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 μg 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 (∼1 cm/h) could be successfully simulated only with the nonequilibrium model, whereas the equilibrium model was adequate for the slower‐velocity (∼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.