Constraining source attribution of methane in an alluvial aquifer with multiple recharge pathways
Charlotte P. Iverach
a,b,c, Dioni I. Cendón
c,b, Sabrina Beckmann
d, Stuart I. Hankin
c, Mike Manefield
e, Bryce F.J. Kelly
a,b,⇑aSchool of Biological, Earth and Environmental Sciences, UNSW Sydney, NSW 2052, Australia
bConnected Waters Initiative Research Centre, UNSW Sydney, NSW 2052, Australia
cAustralian Nuclear Science and Technology Organisation, New Illawarra Rd, Lucas Heights, NSW 2234, Australia
dCollege of Earth, Ocean and Environment, University of Delaware, 700 Pilottown Road, 19958 Lewes, USA
eSchool of Civil and Environmental Engineering, School of Chemical Engineering, UNSW Sydney, NSW 2052, Australia
h i g h l i g h t s
The source of CH4in a freshwater alluvial aquifer was investigated.
A multi-faceted biogeochemical approach was used to attribute the source of groundwater CH4. Different depth zones of CH4
occurrence in the alluvium were identified.
Source of CH4was attributed to underlying artesian discharge, rather thanin situproduction.
g r a p h i c a l a b s t r a c t
a r t i c l e i n f o
Article history:
Received 11 June 2019
Received in revised form 8 October 2019 Accepted 9 October 2019
Available online 2 November 2019 Editor: Patricia Holden
Keywords:
Methane Hydrogeochemistry Microbiology Isotopes Groundwater Discharge
a b s t r a c t
Identifying the source of methane (CH4) in groundwater is often complicated due to various production, degradation and migration pathways, particularly in settings where there are multiple groundwater recharge pathways. This study demonstrates the ability to constrain the origin of CH4within an alluvial aquifer that could be sourced fromin situmicrobiological production or underlying formations at depth.
To characterise the hydrochemical and microbiological processes active within the alluvium, previously reported hydrochemical data (major ion chemistry and isotopic tracers (3H,14C,36Cl)) were interpreted in the context of CH4and carbon dioxide (CO2) isotopic chemistry, and the microbial community composi- tion in the groundwater. The rate of observed oxidation of CH4within the aquifer was then characterised using a Rayleigh fractionation model. The stratification of the hydrochemical facies and microbiological community populations is interpreted to be a result of the gradational mixing of water from river leakage and floodwater recharge with water from basal artesian inflow. Within the aquifer there is a low abun- dance of methanogenic archaea indicating that there is limited biological potential for microbial CH4pro- duction. Our results show that the resulting interconnection between hydrochemistry and microbial community composition affects the occurrence and oxidation of CH4within the alluvial aquifer, con- straining the source of CH4in the groundwater to the geological formations beneath the alluvium.
Ó2019 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).
https://doi.org/10.1016/j.scitotenv.2019.134927
0048-9697/Ó2019 The Author(s). Published by Elsevier B.V.
This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).
⇑ Corresponding author at: School of Biological, Earth and Environmental Sciences, UNSW Sydney, NSW 2052, Australia.
E-mail address:[email protected](B.F.J. Kelly).
Contents lists available atScienceDirect
Science of the Total Environment
j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l oc a t e / s c i t o t e n v
1. Introduction
Interest regarding the occurrence of methane (CH4) in ground- water, including the identification of production and migration pathways, has increased as a result of the global expansion of unconventional gas production [shale gas, coal bed methane, and coal seam gas] (Atkins et al., 2015; Heilweil et al., 2015;
Iverach et al., 2015, 2017a; Mortiz et al., 2015; Li et al., 2016;
Owen et al., 2016; Sherwood et al., 2016; Zhang and Soeder, 2016; Harkness et al., 2017; Jasechko and Perrone, 2017; Barth- Naftilan et al., 2018;Schout et al., 2018; amongst others). Failure to establish natural pathways of water and gas migration prior to gas development and groundwater extraction has resulted in con- siderable debate about the causes of high CH4 concentration within aquifers (Heilweil et al., 2015). Adding to the complexity is the impact that the biogeochemistry of groundwaters from dif- ferent formations has on both the production, degradation, and migration processes acting upon the CH4 and our capacity to use the isotopic signature of CH4as a tracer when these ground- waters mix.
Methane can be present in groundwater as a result of microbial production in situ (via methanogenic archaea), natural upwards migration from deeper geological formations through natural faults and fractures, or induced migration via anthropogenic activ- ity (Barker and Fritz, 1981; Fontenot et al., 2013; Iverach et al., 2017; Kulongoski et al., 2018; Li et al., 2016; Simkus et al., 2016;
Ward and Kelly, 2013). Knowledge of the distribution and source of CH4in groundwater has the potential to be used as a preliminary indicator of water and gas migration (Osborn et al., 2011; Molofsky et al., 2013; Iverach et al., 2015; Cahill et al., 2017), as the occur- rence of CH4produced from different sources in groundwater can be isotopically traced (Whiticar, 1999; Iverach et al., 2015, 2017a; Currell et al., 2017; Botner et al., 2018). This is useful for assessing gas migration, especially when coupled with key geo- chemical indicators, such as the concentration of SO42, isotopes of C in dissolved inorganic carbon (d13CDIC) and isotopic dating techniques (3H,14C and36Cl/Cl).
Changes to geochemical conditions in freshwater alluvial aqui- fers can be a result of both natural processes, such as recharge pathways, and anthropogenic processes, such as groundwater extraction (McLean, 2003; Lower Namoi Groundwater, 2008;
Lamontagne et al., 2015, Iverach et al., 2017a,b). Increased draw- down of groundwater due to groundwater withdrawals, as well as the subsequent recovery of groundwater levels after cessation of pumping can affect the occurrence of both CH4and other bio- geochemical indicators (Kelly et al., 2007; Smith et al., 2016). Addi- tionally, recharge pathways to groundwater systems, such as surface infiltration or artesian discharge, can influence the distri- bution and transport of gas within the aquifer. Therefore, it is important to identify processes occurring in the groundwater that can potentially influence the migration of both water and gas.
Identification of baseline conditions and primary CH4origin and migration pathways allow for a better assessment of both co- located irrigation and gas production impacts.
The objective of this study is to characterise the origin of CH4in an alluvial aquifer where there is substantial artesian discharge occurring (Iverach et al., 2017b). The Great Artesian Basin (GAB) is the largest artesian aquifer system in the world, and it overlies many oil and gas reservoirs that are being exploited for both con- ventional and coal seam gas (CSG) (Queensland Water Commission, 2012;Towler et al., 2016). Where faults or igneous intrusions hydraulically connect the underlying formations to the GAB there is potential for CH4that originates in these underlying formations to be incorporated into discharge from the GAB into the overlying alluvial aquifer.
In this study, using hydrochemical data, microbial population analysis, isotopic tracers, and facies analysis we constrain the source of the CH4in the alluvium that overlies the artesian basin to being predominantly derived from artesian discharge. Addition- ally, we apply in a novel way, a Rayleigh fractionation model to link our alluvial monitoring well observations to the CSG exploration well data. We demonstrate that the rate of oxidation observed is consistent with the hypothesis that the primary source of CH4in the alluvium can be traced back to the formations underlying the GAB.
1.1. Migration pathways and processes affecting methane distribution in shallow groundwater
When determining the source of CH4in an aquifer, considera- tion needs to be given to riparian zone linkages (Boulton et al., 2014), in situ production and oxidation (Van Stempvoort et al., 2005; Kietäväinen et al., 2017; Schout et al., 2018; Raidla et al., 2019), and inputs from underlying formations via upwards migra- tion (Barker and Fritz, 1981; Schoell, 1988; Wilson et al., 2014;
Moya et al., 2016). Processes that can affect the occurrence of the gas upon migration include hydrological controls, such as ground- water level fluctuations and recharge pathways (Wang et al., 2015;
Zhou et al., 2015), the composition and abundance of microbial communities and their relative position along the groundwater flow path (Beyer et al., 2015; Maamar et al., 2015) and the geo- chemical conditions in the groundwater (Zhang et al., 1998;
Whiticar, 1999; Moya et al., 2016; Owen et al., 2016; Currell et al., 2017; Schweitzer et al., 2019).
Hydrochemical parameters have been identified as having a major bearing on the distribution, concentration and isotopic composition of CH4 in groundwater. Previous studies have iden- tified O2 and SO42 concentrations as amongst the most impor- tant chemical parameters in regulating CH4 distribution in groundwater (Zhang et al., 1998; Whiticar, 1999). Additionally, Zhang et al. (1998) found that a positive correlation between CH4 and SO42 could potentially indicate a thermogenic origin for CH4 and that CH4 oxidation may be correlated with SO42reduction in the groundwater studied. A more recent study found that high dissolved SO42 concentrations along with high CH4concentrations could indicate gas from potential contamina- tion or rapid migration (Currell et al., 2017). The utilisation of geochemical parameters is common in studies such as these, and increasingly studies have combined major ion geochemistry, isotopic tracers, microbial community analyses, and Rayleigh fractionation modelling to identify migration and oxidation pro- cesses of CH4in groundwater (Rasigraf et al., 2012; Miller et al., 2016; Nowak et al., 2017; Wegner et al., 2018; McIntosh et al., 2019).
Processes that affect the occurrence and distribution of CH4, such as oxidation, are also constrained by geochemical conditions.
Aerobic CH4oxidation has been coupled to, apart from the pres- ence of O2, denitrification in groundwater (Zhu et al., 2016). It is the relationship between geochemical indicators and the anaerobic oxidation of CH4(AOM) that plays an important role in constrain- ing CH4distribution, especially in anaerobic aquifer conditions. A good indication of the potential for AOM in groundwater is the prevalence of those electron acceptors that anaerobic methan- otrophs can use to oxidise CH4in the place of O2. These include SO42, NO3, NO2, and Fe2+(Valentine and Reeburgh, 2000; Ettwig et al., 2010; Sivan et al., 2011; Green-Saxena et al., 2014; Antler et al., 2015). These major ion data enable better interpretation of the processes affecting the occurrence of CH4in groundwater and can provide evidence for eitherin situproduction or incidences of migration.
The Lower Namoi Alluvium (LNA), New South Wales (NSW), Australia (Fig. 1) was selected to demonstrate that combined hydrogeochemical and microbiological analyses can constrain CH4 source attribution. This alluvial aquifer is known to be recharged by both modern surface water and old artesian dis- charge (Lower Namoi Groundwater, 2008; Smithson, 2009;
Powell and Scott, 2011; Lamontagne et al., 2015; Santos Limited, 2015; Iverach et al., 2017b; DPI Water, 2017). Thus, our study area is well suited to testing our multi-faceted methodological approach to constraining the source of CH4in a setting where there is significant artesian discharge to an alluvial aquifer.
2. Study area
2.1. Hydrogeological setting
The Lower Namoi River Catchment sits within the Coonamble Embayment, which is a structural division of the Surat Basin (McLean, 2003). The Surat Basin is a sub-basin of the GAB. The southernmost portion of the LNA is underlain by Triassic forma- tions, whilst the remainder of the study area is underlain by Juras- sic formations. The oldest outcropping bedrock formation within study area is the early Triassic Digby Formation (Tadros, 1993), outcropping in the south-east of the study area. The Digby Forma- tion is overlain by the Triassic Napperby Formation, which occurs at a depth of 106 m, just below the base of sample 30345. The
Jurassic Formations important to this study are the Purlawaugh Formation, Pilliga Sandstone, and the Orallo Formation (Fig. 2).
The Pilliga Sandstone is the primary aquifer of the GAB in the Namoi region (McLean, 2003).
The coal seams being targeted by Santos Ltd. for CSG production in the Namoi Catchment are in the Permian Maules Creek Forma- tion (Narrabri and Bohena seams) and the Permian Black Jack Group (Hokissons seam). There is no contact between the coal seams targeted for CSG production and the alluvial sequences used for irrigation in the zone studied (Santos Limited, 2015).
From the late Cretaceous to the mid Miocene, a palaeovalley was carved through the early Cretaceous sedimentary rocks (Kelly et al., 2014). This palaeovalley was filled with reworked allu- vial sediments from the mid Miocene to the present. Groundwater abstraction in the study area is mostly from these alluvial sedi- ments. Interbedded clays, sands and gravels form the~140 m thick alluvial sequence of the Lower Namoi Alluvium (Williams et al., 1989; Kalaitzis and Jamieson, 2000). The alluvial sediments have a complex, heterogeneous architecture, which is vertically hydraulically connected in some places due to deposited sediments from meandering rivers (Kelly et al., 2013). Three main non- formally defined aquifers/formations have traditionally been used to describe the LNA: the Narrabri Formation (shallow), the Gunne- dah Formation (intermediate), and the Cubbaroo Formation (deep).
More recent research in the Namoi Catchment suggests that the rigid subdivision of the Narrabri, Gunnedah, and Cubbaroo forma- Fig. 1.Map of the study area highlighting groundwater bore locations where samples were collected and major geological structures. The red bores are the locations of our groundwater samples; the blue wells are locations of gas wells. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
tions is better characterised as a distributive fluvial system with the dominant sediment deposited reflecting a change in climate from relatively wet to dry, resulting in more high energy deposits, sands and gravels at depth, and more low energy deposits, fine sands to clay, in the upper portion of the aquifer (Kelly et al., 2014; Acworth et al., 2015). This interpretation of the sedimentary sequence is also supported by the geochemical mixing shown in Iverach et al. (2017b).
2.1.1. Regional structural geology
The regional structural geology of the Gunnedah Basin is described in (Hamilton et al., 1988), Tadros (1995), and ESG (2004). The Wilga Park Anticline located 5 km to the south of Nar- rabri runs north-south, through our study area (Supplementary Fig. 1). A north-south seismic section along the axis of the anticline maps faults that cut into the Pilliga Sandstone and a volcanic plug that extends from the regional basement and passes upwards through the Maules Creek Formation and Hoskissons coal seam into the base of the Pilliga sandstone. The faults and plug contact zones are potential pathways for groundwater and gas to migrate.
Additionally, structural subdivisions of the Gunnedah Basin (Hamilton et al., 1988;Tadros, 1995; ESG, 2004) show that there is a lineament running parallel to the Namoi River in our study area that may provide an additional pathway for groundwater and gas migration. (Gurba and Weber, 2001) mapped a number of igneous intrusions in the region and demonstrated that near these intru-
sions there is elevated CO2 associated with gas migration from depth. The seismic surveys conducted by ESG traverse some of these mapped igneous intrusions (refer Wilga section inSupple- mentary Fig. 1) (ESG, 2004).
Eastern Star Gas (ESG) conducted seismic surveys throughout the region for the purpose of identifying hydrocarbon prospects.
The primary targets were conventional sandstone reservoirs in the Triassic Digby Formation and the Early Permian lower Porcu- pine Formation sandstone. These seismic surveys identified a num- ber of north-south trending anticlines associated with basement fault reactivation, regional compressional deformation and igneous intrusions (ESG, 2004). One of the seismic lines interpreted near Yarrie Lake, south-west of Narrabri (Supplementary Fig. 1) shows fault planes appearing in the east-west section that continue northwards, parallel to the thrust that is evident in Bellata seismic sections, 20 km north of Narrabri, inTadros (1995). Some of these fractures in the ESG seismic section propagate into the top of the Purlawaugh Formation and into the Pilliga Sandstone.
2.2. Surface water recharge
The Namoi River is the primary watercourse through the region and it flows from the south-east to the north-west of the study area. Groundwater withdrawals associated with irrigated agricul- ture have disconnected the Namoi River from the underlying aqui- fer in the western portion of the study area, and there is now Fig. 2.Stratigraphy of study area from a well drilled within our study area (adapted from Culgoora-2 well completion report (ESG, 2004)). The formations of the GAB and locations where the historic CH4samples were collected are highlighted. The LNA, where our samples are collected, sit above the GAB.
continuous river leakage recharge to the LNA (Kelly et al., 2013, Iverach et al., 2017b). Flooding within the region occurs 1–3 times per decade (Kelly et al., 2013). Water isotope analyses indicate that floodwaters are the primary source of surface water recharge (Iverach et al., 2017b). Diffuse recharge is a minor source of surface water recharge (Kelly et al., 2013).
2.3. Artesian discharge to the lower Namoi alluvium
There have been a series of regional scale water balance groundwater models for the lower Namoi (Williams et al., 1989, Merrick, 2000, CSIRO, 2007; Kelly et al., 2007; Schlumberger, 2012; SANTOS, 2018). All these models have incorporated dis- charge from the GAB into the LNA. These models have used grid cell sizes that range from the order of kilometres to tens of kilome- tres. Artesian discharge from the GAB into the LNA is also sup- ported by hydrogeochemical investigations (Calf, 1978, McLean, 2003, Iverach et al., 2017b).
At the point scale, Iverach et al. (2017b), using geochemical techniques and radioactive isotopic tracers, determined that up to 70% of the groundwater was sourced from the GAB in some places of the LNA. The hydrochemical results from this previous work (major ions,3H, 14C, and 36Cl), as well as our conclusions regarding percentage extent of artesian discharge to the alluvium, form one of the key pieces of evidence that this research uses to constrain the source of CH4 in the LNA. These results are sum- marised in theSupplementary Information. This study builds on these previous findings, incorporating new CH4 concentration and isotopic data from both the alluvium and formations underly- ing the GAB, as well as microbiological community analyses and structural geological information, to constrain the source of CH4 in the LNA.
3. Methods
The methodology below has been summarised with references to where these methods have been presented in the literature.
Additionally, complete details of sample collection and analysis are included in theSupplementary Information.
3.1. Groundwater collection
From 28 January 2016 to 8 February 2016 we collected 28 groundwater samples, from State Government groundwater mon- itoring bores, and 1 surface water sample, from the Namoi River, for geochemical analyses, microbial community composition anal- yses, and CH4concentration and isotopic composition analysis. The groundwater monitoring wells are screened at varying intervals, intersecting aquifers at depths ranging from 9 m to 204 m below ground surface (bgs). Samples for microbial community analysis were collected by filtering 2 L of pumped groundwater using a 0.2
l
m filter (Merck Millipore). The filters with collected biomass were stored at 4°C until DNA extraction.Dissolved gas in the groundwater was extracted using a LGR Dissolved Gas Extraction Unit (DGEU) (LGR Inc., Mountain View, CA). Groundwater was pumped into a sealed 10 L flow cell directly from the formation at a flow rate adapted to match or exceed the DGEU water pumping rate. The water was then pumped into the DGEU from the flow cell using suction strainers and passed through an internal hydrophobic micro-porous membrane. Con- currently, sweep gas (N2) was pumped into the DGEU, which was passed into the membrane from the opposite direction to the groundwater flow. The dissolved gas in the groundwater was stripped through the membrane by the cross flow of the sweep gas. The extracted gas was passed into 3 L SKC FlexFoil bags for
later isotopic analysis. Wastewater and waste sweep gas were passed out of the instrument.
3.2. Biogeochemical analyses
Groundwater samples for major ions and isotopic tracers were analysed at ANSTO following methods described in (Cendon et al., 2014),Cendón et al. (2015), Iverach et al. (2017b),Wilcken et al., 2017. Samples ford13CDOCwere analysed at UC-Davis Stable Isotope Facility with complete methods described in Meredith et al. (2016).
Groundwater samples for microbial community analysis were prepared and analysed at UNSW Sydney following methods described inIverach et al. (2017a). Full details are included in the Supplementary Information.
3.3. Methane analyses
All dissolved gas samples were analysed for CH4concentration using a CRDS Picarro G2201-ianalyser. These results are used to determine the overall distribution of CH4 within the alluvium.
Methane concentration results collected from degassing ground- water are presented here in ppm. Additionally, this allows the CH4 concentration results for the alluvial groundwater to be directly comparable to historic data from formations underlying the GAB, which were reported in ppm.
Ten of the total 29 dissolved gas samples (those with [CH4]
> 500 ppm) were analysed on a GC-IRMS at Intertek Geotechnical Services Pty Ltd, Western Australia. Compositional GC analysis of the samples was used to obtain [CH4] and [CO2], whilst IRMS was used for stable isotope analysis of CH4 and CO2 (d13C-CH4
andd13C-CO2, respectively). The IRMS is equipped with a pyrolysis reactor, which was used for stable hydrogen isotope analysis in CH4 (d2H-CH4). Six samples were analysed for d2H-CH4. For all other samples the CH4concentration was too low for analysis via GC.
4. Results
4.1. Major ion chemistry
The major ion chemistry that we present here are those param- eters measured inIverach et al. (2017b)that most directly affect the occurrence of the groundwater CH4 measured in this study.
The concentration of SO42 for all samples collected ranged from < 0.05 mg/L to 365 mg/L (average: 22.5 mg/L; Supplementary Table 2). The highest SO42concentration measured in the study area was significantly elevated compared to the other samples;
the second highest concentration was 39.1 mg/L. Additionally, other major ions (particularly Cl, Br, and Na) were elevated in this sample compared to the other alluvial samples (Supplementary Table 2). This was likely due to separate evapotranspiration pro- cesses occurring at this site.
Nitrate (NO3) concentrations ranged from below the detection limit (<0.01 mg/L) to 20.9 mg/L (average: 4.3 mg/L; Supplementary Table 2) and nitrite (NO2) concentrations ranged from below the detection limit (<0.01 mg/L) to 0.15 mg/L (average: 0.05 mg/L;
Supplementary Table 2). The concentration of Fe2+in the alluvial groundwater ranged from 0 mg/L to 2.55 mg/L (average 0.32 mg/
L; n = 30; Supplementary Table 2).
The concentration of DO in the groundwater reported in this study was measured in a flow cell at the ground surface and is therefore not an accurate measure of the formation value. How- ever, these measurements do provide a preliminary indication of the conditions in the aquifer. The concentration of DO in all
groundwater samples ranged from 0.07 mg/L to 6.67 mg/L (aver- age: 1.19 mg/L; Supplementary Table 2). Oxidation-reduction potentials in the alluvial groundwater were also measured at the surface and ranged from 0.007 V to 0.324 V (reported as Eh;
Eref= 200 mV). These values generally become more reducing with depth and increasing CH4concentration (Supplementary Fig. 2).
4.2. Carbon stable isotopes (d13CDOC)
Concentrations of dissolved organic carbon (DOC) were gener- ally low (0.05–6.2 mg/L; average: 0.75 mg/L; Supplementary Table 3). As such, in 13 of the 29 groundwater samples, the DOC concentration was below the lower limit for which acceptable iso- topic results were obtainable (0.1 mg/L). All results are presented in Supplementary Table 1a for completeness, but only those iso- topic values above the lower limit are discussed. The remaining 16 samples had an organic carbon isotopic signature (d13CDOC) between12.5‰ and 32.5‰ (average:23.1‰). The majority of groundwater samples generally fit within the expected range of values for alluvial systems in eastern Australia (Duvert et al., 2015).
4.3. Groundwater residence times (3H,14C,36Cl)
The3H,14C, and36Cl results and their implications have been fully described inIverach et al. (2017b). We use them in this study alongside the groundwater CH4data to robustly interpret the ori- gin of, migration pathways and oxidation processes affecting the concentration distribution and isotopic chemistry of CH4 in the LNA. The findings inIverach et al. (2017b)are summarised in the Supplementary Information.
4.4. Microbial community analysis
The microbial community in the groundwater contained mainly bacterial members of the phyla Proteobacteria (Alpha-, Beta-, Gamma-, and Deltaproteobacteria), Bacteroidetes, Actinobacteria, Firmicutes, Planctomycetes, Deinococcus-Thermus, Nitrospirae and archaeal sequences affiliated to the Crenarchaeota and Eur- yarchaeota. Differences in microbial community composition and distribution were observed with depth (Fig. 3).
The shallow groundwater (0–30 m) was dominated by crenar- chaeal members related to Candidatus Nitrososphaera gargensis and Nitrosopumilus maritimus. The deeper groundwater (56–
190 m) was dominated by members within the Betaproteobacteria (Gallionella ferruginea), Gammaproteobacteria (Acinetobacter tjern- bergiae,A. bouvetii,A. lwolffii), Bacteroidetes (Chryseobacterium for- mosense) and Planctomycetes (Panctomyces limnophilus).
Regarding methanogenic archaea, only a few sequences belong- ing toMethanocalculus taiwanensisandMethanosaeta pelagicawere observed in the groundwater between 37 and 60 m. In turn, sulfate- and sulfur-reducing bacteria and archaea (Desulfovibrio sp.,Desulfosporosinus auripigmenti, and Desulfurococcussp.) were abundant throughout the groundwater.
Aerobic methanotrophic bacteria affiliated to the orderRhizo- bialeswere detected in the groundwater. Both species,Methylocella palustrisandMethylosinus acidophilus,have been found in various freshwater aquifers before (Costello et al., 2002; Iverach et al., 2017a) and are facultatively methanotrophic. This means they can utilise methanol, acetate, pyruvate, succinate, malate and etha- nol as well as CH4(Dedysh et al., 2005).
4.5. Occurrence of methane in the LNA
Methane was present in all alluvial groundwater samples, rang- ing from 0.74 ppm to 3,633 ppm (average: 775 ppm, n = 29, Sup-
plementary Table 1a). These results were measured in-field on the Picarro G2301-i isotope analyser. The 10 samples > 500 pm that were measured using GC ranged from 580 ppm to 3,427 ppm (average: 1,946 ppm; n = 10; Supplementary Table 1a).
Throughout the study area, CH4generally increases with depth, with the lowest concentrations measured in the shallow ground- water (<30 m) (Fig. 4). Methane concentrations in these samples were all too low to be analysed for isotopic composition using GC (<500 ppm). In the intermediate groundwater (30–80 m), CH4
ranged from 520 ppm to 3,427 ppm (average: 1,504.4 ppm; n = 5) and in the deep groundwater (>80 m) CH4ranged from 865 ppm to 3,318 ppm (average 2,387.6 ppm; n = 5). All CH4concentrations are detailed in Supplementary Table 1a.
The carbon isotopic signature of CH4in the 10 samples analysed on GC-IRMS ranged from 75.0‰ (deep) to 50.6‰(shallow), becoming more 13C-depleted with depth (Supplementary Table 1a). The isotopic signature of hydrogen in the CH4ranged from306‰ to213‰. The carbon isotopic signature of CO2in the alluvial samples ranged from21.6‰to15.3‰.
5. Discussion
5.1. Distribution and isotopic signature of methane in the LNA
The concentration of CH4 in our groundwater samples varied both with distance away from the river corridor and with depth.
The concentration was low in the groundwater extracted from the shallow alluvium (average: 24 ppm; n = 7) and increased sig- nificantly with depth (average: 1,815 ppm; n = 7) (Fig. 4). This sug- gests CH4 production at depth, followed by dispersion and oxidation as the CH4migrates upwards into the shallow alluvium.
This hypothesis is tested by examining the rate of oxidation using a Rayleigh fractionation model.
There is also a spatial trend with higher CH4concentrations in the south of the study area (labelled inFig. 5). These areas coincide with the locations of important faults, igneous intrusions and per- meable facies identified in the study area (Fig. 1), as well as loca- tions of wells drilled by ESG for gas exploration.
There have been three extensive campaigns in the region to assess if CH4 is leaking from either historical exploration wells, existing production wells, or new test wells (Day et al., 2016;
Hatch et al., 2018; Kelly et al., 2019). These three independent campaigns have all focused on measuring the CH4mole fraction of the ground level atmosphere, including measurements of well- head fittings (Day et al., 2016). No leaks have been detected from these many thousands of kilometres of surveying, that could be directly attributed to either gas exploration wells, production wells, or test wells. In addition, the typical distance from the sam- pled alluvial groundwater monitoring bores to the current region of production is greater than 10 km. The lack of any leak detection from CSG well casing infrastructure combined with the distance between the groundwater monitoring bores and producing CSG field suggest that the gas migration inferred from the groundwater data presented here is via natural geological pathways rather than via leaky CSG wells.
5.1.1. Methane and carbon dioxide concentration and isotopic signature from formations underlying the GAB
Eastern Star Gas carried out CH4concentration,d13C-CH4,d13C- CO2, andd2H-CH4analyses between 2008 and 2011. These analyses are presented in Supplementary Table 1b. Methane concentration analyses were carried out in 4 wells targeting the Hokissons coal seam proximal to our study area and were significantly elevated, ranging from 13,200 ppm (755 m bgs) to 789,900 ppm (948 m bgs) (average: 376,256 ppm) (Supplementary Table 1b). These
Fig. 3.Microbial community composition and abundance in the LNA, ordered from shallow to deep, starting with the surface Namoi River sample (A – overall microbial abundance; B – abundance of microbes without unclassified bacteria and archaea). This shows a clear change in the community composition with depth (particularly at the red markers around 55 m and 85 m bgs). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
analyses were undertaken using GC between 2008 and 2011 and were collected by Eastern Star Gas via gas desorption from wells as deep as~1,000 m bgs in and to the south-west of our study area (ESG, 2008–2011).
Analyses for d13C-CH4 and d13C-CO2 were undertaken in 17 wells, and d2H-CH4 analyses undertaken in 7 wells proximal to our study area at depths ranging from 189 m bgs to 1,028 m bgs.
These samples displayed a CH4carbon isotopic signature between 79.3‰and44.0‰(average:63.9‰; n = 32), suggesting that, as with the LNA, the source of the extremely elevated concentra- tions of CH4 in the formations underlying the GAB is of mixed microbial thermogenic origin. Three of the 4 samples collected below 1,000 m from the Hoskissons and Bohena coal seams had microbial signatures between77.0‰and68.0‰. Analyses for d2H-CH4 were only undertaken in 11 wells and ranged from 148‰ to 275‰ (average:239‰, Supplementary Table 1b).
Thed13C-CO2 values for the wells sampled by ESG have a wide
range, with values ranging from 15.5‰ to + 13.9‰ (average:
2.3‰).
The microbial signature of the CH4observed in the samples col- lected from formations underlying the GAB, and those samples col- lected in the alluvial groundwater suggests that these two areas share a common process for the occurrence of CH4–in situmicro- bial production. Whether the biogeochemical conditions support the production of CH4in each formation, or whether the CH4has migrated from the deeper formations underlying the GAB upwards into the LNA is explored below.
5.1.2. Regional structural geology suggests potential pathways of upward migration for methane
The structural geology underlying our study area provides fur- ther evidence that a common source of CH4 exists between the LNA and deeper formations. The north-south Wilga Park seismic line over the Wilga Park structure (shown in Fig. 1) shows a Fig. 4.Summary of major processes occurring through the vertical profile of the LNA; (a) shows the CH4concentration increasing with depth, whilst; (b) shows the isotopic signature of the CH4becoming more12C-enriched with depth; (c) shows that SO4concentrations conversely decrease with depth and (d) shows36Cl results decreasing with depth, suggesting groundwater with long residence times in the deeper LNA. Shallow samples with low36Cl ratios are labelled.
volcanic intrusion and associated faulting (Hamilton et al., 1988 Tadros, 1995). The location of these structures coincided with higher GAB input and lower36Cl/Cl values as identified inIverach et al. (2017b), and with samples that displayed higher CH4concen- trations as observed inFigs. 1 and 4.
Seismic data from ESG (ESG, 2004) additionally shows numer- ous structures in the study area that propagate into the Purla- waugh Formation and the Pilliga Sandstone from deeper underlying formations. Methane concentration data from ESG (2008–2011)show that there are extremely elevated concentra- tions of CH4 in these deeper formations. Additionally, of the 25 samples collected from formations underlying the GAB, 21 had an isotopic signature indicating that the CH4had a microbial origin.
The structures identified inHamilton et al., 1988,Tadros (1995), andESG (2004)present the most likely pathway for both the arte- sian discharge within the Pilliga Formation identified inIverach et al. (2017b)and a potential migration pathway for CH4sourced from the underlying formations.
5.2. Biogeochemical controls on the occurrence of methane in the alluvial groundwater
Iverach et al. (2017b)used radioactive isotopic tracers (3H,14C,
36Cl) in the alluvial groundwater to show that there are two pri- mary inputs to the LNA; freshwater surface recharge and artesian discharge from the underlying GAB. Groundwater comprised pri- marily of freshwater surface input was characterised by high3H and14C, similar to those associated with modern recharge. Addi- tionally, high36Cl/Cl ratios were also associated with this surface input. Alluvial groundwater that was comprised principally of arte- sian discharge was characterised by extremely low 3H and 14C activities, and low36Cl/Cl ratios, suggesting groundwater with a residence time in the order of hundreds of thousands of years.
Iverach et al. (2017b) established that the groundwater in the LNA represents a continuum with depth between modern surface recharge and old artesian discharge. It is this continuum that forms the basis of our discussion regarding the geochemical and micro- bial processes operating on CH4in the alluvial groundwater.
Fig. 5highlights that it is those alluvial samples with a36Cl/Cl value < 1001015, similar to that of groundwater measured in the GAB (suggesting groundwater that is hundreds of thousands of years old) that have the highly elevated CH4 concentrations (>500 ppm). This suggests that these two primary inputs to the LNA are the major driver of the distribution of CH4that we see through the vertical profile of the LNA. Additionally, the coinci- dence of samples that were located above significant geological structures with elevated CH4concentration and low36Cl/Cl values (highlighted inFig. 5) provided further evidence that inputs to the LNA, particularly artesian discharge, substantially constrain the occurrence of CH4.
Inorganic chemical parameters, such as some major ions, have the potential to elucidate processes affecting the distribution and oxidation of CH4. For example, elevated SO42concentrations do not favour microbial CH4 production in groundwater. This is because sulfate-reducing bacteria outcompete methanogenic archaea for important substrates (such as acetate). Therefore, ele- vated CH4 occurring concurrently with elevated SO42suggests a thermogenic origin of CH4 (Zhang et al., 1998; Currell et al., 2017). In our alluvial groundwater samples, those with the highest CH4concentration were also amongst those that had the lowest SO42concentration (Fig. 4). This suggests that the source of the CH4in the alluvial groundwater is microbially produced, agreeing with the isotopic signature of CH4in the alluvium.
In addition to SO42, other major ions, such as Fe2+, NO3 and NO2, have the potential to provide insights into anaerobic CH4
oxidation processes that may be occurring. This is because some methanotrophic archaea (ANME) are able to use these ions to oxidise CH4 in the absence of oxygen. However, no members of the ANME were characterised in our alluvial microbiological samples (Fig. 3). Thus, there is little potential for CH4oxidation in the deeper groundwater, where dissolved oxygen concentra- tions are almost 0 mg/L. As the concentration of dissolved oxygen increases through the vertical profile of the LNA, aerobic oxidation potential increases. We see evidence of this aerobic oxidation in the significant decrease in concentra- tion and enrichment in 13C in the CH4 in the shallow groundwater.
5.2.1. The impact of changing microbial community composition with depth on groundwater methane
The distribution of the microbial community in the LNA revealed significant changes to the community composition at around 50 m and 75 m (Fig. 3).Kelly et al. (2014)identified a sig- nificant change in the proportion of clay and sand in the alluvium at 49 m and 80 m. This change correlates with the shift in the near surface microbial community composition and step changes in the hydrochemical continuum through the vertical profile of the allu- vium suggesting some interconnection between sediment type, hydrochemistry and microbial community composition. Addition- ally, the shift in microbial community composition in the lower 70 m of the alluvial aquifer coincides with those samples where GAB discharge dominates the groundwater chemistry (Iverach et al., 2017b).
The crenarchaeal members related to CandidatusNitrososphaera gargensisandNitrosopumilus maritimusprimarily identified in the shallow groundwater (0–30 m) are ubiquitous in freshwater habi- tats and mediate the oxidation of NH4+to NO2. These species were accompanied byNitrospira marina, a bacterium that plays a role in the nitrogen cycle by performing NO2oxidation in the second step of nitrification. Due to the area of study being a major agricultural production region, with many groundwater samples taken below irrigation fields, this is most likely a result of N2-based fertilisers leaching into the groundwater during irrigation (Harris et al., 2018). Further research is required to determine if this may be Fig. 5.Plot of36Cl/Cl (1015) vs [CH4] (ppm) showing that the majority of alluvial
samples with elevated CH4have36Cl/Cl values consistent with that of GAB recharge and older GAB groundwater. The circled sample shows the deepest sample, which was identified as a significant outlier inIverach et al. (2017b). This sample is screened in the Purlawaugh Formation, underlying the LNA and is experiencing recharge from the Namoi River at this site. The labelled samples with elevated CH4
show those that are located above significant geological structures.
due to induced recharge as a result of seasonal groundwater extraction for irrigation.
From the species that dominated the deeper alluvial groundwa- ter (between 55 and 70 m), Gallionella ferruginea (of the class Betaproteobacteria) is an Fe2+and reduced sulfur compounds (sul- fide, thiosulfate) oxidising bacterium usually found in low-oxygen groundwater. This correlates with the low Fe2+concentrations, as well as the low DO concentrations that we observe at these depths.
Chryseobacterium formosense were identified primarily in deep groundwater between 83 and 190 m. This species is capable of hydrocarbon degradation and is typically found in oil- contaminated habitats. This agrees with our findings of elevated concentrations of CH4in the deep alluvial groundwater.
The presence of only a few sequences belonging to the metha- nogenic archaea between 37 and 60 m suggests that conditions at this depth are most favourable to the anaerobic archaea. At this depth, SO42 and DO concentrations are significantly lower than they are in the shallow groundwater. Additionally, the presence of these archaea suggests that there is the potential in the interme- diate groundwater forin situCH4production. However, the abun- dance of sulfate-reducing bacteria through the groundwater suggests that methanogenic archaea were outcompeted by these reducers for essential substrates (such as acetate and H2). The absence of methanogens deeper than 60 m, where we observed the highest concentrations of CH4, suggests that microbial produc- tionin situis not the primary source of CH4in the LNA, despite observed isotopic values being compatible with microbial production.
Further evidence for the absence of microbial CH4production is the concentration of SO42in the groundwater alongside the con- centration of DOC and CH4(Fig. 6).Kulongoski et al. (2018)showed that a negative relationship between SO42and DOC, with increas- ing CH4concentration, represented shallowin situmicrobial CH4 production.Fig. 6shows that our samples do not follow this trend, as CH4in our groundwater samples increases as DOC decreases, further suggesting that the CH4 we measure in the LNA is not microbially producedin situ.
The detection of methanotrophic bacteria between 0 and 50 m indicates that chemical and biological conditions are favourable for CH4 oxidation. The absence of these methanotrophic bacteria
between 70 and 180 m indicates that the sub-anoxic to anoxic con- ditions in the deep groundwater inhibit their growth.
5.3. Significance of multiple recharge pathways to the LNA on the occurrence and oxidation of alluvial methane
Evidence for the control of artesian discharge on the occurrence of CH4in the alluvial groundwater is the SO42concentration in the shallow groundwater (Fig. 4). This shallow groundwater is primar- ily comprised of modern surface water input and the SO42concen- tration is too high to facilitate microbial CH4production (Whiticar, 1999). This agrees with the absence of methanogens that we observe in the shallow groundwater, indicating no biological potential for shallow CH4 production. Nonetheless, we see evi- dence of elevated CH4 in some shallow groundwater samples, despite an absence of CH4-producing archaea and high SO42con- centrations that potentially inhibit CH4production. This suggests that the CH4 present in the upper alluvium has migrated there from elsewhere, in this case, from the underlying Permian- Triassic formation into the GAB and eventually into the LNA.
Fig. 4presents a summary of the primary processes operating in the LNA, combining the concentration of CH4and SO4, the isotopic signature of CH4, the depths at which the microbial community composition changes significantly (Fig. 3), the depth at which the sediment composition in the LNA changes significantly (Kelly et al., 2014), and the depths where we see a significant change in the36Cl/Cl ratios (also evident inFig. 5), showing that the deeper alluvial groundwater displays residence times increasingly like those measured in the GAB.Fig. 4shows that the changing geo- chemical conditions, sediment composition, and microbial com- munity composition through the vertical profile of the LNA due to mixing between the two primary groundwater inputs, signifi- cantly affects the occurrence of CH4in the alluvium, constraining its distribution through the LNA.
The interplay between the hydrochemistry, microbial commu- nity composition, and the sediment composition within the LNA significantly affects the occurrence of groundwater CH4. In the LNA, we propose that it is the slow blending of the surface water recharge with the basal artesian discharge into the LNA (as estab- lished inIverach et al., 2017a,b) that is the primary control on the microbial community. It is then the interplay between the upwards migration of CH4sourced at depth encountering different microbial communities within the LNA that influences the locally observed CH4 concentration and isotopic composition. This interpretation is supported by the following observations.
The isotopic composition of the CH4in the LNA suggests that it is primarily microbially-derived, with varying degrees of 13C- enrichment. This is despite our biogeochemical analyses indicating limited potential forin situproduction.Fig. 7a shows a plot ofd13C- CO2vsd13C-CH4, which classifies dissolved gas samples into dom- inant processes according to their isotopic composition (Whiticar, 1999). The majority of the alluvial samples from this study follow the oxidation trend displayed on the graph, suggesting that the CH4
within the LNA is experiencing oxidation, with minimal microbial production. An analysis of the isotopic composition of hydrogen in the CH4(d2H-CH4) allows for further determination of the origin of the CH4 measured in the groundwater (Fig. 7b). Most of the groundwater samples that were analysed ford2H-CH4plot in the zone attributable to microbial production and have similar isotopic signatures to those samples measured in the formations underly- ing the GAB (ESG, 2008–2011). This suggests that these deeper alluvial samples either share a common origin of CH4 with the ESG samples, or that the source of CH4in the deeper LNA is the for- mations that these ESG samples were collected from.
The instances of13C-enrichment in some shallow alluvial sam- ples is most likely associated with oxidation. The general trend of Fig. 6.SO4vs DOC, showing decreasing SO4with decreasing DOC and increasing
depth. AsFig. 4highlights, increasing depth is analogous with increasing CH4. The lower extent of the x-axis is also the DOC detection limit.
13C-enrichment in CH4coupled with13C-depletion in CO2(Fig. 7a;
Supplementary Table 1a), suggests that the CO2is a product of the oxidation of CH4. Additionally, the isotopic values from the forma- tions underlying the GAB (ESG, 2008–2011) show a13C-depletion in CH4with an enrichment in CO2(Supplementary Table 1b). This suggests that the CH4in the formations underlying the GAB has migrated upwards through the GAB and into the alluvium, and as it has migrated into groundwater with different biogeochemical conditions, the CH4has been oxidised (thus becoming more 13C- enriched), producing CO2that is13C-depleted. This explains why our alluvial samples have a more13C-depletedd13C-CO2signature than the ESG samples and provides further evidence that mixing between the two primary inputs to the LNA has a significant effect on CH4distribution and oxidation in the alluvium.
Furthermore, the clustering of samples inFig. 7b, which doesn’t include the shallow oxidised samples due to concentration detec- tion limits of the GC-IRMS, suggests mixing between the deeper LNA, which had more elevated concentrations of CH4, and the GAB. With the extent of GAB discharge identified inIverach et al.
(2017b)(up to 70% in some locations), we suggest that the above evidence of the migration and oxidation of CH4in the alluvium is a result of the deeper groundwater discharging into the LNA and subsequent mixing with surface recharge through the vertical pro- file of the LNA.
The CH4concentration gradient from relatively high at depth to lower in the near surface portion of the alluvial aquifer, in associ- ation with an absence of methanotrophs at depth and abundance in the shallow alluvium, strongly supports the hypothesis that CH4sourced from the underlying formations is oxidised as it rises through the alluvial sequence. If this hypothesis is correct, then the rate of oxidation is likely to be similar to other alluvial settings worldwide. To determine the rate of oxidation aRayleigh (1896) fractionation model was fitted to the alluvial CH4 concentration and isotopic data. The Rayleigh model fitted to the data was:
d13CCH4loc1000f½ð1=
a
Þ 1 lncloc=cstartÞg þd13CCH4startð1Þ wherecstartis starting CH4concentration over the interval modelled, clocis the CH4concentration at any location within the alluvium, d13C-CH4startis starting isotopic composition of the interval mod-
elled, andd13C-CH4locis the isotopic composition at any location within the alluvium. This model has previously been applied to other natural settings (Coleman et al., 1981; Grant and Whiticar, 2002). The fractionation factor (
a
) for the oxidising CH4was deter- mined by fitting Eq.(1)to the data using a non-linear equation fit- ting function in Mathematica (Wolfram Research Inc, 2012). Both cstartandd13C-CH4startwere allowed to float, in addition toa
, thusno assumptions were made about the origin of CH4. The best fit val- ues forcstart,d13C-CH4startand
a
were 4062.71 ppm (95%CI: 4062.7, 4062.72), 72.8‰ (95% CI: 79.9,-65.7) and 1.00738 (95% CI, 1.00078 to 1.01397), respectively (Fig. 8).Reported
a
values for natural landscape settings range from 1.003 to 1.049 (Happell et al., 1994; Chanton and Liptay, 2000;Sansone et al., 2001; Chanton et al., 2008). The value from deep alluvial samples determined ford13C-CH4startfits within the ranges reported by ESG for the isotopic composition of CH4in the ground- water of the formations underlying the GAB (including the Digby Formation, the Black Jack Group, and the Maules Creek Formation (Fig. 2). This insight provides an additional constraint and supports the interpretation that the CH4within the alluvium is sourced from the underlying geological formations.
Fig. 7.(a) Plot ofd13C-CH4vsd13C-CO2(afterWhiticar, 1999) with samples from this study as well as those collected in the GAB byESG (2008–2011); (b) plot ofdD-CH4vs d13C-CH4(afterWhiticar, 1999) suggesting microbial production as the primary origin of CH4in the LNA.
Fig. 8.Best fit Rayleigh fractionation model with 95% confidence interval bounds.
6. Conclusion
In an alluvial aquifer CH4in the groundwater can be sourced from the riverine zone,in situproduction, or underlying geological formations. In the LNA we used a combination of hydrochemical measurements, microbiological community analyses, isotopic trac- ers and fractionation modelling to constrain the source of CH4to being primarily from the underlying geological formations.
Radioactive isotopic tracers (3H, 14C, and36Cl) and major ion chemistry measurements at varying depths within the LNA mapped the zones of surface water recharge and basal artesian dis- charge, plus the percentage extent of mixing of these different water sources within the alluvium. The microbial community com- position shows distinct changes with depth, in alignment with the zones of dominant surface water, mixed surface and artesian water, and artesian water, suggesting that the geochemical condi- tions strongly influence the composition of the microbial commu- nity. The microbial communities present in the aquifer have a low abundance of methanogens between 37 and 60 m, indicating that there is limited microbial potential to produce CH4in this interval of the alluvium. Geochemical data, such as the concentration of major ions, redox conditions, and DOC, also suggest that the conditions in the shallow groundwater favour CH4oxidation. The composition of the microbial community and geochemical obser- vations conflict with the observation of elevated concentrations of CH4within the aquifer, and the trend of increasing CH4concen- tration with depth.
Methane concentration data from the LNA indicate that CH4increases with depth and the isotopic signature suggests that it is primarily microbially-derived with varying degrees of 13C- enrichment. The rate of fractionation determined by a best fit Ray- leigh model was assessed to be similar to other aquifers around the world where the oxidation of CH4is the dominant process acting on the CH4 within the aquifer. The inverse relationship between the isotopic signatures of CH4and CO2in both the LNA and under- lying the GAB suggest that the CH4is being oxidised as it migrates upwards through the alluvium.
Using biogeochemical insights, supported by the determined rate of CH4fractionation and the faults, igneous intrusions and per- meable facies providing pathways for upward migration, the dom- inant source of CH4in the alluvial aquifer has been constrained to being primarily from geological formations underlying the LNA.
There is little evidence to support any significantin situCH4pro- duction in the alluvium or riverine zone. The biogeochemical methods and analyses used in this research are suitable for appli- cation in aquifer systems worldwide where there is concern about CH4source attribution.
Declaration of Competing Interest
The authors declare that they have no financial, personal, or otherwise conflict of interest.
Acknowledgements
This research was funded by the Cotton Research and Develop- ment Corporation (CRDC) (grant number UNSW1601). Charlotte Iverach was supported by scholarships from the Australian Government, ANSTO and CRDC. ANSTO support and analytical staff are thanked for their continuous efforts (Chris Dimovski, Henri Wong, Robert Chisari, Vladimir Levchenko, Klaus Wilcken, Barbora Gallaguer, Jennifer van Holst, Krista Simon, Alan Williams, Simon Varley). Access to groundwater monitoring bores was facilitated by the former Department of Primary Industries (NSW) and
WaterNSW. Landholders who provided access to monitoring wells on their land are also thanked.
Appendix A. Supplementary material
Supplementary data to this article can be found online at https://doi.org/10.1016/j.scitotenv.2019.134927.
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