Investigating radiocarbon isotope anomalies in CO2 for CCS monitoring: Insights from the Aquistore project and the influence of coal mine spoils

• Δ 14 CO 2 anomalies at Aquistore predate injection, challenging its use as a tracer. • Coal seam venting and microbial activity were tested as causes of Δ 14 CO 2 anomalies. • Microbial activity on coal fragments explains Δ 14 CO 2 depletion in dry conditions. • Site-specific monitoring is needed to avoid misinterpreting natural Δ 14 CO 2 signals. • Δ 14 CO 2 should be combined with tracers and emission monitoring for better reliability. The radiocarbon isotope of CO 2 (Δ 14 CO 2 ) is a valuable tool for investigating soil-respired CO 2 and identifying its sources. At Aquistore, a CCS project in Canada, Δ 14 CO 2 is incorporated into surface soil-gas geochemical MMV studies to demonstrate the absence of surface impacts from CO 2 injection. However, some monitoring locations consistently show negative Δ 14 CO 2 values (usually indicative of fossil CO 2 ) that predate CO 2 injections, suggesting these anomalies are natural or linked to soil disturbances. This study investigates the origins of these anomalies. We first hypothesized that CO 2 venting from coal seams underlying parts of the monitoring grid could explain the negative values. A spatial correlation between negative Δ 14 CO 2 values, historic open-pit coal mines, and mine spoils supported this hypothesis. However, Δ 14 CO 2 analysis from a nearby farm (control site) with intact coal seams but no mine spoils showed modern-age signatures (6.8 ± 16 %), ruling out the venting hypothesis. Next, we tested whether microbial decomposition of weathered coal fragments was responsible. A soil survey revealed a strong correlation between coal fragment concentrations (1.85–40.64 % w/w) and Δ 14 CO 2 anomalies. Laboratory incubation experiments further supported this hypothesis, showing that dry soils with weathered coal fragments produced proportionately negative Δ 14 CO 2 values, likely due to microbial activity. These findings underscore the need for a more nuanced and site-specific approach to CCS monitoring. In geochemically complex sites like Aquistore, relying solely on Δ 14 CO 2 may be misleading, and alternative tracers should be considered to ensure accurate soil-gas anomaly interpretations.