Constraining the effectiveness of inherent tracers of captured CO2 for tracing CO2 leakage: Demonstration in a controlled release site.

Geological storage of carbon dioxide (CO2) is an integral component of cost-effective greenhouse gas emissions reduction scenarios. However, a robust monitoring regime is necessary for public and regulatory assurance that any leakage from a storage site can be detected. Here, we present the results from a controlled CO2 release experiment undertaken at the K-COSEM test site (South Korea) with the aim of demonstrating the effectiveness of the inherent tracer fingerprints (noble gases, δ13C) in monitoring CO2 leakage. Following injection of 396 kg CO2(g) into a shallow aquifer, gas release was monitored for 2 months in gas/water phases in and above the injection zone. The injection event resulted in negative concentration changes of the dissolved gases, attributed to the stripping action of the depleted CO2. Measured fingerprints from inherent noble gases successfully identified solubility-trapping of the injected CO2 within the shallow aquifer. The δ13C within the shallow aquifer could not resolve the level of gas trapping, due to the interaction with heterogeneous carbonate sources in the shallow aquifer. The time-series monitoring of δ13CDIC and dissolved gases detected the stripping action of injected CO2(g), which can provide an early warning of CO2 arrival. This study highlights that inherent noble gases can effectively trace the upwardly migrating and fate of CO2 within a shallow aquifer.

Characterization of a NAPL-contaminated site using the partitioning behavior of noble gases.

Noble gases have been used for oil field exploration due to their partitioning behavior in oil-water systems. However, their application to study sites contaminated with non-aqueous phase liquids (NAPL) has been limited, except for 222Rn, which has been traditionally used as a partitioning tracer for contaminated sites. This study applied natural noble gas components such as 222Rn, He, Ne, Ar, Kr, and Xe to the characterization of a field site contaminated with trichloroethylene (TCE) located in Wonju, Korea. Groundwater at the site showed a maximum level of TCE that exceeded 1000 µg/L, with an approximate average of 400 µg/L, indicating the presence of residual TCE in the subsurface system even after remediation. The traditional tracer (i.e., 222Rn) was first used to characterize residual TCE. However, its heterogeneous distribution throughout the fractured bedrock aquifer negated its usefulness as a TCE indicator. The use of radiogenic 4He was also limited by the wide distribution of radiogenic sources on the site. By contrast, changes in the TCE level had clear effects on the conditions of other noble gases, such as Ne, Ar, and Xe, making them useful for characterization of the TCE-contaminated site. Furthermore, calculation of the TCE/water ratio including residual TCE was achieved, but identification of the TCE originating from the vadose zone was relatively hard. The results of this study indicate that based on their partitioning behavior, naturally-occurring noble gases can be used to delineate and quantify residual TCE.