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Rivard et al. 2018. Environ. Monit. Assess., doi: 10.1007/s10661-018-6532-7
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Can groundwater sampling techniques used in monitoring wells 1
influence methane concentrations and isotopes? 2
3
Christine Rivard1*, Geneviève Bordeleau1, Denis Lavoie1, René Lefebvre 2, Xavier 4
Malet1 5
1 Geological Survey of Canada, 490 rue de la Couronne, Québec, Quebec, Canada, G1K 9A9 6
2 Institut national de la recherche scientifique – Centre Eau Terre Environnement, 490 rue de la Couronne, 7
Québec, Quebec, Canada, G1K 9A9 8
*corresponding author :Tel : 418-654-3173, Email : [email protected] 9
10
Abstract 11
12
Methane concentrations and isotopic composition in groundwater are the focus of a 13
growing number of studies. However, concerns are often expressed regarding the integrity 14
of samples, as methane is very volatile and may partially exsolve during sample lifting in 15
the well and transfer to sampling containers. While issues concerning bottle-filling 16
techniques have already been documented, this paper documents a comparison of methane 17
concentration and isotopic composition obtained with three devices commonly used to 18
retrieve water samples from dedicated observation wells. This work lies within the 19
framework of a larger project carried out in the Saint-Édouard area (southern Québec, 20
Canada), whose objective was to assess the risk to shallow groundwater quality related to 21
Rivard et al. 2018. Environ. Monit. Assess., doi: 10.1007/s10661-018-6532-7
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potential shale gas exploitation. The selected sampling devices, which were tested on 10 22
wells during three sampling campaigns, consist of an impeller pump, a bladder pump, and 23
disposable sampling bags (HydraSleeve). The sampling bags were used both before and 24
after pumping, to verify the appropriateness of a no-purge approach, compared to the low-25
flow approach involving pumping until stabilization of field physicochemical parameters. 26
Results show that methane concentrations obtained with the selected sampling techniques 27
are usually similar and that there are no systematic bias related to a specific technique. 28
Nonetheless, concentrations can sometimes vary quite significantly (up to 3.5 times) for a 29
given well and sampling event. Methane isotopic composition obtained with all sampling 30
techniques are very similar, except in some cases where sampling bags were used before 31
pumping (no-purge approach), in wells where multiple groundwater sources enter the 32
borehole. 33
34
Keywords: groundwater, sampling techniques, dissolved methane, shale gas, monitoring 35
36
Introduction 37
38
Public concerns about shale gas development are largely related to groundwater quality, 39
with fear that hydraulic fracturing fluids, methane, or saline brines from deep hydrocarbon 40
reservoirs could contaminate shallow aquifers (Lefebvre 2017). Over the last decade, the 41
oil and gas industry has started collecting groundwater samples from residential, farm and 42
monitoring wells up to 1 km around unconventional energy wells prior to and following 43
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drilling. This sampling work is required by many jurisdictions (Jackson and Heagle 2016), 44
but the industry has also carried out such sampling even if the local jurisdiction does not 45
require it, due to public concerns and eventual legal dispute about the impact of their 46
activities on the groundwater quality of surrounding domestic wells. 47
For baseline studies, the type of wells and sampling techniques should be carefully 48
evaluated when trying to obtain a representative picture of methane concentrations in a 49
given area. For instance, Jackson and Heagle (2016) have highly recommended that 50
dedicated observation wells be used for monitoring, due to potential water quality and poor 51
maintenance issues associated with residential wells. The selection of a sampling technique 52
(with respect to both water withdrawal and bottle filling methods) is especially important 53
for water highly charged with dissolved gases (i.e. effervescing samples), as it can impact 54
concentration results (Humez et al. 2016; Molofsky et al. 2016). Indeed, the amount of gas 55
that may dissolve in groundwater at a certain depth depends on the water pressure, which 56
is related to the height of the overlying water column. When downhole gas concentrations 57
are high, as the sample is being lifted to the surface (progressively lowering water 58
pressure), and then poured into containers at atmospheric pressure, some of the dissolved 59
gas will exsolve and thus be lost from the water sample. Factors affecting exsolution 60
include flow type (laminar versus turbulent), pressure changes in the well due to 61
drawdown, the technique used to lift groundwater to the surface, and the bottle filling 62
technique (Gorody et al. 2005 and 2012; Hirshe and Mayer 2009; Coleman and McElreath 63
2012; Molofsky et al. 2016). 64
Issues related to bottle filling procedures for effervescing groundwater have been reported 65
elsewhere (Humez et al. 2015; Smith et al. 2016; and especially in Molofsky et al. 2016). 66
Rivard et al. 2018. Environ. Monit. Assess., doi: 10.1007/s10661-018-6532-7
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A commonly used technique involves filling and capping bottles at the bottom of a larger 67
container filled with purge water, as first published by the USGS 68
(http://water.usgs.gov/lab/chlorofluorocarbons/sampling/bottles/). While being considered 69
a semi-closed system (not in direct contact with atmosphere), this technique still involves 70
potential sample degassing. Such issues are better controlled in closed systems such as the 71
recently developed IsoFlasks® containers (Isotech Laboratories Inc., Champaign, IL), 72
consisting of single-use flexible plastic pouches connecting directly to a sampling tube in 73
the field, and then to a GC-FID at Isotech Laboratories Inc. This technique allows the 74
quantification of both dissolved and free gas phases, which allows computing the original 75
downhole methane concentration. This type of device is currently only available from one 76
supplier, and not many laboratories are set-up to handle such analyses. Consequently, the 77
materials and related analyses are expensive, which is an important limiting factor, 78
especially in studies involving a large number of samples. Noteworthy, in non-effervescing 79
samples, both semi-closed and closed systems have been shown to give comparable results 80
(Molofsky et al. 2016). 81
Issues related to sample lifting have been comparatively less studied, especially for highly 82
volatile compounds such as methane. When sampling dedicated observation wells, several 83
methods can be used for lifting the sample to the surface. These include various types of 84
pumps (e.g., suction, inertial lift, impeller, bladder) or “no-purge” devices (e.g., downhole 85
samplers), which can all target specific intervals within a well. The selection of a purge 86
versus no-purge approach, and even the pump-specific mechanism for lifting the sample, 87
could potentially have an effect on results. 88
Rivard et al. 2018. Environ. Monit. Assess., doi: 10.1007/s10661-018-6532-7
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The objective of the present study is therefore to compare some commonly used water 89
withdrawal techniques, as well as a purge versus no-purge approach, and to see how these 90
techniques may affect both concentrations and isotopic composition of methane in 91
groundwater. This work was conducted within the framework of a larger environmental 92
project carried out in the St. Lawrence Lowlands (eastern Canada), with the objective to 93
assess the risk to shallow groundwater quality from upward fluid migration related to 94
eventual shale gas development in the Utica Shale (Lavoie et al. 2014; Bordeleau et al. 95
2017; Rivard et al. 2017). Within this larger project, both residential (n=30) and 96
observation (n=14) wells were sampled, often several times, amounting to nearly 250 97
samples. Restrictive conditions in some of these wells (e.g., deep targeted sampling interval 98
or very low yield) precluded the use of a single sampling device for all wells. Additionally, 99
the generally low water yield of open borehole wells in this region led to significant 100
drawdown and long recovery times in some of the observation wells, making a no-purge 101
sampling approach appealing. For these reasons, a specific study to verify whether different 102
sample lifting techniques could be used interchangeably within the project was undertaken. 103
To do so, ten observation wells were selected, and three distinct sampling campaigns were 104
conducted. The selected wells represent a wide range of depths, methane concentrations 105
and isotopic composition. At each of these wells, sampling was done consecutively using 106
three commonly used water withdrawal techniques (impeller pump, bladder pump, and 107
downhole sampling bag). Furthermore, the sampling bag was also used prior to any 108
pumping to study the effect of a purge versus no-purge approach. The first sampling 109
campaign, however, only included the impeller pump and sampling bags after pumping. 110
Preliminary tests were also conducted with a peristaltic (suction) pump in three wells. The 111
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characteristics of each sampling technique, along with the results obtained for 112
concentrations and isotopes, are used to recommend the best technique and to discuss the 113
use of other techniques when well conditions are restrictive. 114
It is noteworthy that sampling groundwater containing dissolved gases in various 115
concentrations, and in particular methane, is needed in other situations than those related 116
to hydrocarbon development. For instance, biogenic methane is often produced during 117
anaerobic in situ biodegradation processes that involve the addition of organic substrates 118
to reduce chlorinated volatile organic compounds (CVOCs), nitrate, hexavalent chromium 119
(CrVI) and perchlorate (EPA 2013). Also, landfill gas produced by the decomposition of 120
organic wastes is made up of methane and CO2, so methane can also be found in 121
groundwater adjacent to landfills (Nastev et al. 2001). The recommendations provided in 122
this paper are applicable to any monitoring program involving groundwater with high 123
concentrations of dissolved gases. 124
125
Previous studies on the sampling of groundwater with high dissolved 126
gas content 127
128
Few field studies have assessed the impacts of sampling techniques in groundwater 129
containing high concentrations of dissolved gas, even though it has long been known that 130
sampling gas-charged water from wells is challenging. Several studies starting in the late 131
1980s investigated the effect of sampling devices (both purge and no-purge) on different 132
chemical components, often including volatile organic compounds (VOCs) (e.g., Muska et 133
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al. 1986; Devlin 1987; Barker and Dickout 1988; Parker and Clark 2004; McHugh et al. 134
2015), but no shorter-chain hydrocarbons such as methane. Early studies were reviewed by 135
Parker (1994) who concluded that significant problems of degassing and loss of VOCs had 136
been encountered with almost all the samplers and that, generally, bladder pumps gave the 137
best overall recovery of sensitive constituents while suction-lift pumps had one of the 138
poorest performances. Suction-lift pumps (such as peristaltic pumps) apply a vacuum to 139
the groundwater sampled that can cause depressurization and degassing of the samples 140
(Parker 1994). It must be emphasized that some of the devices reviewed in Parker (1994) 141
have evolved since then and that low-flow rates, minimizing degassing, is now being 142
routinely used. Of note, McHugh et al. (2015) cited several references where samples 143
collected using no-purge methods showed little or no bias in contaminant concentrations 144
(including VOCs but not methane) compared to samples collected after well purging. 145
The U.S. Interstate Technology Regulatory Council (ITRC 2007) provided guidance for 146
proper deployment and collection of groundwater samples containing a variety of 147
contaminants, including volatile gases, using five no-purge sampling technologies. The 148
ITRC literature review did not include testing. Two grab samplers, namely the 149
HydraSleeve (Las Cruces, NM, USA) bags, and the Snap sampler (ProHydro inc., Fairport, 150
NY), and different types of passive diffusion samplers are discussed in this report. Both 151
the HydraSleeve bags and Snap samplers were recommended for the sampling of dissolved 152
gasses such as methane. A significant disadvantage of the Snap sampler is that it can only 153
collect small samples, the largest being 350 mL for polypropylene bottles (and 40 mL for 154
glass bottles) and only four bottles can be connected in a series. Therefore, this could be a 155
major limitation, as sampling campaigns sometimes require much larger volumes of water 156
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for a series of analyses, including isotopic ratios. HydraSleeve bags come in different sizes 157
(the maximum being 2.5 L for a 10 cm diameter) and are designed for single use, as they 158
should be perforated using the dedicated straw to transfer the water from the bag into the 159
sampling bottles. However, this procedure can be circumvented and the bag can often be 160
reused a few times in the same well, if larger water volumes are needed (see Section 161
“Groundwater sampling techniques”). Nonetheless, the thin plastic bags are somewhat 162
fragile, and can generally only be reused once or twice before being damaged (pierced) if 163
the borehole walls are not completely smooth. These two sampling devices can either 164
provide an immediate sample when deployed, or be left within a well at the desired 165
sampling depth for a few days. The latter allows sufficient time for the water within the 166
well to re-equilibrate after being “disrupted” by the positioning of the sampler and for 167
concentrations inside the sampler to equilibrate with the in situ chemical constituents. The 168
latter case, also called passive sampling, thus requires two visits, one for the installation 169
and one for the removal of the device, a time- and money-consuming exercise. 170
Furthermore, it is now recognized that passive sampling is not particularly useful for 171
collecting samples for methane concentrations, since the latter were often shown to vary 172
considerably over time, even over short periods (Hirshe and Myer 2009; Gorody 2012; 173
Humez et al. 2015; Smith et al. 2016). 174
175
Description of the study area 176
177
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The 500 km2 study area is located in the St. Lawrence Lowlands, in southern Quebec, 178
Canada (Figure 1), where shale gas exploration activities targeting the Utica Shale went on 179
from 2006 to 2010, before a de facto fracking moratorium came into force. It is situated 180
about 65 km south-west of Quebec City and extends from the outer edge of the Appalachian 181
piedmont northward to the St. Lawrence River. Most of our study area is located in the St. 182
Lawrence Platform, mainly composed in this region of black shale that contains variable 183
content of organic matter with subordinate siltstone. The shallow bedrock geology is made 184
of three Upper Ordovician clastic units: the Lotbinière, Les Fonds and Nicolet formations. 185
The Lotbinière and Les Fonds formations are time- and facies-correlative with the Utica 186
Shale, which is present at a depth of 1.5 to 2 km in this area. 187
Most residential wells in the Lotbinière area are drilled into bedrock; their depth is 50 m 188
on average. Bedrock is mainly composed of shale and is thus poorly permeable: rock 189
hydraulic conductivities in the region vary between 10-9 and 10-6 m/s (Ladevèze et al. 190
2016). Dissolved hydrocarbons in groundwater originate from the shallow bedrock units 191
(Lavoie et al. 2016). Figure 1 shows the near-surface geology and location of observation 192
wells specifically drilled for this project. The location of observation wells was selected in 193
order to obtain a good spatial distribution over the three geological formations. These 194
observation wells were found to be either under semi-confined or confined conditions 195
(Ladevèze et al., 2016; Ladevèze, 2017). 196
Methane concentrations in groundwater in the Saint-Édouard region, obtained from 14 197
observation wells and 30 residential wells, vary between the detection limit (0.006 mg/L) 198
to more than 80 mg/L (Bordeleau et al. 2017), with a median of about 4 mg/L. Higher 199
methane concentrations are associated with more evolved waters (Na-HCO3, Na-HCO3-Cl 200
Rivard et al. 2018. Environ. Monit. Assess., doi: 10.1007/s10661-018-6532-7
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and Na-Cl water types). Above laboratory-measured concentrations of ~20 to 25 mg/L of 201
methane, the in situ groundwater is usually considered highly charged or even 202
supersaturated (effervescent). This condition leads to obvious issues related to sampling of 203
groundwater downhole (thus under the pressure of a water column) and bringing it to 204
atmospheric pressure to fill sampling bottles. During sampling, gas bubbles were observed 205
in the tubing for many of the wells. Ten observation wells were selected for this study. 206
They are all open to the bedrock and have a sealed metal casing through the overburden. 207
Their total depth varies from 30 to 60 m and their sampling depth ranges from 7.5 to 54 m. 208
The characteristics of observation wells are provided in Table 1 and their locations are 209
shown on Figure 1. 210
211
Field and laboratory methodology 212
213
When sampling the ten selected observation wells, care was taken to minimize drawdown 214
and water disturbance, and samples were always collected at the same targeted depth within 215
a well, where flowing fractures had previously been identified using borehole geophysics. 216
The goal was to collect groundwater samples from these flowing fractures, which is 217
representative of the surrounding bedrock aquifer. To verify the representativeness of 218
groundwater withdrawn with this technique, physico-chemical profiles were measured at 219
every five meters within four observation wells and they provided very distinct 220
characteristics for pH, electrical conductivity, salinity and dissolved oxygen, indicating 221
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that low-flow pumping in these wells indeed allowed sampling water from the targeted 222
intervals. 223
224
Groundwater sampling techniques 225
226
The selected devices to withdraw water from wells were the Grundfos (Bjerringbro, 227
Denmark) Redi-Flo2 impeller pump, the Solinst (Georgetown, ON, Canada) model 407 228
bladder pump, and the HydraSleeve single-use downhole sampling bags. The impeller and 229
bladder pump models were also those used by Humez et al. (2015) for their 8-year 230
monitoring of a well located in a region with groundwater containing high dissolved gas 231
concentrations. Some preliminary tests performed with a peristaltic pump on three of our 232
monitoring wells provided concentrations that were significantly lower compared to those 233
obtained with the impeller pump, especially for well F4 that has a water level at ~8 m below 234
the top of the casing and high dissolved methane concentrations. This represents the 235
maximum distance above the water level from which the peristaltic pump can lift water 236
(Parker 1994). The suction effect of the pump, coupled to the low water level, likely caused 237
additional degassing, as was observed from the numerous bubbles visible in the sampling 238
tube. This pump was therefore not tested any further. An inertial-lift (Waterra) pump was 239
not considered, as based on its operating principle, it was expected to lead to excessive 240
degassing, as reported by Barker and Dickout (1988), Devlin (1987) and others cited in 241
Parker (1994). Table 2 presents the wells and techniques used for each sampling event. 242
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The submersible impeller (Redi-Flo2) pump was selected for its ease of use and sturdiness. 243
It was equipped with a 90-m long, 6.25 mm (¼”) diameter tubing. The pump was slowly 244
lowered into the well to the targeted sampling depth. The flow rate was adjusted so as to 245
cause minimal drawdown in the well, in order to limit degassing. The EPA 246
recommendation for low-flow purge sampling suggests that drawdown should be limited 247
to 10 cm prior to stabilization of field parameters (Puls and Barcelona 1996). However, in 248
most of the wells, especially the least permeable ones, the minimum flow rate that could 249
be achieved with this pump still caused significant drawdown. The average drawdown for 250
all wells from the three sampling campaigns pooled together is 52 ± 41 cm, with a 251
maximum of 183 cm in well F3, where a nearby residential well was in use. Yields were 252
on average 0.32 L/min, with minimum and maximum values of 0.05 and 2.00 L/min. 253
Between 1 and 2 hours of pumping were usually needed for physico-chemical parameters 254
(temperature, pH, conductivity, redox potential, dissolved oxygen) to stabilize. Samples 255
were collected when parameters had been stable for at least 15 minutes. 256
The bladder pump was also equipped with a 90-m long, 6.25 mm (¼”) diameter tubing. 257
This pump allows sampling at a lower flow rate than the impeller pump, which may be 258
desirable in some very low-yielding wells. However, the bladder pump requires some 259
delicate fine-tuning, is less sturdy and requires the use of an air compressor. The pump was 260
carefully lowered in the well, and purging and sampling procedures were identical as those 261
used with the impeller pump. 262
Finally, disposable HydraSleeve bags were used. These bags simply consist of a 263
polyethylene bag that is sealed at the bottom and has a self-sealing check valve at the top. 264
This method was selected because: 1) it appeared simple to use, 2) it allegedly allows the 265
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collection of representative groundwater samples without the need to purge the wells and 266
3) it was recommended for water containing volatile gasses by ITRC (2007). These 267
sampling bags are especially advantageous for sampling wells that have an extremely low 268
hydraulic conductivity, where purging prior to sampling is not possible without lowering 269
the water level by several meters. In our case, this method also allowed sampling of our 270
deepest well (depth of 147 m), where the targeted sampling interval was too deep for our 271
low-flow pumps. As we wished to test both the bag performance for sampling methane and 272
the purge versus no-purge approach, these bags were used before (no-purge approach) and 273
after the two pumps (purge approach) (Table 2). For sampling, the bags were carefully and 274
slowly lowered in the wells until the targeted sampling depth was reached, and samples 275
were collected without delay, using the recommended standard technique (a description is 276
provided in McHugh et al. (2015) and on the manufacturer website). 277
It was not possible in our study to allow the water to equilibrate for a few days (passive 278
sampling) because: 1) methane concentrations are known to vary significantly over time 279
(Rivard et al. 2017), and were even suspected to vary over very short periods based on the 280
work reported in Hirshe and Mayer (2009), so sampling a few days before or after the other 281
techniques would not have led to comparable values between sampling techniques and 2) 282
the largest HydraSleeve bags available were not large enough to provide the required 283
sample volume for the various analyses. Indeed, the sample volume needed was up to 6 284
times the volume contained in the bag. Furthermore, using 6 disposable bags for each well 285
was not a reasonable option, both for financial and environmental reasons. We therefore 286
developed a way to reuse the bags several times at a single well by carefully inserting a 287
sampling tube through the unsealed end of the bag, instead of piercing the bag with the 288
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intended sampling straw. The unsealed end of the bag is still air-tight as the water pressure 289
forces the plastic double-wall to remain closed; it is possible, although not easy, to force 290
our way inside the bag with the sampling tube. In the best cases, the same bag could be 291
used 3 or 4 times before being damaged by the borehole walls. The physico-chemical 292
parameters were verified in the water collected with HydraSleeve bags. Although multiple 293
fillings of the bags involved repeated lowering and lifting of the bags through the water 294
column, the physico-chemical parameters were very similar from one bag to the other and 295
to those measured during pumping, which indicates that the water sampled with the bags 296
is representative of the water previously sampled with the pumps. Likewise, major ions 297
and trace metal analyses (not discussed in this paper) provided similar results for samples 298
collected using the HydraSleeve bags after pumping than with the impeller pump. 299
300
Bottle filling, storage and analyses 301
302
The method chosen for bottle filling was the one documented by the USGS 303
(http://water.usgs.gov/lab/chlorofluorocarbons/sampling/bottles/). It was selected as it is 304
widely used, affordable, involves readily available materials and generates minimal waste 305
(bottles and vials are reused, only the septa need to be changed). Vials (for alkane 306
concentrations) or bottles (for methane isotope composition) were held upright at the 307
bottom of a larger container. The sampling tube was inserted at the bottom of the 308
vial/bottle, progressively filling it and then the larger container. Once the container was 309
full, the tube was removed and the vial/bottle was capped underwater to avoid contact with 310
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the atmosphere. This technique is similar to the inverted bottle method that has been widely 311
used for sampling groundwater containing gas (Humez et al. 2015; 2016, Siegel et al. 2015; 312
2016; Moritz et al. 2015; Molofsky et al. 2016; Smith et al. 2016). In our case, the upright 313
position was selected because the inverted (upside-down) position seemed more prone to 314
trap gas and create a headspace while being filled with groundwater highly charged with 315
methane. This was recently confirmed by Molofsky et al. (2016) in a comparative study. 316
For each sample, three 40-mL amber glass vials were collected for replicate alkane 317
concentration measurements, along with two 1-L amber glass bottles for single methane 318
isotopic measurements (δ13C and δ2H). The open-top caps were lined with grey butyl septa 319
(1-L bottles) or Teflon-coated silicon septa (40-mL vials). Containers were stored on their 320
side (1-L bottles) or upside down (40-mL vials) in a fridge at 4˚C. Water for alkane 321
concentrations and methane C and H isotope ratios was acidified to pH < 2 to avoid 322
proliferation of microorganisms. 323
Concentrations of dissolved C1-C3 alkanes were determined at the Delta-Lab of the 324
Geological Survey of Canada (Quebec City, QC) using a Stratum PTC (Teledyne Tekmar, 325
Mason, OH) purge and trap concentrator system interfaced with an Agilent (Santa Clara, 326
CA) 7890 gas chromatograph equipped with a flame ionisation detector (GC-FID). The 327
method employed was adapted from Pennsylvania Department of Environmental 328
Protection method PA-DEP 3686 (2012) and US Environmental Protection Agency (EPA) 329
method RSK 175 (Kampbell and Vandegrift 1998). Quantification limits on our samples 330
were 0.006, 0.002, and 0.01 mg/L for methane, ethane and propane, respectively. The 331
uncertainty related to sampling, handling and analysis was estimated at 15% of the reported 332
concentration, based on the 90th percentile in replicate samples (Rivard et al. 2017). 333
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Alkane isotopic composition (δ13C and δ2H) was analyzed at either one of three different 334
laboratories, namely the Delta-Lab of the Geological Survey of Canada (Quebec City), 335
Concordia University (Montréal), or the G.G. Hatch laboratory of the University of Ottawa. 336
The choice of the lab depended on the availability of the analytical instruments, in order to 337
ensure timely analysis. Control samples were sent concurrently to the different labs to 338
ensure that results are comparable. Analysis was performed on a Delta V (Thermo Fisher 339
Scientific, Waltham, MA) isotope ratio mass spectrometer (IRMS) at the Delta-Lab and 340
the G.G. Hatch lab, while a GC Agilent 6890 coupled to an Isoprime 100 (Manchester, 341
UK) was used at Concordia. Results are expressed in the usual per mil notation relative to 342
Vienna Pee Dee Belemnite (V-PDB; δ13C) and Vienna Standard Mean Ocean Water 343
(VSMOW; δ2H). The uncertainty related to sampling, handling and analysis was estimated 344
at 1.7‰ for δ13C and 19‰ for δ2H, based on the 90th percentile in replicate samples (Rivard 345
et al. 2017). 346
347
Results 348
349
Methane concentrations 350
351
Results for methane concentrations obtained through the different sampling techniques 352
during each of the three sampling campaigns are presented in Figure 2 (note that all 353
geochemical results will be available in a public database to be released in 2017). It is 354
noteworthy that a comparison of different techniques could not be done for well F10 in fall 355
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2014, as the well was artesian shortly after its drilling. Artesian flowing conditions then 356
resorbed, so that the well could be included for the following campaigns. Also, in summer 357
2015, well F7 could not be sampled using the bladder pump due to malfunction of the 358
pump, which caused numerous air bubbles to enter the sampling tube, thus compromising 359
results. 360
Figure 2 reveals that methane concentrations obtained through the different sampling 361
techniques for a given well are, with few exceptions, quite similar with nearly all individual 362
values being close, when considering uncertainty related to sampling, handling and 363
analysis. Furthermore, there are no systematic trends for higher or lower values related to 364
a given method or well. Student and Fisher statistical tests performed on these time series, 365
with a 10% level of significance, confirmed that there is no evidence that their statistical 366
properties (mean and variance) are different, as they indicated that the null hypothesis of 367
population equivalency could not be rejected. 368
However, even though there does not appear to be a systematic bias related to any particular 369
method, there are still important differences in absolute concentrations measured for some 370
samples, including well F4 in May 2015 (concentrations varying between 34.75 and 46.33 371
mg/L), F4 in July 2015 (concentrations between 26.15 and 55.35 mg/L) and F6 in July 372
2015 (concentrations between 6.39 and 20.96). When comparing the largest and lowest 373
values (using the maximum/minimum concentration ratios, hereafter called “max/min 374
ratio”) obtained for a given well on a given sampling campaign, ratios are mostly between 375
1 (i.e. no variation between techniques) and 2 (i.e. a 100 % variation between highest and 376
lowest values), with a few higher ratios exceeding 3 (Figure 3). High ratios (> 2) tend to 377
be either associated with high (e.g. > 20 mg/L) or low (e.g. < 1 mg/L) concentrations; 6 378
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results (21%) have such high ratios. High concentrations imply important degassing when 379
the sample is lifted to the surface and degassing may occur differently according to the 380
sampling technique (e.g., wells F2 and F4). In contrast, when concentrations are very low, 381
very small absolute variations result in a high max/min ratio (e.g., wells F3 and F8). The 382
two very high max/min ratios for well F6 (containing intermediate methane concentrations) 383
are puzzling; they may be related to mixing of different waters, as this is the case in the 384
nearby well F7 (see Section “Methane isotopic composition”). For the May and July 2015 385
campaigns, max/min ratios always involve HydraSleeve bags, except for two cases (wells 386
F4 and F6 in July 2015), when values obtained with the two pumps appeared to be 387
abnormal compared to the others (see Figure 4). These abnormally low or high 388
concentrations could be due to technical or human error along the process or to an enhanced 389
methane contribution that can sporadically occur under the form of a “slug” (pulses) as 390
described in Dusseault and Jackson (2014). 391
Figure 4 shows that results obtained with the impeller and bladder pumps generally agree 392
very well, except for two points (F4 and F6 in July 2015), where the result from the Redi-393
Flo2 pump was either higher or lower than with the bladder pump. When discarding these 394
two values, the remaining values are almost perfectly aligned and the determination 395
coefficient (R2) becomes 0.98, indicating that methane concentrations are generally similar 396
when using these two types of pump. 397
The comparisons of concentrations obtained from the four sampling methods two by two 398
(matrix plots in Figure 5) also confirms that there is no general bias and that none of the 399
methods systematically underestimates methane concentrations. Values above 20 mg/L 400
often show more disparity in absolute values of concentrations. At such high 401
Rivard et al. 2018. Environ. Monit. Assess., doi: 10.1007/s10661-018-6532-7
19
concentrations, it is likely that the sum of dissolved gases exerts a pressure above 402
atmospheric pressure, causing effervescence; such conditions were also reported by 403
Molofsky et al. (2016) to cause disparity in measured methane concentrations, depending 404
on the bottle-filling method. However, when methane concentrations are compared using 405
concentration ratios or relative “errors” ([C1-C2]/C1 × 100%, C1 being the concentrations 406
obtained with a given method taken as a reference and C2 the concentrations corresponding 407
to one of the other three methods), a bias for higher concentrations is not obvious. Figure 408
6 provides an example of one of these graphs using the concentrations obtained with the 409
impeller pump as a reference, which is representative of the other similar graphs. The only 410
abnormally high values (e.g. > 70% and < -70%) correspond either to a case for which 411
concentrations were very low (below 1 mg/L, for instance in wells F3 and F8) or to a case 412
for which anomalous values were obtained with one of the sampling techniques in July 413
2015 (wells F4 and F6). 414
Very little ethane and even less propane were found in these 10 wells. C2+ hydrocarbons 415
are rarely present in significant quantity in groundwater, as was noted in many other studies 416
(e.g. Baldassare et al. 2014; Molosfky et al. 2016; Humez et al. 2015; 2016; Currell et al. 417
2017). Ethane and propane concentrations above 10 μg/L occurred in only three wells and 418
one well, respectively, and not in all sampling campaigns. Therefore, they could not be 419
used as a basis for comparison of the sampling methods in this paper. 420
421
Methane isotopic composition 422
423
Rivard et al. 2018. Environ. Monit. Assess., doi: 10.1007/s10661-018-6532-7
20
Methane stable carbon (δ13C-CH4) and hydrogen (δ2H-CH4) isotope ratios were also 424
analysed to investigate whether they could be impacted by the sampling technique. The 425
insight of the effect of sampling technique on methane C and H isotope ratios is essential, 426
as these isotopic ratios can be used as a potential indicator for methane migration (either 427
natural or anthropogenic) in groundwater. Figures 7 and 8 present δ13C-CH4 and δ2H-CH4 428
for the ten observation wells from the three sampling campaigns. This section only 429
discusses similarities or dissimilarities obtained with the different sampling techniques; a 430
discussion on the isotopic results in relation to methane source will be presented in an 431
upcoming paper. 432
Figure 7 confirms that δ13C-CH4 values are very similar for all sampling techniques in most 433
wells, with significant overlaps of individual values, considering the ±2‰ uncertainty 434
associated with sampling, handling and analysis. However, in well F7 (May and July 2015), 435
there is a marked difference of approximately 10‰ between the values obtained with 436
HydraSleeve bags before pumping, and the other techniques. This well has an upward flow 437
bringing in some very old, saline groundwater from the bottom of the well, resulting in 438
uncommonly high salinity in the water column when the well is resting (18-25 PSU or 439
practical salinity unit). The well is very sensitive to pumping, with salinity quickly 440
decreasing as freshwater from the shallow aquifer invades the well under pumping. It is 441
therefore not surprising to obtain markedly different isotopic values in a sample that was 442
collected prior to pumping, as the methane present in the very old groundwater is of a 443
different origin than that in the shallow aquifer. Interestingly, concentrations of methane 444
in both sources of groundwater seem to be similar, as this purge-related effect went 445
unnoticed with methane concentrations (Figure 2). 446
Rivard et al. 2018. Environ. Monit. Assess., doi: 10.1007/s10661-018-6532-7
21
In a few other wells (F3 in May and July 2015, F4 in July 2015, F8 in July 2015, and F10 447
in July 2015), δ13C-CH4 values also exhibit a spread exceeding the uncertainty (maximum 448
spread of 7‰), but in these cases the variability is not clearly related to a specific sampling 449
technique. This higher variability may be due to well-specific factors, such as small 450
variations of different sources of methane in given well when methane concentrations are 451
very low (wells F3 and F8), or an upward flow in the well (well F10). In well F4, the 452
isotopic spread is relatively small (max. spread 4.9‰). The difficulty in stabilizing the 453
physico-chemical parameters in this very low-yield well and the fact that very different 454
concentrations were found may indicate contributions of water from different depths or of 455
gas slugs. 456
Compared to methane carbon isotope ratios, hydrogen stable isotope ratios are known to 457
be subject to a greater uncertainty, which was estimated at ±19‰ in this project. They also 458
span a much larger isotopic range than do carbon stable isotope ratios and are less 459
diagnostic with regards to methane origin (Whiticar 1999). Nonetheless, for most of our 460
wells, the δ2H-CH4 values from the different techniques are very similar, and overlap when 461
considering the uncertainty (Figure 8). The only exceptions are wells F3 (all three sampling 462
campaigns; maximum spread of 72‰) and F10 (July 2015, spread of 54‰), which can be 463
explained by the same mechanisms discussed above for carbon isotopes. More specifically, 464
the very low methane concentration in well F3 is significantly affected by methane 465
oxidation, which causes important variations in concentrations over time (Rivard et al. 466
2017). Oxidation causes stronger isotopic fractionation on the hydrogen than on the carbon 467
atoms (Alperin et al. 1988; Kinnaman et al. 2007), which likely explains the observed wider 468
spread in δ2H-CH4 values for well F3. The water in this well may also be affected by the 469
Rivard et al. 2018. Environ. Monit. Assess., doi: 10.1007/s10661-018-6532-7
22
sporadic pumping of a nearby well, which is deeper (76 m) and which was in use at the 470
time of sampling in July 2015. Contrary to oxidation, late-stage methanogenesis causes 471
isotopic fractionation on the carbon isotopes, but not on the hydrogen isotopes. This occurs 472
because the carbon used by methanogens comes from a limited carbon pool, which may 473
become exhausted over time, when not replenished due to isolated groundwater conditions. 474
In contrast, the hydrogen comes from the ambient water, which is a comparatively very 475
large pool where the supply of “light” hydrogen, preferred by the microbes, is unlimited 476
(Martini et al. 1998). This could thus explain why the purge-related isotopic effect in wells 477
F4 and F7 are visible on the carbon isotope ratios but not on the hydrogen isotope ratios. 478
Figure 9 presents box plots for both δ13C-CH4 and δ2H-CH4 according to each sampling 479
technique when results from the three field campaigns are pooled together. These graphs 480
confirm that median isotopic values are very close for all sampling techniques, being within 481
the uncertainty range of one another. This is particularly surprising for δ2H-CH4 values, 482
which are naturally more variable and have a higher uncertainty related to sampling, 483
handling and analysis; in spite of this, the median δ2H-CH4 values for each sampling 484
technique only vary between -249.0 and -250.8‰. It is worth mentioning that δ13C-CH4 485
and δ2H-CH4 values from the three groundwater samples collected with the peristaltic 486
pump also provided similar results to those of the other two types of pumps. 487
Although the δ13C-CH4 and δ2H-CH4 values do not seem sensitive to the sampling method, 488
results suggest that no-purge methods could sometimes provide different values, for 489
instance if an upward flow is present in the well. Therefore, if the objective of the sampling 490
campaign is to identify the gas origin, one does not have to worry much about the sampling 491
technique as long as the well is pumped long enough to have representative water from the 492
Rivard et al. 2018. Environ. Monit. Assess., doi: 10.1007/s10661-018-6532-7
23
surrounding aquifer. Nonetheless, other sampling devices than those selected for this study 493
should be tested to make sure that they do not result in isotopic fractionation, such as 494
inertial-lift pumps for instance because its mechanism can entrain water turbulence. 495
496
Discussion and recommendations 497
498
The comparison of methane concentrations and stable isotope ratios (δ13C-CH4, δ2H-CH4) 499
made in the present study did not show any systematic bias related to one of the selected 500
sampling techniques (impeller pump, bladder pump, sampling bags). However, important 501
differences (in absolute values) between concentrations obtained via different techniques 502
could sometimes be observed for a given well and sampling date, especially (but not 503
systematically) when concentrations were high, (i.e. when degassing was significant). 504
Nonetheless, unlike McHugh et al. (2015) who had tested different sampling techniques to 505
compare VOC concentrations and had found that HydraSleeve bags provided lower and 506
more variable VOCs concentrations, our results did not show that dissolved methane 507
concentrations were systematically lower nor were especially more variable with 508
HydraSleeve bags than with the other selected techniques. 509
Furthermore, results obtained before and after pumping were usually similar. This suggests 510
that in many cases, using a no-purge, fast method could be appropriate. However, in 511
particular cases, such as when there is an upward flow bringing more evolved water into a 512
well, a no-purge method (such as HydraSleeve bags used before pumping) could provide 513
different results compared to the other techniques, simply because the water being sampled 514
Rivard et al. 2018. Environ. Monit. Assess., doi: 10.1007/s10661-018-6532-7
24
is not the same with and without pumping. In such cases, the choice of no-purge or purge 515
method will depend on the water source that needs to be sampled, and it is crucial to follow 516
the same approach every time to obtain comparable values over time. Generally, unless 517
one is specifically trying to sample a water source that recedes upon pumping, we 518
recommend using a method that involves low-flow pumping until stabilization of the 519
physicochemical parameters, at the depth where flowing fractures have been previously 520
identified through borehole logging. In cases where there is more than one source of 521
groundwater in a well, isotopic results may also be affected by pumping. Unless very 522
detailed geochemical and hydrogeological characterization of each well in a study area 523
have been conducted, it is likely that such mixing of different water sources would go 524
unnoticed. Therefore, low-flow pumping seems the most prudent choice in most cases. 525
Borehole logging can provide important clues regarding the presence of different types of 526
groundwater from fractured intervals of a well open to a rock aquifer. 527
While none of the tested sampling techniques caused significantly more degassing of 528
samples compared to the others, it is known that the selected “water bucket” bottle-filling 529
technique (corresponding to a “semi-closed” system) will cause some degassing in 530
samples with high gas concentrations (Molofsky et al. 2016). “Closed systems”, such as 531
those using Isoflasks®, are promising in that they allow the collection and analysis of both 532
free and dissolved gas phases. This technique is relatively new and is still very costly, as 533
not many laboratories (outside of the corporation’s internal lab) are set-up to conduct theses 534
analyses. Also, the use of disposable pouches (in some cases, several pouches at a well if 535
large sample volumes are needed), is an environmental downside. If this technique keeps 536
Rivard et al. 2018. Environ. Monit. Assess., doi: 10.1007/s10661-018-6532-7
25
developing and becomes more affordable, degassing issues during sample lifting will 537
become less of a concern. 538
Until then, researchers must still acknowledge that their reported concentrations in 539
effervescing samples likely underestimate true methane concentrations in the aquifer when 540
an open or semi-closed sampling system is used; underestimations are expected to increase 541
along with concentrations and sampling depth. However, methane concentrations are 542
known to vary naturally over time in many regions. For instance, in our study area, 543
concentrations in a given well could reach up to six times the smallest recorded value 544
(Rivard et al. 2017), which exceeds by far any dissimilarities observed among results 545
obtained with the different sampling techniques (in the present study and, for example, in 546
Molofsky et al. 2016). Due to such natural variations, methane concentrations are generally 547
not a very robust diagnostic tool of methane provenance compared to isotopic composition. 548
Although all of the sampling devices tested in this study provided similar results, practical 549
considerations must be taken into account when choosing a technique. We strongly 550
recommend the use of an impeller pump, which is easy to use and very robust, and unlike 551
the bladder pump, cannot pump water when damaged; a similar recommendation was also 552
made by Muska et al. (1986). The bladder pump can, indeed, allow entrance of air in the 553
tubing when the bladder is defective, thereby resulting in much further degassing, which 554
compromises the sample. It is also more fragile and requires more fine-tuning than the 555
impeller pump. However, an advantage of the bladder pump is that it can usually achieve 556
very low pumping rates (lower than the impeller pump), which may be critical in wells 557
with a very low yield. Despite their high initial purchase cost, the advantage of using pumps 558
over HydraSleeve bags is that they can be used for a large number of wells and sampling 559
Rivard et al. 2018. Environ. Monit. Assess., doi: 10.1007/s10661-018-6532-7
26
events over several years. HydraSleeve bags may be an interesting option when sampling 560
only a few wells, or when sampling deep wells or wells with extremely low yield. However, 561
the financial (and environmental) costs of these disposable bags rise quickly, especially if 562
large sample volumes (and thus several bags) are required for a suite of analyses. 563
564
Conclusions 565
566
Three groundwater sampling techniques were compared to evaluate their suitability and 567
interchangeability for collection of samples in open bedrock wells to analyze 568
concentrations and stable carbon and hydrogen isotope ratios of methane, which is the most 569
volatile and abundant hydrocarbon in groundwater. The selected techniques were an 570
impeller (Redi-Flo2) submersible pump, a bladder submersible pump, and disposable 571
sampling HydraSleeve bags, which were used both before and after pumping. The latter 572
procedure was performed to examine the effect of purging the wells on methane 573
concentration and isotopic composition. These sampling techniques were tested over three 574
sampling campaigns in 10 observation wells in the Saint-Édouard area, located ~65 km 575
south-west from Quebec City (eastern Canada). In this region, dissolved methane is 576
naturally present in groundwater and concentrations are usually highly variable spatially 577
and temporally. 578
Results showed that methane carbon and hydrogen stable isotope ratios were not sensitive 579
to the selected sampling techniques, with all four techniques usually providing similar 580
results. Methane concentrations were comparatively more sensitive and significant 581
Rivard et al. 2018. Environ. Monit. Assess., doi: 10.1007/s10661-018-6532-7
27
differences were observed in a few wells. However, no systematic technique-related bias 582
was observed. As for the no-purge approach, it was appropriate in some wells but not in 583
others, depending on the hydrogeological conditions, in particular in the presence of 584
vertical hydraulic or salinity gradients within the well. 585
Based on this work, we therefore recommend the following approach for every 586
groundwater sampling program aiming to characterize methane concentrations and stable 587
isotope ratios: 1) carry out a purging period until stabilization of groundwater 588
physicochemical parameters at the depth where flowing fractures are documented; 2) pump 589
the well at a low flow that will keep drawdown to a minimum, to avoid groundwater 590
pressure changes that result in degassing; 3) remain consistent in sampling depth and bottle 591
filling procedure, as well as for the sampling device; and 4) preferably use a low-flow 592
impeller submersible pump, such as the Redi-Flo2 pump, as this kind of device is simple 593
to use and very reliable, and does not involve the use of disposable materials. 594
595
Acknowledgments 596
597
The authors would like to thank Dr. Mathieu Duchesne of the GSC and Pr. Erwan Gloaguen 598
of INRS for their advices and contribution related to the representation of data with Matlab. 599
Authors would like to acknowledge funding support from the Energy Sector (Eco-EII and 600
PERD programs) and the Earth Science Sector (Environmental Geoscience Program) of 601
Natural Resources Canada. Our gratitude goes out to Mrs Marianne Molgat, formely of 602
Talisman Energy, without whom this project would likely not have taken place. We would 603
Rivard et al. 2018. Environ. Monit. Assess., doi: 10.1007/s10661-018-6532-7
28
also like to deeply thank the Ministère du Développement durable, de l’Environnement et 604
de la Lutte contre les Changements climatiques (MDDELCC), land and well owners that 605
allowed work to be performed on their property, the Municipality of Saint-Édouard, the 606
MRC de Lotbinière and the Ministère des Forêts, de la Faune et des Parcs du Québec. The 607
authors also want to sincerely thank Nicolas Benoit of the GSC for his internal review and 608
two anonymous reviewers for their careful review (to be completed). This paper is GSC 609
contribution # 31812. 610
611
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724
Table 1 Characteristics of the observation wells used in this study
Site Drilling
type
Drilling
year
Total
drilled
depth (m)
Static water
level (m
below TOC)
Overburden
thickness
(m)
Sampling
depth (m
below TOC)
Conditions
F-1 Diamond 2013 50 0.63 2.44 7.5 Confined
F-2 Diamond 2013 52 2.235 6.10 21.5 Confined
F-3 Diamond 2013 50 1.425 20.12 22.7 Confined
F-4 Diamond 2013 60 8.54 40.84 54.0 Confined
F-5 Hammer 2014 50 2.12 9.75 14.4 Confined
F-6 Hammer 2014 50 2.17 6.71 10.0 Confined
F-7 Diamond 2014 50 4.485 11.43 17.7 Semi-
confined
F-8 Diamond 2014 50 1.43 1.43 20.2 Confined
F-10 Hammer 2014 30 0.13 15.85 23.8 Confined
F-11 Hammer 2014 50 1.97 6.4 10.3 Semi-
confined
Notes: Diamond: Diamond-drilled well with a 100 mm (4 in.) diameter; Hammer: Hammer-drilled well with a 152
mm (6 in.) diameter. TOC: top of casing.
Table 2 Characteristics of the three sampling campaigns
Sampling event Sampling technique used Sampled observation wells
November 2014 1) Impeller (Redi-Flo2) pump
2) HydraSleeeve bags (after pumping)
F1, F2, F3, F4, F5, F6, F7, F8
and F11
May 2015
1) HydraSleeve bags (before pumping)
2) Impeller (Redi-Flo2) pump
3) Bladder pump
4) HydraSleeve bags (after pumping)
F1, F2, F3, F4, F5, F6, F7, F8,
F10 and F11
July 2015
1) HydraSleeve bags (before pumping)
2) Impeller (Redi-Flo2) pump
3) Bladder pump
4) HydraSleeve bags (after pumping)
F1, F2, F3, F4, F5, F6, F7, F8,
F10 and F11
1
Fig. 1 Location of the study area and the observation wells
2
Fig 2 Comparison of methane concentrations obtained using different sampling
techniques for nine wells in November 2014 (top) and ten wells in May 2015 (middle)
and July 2015 (bottom). Uncertainty of ± 15% is shown with error bars
0
10
20
30
40
50
60
F1 F2 F3 F4 F5 F6 F7 F8 F10 F11
Met
han
e (m
g/L
)
November 2014
0
10
20
30
40
50
60
F1 F2 F3 F4 F5 F6 F7 F8 F10 F11
Met
han
e (m
g/L
)
May 2015
0
10
20
30
40
50
60
F1 F2 F3 F4 F5 F6 F7 F8 F10 F11
Met
han
e (m
g/L
)
July 2015
Redi-Flo 2
Bladder pump
Hydra-Sleeve before pumping
Hydra-Sleeve after pumping
3
Fig. 3 Maximum differences corresponding to ratios of maximum over minimum
methane concentrations obtained at ten sites using either two methods (fall 2014) or four
methods (spring and summer 2015) for groundwater sampling. Note: Concentrations in
well F8 were very low in May 2015 and one value (the one from the HydraSleeve bag
before pumping) fell below the detection limit and was thus attributed half the detection
limit (i.e., 0.003 mg/L). This resulted in a very high max/min ratio of 11.7
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
F1 F2 F3 F4 F5 F6 F7 F8 F10 F11
Max
imu
m d
iffe
ren
ce b
etw
ee
n m
eth
od
s (m
ax/m
in r
atio
)
Nov./Dec. 2014 (2 methods) May 2015 (4 methods) July 2015 (4 methods)
well F8 in May 2015 max/min = 11.7
4
Fig. 4 Comparison between methane concentrations obtained with the impeller (Redi-
Flo2) and the bladder pumps. A few tests were also done outside the three field
campaigns for a total of 24 data pairs (blue dots). The solid line integrates all samples
(R2 = 0.813), while the dotted line excludes two abnormal values (R2 = 0.979)
R² = 0.8133
R² = 0.979
0
10
20
30
40
50
60
0 10 20 30 40 50
[CH
4] w
ith
th
e b
lad
de
r p
um
p (
mg/
L]
[CH4] with the RediFlo2 pump (mg/L)
F4, July 2015
F6, July 2015
When rejecting the
two abnormal values
(i.e. F4 and F6 in July)
5
Fig. 5 Plots of methane concentrations obtained with the four sampling techniques,
presented in pairs, along with each method’s statistical distribution. The 45° line represents
the perfect match. SLV: HydraSleeve bags; Redi-Flo2: impeller pump; Bladder: bladder
pump.
6
Fig. 6 Comparisons of dissolved methane concentrations obtained through HydraSleeve
bags and a bladder pump with those obtained with the impeller (Redi-Flo2) pump
(considered here as a reference)
-250%
-200%
-150%
-100%
-50%
0%
50%
100%
150%
0 5 10 15 20 25 30 35 40 45R
ela
tive
dif
fere
nce
(%
)
Methane concentration (impeller pump)
SLV before
Bladder
SLV after
7
Fig. 7 Comparison of δ13C -CH4 values obtained using different sampling techniques for
nine wells in November 2014 (top) and May 2015 (middle) and ten wells in July 2015
(bottom). Uncertainty of 1.7‰ is shown with error bars. Notes: For November 2014, well
F1 does not have a value for the HydraSleeve technique as the bottle broke. In May 2015,
well F8 did not have enough methane to run isotopic analyses. For July 2015, well F7
does not have a value for the bladder pump as it did not function well
-110
-90
-70
-50
F1 F2 F3 F4 F5 F6 F7 F8 F10 F11
Met
han
e δ
13C(‰
)
November 2014
-110
-90
-70
-50
F1 F2 F3 F4 F5 F6 F7 F8 F10 F11
Met
han
e δ
13C(‰
)
May 2015
-110
-90
-70
-50
F1 F2 F3 F4 F5 F6 F7 F8 F10 F11
Met
han
e δ
13C(‰
)
July 2015
Redi-Flo 2Bladder pumpHydra-Sleeve before pumpingHydra-Sleeve after pumping
8
Fig 8 Comparison of δ2H -CH4 values obtained using different sampling techniques for
ten wells in November 2014 (top), May 2015 (middle), and July 2015 (bottom).
Uncertainty of ± 19‰ is shown with error bars. Notes: For November 2014, well F4 does
not have results for the Redi-Flo2 pump due to broken bottles. For May 2015, wells F2
and F6 do not have results for HydraSleeve bags before pumping again due to broken
bottles. Well F8 did not have enough methane to run isotopic analyses. For July 2015,
well F7 does not have a value for the bladder pump as it did not function well
-400
-300
-200
-100
F1 F2 F3 F4 F5 F6 F7 F8 F10 F11
Met
han
e δ
2H(‰
)
November 2014
-400
-300
-200
-100
F1 F2 F3 F4 F5 F6 F7 F8 F10 F11
Met
han
e δ
2H(‰
)
May 2015
-400
-300
-200
-100
F1 F2 F3 F4 F5 F6 F7 F8 F10 F11
Met
han
e δ
2H(‰
)
July 2015 Redi-Flo 2Bladder pumpHydra-Sleeve before pumpingHydra-Sleeve after pumping
9
Fig. 9 Box plots of the values obtained for δ13C-CH4 (left) and δ2H-CH4 (right) with the
four sampling techniques over the three field campaigns. The band inside the box
corresponds to the 50th percentile (median), the bottom and top of the box correspond to
the 25th percentile (1st quartile, Q1) and 75th percentile (3rd quartile, Q3), while the
whiskers provide the minimum and maximum value. “SLV” stands for HydraSleeve
bags.