Scholarly article on topic 'Groundwater methane in a potential coal seam gas extraction region'

Groundwater methane in a potential coal seam gas extraction region Academic research paper on "Earth and related environmental sciences"

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Abstract of research paper on Earth and related environmental sciences, author of scientific article — Marnie L. Atkins, Isaac R. Santos, Damien T. Maher

Abstract Study region This study investigates dissolved methane distribution in groundwater from the Richmond River Catchment (New South Wales, Australia) before proposed coal seam gas (CSG, or coal bed methane) development. Study focus Unconventional gas exploration has rapidly expanded in recent years. However, the impact of these operations on groundwater systems is poorly understood. A total of 91 groundwater samples were analyzed from 6 geological units. Our observations act as regional baseline research prior to CSG extraction and may assist with long term impact assessment. New hydrological insights for the region Methane was found in all geological units ranging between 0.26 and 4427μgL−1 (median 10.68μgL−1). Median methane concentrations were highest in chloride-type groundwater (13.26μgL−1, n =58) while bicarbonate-type groundwater had lower concentrations (3.71μgL−1). Groundwater from alluvial sediments had significantly higher median methane concentrations (91.46μgL−1) than groundwater from both the basalt aquifers (0.7μgL−1) and bedrock aquifers (4.63μgL−1); indicating geology was a major driver of methane distribution. Methane carbon stable isotope ratios ranged from –90.9‰ to –29.5‰, suggesting a biogenic origin with some methane oxidation. No significant correlations were observed between methane concentrations and redox indicators (nitrate, manganese, iron and sulphate) except between iron and methane in the Lismore Basalt (r 2 =0.66, p <0.001), implying redox conditions were not the main predictor of methane distribution.

Academic research paper on topic "Groundwater methane in a potential coal seam gas extraction region"

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Journal of Hydrology: Regional Studies

journal homepage www.elsevier.com/locate/ejrh

Groundwater methane in a potential coal seam gas extraction region

Marnie L. Atkinsa b *, Isaac R. Santosa b, Damien T. Mahera

a School of Environmental Science and Management, Southern Cross University, Lismore, New South Wales 2480, Australia b National Marine Science Centre, School ofEnvironment, Science and Engineering, Southern Cross University, Lismore, New South Wales 2480, Australia

ARTICLE INFO ABSTRACT

Study region: This study investigates dissolved methane distribution in groundwater from the Richmond River Catchment (New South Wales, Australia) before proposed coal seam gas (CSG, or coal bed methane) development.

Study focus: Unconventional gas exploration has rapidly expanded in recent years. However, the impact of these operations on groundwater systems is poorly understood. A total of 91 groundwater samples were analyzed from 6 geological units. Our observations act as regional baseline research prior to CSG extraction and may assist with long term impact assessment.

New hydrological insights for the region: Methane was found in all geological units ranging between 0.26 and 4427 ^gL-1 (median 10.68 ^gL-1). Median methane concentrations were highest in chloride-type groundwater (13.26 ^gL-1, n = 58) while bicarbonate-type groundwater had lower concentrations (3.71 ^g L-1). Groundwater from alluvial sediments had significantly higher median methane concentrations (91.46 ^gL-1) than groundwater from both the basalt aquifers (0.7 ^gL-1) and bedrock aquifers (4.63 ^gL-1); indicating geology was a major driver of methane distribution. Methane carbon stable isotope ratios ranged from -90.9%o to -29.5%», suggesting a biogenic origin with some methane oxidation. No significant correlations were observed between methane concentrations and redox indicators (nitrate, manganese, iron and sulphate) except between iron and methane in the Lismore Basalt (r2 = 0.66, p < 0.001), implying redox conditions were not the main predictor of methane distribution.

© 2015 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

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Article history:

Received 12 February 2015

Received in revised form 26 May 2015

Accepted 28 June 2015

Available online 24 August 2015

Keywords: Stable isotopes Aquifer

Baseline research Geology Catchment Hydrochemistry Coal bed methane Unconventional gas

1. Introduction

Exploitation of unconventional gas resources (coal seam gas, shale gas and tight sands gas) have significantly expanded in recent decades due to advanced extraction processes such as hydraulic fracturing, horizontal drilling and aquifer depressur-ization (Hamilton et al., 2014; Kargbo et al., 2010; Kerr, 2010; Kinnon et al., 2010; Renet al., 2014). Coal seam gas (CSG), also known as coal bed methane (CBM), is composed primarily of methane (CH4) which is trapped under pressure within coal seam pores and fractures. CSG represents a substantial natural gas resource and the Australian CSG industry has experienced

* Corresponding author at: School of Environmental Science and Management, Southern Cross University, Military Rd., Lismore, New South Wales 2480, Australia.

E-mail address: m.atkins.30@student.scu.edu.au (M.L. Atkins). http://dx.doi.org/10.1016/j.ejrh.2015.06.022

2214-5818/© 2015 The Authors. Published by Elsevier B.V. This is an open access article underthe CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

rapid growth within the last decade. However, there may be significant environmental issues associated with CSG extraction processes (Varade and Meshram, 2010).

Unconventional gas extraction methods require numerous wells over large areas. Faulty or inadequate well casings can lead to stray gas migration into overlying aquifers, and regional groundwater resources may become contaminated (Jackson et al., 2013a; Osborn et al., 2011 ). Aquifer depressurization of CSG target formations may lead to greater gas transfer into the overlying or underlying formations, and nearby surface water zones (Apte et al., 2014). Enhanced aquifer connectivity may possibly dewater aquifers surrounding the CSG target formation and/or deliver constituents within the coal seam water via groundwater transport into adjacent waterways.

Research to date on Australian unconventional gas development has mainly focused on CH4 isotopic composition of CSG (Hamilton et al., 2014; Kinnon et al., 2010), groundwater quality within the coal seams (Kinnon et al., 2010; Owen et al., 2015; Papendick et al., 2011), CSG content in targeted geological formations (Faiz et al., 2007; Hamilton et al., 2012; Scott et al., 2007; Thomson et al., 2014) and changes in atmospheric chemistry associated with CSG development (Maher et al., 2014; Tait et al., 2013). As yet in Australia, the literature lacks baseline studies with a focus on groundwater chemistry within the overlying shallow aquifers that are often used as a regional water source. Lack of sufficient baseline groundwater information renders it difficult to adequately assess the impact of CSG extraction processes within Australia (Tait et al., 2013).

Methane has a global warming potential 72 times greater than carbon dioxide (CO2) over a 20 year period, making it a potent greenhouse gas (Solomon et al., 2007). Methane is produced through organic matter decomposition and can be either biogenic (microbially derived) or thermogenic (thermally derived) in origin (Barker and Fritz, 1981). Biogenic CH4 production processes usually occur at shallow depths and utilize predominantly two metabolic pathways: acetate fermentation and CO2 reduction (Conrad, 1989; Oremland et al., 1988; Schoell, 1988; Whiticar, 1999). Thermogenic CH4 results from diagenesis at greater depth, where increased temperature and pressure provide an optimal environment for subsurface thermal organic matter decomposition (Barker and Fritz, 1981). Thermogenic CH4 generation is unlikely in groundwater systems less than 400 m deep but thermogenic CH4 can be found in shallow aquifers due to upward CH4 migration (Coleman et al., 1977).

Biogenic and thermogenic CH4 result in different carbon isotopic signatures (<513C—CH4) which can be analyzed in combination with geochemical and hydrogeological information in order to assess CH4 origin (Chung et al., 1988; Schoell, 1980, 1988). Biogenic CH4 can have <513C values ranging from -110%o to -40%o while thermogenic CH4 carbon stable isotope values range between -50% and -20% (Schoell, 1980; Whiticar, 1999). For biogenic CH4, isotopically lighter carbon is utilized more readily by methanogens, resulting in 13C depletion in the produced CH4 relative to the substrate (Whiticar, 1999; Whiticar et al., 1986). <513C—CH4 can also be utilized to differentiate between biogenic pathways of CO2 reduction (-110% to -55%) and acetate fermentation (-70% and -40%) (Rice, 1993; Whiticar, 1999; Whiticar et al., 1986). Transitional isotope compositions lie between the two biogenic CH4 fields causing an overlap attributed to CH4 migration, CH4 oxidation and shifts in the isotopic composition of the original organic material (Boreham et al., 1998; Faiz and Hendry, 2006; Whiticar, 1999). Methane oxidation is an important microbial process where microbes oxidize CH4 to CO2, during which 12C is preferentially oxidized which leaves residual CH4 enriched in 13C (Alperin et al., 1988; Coleman et al., 1981; Whiticar and Faber, 1986).

Groundwater may transport CH4 through geological units into adjacent surface waters (Bugna et al., 1996). CH4 concentrations and <513C—CH4 can vary due to factors such as geological and hydrochemical characteristics, organic matter concentration and redox parameters (Aravena et al., 1995; Darling and Gooddy, 2006; Hansen et al., 2001; Jakobsen, 2007). Assessing <513C—CH4 may provide information on groundwater and surface water CH4 origins, and can be utilized to assess connectivity between deeper underlying coal seams and overlying shallow aquifers (DNRM, 2012). Investigating groundwa-ter CH4 dynamics and hydrochemistry are an important component for baseline research as a tool to monitor aquifers and detect potential long term changes brought about by CSG extraction. Cheung et al. (2010) found distinct differences between shallow groundwater and coal bed methane produced fluids in Alberta, Western Canada. Shallow groundwater contained lower total dissolved solids, higher sulphate concentrations and different hydrochemical characteristics in comparison to groundwater in coal bed methane aquifers. Sharma and Baggett (2011) utilized carbon stable isotopes of dissolved inorganic carbon (<513C-DIC) to trace coal bed produced water infiltration from impoundments into shallow groundwater. In both cases, the investigation was performed after gas production commenced which complicates the interpretation of observations.

A recent study by McPhillips et al. (2014), established baseline dissolved CH4 distribution patterns in New York State, USA, a potential future shale gas development region. Numerous studies have reiterated the importance of conducting baseline research before gas extraction commences (Jackson et al., 2013b; Sharma et al., 2014; Vidic et al., 2013). Concerns over the potential impacts of horizontal drilling and hydraulic fracturing extraction methods have instigated shallow ground-water and produced water analysis in shale gas and coalbed methane development zones (Chapman et al., 2012; Sharma and Baggett, 2011; Sharma and Frost, 2008; Vengosh et al., 2013; Warner et al., 2012; Warner et al., 2013). However, globally, the literature lacks baseline studies prior to unconventional gas extraction and many studies are conducted after the development of gas fields. For example, investigations in the Marcellus Shale region, Pennsylvania, concluded stray gas (predominantly CH4) contamination of drinking water resulted from shale gas extraction, likely from leaky well casings (Osborn et al., 2011). However, a later study suggested the presence of CH4 in that region is likely related to surface topography and hydrogeological characteristics rather than shale gas extraction technologies (Molofsky et al., 2013). A follow up study demonstrated that surface topography and hydrogeology were not responsible for stray gas contamination, rather distances <1 km between private drinking water wells and gas wells were significantly related (Jackson et al., 2013a). In Australia, a recent review into bubbling methane gas in the Condamine River, Queensland (a highly productive CSG region), concluded

the biogenic gas is from geologic sources, possibly from shallow coal seams (Apte et al., 2014). However, there is insufficient information to determine whether the gas migration pathways are natural or related to CSG activities, such as aquifer depressurization. These cases reflect the importance of baseline research prior to unconventional gas extraction.

The objective of this study is to perform a geochemical assessment of groundwater, focusing on CH4 concentration and carbon stable isotope ratios in a catchment characterized by alluvial sediments, basalts and sedimentary bedrock aquifers in an Australian setting that is at the pre-production stage of CSG development (i.e., abandoned exploration wells only; no current production). Major ions may also be utilized as a tool to assess possible long term CSG impacts since groundwater present in coal seams often has a unique chemical composition compared with shallower groundwater (Cheung et al., 2010; Van Voast, 2003). We hypothesize that if aquifer connectivity is altered in CSG extraction regions, sparingly soluble gases such as CH4 will move more rapidly between geological units than other solutes and therefore may act as an "early warning" signal of any potential long term groundwater impacts. While we cannot currently test this hypothesis, the information obtained can be utilized when CSG extraction occurs in the study site and will allow regional-scale comparisons of groundwater chemistry 'before and after' CSG drilling.

2. Materials and methods

2.1. Study site

The Richmond River Catchment (RRC) Fig. 1) is located in the far north coast of New South Wales, Australia, and has a catchment area of almost 7000 km2. An area of 800 km2 is protected national parks and reserves, existing mainly in the northern Border Ranges where elevations reach over 1000 m (DPI, 2012). The Alstonville Plateau, north-east of Lismore, is shaped by basaltic flows and is mostly forested. The Richmond River begins in the far north catchment and stretches for 170 km past several townships, meeting the Pacific Ocean at Ballina. The Wilsons River is the main tributary to the Richmond River, entering at Coraki and contributing around 60% flow to the lower Richmond River (DPI, 2012). The region experiences a mild sub-tropical climate and high rainfall with an annual median precipitation greater than 1000 mm in most areas. The highest rainfall is recorded along the coastal fringe during December-April when over 60% of annual rainfall is received (Atkins et al., 2013 and references therein). The southern part is the driest portion of the catchment and the entire region is known to experience long periods of drought. The RRC hosts considerable CSG reserves within the Walloon Coal Measures and is currently targeted as a potential CSG extraction region. Approximately 50 exploration wells with depths from ~620 m to 1520 m have been drilled in this catchment (Fig. 1) with no associated infrastructure, and while operations were suspended by the gas company in March 2013, there are plans for future CSG extraction.

The catchment displays complex geological sequences of which our knowledge is still evolving. The RRC overlies the wider Clarence-Moreton Basin (C-MB) which consists of hard rock sediments, volcanic deposits and alluvial sediments, totaling in excess of 3000 m depth. The basin unconformably overlies lower Palaeozoic sediments which have been subject to extensive deformation and folding while Mesozoic consolidated sediments within the basin are overlain by Cenozoic volcanics and Quaternary alluvial sediments (McElroy, 1962). This work focuses mainly on the shallow aquifer system made of Quaternary alluvial sediments, Cenozoic basalts and late Jurassic/early Cretaceous bedrock in the northern RRC, excluding most of the southern Bungawalbin subcatchment (Fig. 1). Local stratigraphy is derived from proposed formations by Doig and Stanmore (2012) which builds on previous work from Wells and O'Brien (1994). Previous work on groundwater systems in the RRC include Drury (1982) who conducted seminal research on the alluvial sediments, while Brodie and Green (2002) and Budd et al. (2000) reported on the Alstonville Plateau fractured basalt aquifers (Fig. 1). Shallow groundwater resources are important in this region for domestic use, stock and agricultural activities, especially during times of drought.

The Quaternary Sediments represent unconfined to semi-unconfined groundwater systems typically characterized by good water quality. These alluvial units (South Casino Gravel, Fairy Hill Member, Greenridge Formation, Woodburn Sand and Gundurimba Clay) (Fig. 1) are poorly consolidated fluvial deposits, existing beneath a shallow layer of floodplain and sand dune sediments (Drury, 1982). Deep bedrock channels consist of gravel, sand, silt, clay and minor woody fragments with detrital quartz, volcanic sediments and coal fragments from underlying basalt and bedrock units. Woodburn Sand is a coastal alluvial sediment consisting of quartzose sand, heavy mineral deposits and feldspars while the Gundurimba Clay is a thick estuarine deposit containing an abundance of macroshells, corals and foraminifera (Drury, 1982).

The Cenozoic basalt aquifers are characterized by shallow unconfined to deeper semi-confined systems. The basaltic composition varies throughout the catchment due to hot spot volcanism resulting in fine grained to more porphyritic textured rocks (Duggan and Mason, 1978). The Cenozoic basalts can be divided into 2 distinct groups; the Kyogle Basalt and the Lismore Basalt (Fig. 1). The Kyogle Basalt (22.5 million years old) resulted from Focal Peak Volcano eruptions and is composed of hawaiite which consists of alkaline olivine but no silica or quartz (Duggan and Mason, 1978). The Lismore Basalt is associated with the Tweed Shield Volcano and dated between 22.6 and 22.9 million years ago (Ewart et al., 1987). The basaltic composition is theolitic and consists of quartz but no olivine. Irregular flow sequences have resulted in variable basaltic thickness of 50-150 m (possibly thicker in some areas) and include a major topographical feature, the Alstonville Plateau (Drury, 1982).

The Grafton Formation and the Orara Formation represent the youngest sedimentary rocks in the RRC, acting as confined aquifer systems but with limited hydraulic communication beyond outcrop zones and poor aquifer properties, such as low permeability, greater than 150 m depth (Doig and Stanmore, 2012). The Grafton Formation is made up of a thick sandstone

Fig. 1. Map ofthe Richmond River Catchment in northern NSW, showing locations ofgroundwater sample sites and the geological units where groundwater was sampled. The Kyogle Basalt is located north west of the catchment and the Lismore Basalt is located east of the catchment. The main townships are Kyogle, Casino, Lismore, Evans Head and Ballina. Stratigraphic units from which groundwater samples were taken are in bold. The Rappville Member unit and Bungawalbin Member unit are not displayed.

and claystone unit (the Piora Member) with minor carbonaceous material, including coal clasts in some unit boundaries, and conformably underlies the Rappville Member aquitard (Doig and Stanmore, 2012) (Fig. 1). The Orara Formation is divided into the Kangaroo Creek Sandstone Member (aquifer) and the Bungawalbin Member (aquitard) (Doig and Stanmore, 2012). The Kangaroo Creek Sandstone Member (Kangaroo Creek from herein) is a fluvial channel deposit containing quartzose sandstone in calcareous or argillaceous cement (McElroy, 1962). Porosity and permeability are moderately low with minor thin carbonaceous mudstone and siltstone layers existing in the sandstone unit (Doig and Stanmore, 2012). High iron content and garnet rich sandstone have been detected in some sections (Drury, 1982), and the unit thickness is thought to be ~200 m (Doig and Stanmore, 2012). Recently, the Kangaroo Creek formation has become a target for conventional gas exploration.

The Walloon Coal Measures, the CSG target layer, was deposited by low energy alluvial plain sedimentation associated with peat forming wetlands and consists of volcaniclastic detritus and sandstone, interbedded with siltstone, mudstone and claystone (Doig and Stanmore, 2012). The sequence outcrops toward the edges of the catchment and basaltic dykes, sills and plugs are common intrusions (Drury, 1982). Numerous thin coal seams, fossilized wood fragments and carbonaceous shale exists in this unit while low permeability means it can act as an aquitard (Doig and Stanmore, 2012). The Orara Formation conformably overlies the Walloon Coal Measures.

2.2. Experimental strategy

A total of 91 groundwater samples were collected across the catchment between May and September 2013 from a combination of private bores (51) and governmental monitoring bores (40). Private bores mainly sourced groundwater from bedrock and basalts while monitoring bores mainly sourced groundwater from alluvial sediments. The combination allowed sampling with spatial variability within, and between, geological units. There were no groundwater bores sampled from the CSG target formation. Bores were purged at least 3 casing volumes prior to sampling and once temperature, conductivity and pH measurements stabilized, a representative sample was taken (Sundaram et al., 2009). Groundwater bore depths ranged from 5 m to 120 m. Detailed bore log information was obtained from the NSW Office of Water Bore Registry to determine the geological units from where groundwater was extracted. One sample had no depth record and the water bearing aquifer was deciphered using regional geological maps (Brown et al., 2007; Henley et al., 2001).

Samples were collected for CH4, <513C—CH4, CO2, <513C—CO2, dissolved inorganic carbon (DIC), dissolved inorganic carbon stable isotopes ratios (§13C-DIC), dissolved organic carbon (DOC), ions (Na+, Ca2+, Mg2+, K+, Cl-, HCO3-, NO3-, total dissolved Mn, total dissolved Fe and SO42-), pH, specific conductivity, dissolved oxygen (DO) and temperature. Duplicate samples for each parameter were collected every 20 sites to ensure sampling technique consistency. CH4 and CO2 samples were collected in triplicate in 215 mL pre-rinsed gas tight bottles, treated with 200 |L of saturated HgCl2 solution and sealed with a cap and septa, leaving no headspace. A calibrated handheld YSI-85 was used to measure pH, DO, temperature, salinity and specific conductivity. The National Bureau of Standards (NBS) scale was used to perform pH calibrations with 4 and 7 standards. To cover the range of observed groundwater conductivities, calibrations were performed with deionized water and a 1413 |S cm-1 standard. Samples for DIC, 113C-DIC and DOC were collected with a sample rinsed 60 mL polypropylene syringe and filtered through 0.7 mm Whatman GF/F filters, leaving no headspace or bubbles, into 40 mL VOC borosilicate vials (acid rinsed and precombusted at 450 °C for 4h) containing 100 |L of saturated HgCl2 solution for preservation. Major ions were collected through a 0.45 |im syringe filter. All samples were kept on ice until stored in laboratory refrigerators or freezers as appropriate.

2.3. Analytical methods

For CH4 and CO2 samples, a 50 mL zero air headspace (free of CH4 and CO2) was introduced into the bottle and samples were left for overnight equilibration. A 40 mL equilibrated gas sample was extracted and injected into a Tedlar gas bag with 400 mL of CO2- and CH4-free air. The sample was analysed by a cavity ring down spectrometer (Picarro G2201-i) which measured CH4 and CO2 concentrations and carbon stable isotope ratios. CH4 and CO2 concentrations were determined using Henry's law and the carbon stable isotope ratios were recorded as per mil (%o), relative to V-PDB (Gatland et al., 2014). Average analytical uncertainty between triplicate samples was better than 7.21%. DOC samples were treated in the laboratory with 250 |L 85% phosphoric acid to remove DIC prior to analysis. DIC, 113C-DIC and DOC samples were analysed using an OI Scientific Aurora 1030 TOC analyzer coupled to a Thermo Fisher Delta V isotope ratio mass spectrometer (IRMS) with analytical uncertainties better than 5% (Maher and Eyre, 2011). Nitrate samples were analyzed using a Quick Chem 800 Flow Injection Analyzer (FIA) (Lachat, 1994). Major ion samples were pre-treated with 200 |L pure nitric acid and analysed using a PerkinElmer NexION 300D ICPMS. Major ion and nitrate analytical uncertainties were approximately 6%. HCO3-concentrations were calculated using pH and DIC concentrations on the excel macro CO2 system (CO2 SYS), along with the NBS pH scale and the Dickson and Millero (1987) carbonate system constants. AqQA software (Rockware Inc.) was used to generate Piper diagrams and to determine hydrochemical classification. STATISTICA was used to analyze groundwater data. Initially, a correlation matrix determined relationships between water parameters and dissolved methane concentrations. Non-parametric tests were used because dissolved CH4 concentrations were not normally distributed and log transformation did not result in a normal distribution. The Kruskal-Wallis ANOVA was used for grouped variables while the Mann Whitney U-test was used for two independent variables.

3. Results and discussion

Chemical composition and isotopic characteristics of 91 groundwater samples from the RRC are summarized as a function of the six geological formations (Table 1). Groundwater displayed wide variation in CH4 concentration, <513C—CH4, water types, specific conductivity and dissolved Fe (Fig. 2). Here we explore linkages between hydrochemistry, geological formations and redox chemistry indicators with dissolved CH4 concentrations and stable isotopes.

The average CH4 concentration across the catchment was 217.95 ±68.78 |igL-1 (Table 1; uncertainties refer to standard error throughout), spanning 4 orders of magnitude from 0.26 to 4427 |igL-1 (with 9 samples below the detection limit of 0.26 |igL-1) (Fig. 2c). Besides five samples with CH4 concentration exceeding 1000 |igL-1 (WS7 = 1242 |igL-1, WS1 =1881 |gL-1, KC7 = 2702 |gL-1, QS16 = 2891 |gL-1 and QS20 = 4427 |gL-1) (Fig. 2a), all other samples were lower than 804 |gL-1. The CH4 concentration data set was not normally distributed (Shapiro Wilk's W-test, p = 0.005), therefore the median (10.68 |g L-1) was considered a more robust indicator of CH4 central tendency concentrations within the catchment. While there is little published data on CH4 from shallow Australian aquifers, the RRC groundwater CH4 concentrations are within the low range of other shallow groundwater environments. In central Japan, groundwater aquifers up to 180 m deep displayed average CH4 concentrations of 1350 |igL-1 (Watanabe et al., 2008). In UK groundwater, lower CH4 concentrations ranged between <0.05 and 465 |igL-1, averaging 13.64 |igL-1 (Gooddy and Darling, 2005). In Florida (USA) coastal groundwater CH4 ranged from 9.63 to 256 |g L-1 (Santos et al., 2009). Coal seam gas groundwater from inner basins usually contains much higher CH4 levels as in the Elk Valley coalfield groundwater, Canada, which contained 3900-46,500 |igL-1 of CH4 (Aravena et al., 2003).

3.1. Groundwater hydrochemistry and methane production

Identifying groundwater types and possible relationships to dissolved CH4 concentration is an important component of baseline assessments in shallow aquifer systems. Six groundwater types were identified for the RRC; Na-Cl, Ca-Cl, Mg-Cl, Na-HCO3, Ca-HCO3 and Mg-HCO3 (Figs. 3 and 4a). SO42- levels were generally low with SO42- making up less than 20% of the anion charge, except for 7 Na-Cl groundwater samples. Hydrochemistry within the catchment is complex and did not necessarily reflect specific geological characteristics (Fig. 3). Although the Woodburn Sand contained only Na-Cl groundwater, all other geological formations showed a hydrochemical range. The Kyogle Basalt in the upper catchment contained solely HCO3-type groundwater but presented a range of predominant cations (Ca2+, Mg2+ and Na+). The Quaternary Sediments contained varying groundwater chemistry due to spatial variability and multiple recharge zones (Drury, 1982). In the upper catchment, the Quaternary Sediments are substantially recharged through surface water and basalt of HCO3-type groundwater while in the lower catchment, recharge through Cl-dominated bedrock results in predominantly Na-Cl groundwater (Fig. 2a and b). The Quaternary Sediments exhibited low SO42- groundwater, which was one of the only consistent features. Besides one sample (QS15), SO42- made up less than 15% of dissolved anions.

The Lismore Basalt aquifer in the Alstonville Plateau contained a range of Na-Cl and HCO3-type groundwater which may be related to precipitation chemistry, water-rock interactions and residence time (Drury, 1982). Na-Cl groundwater was shallower with depths averaging 32.3 ± 6.7 m (n = 8) while the HCO3 groundwater type was much deeper, averaging 68 ±12.1 m (n = 6). Na-Cl groundwater in the highly vesicular and fractured basalt may result from response to precipitation chemistry at shallow depths (Brodie and Green, 2002). At greater depth, carbonate dissolution of weathered horizons results in more alkaline HCO3-type groundwater, more common at the base ofthe plateau (Brodie and Green, 2002; Budd et al., 2000). Lismore Basalt HCO3-type groundwater in the Alstonville Plateau may be related to the much older proposed Alstonville Basalt (Cotter, 1998).

The five groundwater samples exceeding 1000 |g CH4 L-1 were Na-Cl (2702 |igL-1, 1881 |igL-1 and 1242 |igL-1), Na-HCO3 (4427 |g L-1) and Ca-HCO3 (2891 |g L-1) (Appendix ATable A1). All HCO3-type groundwater contained some CH4 with the highest concentrations measured in Ca-HCO3 groundwater (median 49.39 |ig L-1, n = 8) (Fig. 4a), although removal ofthe high sample (2891 |gL-1) lowered the median to 0.68 |gL-1. Na-HCO3 (4.77 |gL-1 CH4, n = 17) and Mg-HCO3 (0.73 |g L-1, n = 8) were comparatively lower. Most HCO3-type samples were located in the upper catchment, influenced by the Kyogle Basalt lithology while some samples were located on the Alstonville Plateau, influenced by carbonate dissolution in the Lismore Basalt (Budd et al., 2000) (Fig. 2b). Cl-type groundwater was widespread throughout the catchment with a total of 58 samples. Ca-Cl (n = 2) and Mg-Cl (n = 3) groundwater were mainly found in the north-west catchment region with median CH4 concentrations of 0.25 |g L-1 and 0.3 |g L-1, respectively (Fig. 4a). Na-Cl groundwater (n = 53) was mainly located within the coastal sand region, the Alstonville Plateau and the Casino region resulting from sea spray deposition, precipitation chemistry and estuarine clay deposits (Gundurimba Clay) (Drury, 1982), and contained median CH4 concentrations of 16.25 |gL-1 (Fig. 4a). Na-Cl groundwater types often dominate groundwater in marine deposits (Cheung et al., 2010).

There was no significant difference in CH4 concentrations between the six groundwater types (Kruskal-Wallis ANOVA, p = 0.06) (Table 2). Although, Cl-type groundwater had higher median CH4 concentrations (13.26 |gL-1, n = 58) than HCO3-type groundwater (3.71 |g L-1, n = 33) (Table 1), there was no statistical differences in CH4 concentration between these two groundwater type groups (Mann Whitney U-test, p = 0.52) (Table 2). In a baseline groundwater chemistry study by McPhillips et al. (2014), a statistical difference existed between CH4 concentrations in Ca-HCO3 (lower) and other groundwater types (Na-Cl, Na-HCO3-Cl, and Na-HCO3). A correlation matrix (Appendix A Table A2) found no significant relationships between

Table 1

Water quality data for 91 groundwater samples in each geological unit in the Richmond River Catchment including general water parameters, carbon stable isotopes and water types. Average concentrations with standard errors were reported for general water parameters and a range was reported for stable isotopes. The median was also reported for methane concentration due to the skewed distribution. l13C—CH4 was reported for CH4 samples >3.9 |xg L-1. The full data set for individual samples can be found in the supplementary material in Table A1.

Geological unit Number of bores Depth (m) Sp cond (mScm-1) pH DO (mgL-1) CH4 (^gL-1) CH4 (median) (^gL-1) DOC (mM) 813C—CH4 (%„) 813C—CO2 (%„) 813C-D1C (%„) Water types

Lismore Basalt 25 8 to 120 0.37 ±0.06 6.31± 0.2 3.19 ±0.42 9.88 ±5.14 0.69 0.08 ± 0.01 n = 6 -35.08 to -60.45 -15.55 to -29.66 -9.74 to -22.90 Na-Cl, Mg-Cl, Ca-Cl, Na-HCOa, Mg-HCOa, Ca-HCOa

Kyogle Basalt 13 15 to 82 0.91 ±0.1 7.24 ±0.15 2.47 ± 0.59 19.26 ±12.03 1.04 0.18 ± 0.02 n=3 -50.11 to -88.45 -17.87 to -26.41 -9.49 to -17.46 Na-HCOs, Mg-HCO3, Ca-HCO3

Piora Member 10 13 to 52.1 2.21 ± 0.59 6.36 ±0.21 1.54 ±0.6 48.51 ±31.86 7.15 0.19 ± 0.03 n=5 -36.77 to -60.53 -16.05 to -25.34 -10.95 to -18.86 Na-Cl, Mg-Cl, Ca-HCO3

Kangaroo Creek 8 21 to 90 4.13 ± 3.15 6.22 ±0.17 0.63 ±0.14 388.66 ±332.13 4.63 0.19 ± 0.05 n=5 -50.71 to -75.78 -13.97 to -24.82 -6.67 to -19.55 Na-Cl, Ca-Cl

Quaternary Sediments 22 14 to 46 1.94 ±0.35 6.86 ±0.08 0.52 ±0.1 482.90 ± 229.79 56.04 0.11 ± 0.03 n = 20 -29.46 to -87.79 -17.47 to -27.12 -5.34 to -18.28 Na-Cl, Mg-Cl, Na-HCO3, Mg-HCO3, Ca-HCO3

Woodburn Sand 13 5 to 35 2.34 ±1.59 5.40 ±0.13 0.40 ±0.06 393.69 ± 159.67 105.84 0.28 ± 0.09 n =13 -32.50 to -90.88 -15.30 to -26.49 -11.56 to -23.57 Na-Cl

Total for the Richmond River Catchment 91 5 to 120 1.64 ±0.38 6.44 ±0.09 1.70 ±0.2 217.95 ±68.78 10.68 0.15 ±0.02 -29.46 to -90.88 -13.97 to -29.66 -5.34 to -23.57 Na-Cl, Mg-Cl, Ca-Cl, Na-HCO3, Mg-HCO3, Ca-HCO3

Fig. 2. Maps of (a) geological units, (b) water types, (c) methane, (d) l13C—CH4, (e) specific conductivity and (f) dissolved iron within the Richmond River Catchment. Sites mentioned throughout the text are visible on map (a) High CH4 concentrations were at KC7, QS16, QS20, WS1 and WS7. Redox processes were discussed for WS9, WS10 and WS2. There was high SO42- at QS15 and coal fragments in bores at sites QS1 and QS11. Location sites are not to scale but for visual representation only.

Ca --- Cl

Fig. 3. Piper plot illustrating the hydrochemical composition of shallow groundwater in the Richmond River Catchment. Two general groupings are evident, HCO3-type groundwater and Cl-type groundwater.

Table 2

Statistical analysis comparing methane concentrations to both hydrochemistry and geology in the Richmond River Catchment.

Groups Tested Type of Statistical Test p - value Statistical Test Result

Methane concentration (^g L-1) Shapiro Wilk's Wtest <0.01 Skewed distribution

Logarithmic methane concentration (|xg L-1) Shapiro Wilk's Wtest 0.004 Skewed distribution

Individual watertypes: Na-Cl, Ca-Cl, Mg-Cl, Kruskal-Wallis ANOVA 0.06 No difference

Na-НСОз, Ca-HCO3 and Mg-HCOa

Grouped watertypes: Cl and HCO3 Mann Whitney U test 0.52 No difference

Individual geological units: Lismore Basalt, Kyogle Kruskal-Wallis ANOVA <0.01 Statistical difference

Basalt, Piora Member, Kangaroo Creek, Quaternary

Sediments and Woodburn Sand

Lismore Basalt vs Quaternary Sediments Mann Whitney U test <0.01 Statistical difference

Lismore Basalt vs Woodburn Sand Mann Whitney U test <0.01 Statistical difference

Kyogle Basalt vs Quaternary Sediments Mann Whitney U test <0.01 Statistical difference

Kyogle Basalt vs Woodburn Sand Mann Whitney U test <0.01 Statistical difference

Grouped geological units: basalt, bedrock and Kruskal-Wallis ANOVA 0.02 Statistical difference

alluvial sediments

Basalt vs bedrock Mann Whitney U test 0.21 No difference

Bedrock vs alluvial sediments Mann Whitney U test 0.01 Statistical difference

Basalt vs alluvial sediments Mann Whitney U test <0.01 Statistical difference

CH4 andHCO3-, Cl-,Na+, Mg2+, Ca2+, K+, SO42- or SAR (sodium adsorption ratio), implying hydrochemistry is a poor indicator of dissolved CH4 distribution in the catchment. While cluster analysis, principal component analysis and factor analysis are useful to explain hydrochemical evolution of groundwater as in a study in the Eromanga and Galilee basins, QLD (Moya et al., 2015), these multi-variate statistical methods did not explain CH4 variation in the RRC and are not reported here.

In the United States, coal bed methane production waters are characterized by enriched Na+, HCO3- and sometimes Cl-, and depleted SO42-, Ca2+ and Mg2+ (Van Voast, 2003). The same trend occurred in CSG production waters in the

Na-Cl Ca-Cl Mg-Cl Na-HC03 Ca-HC03 Mg-HC03 Groundwater Types

LB KB PM KC QS WS Geological Units

Fig. 4. Bar chart showing median dissolved methane concentrations within (a) groundwater types and (b) geological units. The number of samples is shown above the bar. Exclusion of one high sample in Ca-HCO3 (2891 |ig L-1), lowered the median methane concentration to 0.68 |ig L-1. LB = Lismore Basalt, KB = Kyogle Basalt, PM = Piora Member, KC = Kangaroo Creek, QS = Quaternary Sediment and WS=Woodburn Sand.

Bowen Basin, Queensland, Australia (Kinnon et al., 2010). While in the Surat/Clarence-Moreton basins of eastern Queensland, Australia, variable CSG groundwater hydrochemistry was reported where some samples had dominating Cl- anions and low HCO3- (Owen et al., 2015). Although, some studies have focused on the WCM hydrochemistry in the C-MB in Queensland (Owen and Cox, 2015; Owen et al., 2015), there is little published data on the RRC WCM hydrochemistry in New South Wales. Samples from the WCM south west of Beaudesert in Queensland showed a variable range for specific conductivity of3.13-9.17mScm-1 (ArrowEnergy, 2012). Specific conductivity in the RRC shallow groundwater averaged 1.64 ± 0.38 mS cm-1 (Fig. 2e and Table 1); therefore, if the RRC WCM have similar conductivity ranges, it may be possible to use conductivity as a simple CSG groundwater tracer in parts of the catchment. There was no significant correlation between specific conductivity and CH4 in the catchment (Fig. 5a).

3.2. Geological drivers of methane

The broad range of CH4 concentrations across the RRC are an indication of groundwater geochemistry complexity within the catchment, and highlight the importance of baseline hydrogeochemical investigations before the development of CSG fields (Fig. 2c). In general, groundwater within the alluvial sediments contained the highest median CH4 concentrations (Quaternary Sediments; 56.04 |igL-1, Woodburn Sand; 105.84 |igL-1), bedrock formations displayed lower CH4 concentrations (Piora Member; 7.15 |gL-1, Kangaroo Creek; 4.63 |gL-1) and basalt formations contained the lowest CH4 concentrations (Lismore Basalt; 0.69 |gL-1, Kyogle Basalt; 1.04 |gL-1) (Fig. 4b). CH4 concentrations between individual geological units

o * * A ♦ •

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'°o * â °♦

o 0 • °

0.1 I 10

Sp Cond (mS cm'1)

«Xb) O l.iunofc Baxall

A Kvoglc Basalt

□ Piora Member

+ kangaroo Creel

y Qualcrnufv Sedimenlt

§> ♦ ■ff Woodburn Said

Jo' o

□ o A*

o « O

c 0 -o o

DO (mg L1)

— 100

60 90 Depth (m)

-80 -60 -40

S"C-CH< (%o)

Ä 100 -I

ÔO 0 D

0.4 0.6 0.8 DOC(mM)

^ 100 J

0 V ♦ * v

A * v \ Ao * 4

V o ^ V * V

• O # *

o^ °° 9A

ê® t\ o 0 n

9P O □

0.1 1 Fc(mg L'1)

Fig. 5. Scatter plots illustrating correlations between methane and (a) specific conductivity, (b) dissolved oxygen, (c) depth, (d) ¿13C—CH4, (e) dissolved organic carbon and (f) dissolved iron in varying geological units. Notice the log scale for CH4, specific conductivity and dissolved Fe.

were statistically different (Kruskal-Wallis ANOVA, p<0.01) (Table 2). There was a statistical difference between Lismore Basalt and Quaternary Sediments, Lismore Basalt and Woodburn Sand, Kyogle Basalt and Quaternary Sediments, and Kyogle Basalt and Woodburn Sand (all Mann Whitney U-test, p <0.01) (Table 2). When the aquifers were grouped, there was a statistical difference in CH4 concentrations between the alluvial sediments, bedrock formations and basalts (Kruskal-Wallis ANOVA, p = 0.02). Methane concentrations in the alluvial sediments were statistically different from the basalts (Mann Whitney U-test, p <0.01), and from the bedrock (Mann Whitney U-test, p = 0.01), suggesting geology plays a major role in dissolved CH4 distribution.

Groundwater within the alluvial sediments (the Quaternary Sediments and the Woodburn Sand) contained relatively high CH4 concentrations with the majority of samples (n = 30) between 14.45 |igL-1 and 4427 |igL-1, and 5 samples <10 |igL-1 (Table 1, Fig. 4b). These unconfined aquifers can receive increased organic matter in recharge zones which leads to enhanced organic matter decomposition (Murphy et al., 1992). There was a weak positive correlation between DOC (average; 0.15 ±0.01 mM, Table 1) and CH4 concentration across the RRC (r2 =0.07, p = 0.01) (Fig. 5e), indicating organic matter supply plays an important role in controlling groundwater CH4 concentrations in the catchment. However, in individual geological units, this relationship only occurred in the Woodburn sand (r2 = 0.34, p = 0.045) which contained the highest DOC concentrations (0.28 ±0.09mM), corresponding to high median CH4 concentrations (105.84 |igL-1) (Table 1). Sites QS1 and QS11 contained coal fragments with CH4 concentrations of 200.45 |igL-1 and 687.9 |igL-1, respectively (Fig. 2a). Several sites had connectivity with the Gundurimba Clay (Appendix A Table A1), an estuarine deposit (Drury, 1982). For those samples, the median CH4 concentration was 142.28 |igL-1 (n = 14, median DOC = 0.12 mM), implying there may be organic carbon pockets present in this estuarine sediment. Reducing conditions are prevalent in the Woodburn Sand unit due to a shallow water table and oxygen depletion in the soil zone, providing suitable conditions for methanogenesis (Drury, 1982). Monitoring groundwater chemistry in the alluvial sediments is important since deeper groundwater is likely to pass through these sediments before entering surface water.

The Piora Member and Kangaroo Creek aquifers represent confined sandstone units where groundwater recharge is slow and organic matter delivery is low (Doig and Stanmore, 2012). CH4 concentrations were relatively low (Fig. 4b), indicating low rates of methanogenesis or high rates of CH4 oxidation, or a combination of both processes (<513C—CH4 ranged between -75.78% and -36.77%; refer to Section 3.3 — carbon stable isotopes). In both bedrock formations, most samples contained relatively low CH4 concentrations with 12 samples below 22.4 |gL-1 and 6 samples between 116.25 and 2702 |gL-1. Recharge areas around the basin margins are limited and groundwater would display slow lateral movement through the catchment, contributing to relatively stable conditions within the bedrock (Doig and Stanmore, 2012). Seasonal variations in CH4 concentrations for bedrock at depths up to 180 m are considered minimal as observed in central Japan (Watanabe et al., 2008), suggesting relatively consistent CH4 concentrations throughout the year. Site KC7 (90 m deep) had an unusually high CH4 concentration (2702 |g L-1), possibly resulting from organic carbon pockets associated with minor plant and coal fragments which exist in these units (Drury, 1982) (Fig. 2a).

The Lismore Basalt and Kyogle Basalt, while displaying different hydrochemistry, contained similarly low CH4 concentration (Fig. 4b) throughout the catchment and the Lismore Basalt had the lowest DOC content (0.08 ± 0.01 mM) (Table 1). Fractured horizons within the Lismore Basalt promotes enhanced precipitation infiltration and groundwater moves efficiently to deeper aquifers or outlets at various springs (Brodie and Green, 2002; Budd et al., 2000), which may also apply to the Kyogle Basalt. This may have reduced groundwater CH4 formation in the basalts by possibly preventing organic matter accumulation and degradation, and therefore minimal CH4 production. Alternatively, the dissolved oxygen (DO) concentration was highest in the basalts (Lismore Basalt; 3.19 ± 0.42 mg L-1, Kyogle Basalt; 2.47 ± 0.59 mg L-1) (Table 1), implying that organic matter degradation may have occurred through aerobic processes, leading to minimal methanogenesis. Although, there was a weak negative correlation between CH4 and DO (r2 = 0.05, p = 0.03), there was no correlation between CH4 and DO in individual aquifers (Fig. 5b).

3.3. Carbon stable isotopes

The carbon isotopic composition of groundwater is an important tool in obtaining information regarding CH4 origin (Aravena et al., 1995; Cheung et al., 2010; Coleman et al., 1988; Scott et al., 1994; Strapoc et al., 2007). Methane contained within the Walloon Coal Measures around the Casino region (C-MB, NSW) carried a thermogenic signature with a wide 113C-CH4 range from —48% to -13% (Doig and Stanmore, 2012). While in the Walloon Subgroup, Surat Basin, Queensland, also part of the C-MB, enhanced secondary microbial methanogenesis is responsible for CH4 content, leading to a mixed signature from -58.5% to -45.3% (Hamilton et al., 2014). In this study, <513C—CH4 could only be recorded for CH4 samples >3.9 |gL-1 (n = 52). The <513C—CH4 values ranged between -90.9% and -29.5% within the catchment (Fig. 2d) and each geological unit displayed variability (Table 1).The majority of samples (n = 37) had <513C—CH4 between -67.9% and -41.9% while the most depleted samples were less than -74.7% (n = 10) and the most enriched samples were greater than -36.8% (n = 5) (Appendix A Table A1). Heavily depleted <513C—CH4 samples were measured mainly from alluvial sediments except two samples, from the Kyogle Basalt (-88.5%, 15 m depth, KB8) and the Kangaroo Creek (-75.8%, 90 m depth, KC7) (Fig. 2a). There was no correlation between depth and <513C—CH4.

Methane oxidation is an important consideration when using stable isotope ratios to establish CH4 origin because it is a biogeochemical process which shifts the isotopic value of the residual CH4 due to preferential oxidation of the isotopically lighter CH4 (Bigeleisen and Wolfsberg, 1958). Therefore, <513C—CH4 due to methane oxidation can lead to a residual CH4 pool that may appear similar to unaltered thermogenic CH4 (Coleman et al., 1981). Although, upward thermogenic CSG migration from underlying coal seams in the RRC is possible, it is difficult to determine whether CH4 samples more enriched in 13C are thermogenic. With the exception of one sample (78 m deep), four samples with carbon signatures greater than -40% had slightly elevated dissolved Mn, dissolved Fe and SO42- (0.9, 2.24, 26.7 mg/L, respectively) and were from aquifers 40 m or less in depth. The enriched isotopic composition and slightly elevated reducing agents may reflect CH4 oxidation rather than thermogenic CH4 because shallow aquifers may have greater recharge and microbial activity, and CH4 concentrations were not anomalous (41.34, 105.84, 37.73 and 317.16 |gL-1). By including complimentary tracers such as <52H—CH4 and other

Fig. 6. (a) CH4 concentrations vs distance to CSG exploratory wells, (b) ¿13C—CH4 vs distance to CSG exploratory wells. LB = Lismore Basalt, KB = Kyogle Basalt, PM = Piora Member, KC = Kangaroo Creek Sandstone Member, QS = Quaternary Sediments and WS = Woodburn Sand. No significant correlations are present. Notice the log scale forCH4 concentration.

hydrocarbons, the presence of thermogenic CH4 can be determined (Golding et al., 2013; Wang et al., 2015). However, it was not possible to analyze these tracers in this study. Other processes likely to result in <513C—CH4 within the thermogenic isotopic range are carbon limitation within the aquifer or stagnant water (Hamilton et al., 2014) which may not be applicable in this study since DOC (0.15 ± 0.01 mM) concentrations were likely non-limiting (Table 1).

There was a significant negative correlation between CH4 concentration and <513C—CH4 (r2 =0.3, p<0.001) (Fig. 5d). As CH4 concentration decreased, <513C—CH4 became enriched, providing an indication of possible methane oxidation or a shift in CH4 production from the CO2 reduction pathway to the acetate fermentation pathway. For deeper subsurface environments, CO2 reduction mechanisms appear to be the dominant CH4 production pathway (Aravena et al., 1995; Cheung et al., 2010; Coleman et al., 1988). However, the shallow groundwater systems of the RRC exist predominantly as unconfined systems, creating environmental conditions suitable for CO2 reduction, acetate fermentation and methane oxidation to occur.

The CH4 isotopic composition is highly dependent on the isotopic composition of the carbon source (Martini et al., 1996). Therefore, incorporating CO2 and DIC stable isotopes as part of comprehensive geochemical investigations of CSG water and shallow groundwater can assist in determining CH4 origins (Table 1). In the Powder River Basin, Wyoming, coal bed produced waters had different l13C-D1C values to shallow groundwater, enabling l13C-D1C to be utilized as a tracer of produced waters seeping from impoundments into shallow aquifers (Sharma and Baggett, 2011). In closed systems where biogenic pathways generate CH4, <513C—CO2 (~0-+20%o) and l13C-D1C become more positive due to methanogens preferential removal of 12C, while <513C—CH4 becomes more negative (Botz et al., 1996; Mcintosh et al., 2008; Scott et al., 1994; Sharma and Frost, 2008; Smith and Pallasser, 1996). in contrast, the opposite occurs in thermogenic CH4 systems, resulting in more negative l13C—CO2 (-25% to -10%) and more positive §13C-CH4 (Chung and Sackett, 1979; Irwin et al., 1977). Many investigations comparing biogenic coal bed methane and biogenic shale gas to shallow groundwater or near surface water, have found produced water exhibited positive l13C-D1C while shallow groundwater showed negative l13C-D1C (Aravena et al., 2003; Cheung et al., 2010; Sharma and Baggett, 2011; Sharma and Frost, 2008; Sharma et al., 2014). 1n this study, <513C—CO2 values were characterised by a range from -29.7% to -14% while l13C-D1C values ranged between -23.6% and -5.3% (Table 1). However, comparisons between shallow groundwater and coal seam groundwater are currently not possible because no data is available on the l13C-D1C values of groundwater within the RRC Walloon Coal Measures.

Studies in the Marcellus shale region in Pennsylvania concluded private drinking wells within 1 km of shale gas wells were contaminated with stray gases (Jackson et al., 2013a; Osborn et al., 2011). This highlights the importance of establishing baseline groundwater conditions in relation to CSG well locations. CSG wells in the RRC were exploratory and groundwater pathways between the coal seam aquifers and shallow overlying aquifers are unlikely attributed to these ceased activities. At present, there were no significant relationships between CH4 concentrations and distance to CSG exploration wells (Fig. 6a), and <513C—CH4 and distance to CSG exploration wells (Fig. 6b). 1f CSG commences in the region, monitoring long term changes in those relationships may enable an early detection of any changes in groundwater chemistry.

3.4. Redox indicators

Anaerobic organic matter decomposition occurs via a series of microbially mediated redox reactions in the following order: denitrification, manganese (Mn) reduction, iron (Fe) reduction and sulphate (SO42-) reduction, with methanogenesis being the final reaction (1oka et al., 2010; Stumm and Morgan, 1995). Redox indicators (NO3-, dissolved Mn, dissolved Fe and SO42-) varied widely throughout the catchment (Fig. 2f, Appendix A Table A1). Nitrate (NO3-) was absent from 28 samples while the majority of samples with detectable NO3- had concentrations below 10mgL-1 and SO42- was gener-

ally <300 mg L-1. Dissolved Fe ranged between 0.003 and 79.35 mg L-1 while dissolved Mn also displayed high variability (0.0003-5.5 mg L-1). There was no relationship between redox parameters and CH4 concentration in the catchment (except between CH4 and dissolved Fe in the Lismore basalt). 1n a groundwater study, there was a clear negative correlation between SO42- and CH4 concentrations, indicating reduced competition for methanogens with sulphate reducing bacteria (Zhang et al., 1998). However, those correlations were not apparent in the RRC dataset due to the spatial complexity of groundwater samples and low concentration of CH4 in many samples (Appendix A Table A2).

The variability in redox indicators made it difficult to assign a dominant redox process contributing to CH4 formation in individual geological units. However, some sites could be analysed individually. 1n the Woodburn sand, two sites (WS2 and WS10, Fig. 2a) had lower CH4 (16.25 and 91.46 |ig L-1, respectively) which coincided with high dissolved Fe (36.29 and 79.35 mgL-1, respectively) and SO42- (150.9 and 1048.1 mgL-1, respectively). Therefore, the low concentrations observed may be due to inhibiting conditions for methanogenesis due to sulphate reducing bacteria (Darling and Gooddy, 2006) and iron reducing bacteria outcompeting methanogens since conditions indicating methane oxidation (HCO3- = 17.5 and 136.3 mgL-1, and <513C—CH4 = -50.5% and -56%, respectively) were not apparent. However, in the same alluvial geological unit, 3 sites (WS1, WS7 and WS9, Fig. 2a) provided suitable conditions for methanogenesis (i.e., low dissolved Fe and SO42-) and these sites had much higher concentrations of CH4 (1881.96,1242.04 and 753.71 |gL-1, respectively). No correlations existed between <513C—CH4 and SO42- which may indicate anaerobic CH4 oxidation coupled to SO42- reduction and SO42-reduction occurring simultaneously. This will potentially result in varying CH4 and SO42- concentrations, and isotope values; hence assigning specific processes to sites was difficult.

1n the Lismore Basalt, there was minimal SO42- and dissolved Fe concentrations which usually provide suitable conditions for CH4 generation (Berner, 1981; 1oka et al., 2010). Although, CH4 concentrations were low, there was a strong correlation between CH4 and dissolved Fe (r2 = 0.66, p < 0.001). This may indicate Fe reduction had already occurred, releasing reduced Fe2+ in the process and allowing methanogenesis to proceed.

4. Conclusions

This study has provided a baseline groundwater investigation in the Richmond River Catchment, a potential CSG production zone. Considering that CH4 has low solubility and is generally in concentrations above saturation level in the CSG target formations (i.e., occurs in gaseous form), CH4 may move rapidly if aquifer connectivity is enhanced, thereby providing a useful tracer if CSG extraction activities alter groundwater chemistry in shallow overlying aquifers. Shallow groundwater samples (n = 91) within six geological units displayed varying hydrochemical compositions, CH4 concentrations and stable isotopic signatures. Cl-type groundwater contained the highest median concentrations of CH4 (13.26 |gL-1, n = 58) compared to HCO3-type groundwater (3.71 |gL-1, n = 33). The majority of samples (n = 53) were Na-Cl groundwater type which was present in every geological unit except Kyogle Basalt. However, hydrochemistry appeared to be a poor indicator of methane distribution as there were no significant differences between the various groundwater types and CH4 concentrations. CH4 distribution in groundwater was best explained by the geological units with significant differences between the basalts and alluvial sediments, and bedrock aquifers and alluvial sediments. The basalts contained the lowest CH4 median concentration (0.7 |g L-1), the bedrock contained moderate median CH4 concentration (4.63 |g L-1) and the alluvial sediments contained the highest median CH4 concentration (91.46 |g L-1).

The carbon stable isotope values (<513C—CH4; ranged from -90.9% to -29.5%) may be utilized as a tracer of coal seam gas groundwater potentially upwelling into individual shallow wells or adjacent surface waters because the Walloon Coal Measures CSG carries a thermogenic signature. CH4 is most likely biogenic in origin with some <513C—CH4 values indicative of methane oxidation or thermogenic methane migration (i.e., those samples with a <513C—CH4 > —40%). These samples with enriched <513C—CH4 did not display anomalously high concentrations, and the overall trend in the dataset was for 13C CH4 depletion with increasing concentration, therefore we suggest that methane oxidation rather than thermogenic methane migration was the process responsible for <513C—CH4 enrichment. Redox conditions were variable in and between geological units. No correlations existed between CH4 concentration and redox parameters such as NO3-, dissolved Mn, dissolved Fe and SO42- in the catchment, although a correlation existed between CH4 and dissolved Fe in the Lismore Basalt. 1f CSG production commences in this region, groundwater chemistry baseline information will be readily available and may provide assistance to monitor potential CSG impacts. For future studies, additional analysis including <52H—CH4, other hydrocarbons and fluoride would assist in determining methane sources, and possible interactions between CSG groundwater and shallow aquifers (Dahm et al., 2014; Hamilton et al., 2014; Jackson et al., 2013a; Strapoc et al., 2007).

Acknowledgements

This project was funded by a Northern Rivers Regional Council grant and a Southern Cross University Collaborative Research grant. We thank local landowners for allowing us to take groundwater samples from their properties. We would like to thank Richard Boulton, Rachael Davies, Anita Perkins, Ben Stewart, Ronny Trnorsky, James Sippo, Kitty McKnight, Jeremy Link, Neil Taylor and Mahmood Sadat-Noori for their valuable help during field work. Thanks to Rodney Holland from Rous Water for sharing his geological knowledge of the Richmond River Catchment, and Matheus Carvalho and the Environmental Analysis Laboratory at Southern Cross University for sample analysis. Support from the Australian Research Council (LE120100156, DE140101733 and DE150100581) contributed to enable this work.

Appendix A.

Table A1

Individual results for water parameters measured in 91 groundwater samples in the Richmond River Catchment. Methane concentrations below 0.26 |xgL-1 are below the detection limit. Bores connecting with the Gundurimba Clay and coal fragments are indicated by GC and CF, respectively. Monitoring bores have identification numbers. 813C—CH4 values are unavailable for samples with CH4 <3.9 |xgL-1.

Water parameters

Stable carbon isotopes

Hydrochemical parameters

Redox parameters

Sample ID Latitude Longitude Depth Elevation

_(m) (m)

Temp Sp Cond pH DO (oC) (mScm-1) (mgL-1

)(reL-1

DOC 813C-CH4

)(mM)(%.)

HCO3- NO3- Total Mn Total Fe SO42- SAR Water type Monitoring

Bore ID

№0 (mgL- 1) (mgL-1 ) (mgL-1 ) (mgL- 1 ) (mgL- 1 ) (mgL- 1 ) (mgL- 1 ) (mgL- 1) (mgL- 1) (mgL-1 )

-17.05 38.9 20.28 9.54 4.20 38.0 54.8 60.7 0.011 0.02 8.43 1.78 Na-Cl

-18.53 23.0 3.74 1.94 2.74 19.2 27.8 0.08 0.169 0.22 12.11 2.40 Na-Cl

-16.90 20.8 6.15 2.86 2.02 27.6 22.1 1.03 0.035 0.01 8.65 1.73 Na-Cl

-22.90 10.4 1.43 1.33 0.83 9.6 10.6 0.80 0.007 0.01 5.64 1.50 Na-Cl

-14.08 7.5 0.94 1.24 0.42 12.0 6.9 0.51 0.010 0.03 3.98 1.19 Na-Cl

-18.80 14.2 1.05 1.82 0.26 15.2 4.1 10.8 0.011 0.003 7.39 1.95 Na-Cl

-19.43 13.7 4.58 3.56 0.92 19.8 20.1 1.6 0.056 0.05 6.08 1.17 Na-Cl

-12.60 416 36.75 37.57 2.94 469 385 0.17 0.086 0.36 78.91 11.52 Na-Cl

-12.83 10.7 5.48 2.78 0.92 14.6 23.5 4.2 0.080 1.18 4.49 0.93 Na-Cl

-16.00 8.5 0.91 1.47 0.16 14.3 8.1 0.87 0.007 0.04 5.05 1.28 Na-Cl

-11.90 20.0 2.44 3.71 1.09 33.6 8.8 2.5 0.033 0.31 9.78 1.88 Na-Cl

-13.14 30.5 18.89 4.47 2.02 10.4 163 0.01 0.107 0.23 6.82 1.64 Na-HCO3

-20.60 20.4 17.86 11.25 3.29 21.4 157 0.00 0.050 0.03 6.35 0.93 Mg-HCO3

-20.47 25.1 21.82 5.66 1.42 26.7 104 3.9 0.004 0.01 6.41 1.24 Na-HCO3

-20.51 18.6 13.50 9.46 4.22 21.2 145.0 0.00 0.130 0.45 5.24 0.95 Na-HCO3

-15.60 56.9 7.30 1.82 2.20 37.7 108 7.78 0.013 0.005 5.57 4.89 Na-HCO3

-9.97 30.1 6.88 5.46 0.70 19.3 76.6 3.09 0.043 0.09 7.59 2.08 Na-Cl

-13.54 27.2 25.55 15.88 2.57 12.6 170 0.10 0.057 0.03 5.05 1.04 Mg-HCO3

-12.99 69.6 71.15 43.65 0.79 78.5 332 10.5 0.007 0.01 25.85 1.60 Mg-HCO3

-11.87 57.9 57.64 33.40 0.64 46.7 270 4.8 0.000 0.01 9.96 1.50 Ca-HCO3

-17.34 39.1 2.32 2.43 0.44 19.2 32.5 17.6 0.006 0.03 22.29 4.28 Na-Cl

-14.03 27.7 41.12 25.15 1.90 85.0 144 0.00 0.001 0.01 4.46 0.84 Mg-Cl

-9.74 10.3 4.52 4.21 1.74 13.6 57.4 0.81 0.003 0.06 3.77 0.83 Na-HCO3

-11.16 17.6 40.34 24.35 1.02 51.7 120 9.8 0.104 0.01 6.51 0.54 Ca-Cl

-17.76 97.8 1.69 0.16 1.16 21.2 1668 0.02 0.003 0.005 5.72 19.31 Na-HCO3

-11.15 88.0 82.09 31.44 0.47 78.4 365 10.1 0.001 0.02 16.89 2.09 Ca-HCO3

-13.70 60.0 68.77 42.16 1.15 40.8 411 10.6 0.001 0.01 11.70 1.40 Mg-HCO3

-12.73 146 28.13 37.15 3.61 52.7 532 1.05 0.011 0.01 12.90 4.25 Na-HCO3

-9.49 55.3 56.50 18.23 0.51 19.5 322 7.9 0.000 0.003 7.46 1.64 Ca-HCO3

-17.46 103 18.34 13.98 5.15 35.5 505 0.00 0.093 0.35 4.56 4.39 Na-HCO3

-12.75 79.9 57.11 43.08 1.04 71.9 363 0.00 0.879 0.05 5.87 1.94 Mg-HCO3

-11.69 40.3 44.43 24.13 0.44 37.2 178 9.3 0.000 0.01 6.60 1.21 Ca-HCO3

-11.32 33.1 38.19 20.70 0.86 30.6 133 0.78 0.083 0.62 5.83 1.07 Ca-HCO3

-10.09 288 4.63 1.54 5.95 145 1585 0.19 0.001 0.01 8.77 29.62 Na-HCO3

-11.28 96.1 29.27 68.02 4.95 50.4 617 11.3 0.001 0.01 22.49 2.23 Mg-HCO3

-10.49 55.7 34.94 24.31 0.96 49.2 213 1.2 0.008 0.02 4.76 1.77 Na-HCO3

-10.61 137 76.11 65.00 2.15 96.1 518 16.1 0.009 0.02 11.02 2.79 Na-HCO3

-12.20 129 126.07 73.48 0.85 136 513 12.0 0.085 0.03 12.83 2.26 Ca-HCO3

-14.79 478 53.38 26.45 1.11 418 370 0.03 0.532 0.34 285.13 13.37 Na-Cl

-13.28 17.6 0.88 2.17 0.07 29.2 7.4 2.6 0.001 0.20 4.59 2.30 Na-Cl

-18.86 816 193.08 157.85 2.13 1864 54.4 0.00 4.368 0.78 65.67 10.55 Na-Cl

-11.36 152 26.75 21.34 1.44 175 26 3.7 0.034 0.09 12.87 5.31 Na-Cl

-16.23 312 71.17 60.61 0.48 517 221 0.00 0.052 1.09 71.77 6.56 Na-Cl

-15.73 660 172.27 170.83 6.41 1602 126 0.00 5.500 2.80 30.27 8.52 Na-Cl

-11.10 291 80.27 34.14 3.50 400 280 0.12 0.161 0.23 24.30 6.85 Na-Cl

-10.95 40.9 70.37 25.66 1.19 46.2 211 0.15 1.199 0.67 6.79 1.06 Ca-HCO3

-14.30 125 106.54 68.08 1.14 194 375 4.8 0.002 0.05 41.14 2.32 Mg-Cl

-14.78 148 59.06 57.16 1.62 261 258 0.12 0.861 0.70 28.54 3.29 Na-Cl

LB1 -28.76776 153.4042 32 36 20.5 0.41 5.83

LB2 -28.87914 153.4639 44 129 21.1 0.15 6.02

LB3 -28.80602 153.4602 51 109 21.9 0.17 5.61

LB4 -28.59058 153.4225 60 347 18.7 0.07 5.15

LB5 -28.64805 153.3979 55 157 19.7 0.06 5.03

LB6 -28.64983 153.4557 13 178 19.7 0.10 4.73

LB7 -28.70155 153.4452 27 111 19.7 0.13 5.37

LB8 -28.89456153.1771 39 11 18.7 2.38 7.53

LB9 -28.863 72 1 53.43 3 8 11.2 151 20.5 0.11 5.63

LB10 -28.82489 153.4408 14 114 21.5 0.07 5.16

LB11 -28.88886153.4152 10.5 155 20.1 0.08 5.10

LB12 -28.78423 153.4816 46.5 95 20.0 0.27 7.17

LB13 -28.8262 153.4695 66 156 20.9 0.28 7.28

LB14 -28.80288 153.3763 63 142 20.9 0.28 6.71

LB15 -28.863 74 1 53.43 3 7 38 151 20.6 0.24 7.24

LB16 -28.77224 153.5347 120 92 20.9 0.34 6.79

LB17 -28.76748 153.3996 60 35 22.2 0.22 6.19

LB18 -28.55077 153.3665 30 224 19.3 0.35 6.94

LB19 -28.83759 153.1675 88 52 21.1 0.96 6.93

LB20 -28.71208 153.0503 37 71 18.3 0.75 6.92

LB21 -28.75612 153.3941 30.5 21 21.0 0.24 5.67

LB22 -28.75518 153.0109 8 61 17.5 0.46 6.61

LB23 -28.74052 152.9903 36 45 19.5 0.14 6.58

LB24 -28.73695 152.9896 37 41 21.7 0.46 6.70

LB25 -28.74116153.4859 78 65 22.0 0.43 8.80

KB1 -28.53488 152.9407 35 110 20.9 0.94 6.93

KB2 -28.53283 152.9591 40 80 21.4 0.85 6.97

KB3 -28.53368 152.8875 82 154 21.5 0.97 7.58

KB4 -28.65232 152.9748 44 58 21.6 0.63 7.09

KB5 -28.7388 152.9592 46 64 21.7 0.63 7.68

KB6 -28.59547 153.0075 26.5 61 21.6 0.92 7.40

KB7 -28.56192 153.0107 73 21.7 0.55 6.80

KB8 -28.45477 152.992 15 98 20.9 0.44 6.62

KB9 -28.70982 152.9578 30 79 17.7 1.30 8.56

KB10 -28.78472 152.9979 49 60 22.2 1.00 7.71

KB11 -28.6759 152.8602 33 138 21.6 0.59 6.88

KB12 -28.90079 152.82 61 322 21.0 1.36 7.11

KB13 -28.89976152.8197 56 324 20.5 1.59 6.78

PM1 -28.8146 153.0712 24 40 20.9 2.31 6.92

PM2 -29.03118 153.1043 14 34 19.9 0.12 5.17

PM3 -28.93865 153.1305 25.7 17 21.4 6.08 5.67

PM4 -28.96617 153.1217 22 23 21.0 0.99 5.66

PM5 -28.92388 153.093 13 22 20.5 2.26 6.85

PM6 -28.87098 153.0096 26.2 34 20.6 4.91 5.98

PM7 -28.86755 153.0014 52.1 27 20.6 1.79 7.13

PM8 -28.78054 153.1944 40 16 21.2 0.70 6.65

PM9 -28.87642 153.2302 15.2 13 21.5 1.57 6.83

PM10 -28.90565 153.1478 37.7 12 21.1 1.41 6.69

0.69 1.68 1.12 0.45 0.31 0.23 1.19 36.27

116.24 1.88 12.07 0.69 0.22 4.77 6.54 0.70 0.28 0.41 0.23 0.16 0.13 0.12 3.71 0.15 56.79 0.68 1.04 1.67 0.26 1.43 10.97 0.30 98.10

I.47 0.36 0.55 0.70 132.88 0.17 2.96 3.62 0.24

116.25 10.68 317.16 0.30 22.40

48.81 48.58

51.55 60.45

0.06 0.07 0.06 0.06 0.07 0.07 0.06 0.18 0.06 0.05 0.05 0.08 0.04 0.05 0.05 0.04 0.05 0.11 0.16 0.22 0.12 0.08 0.11 0.07

0.10 -35.08

0.04 -58.04 0.17

0.17 -88.45 0.19 0.22 0.15 0.27 0.30 0.41 0.06 0.13 0.12 0.19 0.24 0.19 0.21

-50.11

-60.53 -47.35 -52.71 -36.77

0.12 -58.25

-23.01 -23.28 -20.76 -25.67 -16.61 -21.07

-22.88 -16.98 -19.35 -15.55 -22.22 -29.66 -27.88 -29.19 -24.84 -17.21 -21.66 -22.94 -19.29 -21.76 -21.82 -16.61 -20.07 -24.38 -20.76 -21.81 -22.13 -18.93 -26.41 -22.85 -18.83 -17.87 -20.14 -21.44 -19.39 -21.13

-24.06 -16.05

-20.20 -25.34

-20.08 -19.90 -24.13 -24.14

GW40505

GW081006

GW081000

13C—CO2 813

C—DIC Na

GW081003

GW039125

Table A1 (Continued)

Water parameters

Stable carbon isotopes

Hydrochemical parameters

Redox parameters

Sample ID Latitude Longitude Depth Elevation Temp Sp Cond pH DO CH4 DOC 813C-CH4 813C—CO2 813C—DICNa+ Ca2+ Mg2+ K+

(m) (m) (oC) (mS cm-1 ') (mgL- 1)(^gL-1 )(mM)(%.) (mgL-1 ) (mgL-1 ) (mgL-1 )(mgL

KC1 -28.66966152.9966 78 75 21.5 0.62 5.50 0.52 3.23 0.14 -19.55 71.6 10.50 12.31 8.05

KC2 -28.72235 152.9767 85 66 21.6 0.41 5.83 0.76 5.34 0.09 -64.93 -24.82 -19.32 40.9 23.55 7.18 2.91

KC3 -28.85503 152.7898 51 113 20.0 1.26 6.560.85 2.76 0.16 -23.74 -15.29 142 71.84 26.70 3.46

KC4 -28.78361 152.9407 41 38 21.4 0.42 5.940.27 3.92 0.05 -50.71 -24.81 -18.54 42.6 14.85 9.21 2.99

KC5 -28.88924153.2828 22 8 21.6 0.20 6.061.43 254.84 0.43 -58.92 -13.97 -8.35 16.5 12.20 6.09 0.65

KC6 -28.97897 153.298 21 5 20.2 26.14 6.240.43 136.61 0.12 -58.56 -22.78 -12.93 2688 1558.28 1063.75 4.32

KC7 -28.82517152.998 90 40 22.0 1.73 7.040.21 2702.18 0.20 -75.78 -16.86 -13.03 278 59.94 22.01 1.04

KC8 -28.56333 153.0107 27 65 20.9 2.30 6.57 0.54 0.35 0.35 -14.73 -6.67 91.2 199.79 109.04 1.13

QS1 (CF) -28.5958 153.0114 19 57 21.7 0.85 7.340.42 200.45 0.04 -53.04 -22.39 -12.01 48.8 58.06 51.24 0.94

QS2 -28.68076153.0006 22 49 21.3 1.00 7.10 0.19 223.47 0.09 -51.97 -20.31 -9.81 111 60.33 33.21 1.27

QS3 -28.68026153.0114 27 48 17.3 0.41 6.87 0.32 486.26 0.79 -67.90 -18.00 -9.21 41.5 18.08 21.23 3.21

QS4 -28.48201 152.931 20 88 22.0 1.03 7.37 0.26 42.61 0.04 -59.08 -17.47 -6.80 86.4 55.53 60.10 1.35

QS5 -28.48478 152.9305 18 84 21.8 1.02 7.27 0.26 803.08 0.04 -60.25 -17.74 -7.67 107 55.01 43.74 1.89

QS6 -28.78362 152.9683 36.5 37 22.3 0.94 6.87 129.93 0.04 -57.60 -21.86 -12.02 98.9 48.45 35.29 1.17

QS7 -28.78368 152.9813 37.5 38 22.2 0.95 6.78 9.74 0.06 -52.10 -23.47 -13.63 39.2 74.69 47.57 3.22

QS8 -28.90391 153.1186 23.7 16 20.5 2.20 6.91 1.59 1.37 0.05 -25.62 -15.61 345 53.55 52.26 1.83

QS9 -28.978 153.1229 17.6 13 20.6 0.99 5.83 0.32 40.96 0.04 -63.65 -23.93 -18.28 139 18.02 18.55 1.89

QS10 -28.93264153.0051 34.5 34 21.6 1.22 6.81 41.34 0.08 -29.46 -25.04 -14.70 197 32.72 20.28 1.64

QS11 (CF) -28.89868153.0318 31 27 22.2 6.69 6.64 687.89 0.09 -87.64 -22.14 -11.85 682 337.64 242.20 5.32

QS12 -28.88725153.1253 29 15 22.2 2.69 6.82 37.73 0.04 -33.06 -24.40 -14.38 409 52.22 67.28 2.25

QS13 -28.88738153.1502 15 11 21.1 0.69 6.48 1.42 15.59 0.05 -87.79 -23.41 -15.27 68.6 25.03 24.48 0.70

QS14(GC) -28.80074 153.2367 36 11 21.4 2.95 7.23 0.58 229.88 0.09 -57.51 -27.12 -17.07 370 65.46 90.48 10.70

QS15 (GC) -29.01535153.2678 29 7 21.2 2.03 6.240.4 37.61 0.15 -44.37 -17.82 260 81.07 44.31 4.78

QS16(GC) -29.09311 153.4 46 7 20.8 0.94 7.01 0.41 2891.36 0.33 -82.24 -24.47 -16.05 58.9 120.94 14.72 5.06

QS17 -28.89555 153.0404 23 26 21.4 2.62 6.87 69.47 0.08 -55.40 -21.80 -10.99 474 59.57 46.68 2.29

QS18 -28.87349153.1311 27 16 21.9 3.44 6.76 32.17 0.07 -56.52 -23.02 -13.00 450 115.30 124.75 2.28

QS19 -28.91479153.1137 13.5 14 20.8 5.75 6.79 0.36 3.22 0.08 -23.95 -13.85 837 135.77 129.58 1.69

QS20 (GC) -28.8632 153.1338 21 14 22.0 1.07 7.27 4427.64 0.11 -85.97 -17.86 -9.82 113 60.57 44.83 7.92

QS21 (GC) -28.8039 153.2349 36 11 21.4 2.62 7.05 0.24 178.72 0.10 -45.02 -26.41 -16.17 343 78.46 98.00 10.44

QS22 -28.62188152.9915 29 57 21.9 0.69 6.56 33.23 0.05 -50.64 -5.34 88.1 37.39 20.40 1.22

WS1 -28.94076153.4719 5 10 21.0 0.18 4.66 0.45 1881.96 0.26 -90.88 -22.55 -23.03 25.1 0.54 2.85 1.63

WS2 (GC) -29.07972 153.3613 9 5 20.5 2.33 5.15 0.35 16.25 0.23 -50.46 -22.57 -19.47 489 7.17 55.55 17.79

WS3 (GC) -28.93085 153.3709 8 10 19.7 0.11 5.63 0.45 7.25 0.08 -54.00 -15.30 -11.56 12.7 2.40 2.39 0.91

WS4 (GC) -28.94725 153.3677 13 4 20.6 0.16 4.95 0.29 36.21 0.01 -67.25 -19.26 -16.39 17.1 1.83 2.26 0.89

WS5 -28.97913 153.4065 25 13 20.5 0.17 5.721.14 135.57 0.13 -51.30 -23.91 -20.47 17.9 6.74 3.24 0.87

WS6 (GC) -29.09311 153.4 35 7 20.7 4.56 6.260.33 336.62 0.39 -74.74 -26.49 -19.54 706 196.99 68.01 15.79

WS7 (GC) -29.09311 153.4 23 7 20.9 0.89 4.80 0.24 1242.04 1.06 -89.68 -24.20 -23.57 158 1.06 12.19 3.11

WS8 (GC) -28.99576153.3601 15 7 20.8 0.18 5.63 0.42 70.58 0.07 -51.33 -22.36 -19.21 29.6 1.46 2.43 1.39

WS9 (GC) -28.94061 153.4694 19.8 7 21.3 0.28 4.99 0.35 753.71 0.76 -85.65 -21.65 43.1 1.47 4.31 1.54

WS10 (GC) -28.89561 153.5416 29 4 21.1 20.93 5.50 0.29 91.46 -55.99 -18.25 3694 252.83 496.96 96.88

WS11 (GC) -29.08019 153.3638 13 6 19.9 0.30 5.72 0.28 105.84 0.12 -32.50 -21.33 -16.85 64.3 2.93 7.64 3.45

WS12 -29.08746153.379 28 11 21.0 0.26 5.52 0.32 425.98 0.16 -64.82 -24.99 -20.14 33.1 1.03 3.09 1.63

WS13 -29.08447153.3711 23 4 21.6 0.13 5.62 0.29 14.45 0.06 -57.67 -16.94 -13.52 22.5 1.88 2.56 1.47

Average 35 64 20.9 1.64 6.44 1.70 217.95 0.15 -58.75 -21.74 -14.56 214.42 65.78 50.48 3.66

Median 30 40 21.0 0.75 6.69 0.80 10.68 0.10 -57.02 -22.14 -14.03 68.59 34.94 22.01 1.63

Minimum 5 4 17.3 0.06 4.660.19 0.12 0.01 -90.88 -29.66 -23.57 7.46 0.54 0.16 0.07

Maximum 120 347 22.3 26.14 8.80 6.62 4427.64 1.06 -29.46 -13.97 -5.34 3693.62 1558.28 1063.75 96.88

St Error 2.24 7.35 0.1 0.38 0.09 0.20 68.78 0.02 1.63 0.35 0.42 51.67 17.70 13.09 1.08

Cl-'1)(mgL-

HCO3- NO3- Total Mn Total Fe SO42- SAR Water type Monitoring

) (mgL-1 ) (mgL- 1) (mgL- ) (mgL-1 ) (mgL-1 ) BoreID

37.2 0.27 5.351 21.22 4.52 3.55 Na Cl

46.3 0.05 0.094 0.38 5.24 1.89 Na Cl

206 0.09 0.348 0.83 40.48 3.63 Na Cl

63.6 0.01 0.589 10.14 8.86 2.14 Na Cl GW039117

37.6 0.19 0.204 1.73 8.41 0.96 Na Cl GW039139

173 0.00 1.385 1.72 1001.38 12.86 Na Cl GW039143

225 0.00 0.072 0.10 5.91 7.81 Na Cl

292 94.8 0.016 0.01 85.14 1.29 Ca- Cl

416 2.5 0.770 0.59 5.51 1.29 Mg-HCO3 GW039124

374 0.00 1.493 0.59 5.21 1.12 Na HCO3 GW039121

194 0.00 0.332 0.55 4.52 2.85 Na HCO3 GW039123

421 0.00 0.083 0.05 4.69 1.57 Mg-HCO3 GW039130

422 0.00 0.468 0.38 4.81 1.91 Na HCO3 GW039129

275 0.00 0.776 0.16 9.57 2.63 Na HCO3 GW039111

182 10.2 1.059 0.12 7.06 2.64 Mg-Cl GW039110

286 0.01 1.035 0.22 30.46 0.87 Na Cl GW039104

59.4 0.00 0.119 9.29 3.91 8.03 Na Cl GW039148

215 0.00 1.387 1.99 18.79 5.49 Na Cl GW039107

423 0.00 2.985 9.96 19.52 6.68 Na Cl GW039114

257 0.00 0.943 0.36 58.50 6.92 Na Cl GW039103

124 1.4 0.036 0.21 6.11 8.81 Na Cl

479 0.00 0.538 0.44 60.20 2.34 Na Cl GW039137

209 0.00 0.302 4.73 131.18 6.95 Na Cl GW039145

292 0.00 0.473 1.23 9.85 5.77 Ca- HCO3 GW039152

564 0.01 0.587 0.29 41.77 1.35 Na Cl GW039113

412 0.00 0.445 1.03 72.58 11.18 Na Cl GW039102

259 0.00 0.426 0.13 43.09 6.91 Na Cl GW039105

414 0.00 0.492 1.16 4.61 12.34 Na HCO3 GW039101

401 0.00 0.714 1.52 30.86 2.68 Na Cl 3 GW039136

207 0.02 0.170 0.96 23.92 6.11 Na HCO3 GW039133

3.7 0.00 0.019 0.53 4.54 2.88 Na Cl

17.5 0.00 0.067 36.29 150.94 3.02 Na Cl BH 1241

13.2 0.00 0.030 3.25 6.60 13.55 Na Cl GW081056

4.0 0.00 0.024 4.38 17.51 1.39 Na Cl

32.3 0.35 0.283 0.50 8.04 1.99 Na Cl

116 0.00 0.777 7.14 165.52 1.42 Na Cl GW039152

6.3 0.01 0.009 0.83 49.35 11.06 Na Cl GW039152

18.3 0.46 0.018 1.39 6.63 9.45 Na Cl GW039165

10.9 0.00 0.031 0.72 3.60 3.49 Na Cl GW039158

136 0.00 1.498 79.35 1048.09 4.05 Na Cl GW039147

35.2 0.00 0.040 5.96 22.68 31.06 Na Cl BH 1242

27.7 0.00 0.056 1.22 5.03 4.49 Na Cl GW039151

15.4 0.01 0.022 1.90 9.48 3.69 Na Cl GW039150

227.53 3.78 0.456 2.49 46.48 4.59

172.62 0.08 0.080 0.31 8.65 2.34

3.68 0.00 0.0003 0.003 3.60 0.54

1668.22 94.78 5.500 79.35 1048.09 31.06

28.08 1.27 0.103 0.99 16.05 0.56

72.6 194 69.3 28.8 8981 334 312.3 51.0 62.6 21.8 86.2 78.9 93.9 136 482 244 184 1871 680 112 566 444 89 413 823 1629 82.6 543

54.2 42.8 837

23.7 1442 231

71.1 6418

92.2 53.2 26.2 392.63 71.13 9.60 8980.77 124.40

Correlation matrix for groundwater parameters in the Richmond River Catchment. Pearson correlation co-efficient (r) is critical above 0.206. Significant relationships are indicated by bold text. Significant relationships with methane concentrations (813C—CH4, dissolved oxygen and dissolved organic carbon) are indicated by bold and larger text.

Elevation Depth Temp EC pH DO CH4 813C—CH4 813C—CO2 813C DIC DOC HCO3- Ca2* Mg2* K* Na* Cl- SAR SO42- Total Fe Total Mn NO3-

(m) (m) (◦C) (ms cm-1 ) (mgL-1) (WgL-1) (%.) (%.) (*•) (mM) (mgL-1) (mgL-1 ) (mgL-1) (mgL-1) (mgL-1) (mgL-1) (mgL-1) (mgL-1) (mgL-1 ) (mgL-1)

Elevation (m) 1.000

Depth (m) 0.334 1.000

Temp (°C) -0.220 0.176 1.000

EC (ms cm-1) -0.215 -0.106 0.024 1.000

pH 0.049 0.324 0.122 -0.016 1.000

DO (mg L-1) 0.582 0.204 -0.047 -0.186 -0.060 1.000

CH^gL-1) -0.189 -0.003 0.150 -0.011 0.028 -0.230 1.000

813C—CH4(%.) 0.154 0.112 -0.032 0.002 0.217 0.204 -0.552 1.000

813C—CO2(%.) -0.004 -0.249 -0.080 -0.129 -0.201 0.139 0.110 -0.071 1.000

813C—DIC(%.) 0.014 -0.071 0.070 -0.008 0.474 0.174 -0.010 0.158 0.703 1.000

DOC(mM) -0.117 -0.059 -0.172 0.011 -0.111 -0.173 0.261 -0.416 0.070 -0.102 1.000

HCO3-(mgL-1) 0.065 0.213 0.058 0.037 0.780 -0.082 0.027 0.215 -0.108 0.306 -0.009 1.000

Ca2+(mgL-1) -0.132 -0.076 0.007 0.855 0.044 -0.116 0.022 -0.042 -0.056 0.101 0.005 0.035 1.000

Mg2+(mgL-1) -0.162 -0.107 0.029 0.960 0.007 -0.141 -0.017 -0.003 -0.073 0.060 -0.013 0.034 0.951 1.000

K+(mgL-1) -0.146 -0.021 0.024 0.602 -0.093 -0.155 0.031 0.008 -0.330 -0.161 0.137 -0.005 0.151 0.406 1.000

Na+ (mgL-1) -0.230 -0.104 0.016 0.960 -0.042 -0.198 -0.019 0.027 -0.179 -0.063 0.033 0.040 0.679 0.848 0.777 1.000

Cl-(mgL-1) -0.214 -0.118 -0.004 0.993 -0.081 -0.180 -0.021 -0.003 -0.114 -0.050 0.005 -0.022 0.872 0.966 0.571 0.942 1.000

SAR -0.270 -0.030 -0.085 0.579 0.171 -0.293 -0.016 0.133 -0.219 -0.123 0.148 0.484 0.263 0.411 0.594 0.693 0.547 1.000

SO42-(mgL-1) -0.187 -0.107 -0.026 0.934 -0.095 -0.163 -0.046 0.038 -0.108 -0.069 0.076 -0.029 0.737 0.867 0.707 0.941 0.932 0.577 1.000

Total Fe (mgL-1) -0.172 -0.067 0.033 0.529 -0.236 -0.178 -0.028 0.041 -0.072 -0.204 0.029 -0.112 0.109 0.350 0.921 0.695 0.513 0.539 0.641 1.000

Total Mn (mgL-1 ) -0.182 0.018 0.170 0.340 -0.065 -0.236 0.004 0.110 -0.238 -0.088 -0.022 -0.043 0.262 0.310 0.184 0.325 0.335 0.214 0.167 0.250 1.000

NO3- (mgL-1) 0.096 0.036 0.011 -0.047 -0.005 0.121 -0.100 0.136 0.196 0.189 0.068 0.004 0.035 -0.003 -0.057 -0.094 -0.070 -0.160 -0.030 -0.081 -0.127 1.000

Appendix A. Supplementary data

Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.ejrh. 2015.06.022.

References

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