A coupled sediment–water–atmosphere Lagrangian model for assessing short−term CO2 leakage from offshore storage

Sub−seabed CO 2 leakage assessment for offshore carbon storage requires models that track leakage mass across sediment, seawater, and the lower atmosphere. Based on a validated Lagrangian water column CO 2 transport and dissolution model, this study develops a coupled sediment–water–atmosphere framework by incorporating sediment transport and atmospheric dispersion processes. The model is applied to two potential offshore CO 2 storage sites in the South China Sea (80 m and 800 m water depth) under winter and summer forcing, with and without a sediment layer. The two sites exhibited markedly different CO 2 partitioning: 3.43–3.55% of the released CO 2 dissolved into seawater at the 80 m site, compared with 81.91–82.62% at the 800 m site, and the corresponding maximum pH decreases were 0.603–1.004 and 2.112–2.523. This contrast is associated with water depth, but site-specific currents and the seawater volume available for dilution also contribute. The sediment layer delayed and redistributed CO 2 before water column entry, reducing and smoothing the near-source pH decrease. Seasonal winds controlled the atmospheric plume patterns: stronger winter winds produced broader and more dilute plumes, whereas weaker summer winds generated compact plumes with higher local concentrations. Sensitivity experiments identified alkalinity as the strongest control on the pH response at both sites, followed by sediment porosity and temperature, while wind speed and atmospheric horizontal diffusivity controlled the atmospheric CO 2 concentration and plume extent. The framework provides a screening tool for assessing short-term leakage and for designing integrated seabed–water column–atmosphere monitoring strategies.