MODELING THE DESORPTION OF ORGANIC CONTAMINANTS FROM LONG-TERM CONTAMINATED SOIL USING DISTRIBUTED MASS-TRANSFER RATES

Simulation models for the fate and transport of groundwater contaminan ts are important tools for testing our understanding of transport phen omena at long-term contaminated sites and for designing remedial actio n plans. A finite difference formulation for contaminant transport inc luding a distribution of contaminant mass-transfer rates between the w ater and soil is developed. Optimal model simulations using both log-n ormal and gamma distributions of mass transfer rates are compared to t he two-site equilibrium/kinetic model. In all cases, optimal sorption parameters were determined by best fit to laboratory data. For desorpt ion of trichloroethene from long-term contaminated soils, the distribu ted mass-transfer rate model provided significantly improved simulatio ns of aqueous concentrations, as compared to the two-site model, for b oth batch and soil column experiments. However, use of an apparent par tition coefficient demonstrated that the performance of the two-site m odel was very sensitive to the value of the partition coefficient, whi le 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 simulat ion capability of the two-site model decreases.