Coupled environmental modulation and source intermittency in gas leakage from a legacy well

Legacy wells represent a long-term risk to the integrity of geological CO₂ storage and other subsurface energy systems, yet leakage detection and interpretation remain difficult because surface gas fluxes are highly variable and strongly modulated by environmental conditions. Here we present a field investigation of gas leakage from an integrity-compromised legacy well in northeastern British Columbia, using high-frequency, multi-species flux monitoring to examine how subsurface leakage is expressed at the surface under variable environmental conditions. We combine continuous CH₄ and CO₂ flux measurements, repeated spatial mapping, and co-located soil and meteorological observations over several days, using multivariate statistics and machine-learning approaches to interpret leakage behaviour. We find that leakage expresses through two contrasting but coupled surface signatures. Methane emerges as a compact and persistent hotspot (~1 m²), consistent with focused, advective transport along pathways near the wellbore. In contrast, excess CO₂ forms a broader and mobile footprint (~30 m²) that shifts between surveys, reflecting strong near-surface modulation as migrating CH₄ is partially oxidised and redistributed laterally within the soil. Emissions are modest yet sustained over the observation period, illustrating how small, chronic leaks may accumulate into climate and containment concerns over storage timescales. Baseline CH₄ variability is dominated by environmental state, whereas short-lived high-flux events contribute disproportionately to total emissions and are not explained by measured surface forcing, indicating transient changes in subsurface source behaviour or flow configuration. Together, these results show that surface leakage signatures reflect coupled environmental modulation and subsurface intermittency rather than source behaviour alone.