Detecting hydraulic connection between fresh water aquifers and coal seam gas production using the isotopes of carbon in methane

Abstract

There is considerable public concern that coal seam gas (CSG) production will affect groundwater quality and quantity in adjacent or overlying aquifers. To assess this risk there is a need to map pathways of hydraulic connectivity using geochemical and hydraulic measurements. We demonstrate that measurements of methane (CH4) concentration and isotopic composition, dissolved organic carbon isotopes (ö13 DOC) and tritium (3 1-1) activity data delineate potential pathways of hydraulic connectivity between the Walloon Coal Measures (WCM - the target formation for CSG production) and the Condamine Alluvium which is the primary freshwater aquifer in the Condamine Catchment, south-east Queensland, Australia. The Walloon Coal Measures is a 700 m thick, low-rank CSG resource that is overlain by sandstone formations, which form part of the Great Artesian Basin (GAB). The Condamine Alluvium fills a paleovalley carved through the above formations.

At 18 locations, both groundwater and degassing air samples were collected from irrigation bores. Degassing air samples were collected using an SKC 222-2301 air pump, which pumped the gas into 3 L Tedlar bags. The groundwater was analysed for 31-1 and ö13DOC. A mobile CH4 survey was undertaken to collect continuous air samples around areas of agricultural and unconventional gas production.

We use isotope mixing plots to identify the isotopic'source signature of CH4 in the air samples from the off gassing irrigation bores and ambient air samples adjacent to CSG water holding ponds. Samples cluster along clear trend lines within the mixing plot and indicate potential local natural hydraulic connectivity between the target WCM and the overlying Condamine Alluvium.