CARBONATE EQUILIBRIA IN LOW IONIC STRENGTH STREAMS – WHEN IS CO2 REALLY CO2?

Carbonate equilibrium appears simple – three ions – but is infinitely complex. Knowledge of carbonate equilibria is critical to understanding how streams and rivers are a source of CO₂ to the atmosphere. Carbonate equilibria calculations are normally based on measured water temperature and pH in the field, followed by Gran titration in the lab to calculate alkalinity. These data are then used in a set of thermodynamic equations to calculate carbonate ion speciation. In the past, teaching carbonate equilibria worked best with spreadsheet calculations, which still holds for introducing this topic. However, three issues arise in dilute, low ionic strength streams. First, standard pH electrodes underestimate the measured pH. Second, dissolved organic carbon (DOC) contributes to alkalinity. Third, iterative carbonate equilibria calculations are required, which is difficult to execute by students in spreadsheets. As a result, the spreadsheet method for calculation of pCO₂ is not accurate for low ionic strength stream water. The advent of freely available and simple to use software, such as Visual Minteq, along with recently published equations that correct for organic alkalinity and pH measurement error, allow the development of alkalinity calculation exercises that illustrate these issues. The calculated pCO₂, after correction for these errors, can be less than half of the estimate in the simple spreadsheet model in a low ionic strength headwater stream. As an example, for 161 samples collected in small streams corrected pCO₂ was 30% to 63% of uncorrected pCO₂, with the greatest correction associated with lowest ionic strengths and/or highest DOC. The combination of spreadsheets and batch processing in Visual Minteq makes the calculations relatively easy and quick to complete and compare. Pedagogically, this allows learning to scaffold from collecting samples and measuring pH in the field, to measuring alkalinity in the lab, to calculating carbonate speciation in a spreadsheet model, to using software such as Visual Minteq to calculate carbonate speciation, to considering how low ionic strength and DOC impacts pCO₂ calculations.