ASSESSING TEMPORAL LOSS OF CO2 FROM NATURAL WATERS USING DISSOLVED INORGANIC CARBON CONCENTRATIONS AND STABLE CARBON ISOTOPES

We exposed solutions of NaHCO₃ and groundwater and lake water samples to the atmosphere over 850 hours in a laboratory setting. We aim to assess the loss of CO₂ from the samples as they interact with atmospheric CO₂ over time. All samples were prepared in duplicate and one set was agitated by aquarium pumps to simulate turbulence and mixing in surface waters. We made concentration measurements of alkalinity and dissolved inorganic carbon (DIC), and measured the stable carbon isotope ratio (δ¹³C) of DIC. The concentrations of alkalinity mirrored that of DIC in the NaHCO₃ and lake samples, which increases (40-175%) in the agitated samples compared to 20% for unagitated samples. In contrast the agitated groundwater samples showed a 30% decrease for the first 50 hours followed by a 6% increase in alkalinity and DIC concentrations. The unagitated groundwater sample showed a slow steady decrease of about 5% in alkalinity and DIC concentration over time. Overall, the δ¹³C of DIC increased for the NaHCO₃ and groundwater samples by 6 to 15 per mill, while groundwater samples increased by 6 to 8 and the lake samples increased by 1 to 2 per mill. Estimates of the partial pressure of pCO₂ of all samples were higher than atmospheric and the sample should potentially loss DIC from solution. The overall increase in alkalinity and DIC in the NaHCO₃ and lake samples was due to evaporative concentration, as water evaporated from the reactors over time. This suggests that DIC loss to the atmospheric reservoir was negligible in these samples. The DIC lost from the groundwater samples was in the form of CO_2(aq) which was quickly depleted from solution, causing alkalinity and DIC concentrations to increase due to evaporative enrichment. The increasing δ¹³C of the NaHCO₃ and groundwater samples with increase time suggest carbon exchange between DIC and atmospheric CO₂. The limited change in the δ¹³C of lake water samples despite increase in DIC concentrations suggest that the carbon in the sample was nearly in isotopic equilibrium with atmospheric CO₂. The results of this study suggest that CO₂ loss from natural samples is related to (1) the concentration of CO_2(aq) in the sample and (2) secondary processes (e.g., turbulence) that increases the rate of CO₂ loss to the atmosphere.