IMPROVED APPROACH TO CHARACTERIZE CHANGING KINETIC CONTROLS ON COMMON REDOX REACTIONS USING IN SITU PUSH-PULL TESTS

Push-pull tests are a commonly used technique to evaluate in situ rate estimates for subsurface processes including microbial respiration, contaminant degradation, and aquifer properties. In most instances, the rate estimates from push-pull test data are made using an analytical solution that provides a single first- or zero-order rate description. This approach works well for reactions that do not vary in space and have a single, unchanging kinetic control throughout the experiment. However, published and unpublished push-pull data often contain additional complexities including a lag phase and changing rate constants that could provide important information on additional kinetic controls for the processes being investigated. A numerical code PPTEST was used to reevaluate previously published push-pull data and describe complexities in rate data. This approach does not require the same assumptions as the analytical solution. For example, the experimental solution being pushed into the aquifer need not be well-mixed (i.e. behave like a batch reactor), a condition that in practice is difficult to satisfy. Quantitatively describing the additional features present in complex rate data will lead to improved understanding of kinetic controls and enhance our ability to apply rates measured to other systems.