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

Abstract

The radiocarbon isotope of CO₂ (Δ¹⁴CO₂) is a valuable tool for investigating soil-respired CO₂ and identifying its sources. At Aquistore, a CCS project in Canada, Δ¹⁴CO₂ is incorporated into surface soil-gas geochemical MMV studies to demonstrate the absence of surface impacts from CO₂ injection. However, some monitoring locations consistently show negative Δ¹⁴CO₂ values (usually indicative of fossil CO₂) that predate CO₂ injections, suggesting these anomalies are natural or linked to soil disturbances. This study investigates the origins of these anomalies. We first hypothesized that CO₂ venting from coal seams underlying parts of the monitoring grid could explain the negative values. A spatial correlation between negative Δ¹⁴CO₂ values, historic open-pit coal mines, and mine spoils supported this hypothesis. However, Δ¹⁴CO₂ 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 Δ¹⁴CO₂ anomalies. Laboratory incubation experiments further supported this hypothesis, showing that dry soils with weathered coal fragments produced proportionately negative Δ¹⁴CO₂ 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 Δ¹⁴CO₂ may be misleading, and alternative tracers should be considered to ensure accurate soil-gas anomaly interpretations.