Stable isotopes as indicators of dissolved methane sources and cycling in the Gippsland Basin, Victoria

Abstract

A suite of environmental tracers were analysed from the Gippsland Basin, Victoria, to determine the origins of methane in groundwater and characterise the biogeochemical and physical processes controlling its occurrence and cycling.

Water samples were collected from a range of depths and lithology, and were analysed for stable isotopes of methane plus a suite of other tracers - radiocarbon, noble gases (He-4, Ne, Ar), δ18O, δ2H and δ13CDIC. The data were analysed within the hydrogeological framework to characterise sources of methane in groundwater and identify possible transport processes.

Methane isotopic compositions ranged widely throughout the system. Two predominant groups of methane were found, both of which were of bacterial origin. One group contained isotopes with typical acetate-fermentation signatures (δ13CCH4 = −45.8 to −66.2‰ and δ2HCH4 = −204 to −311‰), and largely occurred in deep groundwater, near the coast. The other group exhibited unusually depleted δ13CCH4 values by typical global standards (−83.7 to −97.5 ‰) and δ2HCH4 values between −236 and −391‰. This group is associated with relatively shallow groundwater, near areas of extensive lignite. Radiocarbon and He-4 data indicate that groundwater age increases with depth, however inter-aquifer mixing complicates age interpretations. Stable isotopes of water in the deepest parts of the system show relatively depleted δ18O and δ2H, suggesting isotopic modification during methanogenesis and/or depleted signatures associated with palaeo-recharge conditions.

The study provides the first data on dual-isotopic compositions of methane in the Gippsland Basin, and new insights into sources and cycling of methane in a multi-layered sedimentary basin. The basin is one in which extensive groundwater extraction and mining activity occurs, which may be having ongoing effects on inter-aquifer connectivity for both water and gases.