RATE-LIMITED SORPTION AND NONEQUILIBRIUM TRANSPORT OF ORGANIC-CHEMICALS IN LOW ORGANIC-CARBON AQUIFER MATERIALS

The rate-limited sorption and nonequilibrium transport of several hydrophobic organic chemicals in three low-organic carbon (< 0.025%) aquifer materials was investigated. Results of miscible displacement experiments performed at two pore water velocities and with very low solution-phase concentrations (30-60-mu-g L-1) were analyzed using a first-order mass transfer nonequilibrium model, as well as a model employing the local equilibrium assumption. Results of the analyses revealed sorption to be significantly rate limited, possibly by a diffusion-limited mechanism. The impact of rate-limited sorption on transport was dependent upon pore water velocity. The experiments performed at a faster velocity (approximately 1 cm/h) could be successfully simulated only with the nonequilibrium model, whereas the equilibrium model was adequate for the slower-velocity (approximately 0.2 cm/h) experiments. Comparison of experimental results to those reported in the literature revealed that time scale has a significant impact on the degree of nonequilibrium observed in, and on the values of rate constants determined from, experiments. Regression equations were developed for the two kinetics-associated parameters contained in the nonequilibrium model: first-order mass transfer constant and fraction of instantaneous sorption.