Optimising soil gas monitoring at an on-shore geological carbon sequestration site

• Systematic, random, and risk-based soil gas monitoring strategies were evaluated. • Apparent trade-off between sample density and data representativeness. • A strategy-based soil gas baseline is recommended for long-term routine monitoring. Soil gas sampling provides valuable insights for environmental assurance monitoring at carbon capture and storage (CCS) sites. Being a point-source sampling technique, soil gas samples are subject to natural temporal and spatial variabilities. CCS-related soil gas monitoring strategies have evolved over the past decades from intensive sampling to a risk-based approach targeting specific areas. This paper outlines the evolution of monitoring and explores options to optimise monitoring strategies at different spatial resolutions. Sampling approaches were trialled using one year of data collected from an Australian CCS demonstration site, the Otway International Test Centre (OITC), where soil gas was monitored at 124 grid-based locations (100–200 m apart) to achieve multiple monitoring objectives. Results of soil gas CO 2 concentration from the 2022 annual sampling campaign ranged over tenfold (0.04 % to 8.07 %), providing a reasonable characterisation of the natural variability of soil gas composition. This systematic approach was compared with random selections of sampling locations at reduced sampling densities, grid-based sampling at reduced sampling densities, and risk-based monitoring. The combined approach with systematic monitoring and a risk-based approach, depending on the stages of the project, are recommended. Strategy-specific baselines extracted from a comprehensive baseline are recommended to take into consideration spatial variability based on the design and potential changes in long-term monitoring.