Stream Measurements Locate Thermogenic Methane Fluxes in

Article pubs.acs.org/est

Stream Measurements Locate Thermogenic Methane Fluxes in Groundwater Discharge in an Area of Shale-Gas Development Victor M. Heilweil,*,† Paul L. Grieve,‡ Scott A. Hynek,‡ Susan L. Brantley,‡ D. Kip Solomon,§ and Dennis W. Risser∥ †

U.S. Geological Survey Utah Water Science Center, Salt Lake City, Utah 84119, United States Earth and Environmental Systems Institute and Department of Geosciences, Pennsylvania State University, University Park, Pennsylvania 16802, United States § Department of Geology and Geophysics, University of Utah, Salt Lake City, Utah 84112, United States ∥ U.S. Geological Survey Pennsylvania Water Science Center, New Cumberland, Pennsylvania 17070, United States

Downloaded by UNIV OF MICHIGAN ANN ARBOR on September 11, 2015 | http://pubs.acs.org Publication Date (Web): March 30, 2015 | doi: 10.1021/es503882b

‡

S Supporting Information *

ABSTRACT: The environmental impacts of shale-gas development on water resources, including methane migration to shallow groundwater, have been difficult to assess. Monitoring around gas wells is generally limited to domestic water-supply wells, which often are not situated along predominant groundwater flow paths. A new concept is tested here: combining stream hydrocarbon and noble-gas measurements with reach mass-balance modeling to estimate thermogenic methane concentrations and fluxes in groundwater discharging to streams and to constrain methane sources. In the Marcellus Formation shalegas play of northern Pennsylvania (U.S.A.), we sampled methane in 15 streams as a reconnaissance tool to locate methane-laden groundwater discharge: concentrations up to 69 μg L−1 were observed, with four streams ≥5 μg L−1. Geochemical analyses of water from one stream with high methane (Sugar Run, Lycoming County) were consistent with Middle Devonian gases. After sampling was completed, we learned of a state regulator investigation of stray-gas migration from a nearby Marcellus Formation gas well. Modeling indicates a groundwater thermogenic methane flux of about 0.5 kg d−1 discharging into Sugar Run, possibly from this fugitive gas source. Since flow paths often coalesce into gaining streams, stream methane monitoring provides the first watershed-scale method to assess groundwater contamination from shale-gas development.

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INTRODUCTION The widespread application of horizontal drilling and hydraulic fracturing has resulted in significant scientific and public concern about the environmental effects of unconventional gas development on watersheds and ecosystems, including groundwater and surface water resources.1,2 Among other mechanisms, groundwater contamination is possible if fluids or stray gases mobilized during gas development migrate upward along preferential pathways, such as boreholes, faults, and fractures.3−6 Thermogenic methane can migrate from shale reservoirs into overlying aquifers as gas dissolved in upwardly migrating fluids or as a separate gas phase; recent studies have suggested a possible link between increased methane concentrations in overlying aquifers and improperly completed or abandoned oil and gas wells.7−10 Methane-laden groundwater from these aquifers will eventually discharge to wells, springs, or gaining streams. The Middle Devonian Marcellus Formation is currently the largest shale-gas play in the United States.11 It covers an area of 114,000 km2 within the Appalachian Basin12 (Figure 1), with a hotspot of intensive development in northern Pennsylvania. While the presence of methane has been documented in some Pennsylvania homeowner wells,13−15 the extent of stray-gas © 2015 American Chemical Society

contamination in shallow aquifers above the Marcellus is controversial because many wells contained high levels of methane prior to Marcellus development.4 Understanding of the extent of contamination is limited because of the lack of groundwater monitoring networks and publicly available predevelopment baseline data.4,5,16 Furthermore, no assessments have been made of the rate of methane migration into aquifers due to either natural or human-induced causes. Such information is needed to understand how hydrocarbons enter freshwater resources and how often such migration is caused by shale-gas development. While the stream reach mass-balance method has been used with dissolved-gas tracers for evaluating mean groundwater age and solute fluxes,17,18 the approach can also be used to quantify rates of groundwater−methane discharge to streams.19 In essence, gaining streams integrate methane inputs from multiple groundwater flow paths. The method has the potential to not only locate natural methane fluxes but also quantify Received: Revised: Accepted: Published: 4057

August 9, 2014 March 9, 2015 March 18, 2015 March 18, 2015 DOI: 10.1021/es503882b Environ. Sci. Technol. 2015, 49, 4057−4065

Article


Environmental Science & Technology

Figure 1. Maps showing location of the study area in northern Pennsylvania and measured stream methane concentrations.

increased fluxes related to shale-gas development. This streambased monitoring approach provides a broader evaluation than sampling programs based on monitoring wells.20,13 Gaining stream reaches integrate groundwater information over kmscale distances (due to coalescing of groundwater flow paths) that are likely more representative of regional aquifer conditions than point samples from individual wells. In this paper, we summarize previously reported regional methane concentrations in 15 streams21 and utilize hydrocarbon isotopes and noble-gas ratios to determine the source of methane entering one stream (Sugar Run) as thermogenic or biogenic. We then use 1-D stream transport modeling, building upon previously estimated fluxes,21 to evaluate thermogenic groundwater−methane entering the stream at one location. In this manner, we demonstrate that stream methane monitoring can be used to identify and quantify fluxes of thermogenic gas to groundwater and surface water.

from 0.07 to 0.43 pads per kilometer,21 indicating that it would provide a representative location for evaluating stream methane in an area of active shale-gas development. The outcropping bedrock in the Sugar Run watershed is Upper Devonian Trimmers Rock Formation (Lock Haven equivalent), an interbedded siliceous siltstone and shale eroded to a thickness of about 50−300 m, with beds striking about North 30° West (N30W) and dipping 11° to the northeast.22 This surficial bedrock has high angle joint sets (64−87°) striking both N70W and N43E, presumably associated with the Nittany Anticline located about 1 km to the south. The Marcellus Formation, a 120-m thick silty shale, is located at a depth of about 500−1300 m. The Trimmers Rock and Marcellus Formations are separated by shale of the Upper Devonian Harrell Formation (60-m thick) and Middle Devonian Mahantango Formation (400-m thick). Additional geologic information can be found in several publications.23,24 Samples were collected for methane analysis during reconnaissance stream sampling in May and June of 2013. Stream and groundwater samples were collected for hydrocarbon analysis during three synoptic studies of Sugar Run on May 21, June 27, and November 12, 2013 (collected sequentially from downstream to upstream to limit disturbance; Figure 2). The sample spacing in each subsequent campaign was decreased from 800 to 400 to 200 m to pinpoint areas with methane-laden groundwater inflow. Stream parameters were measured, and samples were collected at each site near the bottom of the stream and in the main channel of flow. Groundwater samples from Sugar Run were also collected from

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MATERIALS AND METHODS The study began with reconnaissance-level methane sampling of 15 streams located in areas of ongoing shale-gas development in the Marcellus Formation of northeastern Pennsylvania (Figure 1). These streams were selected based on ease of access for sampling and to represent the diverse physiography, land cover, and geology present in the area. One stream − Sugar Run, Lycoming County, Pennsylvania (in a 16.7 km 2 watershed) − was selected for the more detailed sampling because its 0.18/km2 shale-gas well-pad density was equal to the approximate median of the 15 watersheds, which ranged 4058

DOI: 10.1021/es503882b Environ. Sci. Technol. 2015, 49, 4057−4065

Article



Figure 2. Map showing location of sampling sites and Marcellus Shale gas wells in Sugar Run, Lycoming County, Pennsylvania.

N = (A*0.3861)0.2

both Seep 1.5, a small spring located near the stream channel, and temporary drive-point piezometers installed in the streambed at two sampling sites. These drive points were installed to depths of 0.3 to 1 m beneath the streambed and lined with vinyl tubing. Quantifying Groundwater Inflow. Stream discharge (Q) was measured using a SonTek YSI Flowtracker acoustic Doppler current profiler during the three seasonal synoptic studies in order to determine groundwater inflow or loss from the stream. Inflow or loss was calculated from the difference in Q over a reach, accounting for tributaries. Positive values indicate groundwater inflow and negative values indicate stream loss to the groundwater system or hyporheic zone. To confirm that the stream was at baseflow, the empirical equation25

(1)

was used to estimate the approximate number of days (N) after a storm until base-flow conditions were reached, where A is the basin area in km2. With this equation, base-flow conditions for Sugar Run are calculated to occur by 1.5 days after a storm peak. For the May 21, June 27, and November 12 synoptic studies, samples were collected 10, 13, and 5 days after storm peaks when streamflow along the lower end (Site 1 in Figure 2) was 0.10, 0.04, and 0.02 m3/s, respectively (Table S3). It is important to note that base-flow for a particular stream is not temporally constant but rather varies seasonally and annually, especially in areas such as northern Pennsylvania where the streamflow hydrograph is driven by rain events. Thus, it was 4059



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RESULTS AND DISCUSSION Stream Methane Monitoring. The reconnaissance sampling of 15 streams during May and June of 2013 documented methane concentrations as high as 69 μg L−1 (Table S1) or more than 103 times atmospherically equilibrated values. Four of 15 streams had methane concentrations ≥5 μg L−1. These high concentrations clearly indicate a nonatmospheric source of dissolved methane−either biogenic or thermogenic. Meshoppen Creek had the highest stream methane concentration during the May reconnaissance, but the sampling site was immediately downstream of a marsh, and subsequent hydrocarbon isotopic analysis indicated that the sample was likely a mixture of microbial and thermogenic sources. This stream, therefore, was not targeted for further analysis. Stream methane concentrations during the three detailed synoptic studies conducted in Sugar Run and its tributaries ranged from 0.2 to 67 μg L−1 (Table S3). Methane concentrations were consistently highest near Site 1.5, located 3,520 m from the top of the reach (Figure 3). This is

assumed that streamflow in Sugar Run during each of the three synoptics was steady and at base-flow conditions, even though these base-flow conditions were variable at the monthly-toseasonal time scale. Hydrocarbons and Noble Gases. Stream and groundwater sampling and analyses for hydrocarbons and noble gases are described in detail in the Supporting Information text. Radiogenic Ar (40Ar*) was computed using measured isotopes27 ⎛ 40 Ar Ar* = ⎜⎜ 36 ⎝ Ar

40

meas

⎞ − 295.5⎟⎟ × 36Armeas ⎠


40

36

(2) 40

where 295.5 is the atmospheric Ar/ Ar ratio (i.e., Ar* is corrected for atmospheric Ar by assuming that all 36Ar is of atmospheric origin). The difference between δ13C of methane (δ13CCH4) and δ13C of ethane (δ13CC2H5) is defined herein as Δ13C. Because the 4He/40Ar* versus Δ13C is a function of temperature, this combined hydrocarbon and noble-gas signature is indicative of the thermal maturity of the hydrocarbons.26 1-D Stream Transport Modeling. For dissolved gases such as methane entering a gaining stream, the steady state stream-reach mass balance equation18,19 is ∂QC = IgwCgw − k CH4wC ∂x

(3)

where a stream is characterized over its downstream distance x (L) by discharge Q (L3 T−1), methane concentration C (M L−3), and width w (L). Likewise, the groundwater input is characterized by its volumetric water inflow per unit stream length Igw (L2 T−1), its methane concentration Cgw (M L−3), and the apparent atmospheric gas transfer velocity of methane kCH4, (L T−1). Here we use kCH4 to include methane loss by degassing to the atmosphere as well as microbially mediated oxidation.7 Dissolution of atmospheric methane into the stream is also possible but with current atmospheric methane concentrations of 1800 ppb,27 equilibrium solubility for 5−20 °C water is minimal (0.05−0.07 μg L−1). Using eq 3, groundwater−methane concentration (Cgw) and load (Cgw × Igw) for a given stream reach can be evaluated by measuring or estimating Q, Igw, C, kCH4, and w. The gas transfer velocity (kCH4) can be evaluated with the following empirical equations28,29 k600 = 2841(V *S)

Figure 3. Measured stream methane concentrations in Sugar Run, Pennsylvania, May, June, and November 2013.

concordant with observed gas bubbles emanating from both Seep 1.5 and the stream at this location; such surface expressions of methane exsolution have been previously reported, including stray-gas bubbling in creeks in adjacent Sullivan County.23 Shallow groundwater sampled from drivepoint piezometers and Seep 1.5 had methane concentrations as high as 4,600 μg L−1. Mass-balance calculations indicated that methane-laden groundwater is discharging to the stream. The abrupt increase in stream methane near Site 1.5 indicates that this inflow is likely associated with a focused source. However, while joint and fracture sets are present in outcropping sections of the Trimmers Rock Formation along Sugar Run, there is no observable increase in such fractures just above or at Site 1.5. Specific conductance in the stream was low (100 μg L−1) was not detected on November 12th with the 200-m stream sample spacing. While shorter sampling intervals would theoretically allow for the detection of this maximum value, the lack of complete across-stream mixing just below areas of methane influx make such detection unlikely. The overall model uncertainty could also be reduced in future studies by further refinement of stream parameters using stream tracer injection techniques:19 for example, (i) a conservative ion (e.g., Br) tracer dilution for refining Q(x) and determining more precisely the locations and quantities of Igw and (ii) a dissolved gas injection (e.g., Kr or SF6) for refining kCH4. Our previously published simulations using this 1-D stream transport modeling technique indicated methane loads of 1.8 ± 0.8, 0.7 ± 0.3, and 0.7 ± 0.2 kg d−1 discharging to a small section of Sugar Run during the May, June, and November synoptic studies, respectively.21 This previous effort incorporated more complex scenarios including (1) multiple gaining reaches, (2) variable groundwater−methane concentrations, and (3) variable reach lengths along which methane-laden groundwater discharge was simulated. An important finding was that uncertainty in estimated methane load decreased as sample spacing was decreased from 800 to 200 m. Also, the calculated methane loads decreased during the lower base flow conditions of June and November, compared with May. Because methane load (M/T) entering Sugar Run is the product of groundwater−methane concentration (M/L3) and groundwater discharge (L3/T), part of this decline is attributed to less groundwater entering the stream (base flow at Site 1 declined by 60% between May 21 and June 27, 2013). The same calculated methane load for both June and November (yet differing amounts of Igw) implies that the near-stream groundwater−methane concentration may also vary temporally. For example, if the methane in Sugar Run is due to a stray-gas contamination incident as discussed below, a transient pulse could be moving through the aquifer. Clearly, stream methane sampling is useful in documenting methane influxes but will be most powerful if seasonal variability is evaluated. Utility of Stream Monitoring. Elevated methane concentrations can persist over kilometers within a stream reach.19

The length of persistence depends not only on the methane load entering the stream but also on dilution from groundwater inflow (Igw), stream width (w), and gas exchange (kCH4). When wkCH4/Igw is greater than 50, such as for wide streams with large gas transfer velocities that receive relatively little groundwater inflow,33 synoptic sampling at closely spaced intervals (∼0.1 km) may be necessary to characterize peak stream methane. Alternatively, when wkCH4/Igw is less than 5 (e.g., the stream’s gas transfer velocity is low−for example streams with low turbulence that receive a large fraction of total flow from groundwater), sampling site intervals could be increased perhaps by an order of magnitude (∼1 km). In summary, with an appropriate sampling resolution, reconnaissance sampling and synoptic studies can be used to identify areas of groundwater-laden methane discharge, establish baseline water-quality conditions, and evaluate both spatial and temporal trends. Coupling flux estimates with hydrocarbon-isotope and noble-gas sampling can indicate if the methane is thermogenic or derived from microbial processes (e.g., biogenic), and, in some cases, the geologic source of thermogenic gas may be inferred. Because of the simplicity of reconnaissance stream methane sampling, the first step (identifying areas of methaneladen groundwater discharge) can be readily implemented over large regions. Implications. In this study, the stream methane monitoring approach was coupled with additional geochemical analysis to identify and quantify thermogenic methane discharging to a stream. Importantly, the isotopic and noble-gas characteristics of the dissolved gas in Sugar Run are consistent with a Middle Devonian Shale source. After the sampling at Sugar Run was completed, we became aware of a Pennsylvania Department of Environmental Protection (PA DEP) Notice of Violation letter34 stating that at least five domestic water wells have been impacted by stray gas migration near a gas well (permit API 37081-20292) that was noted to have defective casing or cement. The violation stated that the gas well had “caused or allowed gas from lower formations to enter fresh groundwater in Moreland Township”. This gas well was first drilled vertically and then horizontally in a NW direction (N 22 W), roughly perpendicular to and crossing beneath Sugar Run upstream from Site 4 (Figure 2). Because no baseline stream hydrocarbon samples had had been collected prior to this stray-gas migration incident, we are unable to definitively conclude that the groundwater−methane flux to Sugar Run is associated with stray-gas migration. Documentation of this gas-well methane release and the methane contamination of nearby domesticsupply wells, along with the lack of correlation with increased salinity in the stream, all suggest that the source of methane to the stream may be stray gas rather than natural migration of crustal fluids. Our work, therefore, documents and quantifies a thermogenic (and possibly stray-gas) methane flux directly to a stream in an area of active shale-gas development. The application of the stream methane monitoring method in northern Pennsylvania shows that it can be a valuable tool for evaluating impacts of unconventional shale-gas development. The transient nature of streams (including geometry, streamflow, groundwater inflow, and gas transfer velocity) and groundwater−methane concentrations indicate that time series data may be needed to fully represent the variability in both natural and stray-gas methane fluxes. Ideally, stream monitoring would begin prior to shale-gas development in order to establish seasonal and annual variability in baseline ground4063



Environmental Science & Technology water quality prior to potential impacts from development. Initial reconnaissance stream methane sampling can be undertaken very effectively in terms of cost, time, and training and may be a less controversial option to sampling private domestic-supply wells. The technique may only require grab samples for methane analysis at km-scale downstream intervals and could be appropriate for “citizen science”. Once streams with elevated methane are identified, further investigation would become more intensive with respect to time, cost, and qualified personnel to evaluate potential sources; this would typically include hydrocarbon isotopic composition (δ13CCH4, δ13CC2H6, δ2HCH4), stream discharge measurements, and possibly stream tracer injections. Similarly intensive and costly data collection, however, would also be required for studies using domestic-supply wells. Because predevelopment data is needed to conclusively determine sources of thermogenic methane in shallow groundwater and streams, this method has particular relevance in areas of emerging shale-gas plays, where unconventional gas development may occur in the future. Recent literature has focused on the need for accurate baseline monitoring prior to development;16,35 our stream-based method can be used at the watershed scale as an effective reconnaissance tool for such purposes. Shale-gas development is or will be occurring in many places throughout the world where residential water wells are not densely located or cannot be readily accessed for sampling. If such areas have gaining perennial streams, stream monitoring provides a simple and cost-effective alternative to the installation of new monitoring-well networks. Even in areas with existing domestic-supply wells, it can provide a spatially integrated characterization of shallow groundwater quality that may be more representative and complementary to data from individual wells, which typically have smaller capture zones than gaining streams. Thus, stream methane sampling provides a compelling monitoring tool that can be initiated rapidly and provide high-resolution data in both space and time.

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ACKNOWLEDGMENTS

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REFERENCES

We would like to acknowledge the help of Todd Sowers (Pennsylvania State University) for analytical determination of hydrocarbon concentrations (C1, C2) and carbon isotopes (δ13CCH4, δ13CC2H6), Richard Doucett (University of CaliforniaDavis) for analytical determination of hydrogen isotopes of methane (δ2HCH4), and Randall Conger (U.S. Geological Survey) for the Sugar Run discharge measurements. Susan L. Brantley and Paul L. Grieve acknowledge support from NSF OCE 11-40159. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government.

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ASSOCIATED CONTENT

S Supporting Information *

Details of (a) sample collection and analysis and (b) the 1-D stream transport modeling, one figure, and three tables. Table S1 describes stream sampling sites for a reconnaissance survey in northern Pennsylvania, including water-quality characteristics and measured concentrations of methane in samples collected during May and June, 2013. Table S2 provides hydrocarbon and noble-gas isotope chemistry results of streamwater and shallow groundwater at Sugar Run, Lycoming County, Pennsylvania, November 12, 2013. Table S3 provides streamflow, field water-quality parameters, and dissolved methane concentrations in streamwater and shallow groundwater at Sugar Run, Lycoming County, Pennsylvania, May 21, June 27, and November 12, 2013. This material is available free of charge via the Internet at http://pubs.acs.org.

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Article

AUTHOR INFORMATION

Corresponding Author

*Phone: 801-908-5042. Fax: 801-908-5001. E-mail: Heilweil@ usgs.gov. Notes

The authors declare no competing financial interest. 4064


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gases in water-supply wells in north-central Pennsylvania. Appl. Geochem. 2010, 25, 1845−1859. (32) Gardner, W. P.; Harrington, G. A.; Solomon, D. K.; Cook, P. G. Using Terrigenic 4He to identify and quantify regional groundwater discharge to streams. Water Resour. Res. 2011, 47, W06523 DOI: 10.1029/2010WR010276. (33) Solomon, D. K.; Plummer, L. N.; Busenberg, E.; Cook, P. G. Use of chlorofluorocarbons in hydrology: Practical applications of CFCs in hydrological investigations; International Atomic Energy Agency: Vienna, 2006; Chapter 7. (34) Pennsylvania Department of Environmental Protection Notice of Violation, Gas Migration Investigation, Moreland Township, Lycoming County: Oil and Gas Management Program, Eastern District Oil and Gas Operations, Green Valley GMI File, September 20, 2013. (35) Jackson, R. E.; Gorody, A. W.; Mayer, B.; Roy, J. W.; Ryan, M. C.; Van Stempvoort, D. R. Groundwater protection and unconventional gas extraction: The critical need for field-based hydrogeological research. Groundwater 2013, 51 (4), 488−510.

(15) Sloto, R. A. Baseline groundwater quality from 20 domestic wells in Sullivan County, Pennsylvania, 2012. U.S. Geological Survey Scientific Investigations Report 2013-5085, 2013. (16) Vengosh, A.; Jackson, R. B.; Warner, N.; Darrah, T. H.; Kondash, A. A critical review of the risks to water resources from unconventional shale gas development and hydraulic fracturing in the United States. Environ. Sci. Technol. 2014, 48 (15), 8334−8348 DOI: 10.1021/es405118y. (17) Stolp, B. J.; Solomon, D. K.; Suckow, A.; Vitvar, T.; Rank, D.; Aggarwal, P. K.; Han, L. F. Age dating base flow at springs and gaining streams using helium-3 and tritium: Fischa-Dagnitz system, southern Vienna Basin, Austria. Water Resour. Res. 2010, 46 (7), W07503 (2010) DOI:10.1029/2009WR008006. (18) Cook, P. G.; Lamontagne, S.; Berhane, D.; Clark, J. F. Quantifying groundwater discharge to Cockburn River, southeastern Australia, using dissolved gas tracers Rn-222 and SF6. Water Resour. Res.. 2006, 42 (10), DOI:10.1029/2006WR004921. (19) Heilweil, V. M.; Stolp, B. J.; Susong, D. D.; Kimball, B. A.; Rowland, R. C.; Marston, T. M.; Gardner, P. M. A stream-based methane monitoring approach for evaluating groundwater impacts associated with unconventional gas development. Groundwater 2013, 51 (4), 511−524. (20) Breen, K. J.; Revesz, K.; Baldassare, F. J.; McAuley, S. D. Natural gases in ground water near Tioga Junction, Tioga County, northcentral PennsylvaniaOccurrence and use of isotopes to determine origins. U.S. Geological Survey Scientific Investigations Report 2007-5085, 2007. (21) Heilweil, V. M.; Risser, D. W.; Conger, R. W.; Grieve, P. L.; Hynek, S. A. Estimation of methane concentrations and loads in groundwater discharge to Sugar Run, Lycoming County, Pennsylvania. U.S. Geological Survey Open-File Report 2014-1126, 2014. (22) Faill, R. T. Bedrock geologic map of the Montoursville South and Muncy quadrangles, and part of the Hughesvill quadrangle, Lycoming, Northumberland, and Montour counties, Pennsylvania; Pennsylvania Bureau of Topographic and Geologic Survey: Harrisburg, PA, 1979; Atlas 144. (23) Reese, S. O.; Neboga, V. V.; Pelepko, S.; Kosmer, W. J.; Beattie, S. Groundwater and petroleum resources of Sullivan County, Pennsylvania. Pennsylvania Geological Survey (4th Series) Water Resources Report 71, Harrisburg, PA, 2014. (24) Baldasarre, F. J.; McCaffrey, M. A.; Harper, J. A. A geochemical context for stray gas investigations in the northern Appalachian Basin: Implications of analyses of natural gases from Neogene- through Devonian-age strata. AAPG Bull. 2014, 98 (2), 341−372. (25) Linsley, R. K., Jr.; Kohler, M. A.; Paulhus, J. L. H. Hydrology for engineers, 2nd ed.; McGraw-Hill: New York, 1975. (26) Hunt, A. G.; Darrah, T. H.; Poreda, R. J. Determining the source and genetic fingerprint of natural gases using noble gas geochemistry: a northern Appalachian Basin case study. AAPG Bull. 2012, 96 (10), 1785−1811. (27) CDIAC (Carbon Dioxide Information Analysis Center), Recent greenhouse gas concentrations. DOI: 10.3334/CDIAC/atg.032. http://cdiac.ornl.gov/pns/current_ghg.html (accessed Mar 19, 2015). (28) Raymond, P. A.; Zappa, C. J.; D. Butman, D.; Bott, T. L.; Potter, J.; Mulholland, P.; Laursen, A. E.; McDowell, W. H.; Newbold, D. Scaling the gas transfer velocity and hydraulic geometry in streams and small rivers. Limnol. Oceanogr.: Fluids Environ. 2012, 2, 41−53 DOI: 10.1215/21573689-1597669. (29) Jahne, B.; Heinz, G.; Deitrich, W. Measurement of the diffusion coefficients of sparingly soluble gases in water. J. Geophys. Res.: Oceans 1987, 2 (C10), 10767−10776. (30) Warner, N. R.; Jackson, R. B.; Darrah, T. H.; Osborn, S. G.; Down, A.; Zhao, K.; White, A.; Vengosh, A. Geochemical evidence for possible natural migration of Marcellus Formation brine to shallow aquifers in Pennsylvania. Proc. Natl. Acad. Sci. U. S. A. 2012, DOI: 10.1073/pnas.1121181109. (31) Révész, K. M.; Breen, K. J.; Baldassare, A. J.; Burruss, R. C. Carbon and hydrogen isotopic evidence for the origin of combustible 4065


Stream Measurements Locate Thermogenic Methane Fluxes in

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