hydraulically fractured—as methane escapes from flow-back return fluids—and during drill out following the fracturing. Methane is a powerful greenhouse gas, with a global warming potential that is far greater than that of carbon dioxide, particularly over the time horizon of the first few decades following emission. Methane contributes substantially to the greenhouse gas footprint of shale gas on shorter time scales, dominating it on a 20-year time horizon. The footprint for shale gas is greater than that for conventional gas or oil when viewed on any time horizon, but particularly so over 20 years. Compared to coal, the footprint of shale gas is at least 20% greater and perhaps more than twice as great on the 20-year horizon and is comparable when compared over 100 years.

1,261 citations