Comparing greenhouse gas stable isotopes from different sampling methods: Aquifer groundwater and coal seam gas production waters

Methane and carbon dioxide are ubiquitous in aquifers and are gaining increased attention through their role in global warming. The stable carbon and hydrogen isotopes of methane and carbon dioxide, δ¹³C–CH₄, δ¹³C–CO₂ and δ²H–CH₄, can inform on the sources of gas in groundwater and determine if the origin is via thermogenic or different biogenic (microbial) processes. This can, for example, inform if there is connectivity between gas reservoirs and overlying aquifers. These stable isotopes are also used in monitoring in a range of settings, for example, during CO₂ geological storage. The pumping of groundwater to the surface results in depressurisation and gas effervescence, therefore the sampling method (i.e. if the sample contacts atmosphere) is known to affect measured dissolved gas concentrations; however, the effect of sample collection on the stable isotopes remains unresolved. Gas stable isotopes can be collected in the field in a range of sample containers, including dissolved gases in exetainer vials or Isoflasks; or free gases in gas bags or cylinders. Aquifer groundwaters, coal seam gas (CSG) production waters, and CSG free gases were collected from a range of bores for δ¹³C–CH₄, δ¹³C–CO_2, and δ²H–CH₄ analysis. Sample pairs of dissolved gases from exetainer vials and Isoflasks were mainly compared and generally had excellent low directional percentage variabilities. A subset of sample pairs from other containers (including free gas in gas cylinders or gas bags) were also compared with consistent stable isotope signatures generally measured. This brings confidence for using different sample containers if needed for different field conditions and situations.