CO2 IN GROUNDWATER: THE PAST 50+ YEARS, AND MORE TO LEARN

As the science of belowground CO₂ storage or sequestration develops, methods for detecting escaping CO₂ are also being investigated. Knowledge about CO₂ in groundwater is invaluable to establishing the baseline levels to which escaping sequestered-CO₂ may be added. Awareness that groundwater can contain considerable amounts of CO₂ has a very long history, but, in the mid-1940’s and early-1950’s, two papers demonstrated that oxidation of aquifer organic matter was a source of CO₂ in addition to CO₂ from the soil zone. This changed the outlook that CO₂ was essentially a “closed system” for aquifers out of contact with the vadose zone. In the 1960’s, dating of groundwater inorganic carbon by the ¹⁴C method established a quantitative link between soil- and groundwater-CO₂, and established a way to determine the residence time of groundwater. The 1970’s researchers studied carbonate-aquifer springs and modern carbonates, and clarified the processes of carbonate cementation and dissolution as related to CO₂. In the 1980’s and 1990’s began the quantitative modeling of CO₂ transport in the soil and vadose zone, as a result of the shrinking size and expanding capabilities of computers. The modeling has grown more sophisticated in the 2000’s, as has instrumentation for CO₂ measurement. In addition, the importance of groundwater as baseflow to streams and the consequent degassing of CO₂ has given rise to the need to better understand the terrestrial-hydrologic carbon cycle and its impact on atmospheric CO₂. With the probability that soil-CO₂ production is proportional to atmospheric CO₂ concentrations, quantifying the delivery of soil CO₂ to the saturated zone in many soil types and many climates is crucial to characterizing groundwater CO₂ distribution. The global distribution of CO₂ in groundwater is not well known, being somewhat dynamic and being spatially three-dimensional, but should be an important new integrated research area that can be managed with tools such as GIS, and which should also have practical and important application in the monitoring of belowground CO₂ storage or sequestration.