Assessing Connectivity Between an Overlying Aquifer and a Coal Seam Gas Resource Using Methane Isotopes, Dissolved Organic Carbon and Tritium.

Charlotte P Iverach,Dioni I Cendón,Rebecca E Fisher,James L France,David Lowry,Bryce F J Kelly,Andy Baker,Stuart I Hankin,Euan G Nisbet

doi:10.1038/srep15996

Charlotte P Iverach, Dioni I Cendón + Show 7 more

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Coal seam gas (CSG) production can have an impact on groundwater quality and quantity in adjacent or overlying aquifers. To assess this impact we need to determine the background groundwater chemistry and to map geological pathways of hydraulic connectivity between aquifers. In south-east Queensland (Qld), Australia, a globally important CSG exploration and production province, we mapped hydraulic connectivity between the Walloon Coal Measures (WCM, the target formation for gas production) and the overlying Condamine River Alluvial Aquifer (CRAA), using groundwater methane (CH4) concentration and isotopic composition (δ13C-CH4), groundwater tritium (3H) and dissolved organic carbon (DOC) concentration. A continuous mobile CH4 survey adjacent to CSG developments was used to determine the source signature of CH4 derived from the WCM. Trends in groundwater δ13C-CH4 versus CH4 concentration, in association with DOC concentration and 3H analysis, identify locations where CH4 in the groundwater of the CRAA most likely originates from the WCM. The methodology is widely applicable in unconventional gas development regions worldwide for providing an early indicator of geological pathways of hydraulic connectivity.

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The average background [CH4] was 1.774 ± 0 .002 ppm and the isotopic signature was − 47.0 ± 0.05‰ (n = 8 ; Supplementary Table S3 online). These values are slightly higher than those reported for the Cape Grim Baseline Air Pollution Station in Tasmania, and at Arcturus in central Qld[39,40]
To determine the source δ 13C-CH4 value for the WCM three air samples collected along line A-B (Fig. 3), when the wind was blowing from the west directly across the reservoir, were combined with two other samples downwind of the Coal seam gas (CSG) region
At our case study site in the Condamine Catchment, the isotopic value of − 55.9‰ from the irrigation boreholes with detectable [dissolved organic carbon (DOC)] is not as 13C enriched in CH4 as expected from a classical thermogenic source, due to biological processes occurring in situ

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Introduction

- 3 H activities provide information on groundwater residence times and recharge pathways, - [DOC] provides a measure of the carbon inputs, either from the river recharge or the upward migration of CH4 from a coal bed, and - δ 13C-CH4 can be used to characterise the potential sources of the CH4 within an aquifer. To determine the source δ 13C-CH4 value for the WCM three air samples collected along line A-B (Fig. 3), when the wind was blowing from the west directly across the reservoir, were combined with two other samples downwind of the CSG region.

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