Measuring in situ reaction rate constants in wetland sediments

Abstract

Wetlands are ecologically important and play a key role in many environmentally significant chemical reactions. However, an accurate way of measuring in situ reaction rates in wetland sediments has yet to be established. This study evaluates the feasibility of adapting the push-pull test often used to measure in situ kinetics in subsurface environments, to wetlands. Experiments comparing the rates obtained with two methods, the push-pull test and a steady-state flow-through reactor, were conducted in a constructed wetland microcosm. First-order kinetic rate constants were determined for both sulfate and chromate reduction using both methods. Chromate reduction rates showed good agreement between the two methods, while sulfate reduction rates determined by the two methods differed significantly. Since the analysis for the push-pull test is based on a first-order kinetic, this discrepancy is likely due to the non-first-order behavior of sulfate reduction under the given environmental conditions. The largest obstacle identified prohibiting the use of this method is the availability of a tracer that is conservative in the presence of plants.