Characterizing and quantifying the impact of immiscible-liquid dissolution and nonlinear, rate-limited sorption/desorption on low-concentration elution tailing

Long-term elution tailing of organic contaminants, often observed when water or air is used to flush contaminated porous media, has been attributed to several factors. Characterization of this tailing behavior through the quantitative analysis of multiple coupled factors is necessary to enhance our understanding of contaminant transport. The objective of this study was to investigate the transport and elution behavior of trichloroethene in a naturally heterogeneous (poorly sorted) aquifer material, with a specific focus on characterizing and quantifying the relative contributions of rate-limited immiscible-liquid dissolution and nonlinear, rate-limited sorption/desorption to low-concentration elution tailing. A comparison of trichloroethene elution behavior for systems with and without immiscible-liquid phase present suggests that the low-concentration elution tailing observed in the former experiments is associated primarily with nonlinear, rate-limited sorption/desorption. The transport and elution of trichloroethene was successfully simulated using a mathematical model that combines independent, coupled descriptions of rate-limited dissolution and nonlinear, rate-limited sorption/desorption. Specifically, immiscible-liquid dissolution was described using a first-order mass transfer approach with a temporally variable dissolution rate coefficient, and sorption/desorption was described using an approach incorporating a continuous distribution of rate-limited domains. The results of this study indicate that multiple processes contributed to trichloroethene elution behavior when immiscible-liquid phase was present and that a multiprocess model was required to accurately simulate the measured data.