MEASURING DISSOLVED INORGANIC CARBON IN COAL MINE WATERS: A COMPARISON OF METHODS

Dissolved inorganic carbon (DIC) can be present at high concentrations in coal mine waters due to sulfate-driven dissolution of carbonate strata. Depending on the pH, the DIC can be present as dissolved CO₂ or the bicarbonate ion. It can be difficult to accurately measure DIC concentrations in these waters because degassing, oxidation, and precipitation reactions rapidly destabilize the water as it discharges at the surface. In this study we compared various methods for measuring DIC in coal mine waters to evaluate their precision and potential error. The techniques that were used to measure DIC included (1) field measurement of CO₂ using a commercial beverage CO₂-analyzer (CarboQC^TM) and calculating DIC from CO₂, pH, and temperature; (2) laboratory measurement using a Shimadzu Carbon Analyzer; and, (3) calculation from alkalinity and acidity titrations. The first two methods measure only DIC. In the third method, the final result can be biased because the calculation assumes that alkalinity and acidity are dominated by carbonate species; this is not always true in coal mine waters. The measured DIC concentrations were up to 6.5 mmol/L. Results from the CO₂ and Shimadzu methods were comparable although precipitation-related issues arose for the latter due to the time lag between sample collection and analysis. The alkalinity and acidity calculations were the least accurate of the three methods in the coal mine waters, likely due to the presence of non-carbonate alkalinity and acidity in the water. Although less accurate than the other methods, the titrations can be completed in-situ, limiting the impacts of sampling handling and storage. In contrast, all three methods provided comparable DIC data in carbonate-dominated waters.