Evaluating the effects of sample processing treatments on alkalinity measurements

The concentration of bicarbonate, the dominant anion in river water, is typically calculated using measured alkalinity concentrations. Alkalinity concentrations also are necessary for analytical charge balance, carbonate speciation, and PCO2 calculations. Although the Gran titration is the standard method for measuring alkalinity, previous studies have varied in how water samples have been processed prior to analysis. The purpose of this study was to determine whether variation in filtering, refrigeration, and sample storage time leads to statistically significant differences in alkalinity concentrations in oxic, low-turbidity surface waters and groundwaters ranging from undersaturated to supersaturated with respect to calcite. Samples were collected from the Blue Ridge and Piedmont Provinces of northwestern South Carolina and the Valley and Ridge Province of eastern Tennessee. Each sample was analyzed using four processing treatments: filtered and refrigerated, filtered and unrefrigerated, unfiltered and refrigerated, and unfiltered and unrefrigerated (n = 3 analyses per treatment combination). All replicates were analyzed within 24 h using the Gran titration method. Samples then were reanalyzed after 17–194 days of storage to determine if storage time affects alkalinity measurements. Overall, alkalinity concentrations ranged from 0.8 to 107 mg/L CaCO3. Statistically significant differences among the treatments and significant interactions between filtration and refrigeration were limited. For samples in which treatment means differed significantly or in which interactions were significant, differences generally were less than 10% and probably of little biogeochemical importance. For all but one sample locality, storage time did not affect alkalinity. Our results show that for oxic waters that have low concentrations of DOC and reduced metals, filtration, refrigeration and storage time are unlikely to affect measured alkalinity concentrations. However, additional research is needed to determine the degree to which sample processing treatment is important to interpreting the alkalinity concentrations of DOC- and metal-rich samples.