Coal seam gas (CSG), or coal bed methane, developments in sedimentary basins such as the Great Artesian Basin (GAB) in Australia, have the potential to impact on aquifers overlying and underlying the target coal formations. The extent to which this may occur depends upon the degree of hydrogeological connectivity between the coal formations and the surrounding aquifers or aquifer systems, with general implications for groundwater management. In southeast Australia, one such aquifer system, the Condamine Alluvium (CA), overlies the Walloon Coal Measures (WCM), which is a formation of the GAB and also a target for CSG production. To investigate the connectivity between the two systems, multiple lines of investigation were employed involving field investigations, data gathering and analysis (including reinterpretation of geology, multivariate hydrochemistry analysis, regional water-level mapping, drilling and coring across the contact zone, multiple piezometer installations, long-term pumping tests, groundwater-level monitoring and local-scale modelling). The study found a low level of connectivity between the GAB and the overlying CA. A layer of undifferentiated basement clay (referred to as the 'transition zone')-a mixture of alluvial clay and weathered basement-provides an effective impediment to flow across the CA and the underlying GAB formations. Results from the study potentially have wider application across the GAB and sedimentary basins where younger alluvial sediments associated with river systems frequently overlie the erosional surface.

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The groundwater resources of the Condamine River Alluvial Aquifer (Condamine Alluvium) in Queensland have been developed in the past 60 years for irrigation, stock, domestic and other uses. Extraction of more than 55,000 mL/yr from this aquifer is critical to supporting the local irrigation industry. Existing and proposed coal seam gas development in the Surat Basin extends to underneath the western edge of the CA footprint and involves depressurisation of coals that form up to 10% of the Walloon Coal Measures (WCM) at some depth below the alluvium. Reduced groundwater availability from the already-stressed Condamine Alluvium is thus an oft-quoted concern of landholders when further development of the CSG industry is considered. An extensive study, led by the Queensland Office of Groundwater Impact Assessment (OGIA) and supported by Arrow Energy, was carried out to provide quantitative assessments of vertical hydraulic conductivity between the alluvium, coal measures and intervening formations. This extended abstract focuses on one element of the aforementioned study led by Arrow Energy, specifically, aquifer testing undertaken at two representative sites overlying the Condamine Alluvium. This included drilling and collection of core; geophysical, geomechanical and geochemical testing; test pumping and monitoring; and, modelling. An important element of this project, aside from sharing of data and results—was the involvement of local landholders in development of the investigations, and attendance at field days during drilling and presentation of results. Key findings included estimated ranges of vertical hydraulic conductivity derived from parameter estimation modelling that were lower than previously suggested.

The Surat Basin hosts various industries that extract groundwater including coal seam gas (CSG), feedlots, and agriculture. With water drawdown, gas has been observed in some bores drawing groundwater from different aquifers across the Basin. While methane can occur naturally in aquifers, biogenic CSG has been extracted from the Walloon Coal Measures raising questions on the sources of gas in overlying aquifers. Current standard monitoring uses a direct fill approach to measure dissolved methane concentrations in vials; however, this approach may lose gas present above solubility levels. Water and gas sampling was performed on bores in the Springbok, Gubberamunda, Mooga, Hutton, and Precipice Sandstones, the Orallo Formation, and the Condamine Alluvium. Water bores and CSG production wells from the Walloon Coal Measures were also sampled. We compared direct fill with a closed sampling method for dissolved gas, plus a method for sampling the total free and dissolved gas in the field. Higher dissolved and total methane concentrations were measured using closed sampling methods, especially in gassy bores. The majority of our sampled aquifer gases and waters have stable isotopic signatures distinct from CSG, where methane had likely been formed insitu in shallow aquifers by primary microbial CO2 reduction or fermentation processes. In several gassy bores, the source of the methane could not be clearly identified. This study indicates that (1) current monitoring methods may be underestimating methane concentrations above identified thresholds especially in gassy bores and (2) a combination of isotopic techniques may distinguish methane sources and interaquifer disconnectivity in the majority of cases.