hydrocarbon-rich groundwater above shale-gas formations: …...hydrocarbon-rich groundwater above...

Hydrocarbon-Rich Groundwater above Shale-GasFormations: A Karoo Basin Case Studyby William K. Eymold1, Kelley Swana2, Myles T. Moore1, Colin J. Whyte1, Jennifer S. Harkness1, Siep Talma3,Ricky Murray4, Joachim B. Moortgat1, Jodie Miller2, Avner Vengosh5, and Thomas H. Darrah6

AbstractHorizontal drilling and hydraulic fracturing have enhanced unconventional hydrocarbon recovery but raised environmental

concerns related to water quality. Because most basins targeted for shale-gas development in the USA have histories of both activeand legacy petroleum extraction, confusion about the hydrogeological context of naturally occurring methane in shallow aquifersoverlying shales remains. The Karoo Basin, located in South Africa, provides a near-pristine setting to evaluate these processes,without a history of conventional or unconventional energy extraction. We conducted a comprehensive pre-industrial evaluationof water quality and gas geochemistry in 22 groundwater samples across the Karoo Basin, including dissolved ions, water isotopes,hydrocarbon molecular and isotopic composition, and noble gases. Methane-rich samples were associated with high-salinity,NaCl-type groundwater and elevated levels of ethane, 4He, and other noble gases produced by radioactive decay. This endmemberdisplayed less negative δ13C-CH4 and evidence of mixing between thermogenic natural gases and hydrogenotrophic methane.Atmospheric noble gases in the methane-rich samples record a history of fractionation during gas-phase migration from sourcerocks to shallow aquifers. Conversely, methane-poor samples have a paucity of ethane and 4He, near saturation levels of atmosphericnoble gases, and more negative δ13C-CH4; methane in these samples is biogenic and produced by a mixture of hydrogenotrophic andacetoclastic sources. These geochemical observations are consistent with other basins targeted for unconventional energy extractionin the USA and contribute to a growing data base of naturally occurring methane in shallow aquifers globally, which provide aframework for evaluating environmental concerns related to unconventional energy development (e.g., stray gas).

IntroductionAlthough horizontal drilling and hydraulic fracturing

have enhanced unconventional hydrocarbon recoverythroughout the last decade, there have been significantcontroversies regarding the cause and environmental

1School of Earth Sciences, The Ohio State University, 275Mendenhall Laboratory, 125 South Oval Mall, Columbus, OH 43210;[email protected]; [email protected]; [email protected];[email protected]; [email protected]

2Department of Earth Sciences, Stellenbosch Univer-sity, Private Bag XI, Matieland, 7602, South Africa; [email protected]; [email protected]

3Natural Resources and the Environment, CSIR Pretoria, P.O.Box 395, Pretoria, 0001, South Africa; [email protected]

4Groundwater Africa, 38 Disa Ave., Kommetjie, 7975, SouthAfrica; [email protected]

5Division of Earth and Ocean Sciences, Nicholas Schoolof the Environment, Duke University, Durham, NC 27708;[email protected]

6Corresponding author: School of Earth Sciences, The OhioState University, 275 Mendenhall Laboratory, 125 South Oval Mall,Columbus, OH 43210. [email protected]

Article impact statement: Evaluation of Karoo Basingroundwater baseline prior to hydraulic fracturing.

Received December 2017, accepted December 2017.© 2018, National Ground Water Association.doi: 10.1111/gwat.12637

implications of elevated hydrocarbon gas levels indrinking-water wells in various petroleum basins in NorthAmerica (e.g., Osborn et al. 2011; Warner et al. 2012a,2014; Jackson et al. 2013; Molofsky et al. 2013; Darrahet al. 2014, 2015b; Vengosh et al. 2014; Siegel et al.2015; Humez et al. 2016; Nicot et al. 2017a; Harknesset al. 2017b; Darrah 2018). Several studies suggest thatshale-gas drilling leads to fugitive gas contamination ina subset of drinking-water wells near drill sites (Osbornet al. 2011; Jackson et al. 2013; Brantley et al. 2014;Darrah et al. 2014; Heilweil et al. 2015; Sherwood et al.2016), while others suggest that methane is natural andunrelated to shale-gas development (Molofsky et al.2013; Warner et al. 2013; Baldassare et al. 2014; Siegelet al. 2015; Darrah et al. 2015b; Harkness et al. 2017b).

In order to evaluate the environmental effects ofhydrocarbon extraction in petroleum basins targetedfor drilling, baseline groundwater quality has beenevaluated using a multiple geochemical tracer approach(e.g., Molofsky et al. 2013; Darrah et al. 2014, 2015b;McMahon et al. 2015; Nicot et al. 2017a, 2017b, 2017c).These studies have identified elevated levels of methaneat concentrations up to its saturation point in shallowaquifers and revealed an association between methaneand NaCl-type waters, valley bottoms, faulting, and

204 Vol. 56, No. 2–Groundwater–March-April 2018 (pages 204–224) NGWA.org

highly reducing aquifers (Warner et al. 2012a; Jacksonet al. 2013; Darrah et al. 2015a, 2015b; Harkness et al.2017b, 2018). In general, this natural methane displayedlow levels of ethane or higher aliphatic hydrocarbons(i.e., high ratio of methane to higher order hydrocarbonsdesignated as C1/C2+), intermediate to less negativevalues for stable isotopes of methane (e.g., δ13C-CH4),and a long history of multiphase hydrocarbon migrationfrom source rocks (e.g., Marcellus Shale, Barnett Shale)to shallow aquifers based on noble gas evidence. Bycomparison, groundwater samples associated with eitherdocumented or interpreted anthropogenic stray gascontamination events occur across a broad range of saltcontents and water types, display highly elevated levels ofethane and higher aliphatic hydrocarbons (lower C1/C2+)consistent with deeper hydrocarbon sources, more posi-tive δ13C-CH4, and stripping of atmospheric noble gases(Jackson et al. 2013; Brantley et al. 2014; Darrah et al.2014, 2015a; Sherwood et al. 2016; Wen et al. 2016).

While these and other important contributions haveaddressed various aspects of the controversy surroundingelevated levels of hydrocarbon gases in shallow aquifers,the ongoing debate on the benefits and consequencesof shale-gas development is stymied further until thegroundwater community reaches a consensus on thehydrogeological context of naturally occurring methanein shallow aquifers overlying shales targeted for drilling.To this point, although it has been nearly seven yearssince the initial Osborn et al. (2011) paper, there isstill a dearth of published genuinely predrill, baselinedatasets. Although the baseline studies that have beenpublished are of great value, the majority of thesestudies did not start until after unconventional shale-gasdrilling had commenced in these areas (Molofsky et al.2013; Baldassare et al. 2014; Siegel et al. 2015; Darrahet al. 2015b; Humez et al. 2016; Nicot et al. 2017a;Harkness et al. 2017b; Nicot et al. 2017b, 2017c). Thus,it remains uncertain if observations from those datasetsactually represent the natural background. To furthercomplicate the issue, most sedimentary basins targetedfor unconventional development in the USA and Canadawere already exposed to an extensive history of coalmining as well as legacy oil and gas exploitation, leavingan uncertain, or at least unconstrained, potential forcontamination related to those anthropogenic activities.Each of these factors has led to considerable confusionabout the hydrogeological context of genuinely baselineconditions for naturally occurring methane and salinewaters in shallow aquifers overlying shales targeted forunconventional hydrocarbon development.

Recent work has attempted to address this issue byconducting directly paired, longitudinal studies of waterquality both before and after unconventional drilling in thenorthern Appalachian Basin (e.g., Harkness et al. 2017b).Here, we employ a different study design strategy byexamining water quality and the hydrogeological contextfor the occurrence of hydrocarbon gases and salts ina nearly pristine setting with neither a local historyof mining nor conventional or unconventional energy

extraction in the Karoo Basin, South Africa. We designedthis study to improve the groundwater community’sknowledge regarding not only the presence of dissolvednatural gases but also to gain a better understandingof the source and migration mechanisms of subsurfacenatural gas-rich fluids by examining their occurrence andgeochemistry in a region unaffected by coal mining,conventional petroleum extraction, or unconventionalshale-gas development. While it is unclear if thesegenuinely baseline data are globally representative, theycomprise an important piece of evidence with which tocompare existing and future datasets.

Geological and Study BackgroundThe planned exploitation of unconventional hydrocar-

bon resources within the semiarid, water-restricted KarooBasin in South Africa has necessitated a comprehen-sive evaluation of predrill baseline water quality datathroughout the region (Murray 2015; Swana 2016). Insuch an environment, the consequences of compromiseddrinking-water quality can have significant environmental,agricultural, and human health implications. While scien-tists have researched the shallow groundwater networksthroughout the Karoo Basin over the past five decades,major uncertainties remain about the hydrogeological con-text of naturally occurring methane- and salt-rich ground-water throughout the region (Murray 2015; Swana 2016).

In this study, groundwater is evaluated using a com-prehensive suite of tracers, including water parameters(pH, Eh, electrical conductivity, temperature), dissolvedion chemistry (e.g., Na, Ca, Cl, Br), water isotopes(δ2H-H2O, δ18O-H2O), major gas geochemistry (e.g.,N2), hydrocarbon gas geochemistry (C1-C5), compound-specific stable isotopes of hydrocarbons (δ2H-CH4, δ13C-CH4, δ13C-C2H6), noble gas elemental (He, Ne, Ar) andisotope geochemistry (3He/4He, 20Ne/22Ne, 21Ne/22Ne,20Ne/36Ar, 40Ar/36Ar), and tritium (3H) following stan-dard methods reported previously (Darrah et al. 2015b;Moore et al. 2018; Harkness et al. 2017b).

These data were collected from two subsets ofsamples: (1) methane-rich springs or (2) groundwaterboreholes sampled near bubbling springs from across theKaroo Basin. This suite of geochemical tracers allowsus to provide the first comprehensive baseline reportof the groundwater characteristics for this hydrocarbon-rich region which will be followed up in a longitudinalstudy during and after industrial activity, develop a database of genuinely predrill data, and expand the growinggeochemical data base of methane-rich groundwatersglobally.

Brief Geologic/Hydrologic Overview of the Karoo BasinThe Karoo Basin, located in South Africa, spans an

area of more than 200,000 km2 and underlies a majority ofthe country (Aarnes et al. 2011; Figure 1). This sedimen-tary basin developed as a large retro-arc foreland basinbehind the Cape Fold Belt between the late-Paleozoicto mid-Mesozoic (Catuneanu et al. 2005). The Karoo

NGWA.org W.K. Eymold et al. Groundwater 56, no. 2: 204–224 205

Figure 1. Map of Karoo Basin. Sampling site locations (n = 22) are shown as yellow circles and are associated with namedsampling regions (n = 8). General geologic descriptions for surface lithology are indicated via color and in the legend. Notethe presence of dolerite sills, dikes, and intrusions (red) throughout.

Supergroup, which includes the primary and secondarytarget formations for unconventional hydrocarbon devel-opment (i.e., Whitehill and Prince Albert formations), con-tains Carboniferous-age marine glacial deposits at its baseand is comprised dominantly of interbedded terrestrialsandstones, silty carbonates, and shales deposited fromthe Carboniferous to mid-Jurassic (Johnson et al. 1996;Johnson et al. 2006) (Figure 2). The Permian-age White-hill Fm., which is dominantly shale with up to 14% totalorganic carbon (TOC) and contains abundant carbonatesand pyrite, has a thickness of 10 to 80 m and lies betweenthe underlying Prince Albert Fm. and the superior Colling-ham Fm. (Johnson et al. 1996, 2006). Together with theRipon Fm., this group of Permian-age rocks is collec-tively called the Ecca Group (Johnson et al. 1996, 2006;Catuneanu et al. 2005).

Most active groundwater wells in this region areproducing groundwater from shallow aquifers, usuallyless than 200 m from the surface, while the depth tothe water table varies both locally and seasonally with atypical range between 5 and 15 m from ground surface(Rosewarne et al. 2013). While preferential regional-scale groundwater flow is topographically controlled,the dominant direction varies across the Karoo Basin(Adams et al. 2001; Rosewarne et al. 2013). In thewestern Karoo, regional flow direction in shallow aquifersis mainly to the north, while in the central Karoo,regional flow direction in shallow aquifers is dominantlyfrom the north down to the southwest, south, andsoutheast. The eastern Karoo is the most complex section,having variable flow directions in shallow aquifers inareas of the eastern sections of the Karoo (Adams

et al. 2001; Rosewarne et al. 2013). By comparison,heterogeneous, localized groundwater recharge occursprimarily during seasonal flooding events (Le Maltre et al.2009). Most of this localized recharge occurs withinfractured top-of-rock aquifers and fractured areas neardolerite dike intrusions, which dominate actively producedgroundwater flow systems in the area (Mahed 2016).The actively produced aquifers include Permian-agesandstones (e.g., Waterford Fm.) and mudstones, primarilyfrom the Ecca Group (e.g., Collingham and Fort Brownformations) and Beaufort Group (e.g., Koonap, Middleton,and Balfour formations), as well as along fracturedcontacts with Jurassic dolerite dykes (Adams et al. 2001).Faulting, specifically in the northern and northwestportions of the basin, provides additional pathways forheterogeneous groundwater recharge and groundwatermigration (Oesterlen 1991; Johnson et al. 1996; Johnsonet al. 2006). Areas of intense faulting, or fracturing nearfaults or dolerite intrusions, may be preferential pathwaysfor natural gas migration, as well as zones of intenselocal meteoric water recharge. The groundwater residencetimes, flow conditions, and geochemistry of deeper wells(more than 1000 m) are relatively poorly understood(Woodford and Chevallier 2002) but recent work suggeststhat extensive episodes of freshwater recharge duringwetter climatic conditions reduced the salinity of basinalbrines (Harkness et al. 2018).

Design of the StudyThe Water Resource Commission (WRC) of South

Africa proposed a study to develop the baseline geochemi-cal measurements of groundwater quality within the Karoo

206 W.K. Eymold et al. Groundwater 56, no. 2: 204–224 NGWA.org

Figure 2. Stratigraphic column for the Karoo Basin. Lithologic symbols are used to describe each formation name; formationage is provided in the right column. Target formation for shale-gas development is the Whitehill Formation (shown with theCollingham Fm.) in the Ecca Group. This formation of shales and sandstones (with TOC ∼ 14%) varies in thickness between10 and 80 m and underlies the basin between 2 and 4 km depth.

Basin prior to shale-gas exploration (Murray 2015). Theysought to characterize both deep and shallow sources ofgroundwater by collecting samples from known bubblingmethane-rich springs and nearby groundwater boreholesand then evaluating them for major ions, metals, stableisotopes of water and gas, and gas geochemistry (Murray2015).

Our study targeted eight (n = 8) sampling regionsacross the Karoo Basin that were evaluated for gasand water geochemistry (Figure 1). The sampling regionswere selected based on prior knowledge of the presenceof geothermal springs/artesian boreholes or the presenceof deep boreholes. The study also included other nearbydrinking-water or agricultural wells for comparison fora total of n = 22 samples. Samples included two deepboreholes (at depths of ∼1400 and ∼4200 m), ninesprings/artesian wells, and 11 groundwater samples fromshallow boreholes (Murray 2015; Swana 2016). Sampling

occurred over a 2-week period between June and July2014.

Geochemical Techniques ImplementedA comprehensive evaluation of groundwater aqueous

and gas geochemistry is necessary to establish baselinegeochemical measurements prior to shale-gas extraction.Data generated as part of this study are used to evaluatethe content and context of methane- and salt-rich ground-waters throughout the region, and better understand thesource and mechanism of hydrocarbon transport to andwithin shallow aquifers. This study area was chosen toexpand our knowledge of the hydrogeological and geo-chemical context for hydrocarbon gases in an area thatserves as an example of a genuinely predrill, baseline set-ting, thus improving our understanding of naturally occur-ring methane in shallow aquifers globally. Additionally,this work will help provide a framework for evaluating


environmental concerns related to unconventional energydevelopment (e.g., stray gas). Various aspects of waterstable isotopes, sources of salinity, and other aspects ofwater quality were presented previously (Murray 2015;Swana 2016) and elsewhere in this volume (Harknesset al. 2018). Thus, we provide only a summary on thewater quality parameters in the Results and Discussionsections. The current study will focus on placing gasgeochemical data, including hydrocarbon molecular andisotopic composition, and noble gas geochemistry, withinthe context of the hydrological setting.

The molecular composition of hydrocarbons (e.g.,C1/C2+) and stable isotopes for hydrogen (H) andcarbon (C) in methane (δ2H-CH4; δ13C-CH4) and ethane(δ13C-C2H6) present in groundwater can be used todetermine the genetic source (i.e., biogenic vs. thermo-genic) of natural gases (Bernard et al. 1976; Schoell1983; Whiticar et al. 1986). Thermogenic natural gasesare produced by the thermocatalytic degradation oforganic matter. The generation of thermogenic naturalgas produces ethane and heavier aliphatic hydrocarbons(i.e., termed “wetter”) in addition to methane. Thermo-genic gases display less negative stable isotope valuesof δ2H-CH4 and δ13C-CH4 (e.g., δ13C-CH4 > −55‰)(Bernard et al. 1976; Whiticar et al. 1986; Whiticar1999). By comparison, biogenic natural gas is producedby methanogens and consists almost exclusively ofmethane and CO2 (more than 99.9%). Biogenic CH4displays more negative values for δ13C-CH4 between−75‰ and −55‰ (Schoell 1983). The various pathwaysof microbial methanogenesis impart characteristic butsometimes overlapping ranges of δ13C-CH4 and δ2H-CH4for both hydrogenotrophic and acetoclastic methanogen-esis (Whiticar 1999). Hydrogenotrophic methanogenesisproduces CH4 with a larger fractionation from the sourceof organic matter (i.e., more negative δ13C-CH4) thanacetoclastic methanogenesis (Whiticar 1999), whereasacetoclastic methanogenesis strongly fractionates δ2H-CH4. Thus, the δ2H-CH4 serves as an important tracerfor distinguishing between natural gas produced byhydrogenotrophic and acetoclastic methanogenesis.

Following the formation of biogenic and thermogenichydrocarbon gases, additional postgenetic processes canalter the original composition of natural gases. Theseprocesses include: (1) mixing with either other biogenicor thermogenic natural gases; (2) secondary biodegrada-tion that consumes wet gas components, while yieldingCH4 via methanogenesis; (3) anaerobic oxidation (e.g.,sulfate reduction); (4) aerobic oxidation; (5) diffusivefractionation; and (6) fractionation by solubility parti-tioning during fluid migration (e.g., Etiope et al. 2009;Etiope and Ciccioli 2009; Darrah et al. 2015b). Each ofthese processes can alter the original composition andobfuscate the genetic source of natural gas. Alternatively,these changes provide important insights on the historyof natural gas in the subsurface.

In addition to hydrocarbon composition, the inertnature and well-constrained elemental and isotopic com-positions of noble gases found in the atmosphere, crust,

and mantle allow them to serve as reliable geochem-ical tracers of fluid mean residence time, hydrocarbonsource, and transport mechanisms in groundwater (Ballen-tine et al. 2002). The ratios and isotopes of noble gases aredistinct enough to not only discern naturally sourced fluidsfrom those contaminated by drilling, but also to iden-tify the specific migration mechanisms that resulted in theobserved signature (Darrah et al. 2014). This consortiumof analyses provides a robust set of baseline data, servesas a guideline for comparison to potentially contaminatedwater in future studies, and allows us to evaluate the influ-ence and genetic source of hydrocarbon-rich fluids formedwithin or that migrated into shallow aquifers throughoutthe Karoo Basin.

Methods

Sample Collection and Field MeasurementsWater samples were collected after 30 min of flushing

or when the field the parameters stabilized, prior toany treatment systems, and were filtered and preservedin airtight, high density polyethylene (HDPE) bottlesfollowing USGS protocols (USGS 2011). Samples werefiltered through 0.45 µm filters for dissolved ions. Waterchemistry samples were stored on ice or refrigerated untilthe time of their analysis.

At each of these locations, field measurements wereconducted including temperature, pH, oxidation-reductionpotential, alkalinity, and electroconductivity (EC) usinga YSI multiprobe calibrated daily. Hydrocarbon gassamples were collected in closed volume, refrigeration-grade copper tubes that were flushed in-line with atleast 50 volumes of sample water prior to sealingwith stainless steel clamps following methods reportedpreviously (Darrah et al. 2015b; Kang et al. 2016). Noblegas samples were collected in duplicate copper tubesfollowing the same methods established above. Bothsample splits were shipped to the Ohio State University(OSU) for sample analysis.

Analytical MethodsMethods and data related to major dissolved ions,

water stable isotopes, and trace element isotopes (e.g.,B, Sr) were analyzed by standard methods (Warneret al. 2012a, 2014) and presented elsewhere in this vol-ume (Harkness et al. 2018). Dissolved gas samples weremeasured by extracting the fluid from the copper sam-pling tubes on a vacuum line following standard meth-ods (Darrah et al. 2013; Kang et al. 2016; Harknesset al. 2017a). From this gas volume, splits of sampleswere taken for the measurement of major gas compo-nents (e.g., N2, O2, CO2, CH4 to C5H12), noble gases(e.g., He), and stable carbon and hydrogen isotopes(e.g., δ13C-CH4).

The major and hydrocarbon gases for each samplewere measured using an Scientific Research Instrumentsgas chromatograph (GC) at OSU following methodsreported previously (Darrah et al. 2015b; Kang et al.


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Table 2Hydrocarbon Molecular and Isotopic Signatures for Groundwater Samples from the Karoo Basin.

Town Date LocationSample

No.CH4

ccSTP/kgC2H6

ccSTP/kgC3H8

ccSTP/kg C1/C2+

δ2H-CH4per mille

(‰)

δ13C-CH4per mille

(‰)

δ13C-C2H6per mille

(‰)

Florisbad 06/28/14 Florisbad Spa FLS1 32.0 1.32 × 10−2 4.70 × 10−4 2334 −197.5 −50.90 −36.2606/28/14 Farm FLB5 3.40 1.14 × 10−3 5.70 × 10−5 2845 −201.1 −51.60 −36.31

Trompsbur 06/29/14 Vlakfontein VFB1 1.00 b.d.l. — 5915 −188.0 −65.65 −35.9406/29/14 Vlakfontein VFB2 1.21 b.d.l. — 9869 −282.7 −72.14 −39.8406/29/14 Vlakfontein VFB4 2.02 b.d.l. — 32,414 b.d.l. — —

Aliwal 06/30/14 Aliwal North ANS1 35.3 7.72 × 10−3 5.40 × 10−4 4271 −204.0 −64.98 −37.6106/30/14 Farm ANBH1 39.1 7.59 × 10−3 b.d.l. 4877 −196.0 −65.61 −36.75

Venterstad 07/01/14 Rooiwal RWB1c 2.91 7.95 × 10−4 — 3655 −199.9 −50.68 —07/01/14 Rooiwal RWB5 0.01 — — 36,683 b.d.l. — —07/02/14 Vaalbank VBB1 5.31 1.85 × 10−3 5.37 × 10−4 2228 −206.4 −49.54 −35.8707/02/14 La Rochelle LRB1 2.24 b.d.l. — 51,470 −278.6 −71.49 —07/02/14 La Rochelle LRB2 1.61 b.d.l. — 16,547 −269.5 −73.14 −42.15

Fort 07/04/14 Sulphur Baths BFB1 0.01 — — — b.d.l. — —07/04/14 Sulphur Baths BFB2 23.5 4.56 × 10−3 9.74 × 10−4 4255 −207.7 −55.36 −35.2907/14/14 Rocky Ridge RRB1 3.67 7.53 × 10−4 — 4879 −211.7 −55.88 −36.15

Cradock 07/06/14 Cradock Spa CRS1 0.22 b.d.l. — 15,247 −248.4 −72.11 −41.0607/06/14 Waaikraal farm DRB4 3.07 1.89 × 10−4 — 16,247 −258.0 −70.14 −40.84

Leeu 07/08/14 Kruidfontein WP508 0.07 b.d.l. — 7989 b.d.l. — —07/08/14 Groot WP502 1.14 b.d.l. — 12,484 −263.1 −74.45 −42.6507/08/14 Groot WP505 0.27 b.d.l. — 9451 −209.2 −67.64 —

Merweville 07/09/14 Farm SA1/66 — — — — — — —07/09/14 Farm MWB2 1.05 b.d.l. — 14,120 −270.5 −74.61 —

MIN 0.01 7.53 × 10−4 5.70 × 10−5 2228 −282.7 −74.61 −42.65MAX 39.1 1.32 × 10−2 9.74 × 10−4 51,470 −188.0 −49.54 −35.29MEAN 7.57 4.70 × 10−3 5.00 × 10−4 12,889 −229.0 −63.88 −38.21STD 12.7 4.49 × 10−3 2.93 × 10−4 13,078 34.2 9.36 2.68

Samples With Concentrations Below Detection Limits Are Signified by “b.d.l.,” Whereas Samples That Were Not Analyzed Are Denoted by a Hyphen.

2016). The average external precision was determined bymeasurement of a “known-unknown” standard, includingan atmospheric air standard (Lake Erie, Ohio Air) and aseries of synthetic natural gas standards obtained fromPraxair. The results of the “known-unknown” averageexternal precision analysis are as follows: CH4 (1.27%),C2H6 (1.68%), C3H8 (1.34%), N2 (1.25%), and CO2(1.06%). CH4 concentrations are reported as ccSTP/kg ofwater (the SI molar unit for gas abundance in water) atstandard temperature and pressure (STP), where 1 mg/kgof gas is equivalent to 1.4 ccSTP/kg.

Procedures for analyses of stable isotopic values ofcarbon in methane and ethane were described previously(Jackson et al. 2013; Darrah et al. 2015b). Chromato-graphic separation of gas compounds was performedusing a Thermo Fisher Trace Ultra GC and the isotopicvalues were measured using a dual-inlet Thermo FisherDelta V Plus isotope ratio mass spectrometer (ThermoFisher Scientific, Waltham, Massachusetts). With thiscombination, the detection limits for δ13C-CH4, δ2H-CH4,and δ13C-C2H6 were 0.001, 0.005, and 0.001 ccSTP/kg,respectively. The δ13C-CH4 and δ13C-C2H6 values areexpressed in per mille (designated hereafter as ‰) versusthe international standard Vienna Peedee belemnite(VDPD), with a standard deviation of ±0.1‰, whileδ2H-CH4 are expressed in per mille vs. the international

standard Vienna Standard Mean Ocean Water (VSMOW)with a standard deviation of ±0.5‰.

The abundance and isotopic composition of noblegases were determined using a Thermo Fisher HelixSFT Noble Gas MS at OSU following methods reportedpreviously (Darrah and Poreda 2012; Darrah et al. 2013).The average external precisions based on “known-unknown” standards were all less than ±1.46% for noblegas concentrations with values reported in parenthe-ses (4He (0.78%), 22Ne (1.46%), and 40Ar (0.38%)).These values were determined by measuring referencedand cross-validated laboratory standards, including anestablished atmospheric standard (Lake Erie Air) and aseries of synthetic natural gas standards obtained fromPraxair, including known and validated concentrationsof CH4 to C5H12 hydrocarbons, N2, CO2, H2, O2, andeach of the noble gases. Noble gas isotopic standarderrors were approximately ±0.0091 times the ratio ofair (or 1.26 × 10−8) for the 3He/4He ratio, less than±0.402% and ±0.689% for 20Ne/22Ne and 21Ne/22Ne,respectively, and less than ±0.643 and ±0.427%for 38Ar/36Ar and 40Ar/36Ar, respectively (higherthan typical because of interferences from C3H8 onmass = 36 and 38).


Figure 3. Ternary diagrams of the major cations, Ca vs. Mg vs. Na (A) and major anions, HCO3 vs. SO4 vs. Cl (B) in KarooBasin groundwater. The majority of samples appear to fall along a mixing line between two groundwater types: (1) a freshgroundwater type with a Ca-Mg-HCO3 composition and (2) a saline water type with a NaCl composition. Samples with[CH4] exceeding 14 ccSTP/kg (the U.S. Department of Interior limit for methane in groundwater) were observed exclusivelyin NaCl-type groundwaters.

Presentation of DataAll maps, cross sections, and well coordinates

are plotted using ArcMap GIS 10.3 (ESRI, Redlands,California). Additional published hydrocarbon data fromthe Karoo region were taken from (Talma and Esterhuyse2015). All graphics are plotted using Sigma Plot 12.3.Statistical evaluations including mean, minima, maxima,standard deviation, and Pearson correlations were per-formed using SPSS v. 24 (IBM, Armonk, New York).The correlation coefficient r2 reported in the text wascalculated as Pearson’s correlation coefficient, and corre-sponding p-values are provided throughout the text.

We present data from this study with color-codedsymbols based on the abundance of methane, where lowmethane concentrations close to 0 ccSTP/kg are blue andrange up to red for methane concentrations of 40 ccSTP/kg(i.e., the level of methane saturation in groundwater at10 ◦C). Data from previous studies are identified by emptygreen shapes which reflect the source of the data (namely,circles for boreholes, squares for springs, and triangles forcoalbed gases).

Results

Dissolved Ion ChemistryField parameters and water chemistry data are

presented in Table 1. Groundwater pH values variedbetween 7.1 and 10.0. Groundwater temperatures rangedfrom 16.2 to 30.0 ◦C. Redox state ranged from oxidizing tohighly reducing conditions, with field oxidation-reductionpotentials (ORP) from −232 to +197 mV. Salinity of thegroundwater varied over three orders of magnitude, whileelectrical conductivity ranged from 208 to 10,190 µS/cmand total dissolved solids (TDS) ranged from 123 to6100 mg/kg.

Dissolved ion and water stable isotope data arereported elsewhere in this volume (Harkness et al. 2018).Briefly, the Cl content of the samples ranged from rela-tively freshwater (10.1 mg/L) to saline water (4100 mg/L)(Table 2), while Br concentrations ranged from 0.1 to23.4 mg/L, and DIC (as bicarbonate) from 20 to 764 mg/L(Harkness et al. 2018). Overall, samples from this studyarea are dominantly comprised of three endmembers:(Type A) low salinity Ca-HCO3 water, (Type B) low salin-ity Na-HCO3-Cl water, and (Type C) a relatively highsalinity, NaCl water endmember (Swana 2016; Harknesset al. 2018) (Figure 3).

Hydrocarbon and Major GasesHydrocarbon data are presented in Table 2. CH4

concentrations varied from near method detection lim-its (0.01 ccSTP/kg) to near the saturation limits ofmethane in water (39.1 ccSTP/kg). Ethane (C2H6) rangedfrom below method detection limits (0.0005 ccSTP/kg)to 0.0132 ccSTP/kg. Propane (C3H8) was above detec-tion limits (5 × 10−5 ccSTP/kg) in seven samples inwhich the concentrations ranged between 5.70 × 10−5

and 9.74 × 10−4 ccSTP/kg. The corresponding ratiosof C1/C2+ ranged from 2228 to 51,470 (Table 2;Figure 4).

CH4 was strongly correlated to C2H6 (r2 = 0.930,p < 0.01) and all methane-rich waters (defined here asmethane concentrations exceeding 14 ccSTP/kg based onthe U.S. Department of Interior action level) displayedpropane above the method detection limits. While therewas a statistically significant positive correlation betweenCH4 and Cl (r2 = 0.81, p < 0.01) that correlation isdriven by a strong correlation between these parametersin the top quartile of methane concentrations (i.e., thereis significantly greater scatter in samples with lower


Figure 4. Ratio of methane to higher-order aliphatic hydro-carbons (C1/C2+) vs. stable isotopic composition of carbonin methane (δ13C-CH4) of gases dissolved in groundwaterscollected in this study and those from previously publishedgroundwater studies in the Karoo Basin (Talma and Ester-huyse 2015; denoted by green outlined symbols used hereand in subsequent figures). The typical ranges for thermo-genic and biogenic methane compositions are shown as blackand blue boxes, respectively (Bernard et al. 1976). The greenand orange arrows show postgenetic, microbial alterationby aerobic and anaerobic oxidation, respectively. The bluetrend line displays postgenetic alteration from the theoreticalthermogenic endmember following fractionation by solubil-ity partitioning of the hydrocarbon gas components assuminga GGS-R fractionation model (adapted from Darrah et al.2015 and Harkness et al. 2017a). The black mixing linesdepict hypothetical mixing between (1) a theoretical ther-mogenic endmember that was progressively modified by sol-ubility partitioning during fluid migration and (2) a biogenicgas. The majority of samples appear to fall along a mixingtrend between biogenic sources and a migrated thermogenicgas that previously altered by solubility partitioning. All highmethane samples appear to fall on a mixing line betweenfluids that have undergone two-phase advection (most likelyfrom a deeper thermogenic source) that later mix with abiogenic endmember.

methane concentrations, including one anomalous sample[VFB1] that has an exceptionally high Cl concentration[4100 mg/kg], but little methane [1.0 ccSTP/kg]) (Tables 1and 2; Figure 3).

Hydrocarbon stable isotope data are presented inTable 2 and Figures 4–6. The carbon isotopic signaturefor CH4 (δ13C-CH4) ranged from −74.61‰ to −49.54‰,while the hydrogen isotope values for CH4 (δ2H-CH4) ranged from −282.7‰ to −188.0‰ (Figures 4and 5). The δ13C-CH4 values correlated negativelywith C1/C2+ (r2 = −0.566, p < 0.01; Figure 4). The

Figure 5. Stable isotopic values of hydrogen (δ2H-CH4) vs.carbon (δ13C-CH4) in methane from samples collected inthis study and those from previously published groundwa-ter studies in the Karoo Basin (Talma and Esterhuyse 2015).The black oval represents the typical range of thermogenicnatural gases, with the red arrow indicating the trend ofincreasing thermal maturity (Schoell 1983; Whiticar et al.1986). The blue and green boxes represent methane pro-duced by hydrogenotrophic methanogenesis and acetoclasticmethanogenesis, respectively (Schoell 1983; Whiticar et al.1986). The dashed black arrow represents two-componentmixing between methane produced from thermogenic andhydrogenotrophic methanogenesis, while the dashed greenarrow represents two-component mixing between methaneproduced from thermogenic and acetoclastic methanogen-esis. The purple dashed arrow represents mixing betweennatural gases produced by acetoclastic methanogenesis andhydrogenotrophic methanogenesis. The majority of methane-rich samples appear to fall along a mixing line betweenthermogenic natural gas and hydrogenotrophic microbialmethane, while low-methane samples plot as an ambiguousmixture of methane produced by biogenic sources.

carbon isotope values for C2H6 (δ13C-C2H6) rangedfrom −42.65‰ to −35.29‰ and correlated negativelywith δ13C-CH4 (r2 = −0.566, p < 0.01) and C1/C2+(r2 = −0.895, p < 0.01) (Figure 6).

Nitrogen (N2) and noble gas data are presented inTable 3. Dissolved N2 concentrations ranged between11.03 and 24.76 ccSTP/kg (average of 14.42 ccSTP/kg,excluding the anomalously high value of 24.76 ccSTP/kgin sample VFB1; Figure 7). Except for this anomaloussample (VFB1), which also yielded a highly nonat-mospheric 40Ar/36Ar of 967.16, these N2 values arewithin approximately 30% of the anticipated air-saturatedwater (ASW) values assuming recharge at 1200 m,15 ◦C, and a salinity of 10 mg/kg chloride (Figure 7;Table 3). The 36Ar (a noble gas isotope incorporatedinto groundwater exclusively from atmospheric sources)concentrations for the majority of samples rangedfrom 828 to 1562 × 10−6 ccSTP/kg (average [36Ar]of 1184 × 10−6 ccSTP/kg) (Table 3; Figure 7). Thesample with an anomalously high N2 concentrationdisplayed partially stripped values of atmospheric 36Ar([36Ar] = 561 × 10−6 ccSTP/kg). Excluding this sam-ple, the 36Ar values are within approximately 35%of the anticipated ASW values assuming recharge at1200 m, 15 ◦C, and a salinity of 10 mg/kg Cl. The


Figure 6. The carbon isotopes of methane (δ13C-CH4) vs.ethane (δ13C-C2H6) from samples collected in this study.The black dotted trend line shows the anticipated change ingas isotopic composition as a source rock undergoes thermalmaturation, with vitrinite reflectance values indicative ofmaturity. The orange arrow displays the anticipated trendsfor aerobic oxidation inorganically or by microbes. The greenarrow displays the anticipated trends for anaerobic oxidationby methanotrophs. The black arrow displays mixing thatoccurs following the addition of biogneic methane. The datasuggest that dissolved gases in the current study area consistof natural gas produced across a range of thermal maturitiesthat later mixed with varying proportions of microbialmethane.

[20Ne] (also incorporated into groundwater exclu-sively from atmospheric sources) ranges from 131 to527 × 10−6 ccSTP/kg (average of 272 × 10−6 ccSTP/kg),excluding the anomalously high N2 sample that is par-tially stripped and contained only 121 × 10−6 ccSTP/kg(Table 3; Figure 7). The upper range of 20Ne concen-trations is nearly three times the anticipated values forASW recharged in this region. Neon concentrationscorrelate positively with CH4 (r2 = 0.780, p < 0.01)and C2H6 (r2 = 0.817, p < 0.01) concentrations andnegatively to C1/C2+ (r2 = −0.500, p = 0.015).

Helium concentrations ranged from [He] =48.2 × 10−6 ccSTP/kg, which is near the ASW valueof 40.5 × 10−6 ccSTP/kg to 0.483 ccSTP/kg ([He] =483,493 × 10−6 ccSTP/kg) or 11,938 times the antici-pated ASW values of helium (Table 3). These valuescorresponded to He/Ne ratios from 0.190, which is nearASW value of 0.230 to 3011 (Figure 8). Helium concen-trations were highly correlated to methane (r2 = 0.623,p < 0.01), ethane (r2 = 0.740, p < 0.01), Cl (r2 = 0.798,p < 0.01), and Br (r2 = 0.7, p < 0.001) (Figure 9). Exceptfor sample VFB1 from Trompsberg, groundwater sampleswith the highest concentrations of 4He also displayed thehighest CH4 concentrations (Figure 9).

Helium isotopic values (i.e., 3He/4He), presentedas R/RA, where R is the ratio of 3He to 4He ina sample compared to the ratio of 3He to 4Hein air (RA = 1.384 × 10−6), ranged between 0.0114RA and 1.1562 RA (Table 3; Figure 8). The upperlimit of this range suggests that the helium contentis dominated by contributions from ASW and small

Figure 7. The concentrations of N2 (A) and 20Ne (B) vs.36Ar and the concentration of methane vs. the ratio of20Ne/36Ar (C) from samples collected in this study. Themajority of samples display N2 and 36Ar within 30% and35% of their anticipated ASW levels, respectively, based onlocal groundwater recharge conditions with the exceptionof one anomalous sample which displays nearly twice theanticipated ASW levels of N2 and partial stripping of 36Arand 20Ne. The subset of methane-rich samples also displayselevated levels of exogenous 20Ne.

contributions of 3He ingrowth following the decay oftritium (Harkness et al. 2018). There is a trend ofdecreasing 3He/4He values with both increasing con-centrations of 4He and increasing ratios of He/Ne.Although four samples (VFB4, BFB1, LRB1, andRWB5) contain quantifiable mantle contributions (∼1%)and elevated methane concentrations, the correlation ofincreasing 4He/20Ne with decreasing 3He/4He indicatesthat the majority of helium is predominantly from acrustal/radiogenic source (Figure 8). The 3He/4He ratioalso displays significant negative correlations to δ13C-CH4 (r2 = −0.686; p < 0.01), δ13C-C2H6 (r2 = −0.797;p < 0.01), and C1/C2+ (r2 = 0.797; p < 0.01).

Neon and argon isotope ratios provide additionalinformation about the source, water-rock interactions, and


Tabl

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480

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n.


Figure 8. A comparison of the helium isotopic ratio 3He/4He(shown as R/RA, where RA is the ratio of 3He/4He inatmospheric air; RA = 1.384 × 10−6) vs. the ratio of He/Neof gases dissolved from groundwater collected in the KarooBasin. The green and black boxes denote the globallyconstrained values for ASW (green) and tritiated ASW(black) endmembers, respectively. The dashed lines representmixing between these endmembers and crustal helium acrossa range of mantle contributions. The majority of samples fallalong two-component mixing lines between meteoric water(with varying degrees of tritiogenic 3He contributions) andcrustal helium. Nonetheless, we note a resolvable mantlecontribution in a subset of samples with elevated methaneand helium concentrations.

residence time of groundwater (Figures 10 and 11). The20Ne/22Ne and 21Ne/22Ne ratios ranged from 9.412 to10.027 and from 0.0276 to 0.0472, respectively (Table 3;Figure 11). The 40Ar/36Ar and 38Ar/36Ar ratios rangedfrom 294.88 (where ASW is 295.5) to 967.16 and from0.1829 to 0.1930, respectively (Table 3; Figure 11). Boththe neon and argon isotope values range from near ASWlevels (i.e., 21Ne/22Ne = 0.0289 and 40Ar/36Ar = 295.5)to quantifiable excesses of noble gases produced fromradioactive decay, where the abundance of 21Ne and40Ar produced by radioactive decay are denoted by21Ne* and 40Ar*, respectively (Hunt et al. 2012). The21Ne/22Ne correlates significantly to the proportion of4He (r2 = 0.818, p < 0.001), salinity (Cl: r2 = 0.831,p < 0.001; Br: r2 = 0.767, p < 0.001), the concentrationsof methane and other hydrocarbon gases (e.g., CH4:r2 = 0.582, p = 0.003; C2H6: r2 = 0.510, p = 0.015),and hydrocarbon stable isotopes of hydrogen (δ2H-CH4:r2 = 0.666, p = 0.001) (Tables 2 and 3; Figure 11). The40Ar/36Ar also correlates strongly to 4He (r2 = 0.695,p < 0.001) and salinity (Cl: r2 = 0.98, p < 0.001; Br:r2 = 0.99, p < 0.001). While we do observe a generaltrend of increasingly radiogenic 40Ar/36Ar ratios withincreasing concentrations of CH4 (Figure 9) and hydro-carbon stable isotopes of carbon (Figure 11), these trendsare not statistically significant.

There are other gas isotope ratios that provideimportant clues about the ratios of gas and water,gas and groundwater transport mechanisms, and gas-water and water-rock interactions that occur in thesubsurface (Darrah et al. 2014, 2015b; Moore et al.2018; Figure 12). The CH4/36Ar serves as a proxy for

the relative abundance of natural gas and ASW irre-spective of any gas-water partitioning that may haveoccurred. The CH4/36Ar (gas/water ratio) in this studyranges from 13.8 to 42,636 and correlates to C2H6(r2 = 0.913, p < 0.01), Cl (r2 = 0.819, p < 0.01), and4He (r2 = 0.833, p < 0.01). The 4He/CH4 ratio is a sen-sitive indicator of biogenic vs. thermogenic contributionsand water-gas interactions. The 4He/CH4 ratios rangedfrom 24 to 406,302 and correlate significantly to Cl(r2 = 0.724, p < 0.01) and Br (r2 = 0.775, p < 0.01).The fractionation of the 20Ne/36Ar ratios record diagnos-tic partitioning between the gas and water phases duringfluid transport. The 20Ne/36Ar ratios ranged from 0.112to 0.480 and correlated to CH4 (r2 = 0.852, p < 0.01),C2H6 (r2 = 0.912, p < 0.01), δ13C-CH4 (r2 = 0.547,p = 0.012), δ13C-C2H6 (r2 = 0.517, p = 0.048), C1/C2+(r2 = −0.509, p = 0.013).

DiscussionThe nearly pristine nature of the Karoo Basin, South

Africa, provides a rare opportunity to determine thehydrogeological context for the presence of naturallyoccurring hydrocarbon gases in a setting with neither alocal history of mining nor conventional or unconven-tional energy extraction. In this volume, Harkness et al.(2018) demonstrate that groundwater in this region rep-resents a mixture between multiple sources of shallowfreshwater (i.e., defined therein as Type A: Ca-HCO3 andType B: Na-HCO3-Cl) and various ranges of contribu-tions from an exogenous source of high-salinity, NaClwaters (defined therein as Type C). Similarly, we observebroad ranges in levels of hydrocarbon gases and levelsof exogenous noble gases produced by radioactive decay.The levels of each of these parameters provide importantclues that inform us about (1) the occurrence of natu-rally occurring hydrocarbon gases in a genuinely predrill,baseline study; (2) the genetic sources of hydrocarbongases; (3) the relative contributions (i.e., mixtures) ofdeep, hydrocarbon-rich saline fluids in shallow aquifers;and (4) the pathways and mechanism(s) for the migrationof these fluids to shallow aquifers.

The Presence of Naturally OccurringHydrocarbon Gases in a Pristine ShallowAquifer

All samples contained measurable and quantifiableconcentrations of methane. Methane concentrations fromthe current study ranged up to 39.1 ccSTP/kg. Impor-tantly, this value approaches, but does not exceed, thecalculated saturation conditions for methane in freshwa-ter in this region (saturation for [CH4] is ∼40 ccSTP/kgat P(CH4) = 1 atm with normal levels of air-saturatedwater gases [predominantly N2] at 10 ◦C at 1200 m ele-vation). These data support previous observations ofmethane concentrations at or below saturation conditionsin baseline samples thought to be unaffected by shale-gas


Figure 9. The concentration of [Cl] vs. [4He] (A); 21Ne/22Ne (B); and 40Ar/36Ar (C) and [Br] vs. [4He] (D); 21Ne/22Ne (E);and 40Ar/36Ar (F) from samples collected in this study. Linear regressions (dashed lines), r2, and p-values of the regressionfor all data are shown in each plot. The majority of samples display a significant correlation between noble gases producedby radioactive decay (4He, 21Ne*, and 40Ar*) and brine components (i.e., Cl, Br) indicating mixing between a regional brineand relatively fresh meteoric water.

drilling in other areas. Furthermore, this upper range ofconcentrations reinforces the premise that conditions ofgas saturation (i.e., bubble point) in regional groundwaterboth regulate the concentration of methane and physi-cally control the migration of naturally occurring methanewithin shallow aquifers.

Typical of all petroleum basins, the complex tec-tonic history of the Karoo Basin has led to a diversesuite of hydrocarbon sources and the occurrence of pref-erential fluid migration pathways. As a result, there is acorrespondingly diverse nature of groundwater geochem-ical compositions and groundwater quality in the shallowaquifers from this region. Similar to observations in otherbasins around the world, elevated methane concentrations

(defined here as greater than 14 ccSTP/kg or 10 mg/L)corresponded to samples that also displayed elevated lev-els of salts with a NaCl composition (defined as Type Cin Harkness et al. 2018), ethane, and 4He concentrations.In other basins, the relationship between methane, ethane,chloride, and 4He has been attributed to the migrationof a hydrocarbon-rich brine from source rocks to shallowaquifers over unknown periods of geological time (Warneret al. 2012a; Warner et al. 2012b; Jackson et al. 2013).These data reinforce our hypothesis for a common originand migration process for an exogenous hydrocarbon-rich brine that apparently migrated to and mixed withfresh groundwater within shallow aquifers across thestudy area.


Figure 10. A comparison of methane concentration [CH4]vs. the ratio of 21Ne/22Ne (A); the concentration of helium[4He] vs. the argon isotopic composition 40Ar/36Ar (B); andthe ratio of 21Ne/22Ne vs. the argon isotopic composition40Ar/36Ar from samples collected in this study. The solidblack lines depict anticipated ASW values. We note asignificant increase in radiogenic noble gases (4He, 21Ne*,40Ar*) in the methane-rich endmembers, supporting thehypothesis for mixing between biogenic and thermogenicnatural gas.

In comparison to the methane- and salt-rich NaClwaters, the two low-chloride, fresh water endmembersdisplayed significantly lower methane, helium, and ethaneconcentrations. One notable exception from this study isa sample collected from a deep borehole in Trompsburg(VFB1), which displayed the highest salinity and one ofthe highest 4He concentrations recorded in the study butonly contained 1 ccSTP/kg of methane.

The Sources of Naturally OccurringHydrocarbon Gases in Shallow Aquifers

Traditional Hydrocarbon TracersHydrocarbon gas within groundwater can be pro-

duced in shallow aquifers by biogenic (via microbes) or

thermogenic processes, or result from subsurface migra-tion from an exogenous source of biogenic or thermo-genic gas (Darrah et al. 2015a, 2015b). Typically, thesource or genetic origin of hydrocarbon gases in shallowaquifers is determined by a combination of the molecularcomposition of gases (e.g., the ratio of methane [C1] tohigher-order hydrocarbons [C2+], or C1/C2+), the stableisotopic compositions of methane (e.g., δ2H-CH4, δ13C-CH4), and where possible the stable isotopic compositionof heavier aliphatic hydrocarbons (e.g., δ13C-C2H6) (Jack-son et al. 2013; Harkness et al. 2017b). Here, we inte-grate hydrocarbon molecular and isotopic compositionsand noble gases to provide important constraints on thegenetic sources and proportions of biogenic and thermo-genic natural gas, and to validate the use of noble gasesas a source tracer in shallow aquifers.

In the current study, most samples with low methaneconcentrations (<14 ccSTP/kg) display extremely highC1/C2+ and very negative δ13C-CH4 (≤ ∼70‰)(Figure 4). Data from these samples are consistent withcontributions from a purely microbial source of naturalgas in shallow aquifers (blue box in Figure 4). It shouldbe noted that the subset of samples with a nearly puremicrobial molecular and isotopic hydrocarbon compo-sition displays low salt contents (fresh water) and aredominantly found in the low salinity water types (Types Aand B from Harkness et al. 2018). Conversely, the subsetof methane-rich contains higher levels of ethane (and insome cases propane), displayed C1/C2+ ratios between2334 and 4877, and relatively more enriched isotopicsignatures for carbon (δ13C-CH4 = ∼ − 65 to −50‰)(Figure 4). Because ethane is not produced by microbialprocesses, its presence in the Karoo Basin suggests that aresolvable mixture of thermogenic gases likely influencesthese samples. However, we note that the combinationof high C1/C2+ and intermediate δ13C-CH4 values isinconsistent with any known thermogenic sources ofnatural gas.

A comparison of the stable isotopes of hydrogen(δ2H-CH4) and carbon (δ13C-CH4) in methane also distin-guishes low-methane and low-salinity samples from theirhigh methane counterparts. The subset of low methaneplots between hydrogenotrophic and acetoclastic micro-bial endmembers, and indicates a nearly pure biogenicsource of methane present in the low salinity ground-water (Figure 5). While these low-methane samplesappear to cluster near the hypothetical hydrogenotrophic-acetoclastic mixing line, they display more negativeδ13C-CH4 than a simple two-component mixture fromthese two sources (Figure 5). Conversely, while themethane- and salt-rich samples display significant propor-tions of hydrogenotrophic (microbial) methane, most sam-ples plot along a two-component mixing trend betweenhydrogenotrophic (microbial) methane and an unidenti-fied thermogenic natural gas endmember (Figure 5). Sim-ilar observations have been reported in shallow aquifersaround the USA and Canada, reinforcing the interpre-tation that (1) there is a mixture of thermogenic natu-ral gas in shallow aquifers overlying the Karoo and (2)


Figure 11. A comparison of [4He] (A to C); 21Ne/22Ne (D to F), and 40Ar/36Ar (G to I) vs. δ13C-CH4 (A; D; G); δ13C-C2H6(B; E; H); δ2H-CH4 (C; F; I) from samples collected in this study. We note a general trend of corresponding increase inradiogenic noble gases and less negative values of δ13C-CH4, δ13C-C2H6, and δ2H-CH4. These data suggest that increasedabundances of radiogenic noble gases correspond to more thermogenic-like stable isotopes of hydrocarbon gases.

mixing between hydrogenotrophic (microbial) methaneand thermogenic natural gas occurs commonly and nat-urally in shallow aquifers overlying petroleum systems(Revesz et al. 2012; Jackson et al. 2013; Darrah et al.2015b; Harkness et al. 2017b).

A comparison of δ13C-CH4 and δ13C-C2H6 reinforcesthe interpretation that a low abundance of thermogenicnatural gas is present in these samples (Figure 6).When thermogenic natural gas is generated by increasingheat and pressure, the isotopic signatures for bothmethane and ethane evolve simultaneously with increasingtemperature along a generally linear trend and recordimportant clues about the genetic source, thermal maturity,and subsequent postgenetic alteration of natural gases(Whiticar et al. 1994). In general, the δ13C-C2H6 valuesfrom this study area are consistent with the expectedcomposition of thermogenic gases derived from eithermarine (e.g., shale) or terrestrial (e.g., coal) organic matter(Whiticar et al. 1994). The broad range of δ13C-C2H6(−42.65‰ to −35.29‰) equates to vitrinite reflectancevalues of approximately 0.33%Ro to 0.72%Ro (solidblack line in Figure 6) (Whiticar et al. 1994). Thesevitrinite reflectance values correspond to temperatureranges between thermally immature (∼50 ◦C) to the peakof the oil window (∼80 ◦C) and an apparent burial depthof approximately 1.75 to 3 km. The methane-rich samplestypically plot on the higher end of the apparent thermal

maturity range and display δ13C-C2H6 values equivalentto a vitrinite reflectance of approximately 0.5%Ro to0.72%Ro, which is within the oil window (60 to 80 ◦C).

Although the δ13C-C2H6 values provide importantclues about both the presence and thermal character ofthermogenic natural gas, our data are inconsistent withthe simplistic evolution of a thermogenic natural gas(Figure 6). Because all of our samples fall below theanticipated thermal maturation line, we suggest that thecombination of δ13C-CH4 and δ13C-C2H6 data indicatesignificant mixtures of biogenic methane with variablecontributions of natural gas derived from an unidentifiedthermogenic source, consistent with the other hydrocarbondata mentioned above.

The Utility of Noble Gas Tracers for Determiningthe Genetic Source of Natural Gas

The application of traditional molecular and isotopichydrocarbon tracers can be complicated by uncertainties inoverlapping isotopic composition of endmembers, mixing,and postgenetic modification (Whiticar 1999; Etiope et al.2009; Darrah et al. 2015b; Vinson et al. 2017). Becausethey are not altered by chemical reactions or microbialoxidation, noble gases present an advantageous additionto studies of natural gas geochemistry. Still, there are alimited number of studies that have attempted to integratenoble gases with traditional hydrocarbon molecular and


Figure 12. A plot of 4He/CH4 vs. 20Ne/36Ar from samplescollected in this study. The green square represents atheoretical thermogenic endmember for the Collingham Fm.based on age and U/Th concentrations. The dashed grayarrow represents one-phase advection of a thermogenic-like gas source. The solid green arrow represents mixingof thermogenic-like production gas with a “young” biogenicsource. The dashed blue trend lines represent temperature-dependent two-phase advection of a thermogenic-like gasfrom depth to the surface. The black dashed line representsa hypothetical mixing between a migrated thermogenicsource with a young biogenic source. Two-phase solubilityfractionation and advection trends are based on the two-stage groundwater gas stripping and redissolution (GGS-R) model developed in Darrah et al. (2015b). Methane-rich samples appear consistent with two-phase migration ofnatural gas from depth to the shallow aquifers.

isotopic geochemistry in groundwater studies (Jacksonet al. 2013; Darrah et al. 2014, 2015a, 2015b; Wen et al.2016; Harkness et al. 2017b).

Similar to ethane, 4He and heavier crustal noblegases (21Ne*, 40Ar*) occur at elevated levels in older,higher temperature thermogenic natural gases, whereasrecently formed microbial gases are nearly devoid ofethane or radiogenic noble gases (Darrah et al. 2015b;Harkness et al. 2017b; Hunt et al. 2012; Moore et al.2018). Our data reveal significant correlations betweenvarious hydrocarbon tracers and the levels of the crustalnoble gases 4He, 21Ne*, and 40Ar* in groundwater sam-ples from this region. Each of the noble gases producedby radioactive decay (4He, 21Ne*, and 40Ar*) included inthis study correlate to each other and to various hydro-carbon tracers, suggesting a clear presence of thermo-genic methane. Radiogenic 4He correlates to both CH4and C2H6 (r2 = 0.623, p = 0.001; r2 = 0.740, p < 0.01),salinity (Cl [r2 = 0.798, p < 0.001]), as well as δ2H-CH4 (r2 = 0.603, p = 0.005) (Figures 9 and 11). Simi-larly, 21Ne* correlates strongly with CH4 (r2 = 0.909,p < 0.001) and C2H6 (r2 = 0.893, p < 0.001) concen-trations, whereas 40Ar* correlates with salinity (Cl[r2 = 0.591, p = 0.005]), and displays generally higherδ13C-CH4, δ2H-CH4, and δ13C-C2H6 values (Figures 9and 11). Consistent with our interpretations of hydro-carbon molecular and isotopic composition, each ofthese observations supports the presence of thermo-genic methane and validates the use of radiogenic noble

gases as a tracer for thermogenic natural gas in shallowaquifers.

Noble gases can also be a powerful suite of tracersfor resolving the presence of exogenous fluids, discerningthe processes that bring salt-rich natural gases to shallowaquifers, and providing a genetic fingerprint of migratedgases. As discussed in Harkness et al. (2018), thepresence of extremely high 4He concentrations up to483,493 × 10−6 ccSTP/kg (or approximately 1.07 × 104

times atmospheric levels) greatly exceed atmosphericcontributions and/or the maximum plausible endogenoussources. Moreover, the presence of these extremely high4He concentrations that correlate with hydrocarbon tracersof thermogenic gases (ethane, less negative δ13C-CH4 andδ2H-CH4) and δ13C-C2H6) in groundwater samples withelevated (thermogenic) methane and salts suggests that anexogenous source of relatively old groundwater migratedinto the shallow aquifers within the Karoo Basin andmixed with an apparently young, tritium-active source ofmeteoric waters.

While the extremely high 4He concentrationsunequivocally indicate the presence of exogenous fluidsin shallow aquifers from the Karoo Basin, other majorgas and noble gas isotope data provide important cluesabout the source and migration processes that broughtexogenous fluids to the shallow aquifer. Because atmo-spheric gases, namely 20Ne, N2, and 36Ar, have a nearlyuniform starting composition, with small differencesrelated to changes in temperature, salinity, and elevation(atmospheric pressure), they have proven to be a usefultracer for identifying the history of hydrocarbon-richbrine migration (Darrah et al. 2014; Barry et al. 2016)and serving as a diagnostic indicator of migrated gases(Darrah et al. 2015a, 2015b; Harkness et al. 2017b;Moore et al. 2018). For example, elevated concentrationsof 20Ne or an increase in the 20Ne/36Ar without acorresponding increase in other atmospheric noble gases(i.e., 36Ar) are diagnostic of solubility fractionationduring two-phase (free gas + brine) fluid migration.Notably, we observe a marked excess in 20Ne and highlyelevated ratios of 20Ne/36Ar in samples with elevatedsalt, 4He, and methane and ethane levels, and lessnegative δ13C-CH4 and δ2H-CH4 values in this study.The thermogenic methane-rich endmember displays20Ne/36Ar ratios significantly above the ASW equilibriumvalue of approximately 0.147 (up to 0.480; Figure 7).Importantly, the increase in 20Ne/36Ar relates almostexclusively to an increase in [20Ne] concomitantly with[CH4], [C2H6], and [4He] but without any observableincreases (small decrease is observed) in [36Ar], preclud-ing increased levels of air in these samples (Figure 7).Based on these observations, we conclude that the sourceof thermogenic natural gas is an exogenous fluid that hasexperienced multiple-stage, two-phase partitioning duringfluid transport (secondary hydrocarbon migration) fromits source (yet to be identified) to shallow aquifers acrossthe region.

Similar processes of solubility partitioning that frac-tionated the 20Ne/36Ar ratio during fluid transport can also


account for the range of 4He/CH4 in the thermogenic end-member. The relative concentration of exogenous 4Hein crustal fluids (e.g., 4He/CH4, 4He/20Ne) results froma combination of the 4He released within the originalthermogenic natural gas source and the subsequent modi-fication that occurs during postgenetic alteration processes(e.g., fluid migration, oxidation, and methanogenic inputs)(Darrah et al. 2014, 2015a; Moore et al. 2018; Harknesset al. 2017b). While one-phase (gas dissolved in brine)migration or mixing between thermogenic and biogenicgases would be associated exclusively with enrichmentof 4He and hence 4He/CH4 (Darrah et al. 2014, 2015a;Harkness et al. 2017b), the paired enrichment of 4He/CH4and 20Ne/36Ar is consistent with two-phase fluid migra-tion (Darrah et al. 2014). These data further support ourconclusion that the exogenous thermogenic natural gasmigrated to the shallow aquifers, presumably along pref-erential pathways, as a two-phase fluid.

Resolving the Unidentified ThermogenicEndmember

While both traditional molecular and isotopic hydro-carbon and noble gas approaches individually point to thenaturally occurring presence of thermogenic natural gas inshallow aquifers with a highly ambiguous composition, aparsimonious explanation requires careful integration ofall data. The simultaneous presence of higher C1/C2+values than any known (or at least documented) ther-mogenic endmembers, yet heavier δ13C-CH4 values thanany known (or at least documented) biogenic endmem-bers is confounding. Interestingly, these observations fromKaroo are consistent with previous observations fromother petroleum basins around the world (Darrah et al.2014, 2015b; Moore et al. 2018; Harkness et al. 2017b).

The need to understand the source of this apparentlythermogenic methane is not simply esoteric or academicas it is becoming increasing apparent in the publishedliterature that this poorly defined source of naturalgas is common in many aquifers overlying petroleumsystems and provides a useful template understand thediagnostic geochemical signatures of natural hydrocarbongas migration to shallow aquifers and conduct stray gasinvestigations (Etiope et al. 2009; Darrah et al. 2014,2015b; Harkness et al. 2017b). Yet, despite the importanceand preponderance of these trends, there is relatively littlediscussion about the implications of these data in theliterature (Jackson et al. 2013; Darrah et al. 2014, 2015b;Moore et al. 2018; Harkness et al. 2017b) and previousstudies that have identified this trend have struggled toconverge on a consensus for the cause of natural gas withthis composition.

Because of the presence of elevated C1/C2+ values,the most common interpretation for this compositionis mixing between biogenic methane and thermogenicnatural gas. The issue with this interpretation is thatthe observed composition falls along a convex mixingline (rapid increase of C1/C2+ followed by a decreasing

δ13C-CH4) as opposed to the concave mixing line(rapid decrease of δ13C-CH4 following by an increasingC1/C2+), the latter of which fits simple two-componentmixing between biogenic and thermogenic sources(Darrah et al. 2015b; Moore et al. 2018; Harkness et al.2017b). Previously, this convex trend has been variablyattributed to a complex three-component mixing (Osbornet al. 2011; Revesz et al. 2012), diffusion (Prinzhoferand Pernaton 1997), solubility partitioning (Etiope et al.2009; Darrah et al. 2014, 2015b; Harkness et al. 2017b),or oxidation by various potential mechanisms (Etiopeet al. 2009; Etiope and Ciccioli 2009; Vinson et al. 2017).

We propose that it is useful to view the traditionalhydrocarbon molecular and isotopic tracers in light ofall of the geochemical data and consider the potentialfor postgenetic modification during fluid transport. Thecombination of elevated C1/C2+, intermediate to heavyδ13C-CH4, exogenous 4He (and other noble gases pro-duced by radioactive decay), and elevated 20N/36Ar inthis dataset, which is consistent with a growing bodyof data from aquifers overlying other petroleum basins,sheds light on this poorly defined thermogenic endmem-ber. Here, we describe an ad hoc interpretation based ontwo-phase (gas + water) solubility partitioning to explainthis endmember. While this interpretation is clearly adhoc, it does attempt to find a parsimonious interpreta-tion that integrates all of the geochemical results andproduce a viable explanation for both elevated C1/C2+and intermediate to heavy δ13C-CH4 in naturally occur-ring hydrocarbon gases in shallow aquifers.

During the multiple-phase fluid migration(gas + water) of thermogenic natural gas, a two-phase thermogenic natural gas and formational brinewould initially migrate out of the source rocks (primaryhydrocarbon migration) and into nearby formations as aresult of buoyancy and hydrodynamics processes. Thegeochemistry of these fluids would record an initialC1/C2+, δ13C-CH4, δ2H-CH4, δ13C-C2H6, and noblegas composition, which varies according to thermalconditions of gas generation. As two-phase fluid migra-tion proceeds from the hydrocarbon source to reservoirsand eventually shallow aquifers, solubility partitioningwill fractionate the gas components according to theirrespective solubilities. Solubility partitioning wouldfractionate the gas components leading to increasesin the initial C1/C2+ but negligible changes in theδ13C-CH4, δ2H-CH4, or δ13C-C2H6, while also leading toincreases in diagnostic noble gas tracers (e.g., 20Ne/36Ar,4He/CH4). Once this postgenetically modified naturalgas reached shallow aquifers, it can then mix withvariable amounts of biogenic methane. This late stage ofmixing with biogenic methane in shallow aquifers woulddecrease the δ13C-CH4 of methane in shallow aquifers.

Alternative theories may include oxidation or otherforms of transport-induced fractionation (e.g., adsorption,diffusion, and dispersion). We suggest that the key toevaluating these processes is understanding the degreeof fractionation imparted by various transport phenomenaand/or oxidation. Significant volumes of previous work


have demonstrated the diffusion and dispersion are notviable mechanisms for the fractionation of the C1/C2+(reviewed in Darrah et al. 2015b). However, becauseboth solubility partitioning and oxidation remain viablemechanisms, we briefly compare them here.

Because the relative solubility of C1 and C2+ varyby approximately 2× (i.e., 200%), while 12C1H4 isonly 0.05% to 0.1% (0.5‰ to 1.0‰) less soluble than12CD1H3 or 13C1H4, one would anticipate the C1/C2+composition of a natural gas could increase dramaticallyrelative to δ13C-CH4 if solubility partitioning occurredduring fluid migration. Similar increases in C1/C2+ couldresult from aerobic oxidation, while anaerobic oxidationwould actually decrease the C1/C2+ and therefore canbe discounted as a mechanism to describe natural gaswith this composition (Etiope et al. 2009; Etiope andCiccioli 2009). Therefore, it is reasonable to conclude thatmixing with biogenic methane following either solubilitypartitioning or oxidation could produce a natural gas withboth elevated C1/C2+ and more negative δ13C-CH4 (12Cenriched) in the resulting mixture.

The challenge is distinguishing fractionation inducedby solubility partitioning from that induced by aerobic oxi-dation. To do this, we must again consider the consortiumof data simultaneously. Solubility fractionation would notappreciably change the residual the δ13C-CH4, δ2H-CH4,or δ13C-C2H6, whereas aerobic oxidation would lead to aprogressive enrichment in these components (and increasethe C1/C2+ in the residual natural gas). Conversely, sol-ubility partitioning would increase the ratio of diagnosticnoble gas tracers (e.g., 20Ne/36Ar), while aerobic oxi-dation would leave this ratio unchanged. Because datafrom the current study display elevated δ13C-CH4, δ13C-C2H6, and C1/C2+ in the inferred migrated thermogenicendmember, it is unclear if we can definitively distin-guish between solubility partitioning and oxidation pro-cesses. However, because the thermogenic endmemberalso demonstrates elevated 20Ne/36Ar values, we suggestthat solubility partitioning must account for at least someof the observed postgenetic modification of the thermo-genic endmember, specifically the C1/C2+. It is inter-esting to note that, like other petroleum basins aroundthe world, if one considers postgenetic modification ofa migrated thermogenic gas, the majority of the datacan be accounted for by simple two-component mixingbetween a biogenic endmember and a thermogenic end-member that previously experienced postgenetic modifi-cation (Figure 4B).

The Migration and Influence of Fluids fromDepth

Previous work from the Karoo Basin suggests that,like other petroleum basins, discrete conduits of hydrocar-bon and salt water discharge throughout the Karoo Basinare often located in close proximity to conventional strati-graphic and structural hydrocarbon traps (Rowsell and DeSwardt 1976; Talma and Esterhuyse 2015). These observa-tions suggest that regional-scale structural geology and the

occurrence of planar discontinuities (i.e., faults and frac-ture) likely regulate the migration of hydrocarbon gasesand salts from depth to the shallow aquifers.

Many aspects of the geochemistry of hydrocarbongases and saline fluids in the Karoo Basin are also similarto other petroleum basins that have been examined aroundthe world. For example, remnants of more deeply-sourcedhigh-salinity formation waters from the Karoo Basinobserved in shallow groundwater as part of this study havebeen diluted by meteoric water in multiple stages (i.e.,both prior to and during the migration of hydrocarbon- andsalt-rich fluids to near-surface formations), leading to theparadoxical observation of Modern tritium-active waterin the presence of highly saline, thermogenic methane-rich groundwater with extremely long apparent meanresidence times (Harkness et al. 2018). It is important toremember that these observations from the Karoo Basinare consistent with a growing number of observationsfrom shallow aquifers in other petroleum basins aroundthe world (Darrah et al. 2014; Moritz et al. 2015; Darrahet al. 2015a, 2015b; Humez et al. 2016; Wen et al. 2016;Harkness et al. 2017b; 2018).

Consistent with Harkness et al. (2018), we observea striking spatial relationship between samples withhighly elevated 4He concentrations, 4He/CH4, 4He/20Ne20Ne/36Ar, and zones of intense fracturing related to theintrusion of the dolerite sills. The significant excessesof 4He distinguish the presence of quantifiable fluxesof exogenous fluids to shallow aquifers across theKaroo and suggest that the highly fractured zones neardolerite sills likely provided preferential pathways forthe transmission of deeper, saline- and methane-richfluids to the shallow aquifers on yet unknown geologicaltimescales. In addition to serving as pathways for fluidmigration from depth, the zones of intense fracturingnear dolerite intrusions also likely remain as preferentialpathways for fluid migration within shallow aquifersin this region today. The latter point stems from thefact that shallow aquifers in the region consist ofdominantly low porosity and low permeability rocks(Le Maltre et al. 2009; Rosewarne et al. 2013), whichis consistent with recent numerical simulations andhighlights the importance of fractures in controlling therate and distribution of gas migration in low permeabilityformations (Moortgat et al. 2018).

The presence of mantle-derived helium (up to 1%)in groundwater samples rich in methane and salt furthersupports our interpretation for the presence of exogenoussources of natural gas in the shallow aquifers in theKaroo Basin (Figure 8). Each of the samples withmethane concentrations exceeding 14 ccSTP/kg displayedhelium isotopic compositions (i.e., 3He/4He reported asR/RA) above the anticipated crustal production ratio(0.02RA) commonly observed in thermogenic naturalgas (Ballentine et al. 2002; Ballentine and Burnard2002). Importantly, samples with elevated helium isotopiccompositions also displayed He/Ne more than 100 timesatmospheric levels precluding atmospheric contaminationor tritiogenic 3He as a viable source for the elevated levels


of 3He. Additionally, the presence of elevated 3He/4He,specifically within helium-rich samples, provides supportfor the hypothesis that thermogenic methane- and salt-rich fluids migrated to the near-surface aquifers alongpreferential pathways induced by the presence of doleriteintrusions.

Mantle-derived fluids (e.g., 3He and CO2) can bedegassed from an intrusive igneous body, such as thedolerite intrusions commonly observed across the KarooBasin (Ballentine and Sherwood Lollar 2002; Darrah et al.2015b; Moore et al. 2018). Following the emplacementof magmatic intrusions, mantle-derived 3He can bestored within sedimentary reservoirs, such as structuralor stratigraphic hydrocarbon traps, for extended periodsof geological time and can subsequently mix with ormigrate into regions of locally produced fluids (Ballentineand Sherwood Lollar 2002). It remains unclear if theseresolvable mantle contributions are related to the doleriteintrusions or serve as evidence for active fluid migrationalong preferential migration pathways. We do note that itis intriguing that the samples with the highest 3He/4Heratios are those with the highest proportions of methaneand salinity, which could suggest that the same deep-reaching conduits that provided preferential pathways formagmatically derived volatiles also permit the migrationof thermogenic natural gas and brines to the shallowaquifers in the region.

Conclusion

Global Context for the Occurrence of Biogenicand Thermogenic Natural Gas

Within the Karoo Basin, the subset of methane- andsalt-rich samples displays a higher proportion of thermo-genic natural gas, whereas the low-salinity freshwatersdisplay almost exclusively low abundances of microbialmethane derived via a variety of methanogenic pathways.These trends, recorded in pristine aquifers without anyhistory of conventional or unconventional energy extrac-tion or mining, are broadly consistent with a growingbody of data that document a naturally occurring back-ground of elevated levels of thermogenic natural gas inaquifers overlying petroleum systems in the USA andCanada (e.g., Warner et al. 2013; Moritz et al. 2015; Dar-rah et al. 2015b; Humez et al. 2016; Nicot et al. 2017a;Harkness et al. 2017b; Nicot et al. 2017b, 2017c). Fur-thermore, these data confirm the relationship betweena naturally occurring background of thermogenic natu-ral gas and elevated levels of salts with a NaCl-typecomposition. By comparison, the low, natural presenceof microbial methane occurs in freshwater and can beclearly distinguished by the molecular and isotopic com-position of hydrocarbons and the elemental and iso-topic abundances of noble gases. Each of these factorsis important to consider in future studies that exam-ine the occurrence of methane in shallow aquifers orstray gas investigations. In addition to evaluating thecharacteristics of methane occurrence and source, we

document the utility of integrating multiple geochemicaltracers (hydrocarbon molecular and isotopic composition,noble gases, and water chemistry) when evaluating theoccurrences, sources, and processes that regulate naturallyoccurring methane contamination in shallow aquifers. Theintegration of these techniques is specifically important inlight of the growing realization that the geochemical com-position of naturally occurring methane has experiencedalteration by postgenetic modification in the subsurface.

AcknowledgmentsWe acknowledge financial support from NSF EAGER

(EAR-1249255), NSF SusChem (EAR-1441497), andDOE (DE-FE0024357) to T.H.D. We would also liketo thank the members of the School of Earth Sci-ences (OSU) and the Nicholas School of the Environ-ment (Duke) for assisting in data analysis and construc-tive feedback on this project. Additionally, this workwould not be possible without the cooperation withfarmers and property owners across the Karoo Basinand their hospitality during sample collection. Finally,we’d like to express gratitude to Ryno Botha andAndrew Watson who were essential for completing thefield work.

Authors’ Note:The authors do not have any conflicts of interest or

financial disclosures to report.

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hydrocarbon-rich groundwater above shale-gas formations: …...hydrocarbon-rich groundwater above...

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