Simulation models for the fate and transport of groundwater contaminants are important tools for testing our understanding of transport phenomena at long-term contaminated sites and for designing remedial action plans. A finite difference formulation for contaminant transport including a distribution of contaminant mass-transfer rates between the water and soil is developed. Optimal model simulations using both log-normal and γ distributions of mass transfer rates are compared to the two-site equilibrium/ kinetic model. In all cases, optimal sorption parameters were determined by best fit to laboratory data. For desorption of trichloroethene from long-term contaminated soils, the distributed mass-transfer rate model provided significantly improved simulations of aqueous concentrations, as compared to the two-site model, for both batch and soil column experiments. However, use of an apparent partition coefficient demonstrated that the performance of the two-site model was very sensitive to the value of the partition coefficient, while the performances of the distributed models were robust over a wide range of partition coefficients. Desorption studies in continuous-flow stirred tank reactors with laboratory-contaminated soils demonstrated that as the length of the contamination period increases, the simulation capability of the two-site model decreases.

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Rate-limited sorption and desorption strongly influence the fate, transport, and remediation of organic pollutants in subsurface environments. In this study, the rates of sorption and desorption were quantified for 1,2-dichlorobenzene to and from five natural sorbents using a batch methodology. Solute/sorbent contact times of 3, 7, 14, 49, and 99 d were studied for the desorption rate experiments. The sorption and desorption data were simulated with a distributed-rate model that used the Γ probability density function to generate the distribution of first-order rate coefficients. Ninety-five percent confidence intervals on the optimal model parameters were developed and translated into 95% confidence intervals on the various rate-coefficient distributions. Development of the confidence intervals on the rate coefficients facilitated a statistically rigorous evaluation of whether the rate of desorption was equal to the rate of sorption for the different sorbents. For three of the five sorbents studied, the rates of desorption were significantly slower than the rates of uptake for all solute/sorbent contact times studied. For the remaining two sorbents, the rates of desorption were significantly slower than the rates of uptake for solute/sorbent contact times greater than 2−3 d. For contact times greater than 2 d, a significant fraction of the 1,2-dichlorobenzene appeared to be resistant to desorption. However, the rate of desorption and the magnitude of the resistant fraction were independent of contact time for all but one sorbent. The rate observations for this study were consistent with an intraorganic matter diffusion mechanism.

The sorption and desorption rates of carbon tetrachloride (CCl4) and 1,2-dichlorobenzene (1,2-DCB) to and from three organobentonites of varying alkyl-chain length were quantified. The effects of solute structure, organobentonite structure, and solute-sorbent contact time on the rates of solute mass-transfer were investigated. For each solute, the rate of sorption was compared to the rate of desorption. In addition, the rate of 1,2-DCB sorption to a natural peat soil was quantified. The experimental data were simulated using a two-site model and a model incorporating a continuous distribution of mass-transfer rate coefficients. Based on a statistical analysis of the model simulations, the following conclusions were made:  (1) The rate of 1,2-DCB sorption to the organobentonites was significantly faster than the rate of 1,2-DCB sorption to the peat soil. (2) The rate of mass-transfer during sorption and desorption was greater for CCl4 than for 1,2-DCB. (3) The alkyl-chain lengths of the organobentonites did not affect the rate of mass-transfer during sorption; however, the rate of 1,2-DCB desorption decreased as the length of the organobentonite alkyl-chain increased. (4) The rate of solute desorption was slower than the rate of solute uptake for two of the three organobentonites. (5) For most environmental applications using the organobentonites studied here, a local equilibrium assumption will satisfactorily describe CCl4 and 1,2-DCB sorption and desorption.

Abstract-The rates of sorption and desorption were quantified for 1,2-dichlorobenzene to and from a peat soil using a batch methodology. Solute-sorbent contact times of 2, 7, 14, 49, and 99 d were studied for the desorption rate experiments. The experimental data were simulated using a distributed rate parameter model. The optimal model fitting parameters and their 95% confidence intervals were computed. The rates of desorption were significantly slower than the rate of uptake for solute-sorbent contact times greater than 2 d. For contact times greater than 2 d, a significant fraction of the 1,2-dichlorobenzene seemed to be irreversibly sorbed. However, the rate of desorption and the magnitude of the irreversible fraction were independent of contact time for contact times greater than 2 d. The rate observations for this study were consistent with an intraorganic matter diffusion mechanism.