COMPARISON OF METHODS USED FOR CO2 LEAK DETECTION DURING PULSED RELEASE TESTS INTO POTABLE AQUIFER WELLS

One approach proposed to reduce the release of anthropogenic CO₂ to the atmosphere is deep geological storage. Monitoring for potential upward migration of CO₂ to surface and near-surface aquatic environments is therefore necessary. Conventional alkalinity titration methods used to measure aqueous CO₂ in the field have the significant shortcomings of being tedious, potential interference due to non-carbonate alkalinity, and being highly susceptible to error in pH measurements. In this study, we compared the effectiveness of a novel CO₂ detection method using multiple volume expansion with conventional pH/alkalinity titrations. Shallow (ca. 6 m) groundwater monitoring wells in a siliclastic-dominated alluvial aquifer were injected with 10, 60 and 100% volumes of CO₂ gas and geochemical responses were monitored during the injection and retreat phases. A 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. For each pulsed test, gas from a CO₂ tank was bubbled into the bottom of the test well. Water samples were collected from various depths in the well by peristaltic pump for alkalinity titration, chemical analysis, and direct CO₂ analysis. Results showed that the CarboQC method provided rapid (< 3 min) and repeatable results under field conditions within a measured concentration range of 1.6 - 31 mM CO₂. Good correlation was observed between CarboQC and alkalinity/pH CO₂ analysis, but 10 times the number of samples could be processed in the field with the CarboQC, and CO₂ based on alkalinity titration required the further analysis of anions and cations. The ability to analyze water samples quickly and frequently allowed for a more accurate assessment of CO₂ concentration and distribution in the well under field conditions. Depth analysis indicated that a strong vertical gradient of dissolved CO₂ developed, either due to the natural rise of different-sized gas bubbles, or the influx of low-CO₂ groundwater at depth.