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 (10−50%).
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 fingerprint.
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 flow 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
flow of hydraulic fracturing fluids and saline formation waters to
overlying shallow aquifers. Given the buoyancy of gas, the flow
rate of denser saline water would be substantially slower than
the flow 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
confirmed elevated levels of specific conductance (1500 mS/
cm), pH (10−11), methane (25−27 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 flow 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 (2000−2007)
increases in the sali nity of groundwater as sociated with
conventional oil and gas drilling was reported from Gar field
County, CO. There, a rise of chloride concentrations in
drinking water wells was associated with an increase of methane
with a thermogenic isotopic fingerprint, 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 Garfield 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 Garfield
County could reflect 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
fluids in the shallower or intermediate units (that typically have
a different water chemistry). While there might be evidence for
water contamination in some areas of conventional oil and gas
exploration, groundwater sites in areas affected 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 fluids flow 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 sulfide
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, specifically 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, XXX−XXXE