Biogeosciences, 14, 215–228, 2017 www.biogeosciences.net/14/215/2017/ doi:10.5194/bg-14-215-2017 © Author(s) 2017. CC Attribution 3.0 License. Biogeochemical constraints on the origin of methane in an alluvial aquifer: evidence for the upward migration of methane from underlying coal measures Charlotte P. Iverach1,2, Sabrina Beckmann3, Dioni I. Cendón1,2, Mike Manefield3, and Bryce F. J. Kelly1 1Connected Waters Initiative Research Centre, UNSW Australia, UNSW Sydney, NSW, 2052, Australia 2Australian Nuclear Science and Technology Organisation, New Illawarra Rd, Lucas Heights, NSW, 2234, Australia 3School of Biotechnology and Biomolecular Sciences, UNSW Australia, UNSW Sydney, NSW, 2052, Australia Correspondence to: Charlotte P. Iverach ([email protected]) Received: 26 August 2016 – Published in Biogeosciences Discuss.: 5 September 2016 Revised: 12 December 2016 – Accepted: 24 December 2016 – Published: 17 January 2017 Abstract. Geochemical and microbiological indicators of 1 Introduction methane (CH4/ production, oxidation and migration pro- cesses in groundwater are important to understand when at- Interest in methane (CH4/ production and degradation pro- tributing sources of gas. The processes controlling the natural cesses in groundwater is driven by the global expansion of occurrence of CH4 in groundwater must be understood, es- unconventional-gas production. There is concern regarding pecially when considering the potential impacts of the global the potential impacts of gas and fluid movement, as well as expansion of coal seam gas (CSG) production on ground- depressurisation, on groundwater quality and quantity in ad- water quality and quantity. We use geochemical and micro- jacent aquifers used to support other industries (Atkins et al., biological data, along with measurements of CH4 isotopic 2015; Heilweil et al., 2015; Iverach et al., 2015; Moritz et al., 13 composition (δ C-CH4/, to determine the processes acting 2015; Owen et al., 2016; Zhang and Soeder, 2016). upon CH4 in a freshwater alluvial aquifer that directly over- In groundwater, CH4 can originate from numerous sources lies coal measures targeted for CSG production in Australia. (Barker and Fritz, 1981). The two main sources of CH4 in Measurements of CH4 indicate that there is biogenic CH4 in shallow groundwater are in situ biological production (bio- the aquifer; however, microbial data indicate that there are genic) and upward migration of CH4 from deeper geolog- no methanogenic archaea in the groundwater. In addition, ical formations (thermogenic to mixed thermo-biogenic to geochemical data, particularly the isotopes of dissolved in- biogenic) (Barker and Fritz, 1981; Whiticar, 1999). This up- organic carbon (DIC) and dissolved organic carbon (DOC), ward migration is via natural pathways such as geological 2− as well as the concentration of SO4 , indicate limited poten- faults and fracture networks (Ward and Kelly, 2007); how- tial for methanogenesis in situ. Microbial community analy- ever, it can also be induced via poorly installed wells and sis also shows that aerobic oxidation of CH4 occurs in the al- faulty well casings (Barker and Fritz, 1981; Fontenot et al., luvial aquifer. The combination of microbiological and geo- 2013). The main focus of the debate about the occurrence of chemical indicators suggests that the most likely source of CH4 in groundwater is whether it is naturally occurring or CH4, where it was present in the freshwater aquifer, is the up- has been introduced by human activities. This research tests ward migration of CH4 from the underlying coal measures. the hypothesis that a combination of geochemical indicators and microbiological data can inform production, degradation and migration processes of CH4 in the Condamine River al- luvial aquifer (CRAA) in Australia. This freshwater aquifer directly overlies the Walloon Coal Measures (WCM), the tar- get coal measures for coal seam gas (CSG) production in the study area. Thus, our study has ramifications for global Published by Copernicus Publications on behalf of the European Geosciences Union. 216 C. P. Iverach et al.: Biogeochemical constraints on the origin of methane in an alluvial aquifer unconventional-gas studies that investigate connectivity is- because sulfate-reducing bacteria (SRB) often outcompete sues of freshwater aquifers. methanogenic archaea for reducing equivalents (Lovley and Methane is subject to many production and degradation Klug, 1985; Struchtemever et al., 2005). processes in groundwater (Whiticar, 1999). The carbon iso- CH COOH ! CH C CO (R1) topic composition of CH (δ13C-CH / gives insight into the 3 4 2 4 4 C − source (Quay et al., 1999), but oxidation processes may en- CO2 C 8H C 8e ! CH4 C 2H2O (R2) rich or deplete this signature (Yoshinaga et al., 2014). There- 2− Therefore, the presence or absence of [CH4] and [SO4 ] fore, it is very difficult to determine the potential source of are good preliminary indicators of the potential for in situ CH4 and processes occurring using CH4 concentration and methanogenesis. isotopic data alone. In addition, the δ13C-CH of the underlying WCM in Previous studies have used geochemical indicators, such as 4 2− − and around the study area has been characterised. Draper the concentration of sulfate [SO4 ], nitrate [NO3 ] and nitrite − and Boreham (2006) characterised the isotopic signature of [NO2 ], and the carbon isotopic composition of dissolved the WCM to be between −57.3 and −54.2 ‰. Hamilton et 13 inorganic carbon (δ C-DIC) and dissolved organic carbon al. (2014) and Baublys et al. (2015) expanded this range 13 (δ C-DOC) to attribute the source of CH4 in groundwater to be from −58.5 to −45.3 and −57 to −44.5 ‰, respec- (Valentine and Reeburgh, 2000; Kotelnikova, 2002; Antler, tively. Recently, Owen et al. (2016) have established a “shal- 2014; Green-Saxena et al., 2014; Antler et al., 2015; Hu et low” WCM directly underlying the alluvium and a deeper al., 2015; Segarra et al., 2015; Sela-Adler et al., 2015; Cur- “gas reservoir”. The isotopic signatures of these range from rell et al., 2016). Other studies have shown that the presence −80 to −65 and −58 to −49 ‰, respectively. These values of active methanogenesis can be determined using isotopes are summarised in Table 1, along with available ranges of 2 13 of hydrogen in the CH4 (δ H-CH4/ and the surrounding for- δ CDIC for the study area. Thus, the isotopic signature can 2 mation water (δ H-H2O) (Schoell, 1980; Whiticar and Faber, be used to identify the potential source of the CH4; however, 1986; Whiticar, 1999; Currell et al., 2016). Additionally, re- localised formation and oxidation processes that may occur cent studies have used clumped isotopes of CH4 and their either in the aquifer or during transport can confound the in- temperature interpretations to ascribe a thermogenic versus terpretation of mixing versus oxidation processes. biogenic source in groundwater (Stolper et al., 2014). How- The isotopic compositions of DIC and DOC are also use- ever, non-equilibrium (kinetic) processes may be responsible ful indicators of CH4 processes, as they can be used to deter- for an overestimation of CH4 formation temperatures (Wang mine the occurrence of methanogenesis (Kotelnikova, 2002; et al., 2015). Microbiological indicators (in addition to geo- Wimmer et al., 2013). Kotelnikova (2002) found that 13C chemical data) may resolve some of the uncertainties asso- depletion of δ13C-DOC in combination with a 13C enrich- ciated with the determination of CH4 origin, as they directly ment of δ13C-DIC was characteristic of methanogenesis in 12 discriminate between microbiological communities involved groundwater, consistent with the reduction of CO2 by au- in either production or degradation processes. There are no totrophic methanogens. Conversely, δ13C-DIC data are use- studies using combined geochemical and microbiological in- ful because DIC produced during CH4 oxidation was found dicators to assess CH4 production and degradation processes to have a characteristically 13C-depleted signature (as de- in a freshwater aquifer. We aim to fill this gap in the litera- pleted as −50 ‰) (Yoshinaga et al., 2014; Hu et al., 2015; ture. Segarra et al., 2015). Throughout the world the occurrence of freshwater aquifers adjacent to unconventional-gas production is com- 1.2 Methane oxidation in freshwater mon (Osborn et al., 2011; Moore, 2012; Roy and Ryan, 2013; Vidic et al., 2013; Vengosh et al., 2014; Moritz et al., 2015). In groundwater, CH4 is oxidised by methane-oxidising bac- We have previously shown that there may be local natural teria (MOB; methanotrophs) that can utilise CH4 as their sole connectivity between the WCM and the CRAA (Iverach et carbon and energy source. These methanotrophs are grouped al., 2015). Here we show that a combination of geochemi- within the Alpha- and Gammaproteobacteria (comprising 2− − − 13 13 type-I and type-II methanotrophs) and the Verrucomicrobia cal data ([CH4], [SO4 ], [NO3 ], [NO2 ], δ C-CH4, δ C- 13 2 (Hanson and Hanson, 1996). The first step of aerobic CH DIC, δ C-DOC and δ H-H2O), as well as characterisation 4 of microbiological communities present, can inform the dis- oxidation is the conversion of CH4 to methanol. This is catal- ysed by the particulate CH monooxygenase (pMMO) en- cussion surrounding the occurrence of CH4 and its potential 4 for upward migration in the groundwater of the CRAA. coded by the pmoA gene, which is highly conserved and used as a functional marker (Hakemian and Rosenzweig, 2007; 1.1 Geochemical indicators of methanogenic processes McDonald et al., 2008). All known methanotrophs contain the pmoA gene, with members of Methylocella the exception Methanogenesis via acetate fermentation (Eq. 1) and carbon- (Dedysh et al., 2000; Dunfield et al., 2003). Type-II methan- ate reduction (Eq. 2) can be restricted in groundwater with otrophs and some type-I members of the genus Methylococ- 2− −1 abundant dissolved SO4 (> 19 mg L ) (Whiticar, 1999) cus contain the mmoX gene, which encodes a soluble CH4 Biogeosciences, 14, 215–228, 2017 www.biogeosciences.net/14/215/2017/ C.