Spatial and temporal variation of CO₂ and CH₄ emissions from a septic tank soakaway

CO₂ and CH₄ flux measurements over a septic tank soakaway located in a northern maritime climate (Ireland) were conducted for a period of 81 days using a multi-chamber automated soil gas flux chamber system with high spatial and temporal resolution. Overall median CO₂ fluxes were 7.28; 6.40 μmol CO₂ m^-2 s^-1 from the soakaway and control soil, respectively. Overall median CH₄ fluxes were - 0.28; -0.67 nmol CH₄ m^-2 s^-1 from the soakaway and control soil, respectively. While CO₂ fluxes expressed strong diurnal variability driven by soil temperature, CH₄ fluxes were less affected by environmental factors and effectively limited to the first few meters from the septic tank. However, localised CH₄ degassing events were observed during drying conditions with up to 60-times higher fluxes compared to the overall median. The soakaway was found to be a net emitter of both CO₂ and CH₄, releasing a total of 7.327 kg CO₂ yr^-1 and 0.033 kg CO_2Eq. yr^-1, respectively. The apparent spatio-temporal heterogeneity of observed soil gas fluxes identified in this study emphasises the importance of integrating measurements with both high spatial and temporal resolution from on-site installations as engineered nature-based solutions.