Seasonal and Diurnal Dynamics of Atmospheric Radon, Carbon Dioxide, Methane, δ13C-CO2 and δ13C-CH4 in a Proposed Australian Coal Seam Gas Field
2015
Tait, Douglas R. | Maher, Damien T. | Santos, Isaac R.
The expected growth of the coal seam gas industry in Australia requires baseline information for determining any potential long-term impacts of the industry. As such, a 1-year atmospheric time series measuring radon (²²²Rn), methane (CH₄), carbon dioxide (CO₂), δ¹³C-CO₂ and δ¹³C-CH₄ was conducted in an area where coal seam gas (CSG; also referred to as coal bed methane) extraction is proposed (Casino, New South Wales, Australia). We hypothesise that ²²²Rn can be used as a tracer of soil-atmosphere CH₄ and CO₂ exchange, and that carbon stable isotope values of atmospheric CH₄ and CO₂ can be used to identify the source of greenhouse gases. Radon, CO₂ and CH₄ followed a diurnal pattern related to increased concentrations during the formation of a nighttime inversion layer. The study found a significant inverse linear relationship between ²²²Rn concentrations and both rainfall (r ² = 0.43, p < 0.01) and temperature (r ² = 0.13, p < 0.01), while atmospheric pressure, wind speed and wind direction affected concentrations to a lesser degree over seasonal time scales. ²²²Rn had a significant, but weak positive correlation with both seasonal CO₂ (r ² = 0.15, p < 0.01) and CH₄ (r ² = 0.11, p < 0.01) concentrations. The uncoupling between ²²²Rn and CO₂ and CH₄ was likely due to biogenic sources and sinks of CO₂ and CH₄. δ¹³C values of CO₂ and CH₄ indicated variability in the source and sinks of the gases that seems to be linked to different seasonal, soil and spatial sources. This study provides baseline data from a proposed coal seam gas field from which future comparisons can be made.
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