Calcite precipitation rates in the field: Measurement and prediction for a travertine-depositing stream

Rates of calcite precipitation from a travertine-depositing stream were determined from changes in stream composition between consecutive sampling points and were compared with rates predicted from a laboratory-derived rate law ( Plummer et al., 1978). The agreement in rates was generally within an order of magnitude and routinely within a factor of 3. Least agreement between measured and predicted rates was obtained for sections of flowpath where relatively little change in bulk chemical composition occurred, which were the sections with the greatest mass transfer calculation error, and for the stream segment including a waterfall, which was the section with the greatest error in estimated surface area. Reaction rate obtained from the mass of calcite precipitated onto seed crystals placed in the stream significantly underestimated the mass transfer rate. For the travertine-depositing stream of Warm River Cave and Falling Spring Creek, Virginia, the coupling of equilibrium speciation models with mass balance calculations and simple field measurements allowed successful field-based quantification of reaction rates.