CO2 FROM SPRINGS AND SPRING-LIKE PORTALS TO THE UNDERWORLD: CHALLENGING METHODS AND CONVENTIONS (Invited Presentation)

Karst scientists have used estimated values of the CO₂ partial pressure (P_CO2) extensively in their characterization and interpretation of karst waters. Typically, the P_CO2 is calculated from the pH and alkalinity with or without further refinement based on solution speciation, temperature corrections of thermodynamic values, or activity coefficient adjustments. The approach works well in carbonate waters but the inherent assumptions in the calculation may not be as appropriate in waters with a different geochemistry. Even in carbonate waters, the estimated number may depend on pH accuracy, the choice of alkalinity endpoint, and if the sample if filtered. Alternative approaches include direct gas measurement and analysis of total dissolved inorganic carbon, both of which have potential interferences and require some knowledge of the water chemistry. Many of these problems are compounded when sampling non-carbonate waters such as discharge from coal mines, thermal-mineral springs, or reducing waters that change rapidly once the sample is collected. For example, samples collected from high-CO₂ waters for DIC analysis lost more than 45 percent of their CO₂ during the holding time even though standard methods and recommended containers were used. Unfiltered samples and samples that are actively oxidizing and precipitating solids may contain non-carbonate alkalinity and thereby invalidate the assumption that alkalinity is approximately equal to the bicarbonate concentration. Furthermore, samples with low pH can contain high concentrations of CO₂ but are often excluded from calculations regarding CO₂ because there is no measureable alkalinity. Knowing the assumptions, conventions and limitations of each method is critical if we want to understand the role springs and ground water discharges play in carbon cycling and the flux of CO₂ from the terrestrial system.