DIRECT AND CONTINUOUS MEASUREMENT OF DISSOLVED CO2 IN KARST SPRINGS

Deep geologic storage is one approach proposed to reduce the release of anthropogenic CO₂ to the atmosphere by power plants and other industries. The ability to accurately monitor the potential impacts of unanticipated upward migration of injected CO₂ on surface and near-surface aquatic environments is therefore necessary. Prior research has shown the value of non-dispersive infrared (NDIR) CO₂ sensors in determining instantaneous dissolved CO₂ in groundwater monitoring wells and surface springs, but less is known about their robustness in the field during long-term monitoring activities. In this study, NDIR-based sensors were deployed long-term in three central Pennsylvania karst springs. These springs are part of an on-going study monitoring numerous water quality variables to better understand the associated complex hydrology.

The infrared gas analyzers used are designed to measure CO₂ concentrations up to 20% in humid air using a single-beam dual-wavelength NDIR light source and a silicon-based sensor. A waterproof poly-tetrafluoroethylene (PTFE) sleeve that is highly-permeable to CO₂ covers the sensor and is sealed to the meter cable. Data are collected hourly on a logger, and the entire system is powered by marine battery. Energy consumption by the sensor requires that the battery be changed monthly based on the hourly data collection scenario. Specific issues under study include the accuracy of the CO₂ data, the integrity of the waterproof membrane over time, the influence of hydrostatic pressure on sensor operation, potential influence of biofouling on CO₂ diffusion across the sensor membrane, and direct comparison of NDIR measurements with CO₂ determined via grab samples and geochemical modeling. Initial results and problems encountered during the first two months of sensor operation under field conditions will be discussed.