Fractionation processes of δ13C in a soil-shallow groundwater system of a potential CO2 sequestration site: Wandoan, QLD, Australia

Abstract

Near-surface leakage detection is often a crucial part of verifying the success of CO2 sequestration projects, and it cannot be achieved without a detailed description of the natural state of an operational site prior to injection. Without baseline studies, it is difficult to define CO2 anomalies outside natural variation in the subsurface that may be related to leakage. CO2 concentrations and δ13C-CO2 values (of DIC, dissolved gas, and soil gas) were investigated to establish a baseline modelling of natural chemical processes leading to CO2 occurrence and δ13C-CO2 fractionation in the soil-groundwater interface on an example at the Glenhaven site in Queensland, Australia. This was analysed within the context of the hydrogeology and hydrogeochemistry of a shallow aquifer system (<20 m depth). The Glenhaven site and its five monitoring station are located in an area which is a candidate for a future small scale carbon storage pilot project. We combine major ion, stable isotope data of groundwater and soil gas as well as physical hydrogeology data with extensive isotope models to explain the pathways and origins of groundwater and CO2. The results indicate shallow aquitard claystone units at the site restrict vertical groundwater leakage and inter-aquifer mixing. Groundwater chemistry is controlled by transpiration of plants, cation exchange, weathering and meteoric recharge to produce Na-Cl and Na-HCO3-Cl type waters. δ18O and δ2H values suggest a shallow meteoric recharge source largely unaffected by evaporation. CO2 concentrations in soil and dissolved in groundwater range between 631 and 47,516 ppm and δ13C values average −17 and −21‰. The mixing of three potential CO2 sources is discussed: 1) the aerobic decay of C3 plant matter, 2) the aerobic decay of C4 plant matter and, 3) shallow coal related CO2. Despite this, the variability of CO2 concentrations and equivocal sources is not likely to hinder leakage detection in the future of site monitoring.