INVESTIGATING POTENTIAL LOSS OF CARBON DIOXIDE FROM KARST SPRINGS: CONSTRAINTS FROM DISSOLVED INORGANIC CARBON (DIC) AND STABLE CARBON ISOTOPE RATIOS OF DIC (Δ13CDIC)

The potential loss of CO₂ from karst springs were investigated by making measurements of pH, alkalinity, dissolved inorganic carbon (DIC) concentrations, the stable carbon isotope ratios of DIC (δ¹³C_DIC) and calculating partial pressures of CO₂ (pCO₂) along channels from springs discharges. Temporal measurements of these parameters were made for samples collected from the source of the spring and agitated in the laboratory in reactors. Downstream measurements over a distance of 895 m showed a pH increase from 6.81 to 7.77 (14.1%), alkalinity decreased from 5.7 to 5.6 mM C/L (1.75%), DIC decreased from 8.7 to 7.04 mM C/L (18.1%), δ¹³C_DIC increased from -8.4 to -7.3‰ (13.1%) and the log pCO₂ decreased from -1.3 to -2.3 atm. (76.9%). Measurements of the spring samples in the laboratory for 400 hours showed pH increase from 6.81 to 8.33 (22.3%) for the first 147 hours and from 8.33 to 8.6 (3.24%) to the end of experiment, the alkalinity deceased from 5.7 to 3.44 mM C/L (10.5%) for the first 147 hours and increases from 3.44 to 5.1 mM C/L to the end of the experiment, the DIC concentrations decreased from 8.8 to 3.42 mM C/L (61%) for the first 147 hours and followed by and increase 5.7 mM C/L to the end of the experiment, the δ¹³C_DIC increased from -8 to -3‰ (62.5%) for the first 147 hours and remained nearly constant at -3‰ until end of experiment and the log pCO₂ values decreased from -1.3 to -2.9 atm (123%) from 0-147 hours and increased from -2.9 to -3.1 atm. (6.9%) from 147 hours to end of experiment. The increasing pH and δ¹³C_DIC, and decreasing alkalinity, DIC, and pCO₂ along spring channel and over time in the reactors for the first 147 hours are due to CO₂ outgassing. Our result suggest that head waters in karst springs could potentially lose up to 61% of its DIC by outgassing and its δ¹³C_DIC could be enriched by as much as 62.5% during the process. However, field measurements in channels from the spring discharges showed between 18.1% and in DIC and 13.1% enrichment δ¹³C_DIC over distance. The δ¹³C_DIC shows a positive correlation to log pCO₂ for both the field and laboratory samples with the laboratory measurements showing about a 3 times CO₂ fluxes higher than the spring channel measurements. Our results show that combined field and laboratory measurements can give better estimates of the amount of CO₂ that can be lost from karst spring systems.