Seasonal and Diurnal Dynamics of Atmospheric Radon, Carbon Dioxide, Methane, δ13C-CO2 and δ13C-CH4 in a Proposed Australian Coal Seam Gas Field

Abstract

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.