COMPARISON OF CO2 DETECTION METHODS TESTED IN SHALLOW GROUNDWATER MONITORING WELLS AT A GEOLOGICAL SEQUESTRATION SITE

The geological storage of anthropogenic carbon dioxide (CO₂) is one method of reducing the amount of CO₂ released into the atmosphere. Monitoring programs typically determine baseline conditions in surface and near-surface environments before, during, and after CO₂ injection to evaluate if impacts related to injection have occurred. Because CO₂ concentrations in groundwater fluctuate naturally due to complex geochemical and geomicrobiological interactions, a clear understanding of the baseline behavior of CO₂ in groundwater near injection sites is important. Numerous ways of measuring aqueous CO₂ in the field and lab are currently used, but most methods have significant shortcomings (e.g., are tedious, lengthy, have interferences, or have significant lag time before a result is determined). In this study, we examined the effectiveness of two novel CO₂ detection methods and their ability to rapidly detect CO₂ in shallow groundwater monitoring wells associated with the Illinois Basin – Decatur Project geological sequestration site.  

The CarboQC beverage carbonation meter was used to measure the concentration of CO₂ in water by monitoring temperature and pressure changes and calculating the P_CO2 from the ideal gas law. Additionally, a non-dispersive infrared (NDIR) CO₂ sensor enclosed in a gas-permeable, water-impermeable membrane measured CO₂ by determining an equilibrium concentration. Results showed that the CarboQC method provided rapid (< 3 min) and repeatable results under field conditions within a measured concentration range of 15 – 125 mg/L CO₂. The NDIR sensor results correlated well (r² = 0.93) with the CarboQC data, but CO₂ equilibration required at least 15 minutes, making the method somewhat less desirable under field conditions. In contrast, NDIR-based sensors have a greater potential for long-term deployment. Both systems are adaptable to in-line groundwater sampling methods. Other specific advantages and disadvantages associated with the two approaches, and anomalies associated with specific samples, are discussed in greater detail.