Occurrence of greenhouse gases in the aquifers of the Walloon Region (Belgium)

This work aims to (1) identify the most conductive conditions for the generation of greenhouses gases (GHGs) in groundwater (e.g., hydrogeological contexts and geochemical processes) and (2) evaluate the indirect emissions of GHGs from groundwater at a regional scale in Wallonia (Belgium). To this end, nitrous oxide (N2O), methane (CH4) and carbon dioxide (CO2) concentrations and the stable isotopes of nitrate (NO3−) and sulphate were monitored in 12 aquifers of the Walloon Region (Belgium). The concentrations of GHGs range from 0.05 µg/L to 1631.2 µg/L for N2O, 0 µg/L to 17.1 µg/L for CH4, and 1769 to 100,514 ppm for the partial pressure of CO2 (pCO2). The highest average concentrations of N2O and pCO2 are found in a chalky aquifer. The coupled use of statistical techniques and stable isotopes is a useful approach to identify the geochemical conditions that control the occurrence of GHGs in the aquifers of the Walloon Region. The accumulation of N2O is most likely due to nitrification (high concentrations of dissolved oxygen and NO3− and null concentrations of ammonium) and, to a lesser extent, initial denitrification in a few sampling locations (medium concentrations of dissolved oxygen and NO3−). The oxic character found in groundwater is not prone to the accumulation of CH4 in Walloon aquifers. Nevertheless, groundwater is oversaturated with GHGs with respect to atmospheric equilibrium (especially for N2O and pCO2); the fluxes of N2O (0.32 kg N2O-N Ha-1 y-1) and CO2 (27 kg CO2 Ha-1 y-1) from groundwater are much lower than the direct emissions of N2O from agricultural soils and fossil-fuel-related CO2 emissions. Thus, indirect GHG emissions from the aquifers of the Walloon Region are likely to be a minor contributor to atmospheric GHG emissions, but their quantification would help to better constrain the nitrogen and carbon budgets.