A METHOD TO INFER IN-SITU REACTION-RATES FROM PUSH-PULL EXPERIMENTS

A method to evaluate first-order and zero-order in situ reaction rates from a push-pull test is presented. A single-well push-pull test star ts with the rapid injection of a well-mixed slug containing a known qu antity of a conservative tracer and a reactive solute into the saturat ed zone, The slug is then periodically extracted and sampled from the same well. For zero- or first-order reactions, in the absence of sorpt ion and assuming negligible background concentrations, these measureme nts can be used to evaluate reaction rate coefficients directly. The m ethod does not involve computer-based solute transport models and requ ires no knowledge of regional ground water now or hydraulic parameters , The method performs well when the dominate processes are advection, dispersion, and zero- or first-order irreversible reactions. Regional now velocities must be sufficiently low such that the slug stays withi n the area of the well during the sampling phase, In the case of zero- order reactions, results using the method proposed here are compared w ith those obtained through the traditional method of calibrating a com puter-based transport model. The two methods give similar estimates of the reaction rate coefficient, The method is general enough to work w ith a broad range of push-pull experiment designs and sampling techniq ues.