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.

Laboratory batch and column experiments were conducted to study the effect of relatively low concentrations of Triton X-100 and pH increases on trichloroethene (TCE) desorption from field-contaminated soil to water. TCE desorption from the contaminated soil could not be described by a model that assumes a localized equilibrium between the aqueous- and sorbed-phase concentrations of TCE. A kinetic desorption model, the multi-site model with a γ-distribution of rate constants, was used to interpret the data and to determine the mass-transfer coefficients. In both batch and column experiments, the multi-site model performed well in simulating TCE desorption. In laboratory batch and column experiments, the addition of Triton X-100 (at concentrations close to critical micelle concentration) to the soil-water system increased the rate of TCE desorption from the soil at early times, although only by a small amount. Similar results were obtained by increasing the solution pH from 7 to 10. In experiments with Triton X-100, the mean mass-transfer coefficient increased by 11.2% in batch tests and 16.5% in column tests relative to experiments without Triton X-100. The mean mass-transfer coefficient increase caused by increasing pH from 7 to 10 was 53% in batch tests and 7% in column tests.

Transport of a nonionic surfactant (Triton X-100) at aqueous concentrations less than 400 mg/L through a trichloroethene-contaminated sand-and-gravel aquifer at Picatinny Arsenal, NJ, has been studied through a series of laboratory and field experiments. In the laboratory, batch and column experiments were conducted to quantify the rate and amount of Triton X-100 sorption to the aquifer sediments. In the field, a 400 mg/L aqueous Triton X-100 solution was injected into the aquifer at a rate of 26.5 L/min for a 35-d period. The transport of Triton X-100 was monitored by sampling and analysis of groundwater at six locations surrounding the injection well. Equilibrium batch sorption experiments showed that Triton X-100 sorbs strongly and nonlinearly to the field soil with the sharpest inflection point of the isotherm occurring at an equilibrium aqueous Triton X-100 concentration close to critical micelle concentration. Batch, soil column, and field experimental data were analyzed with zero-, one-, and two-dimensional (respectively) transient solute transport models with either equilibrium or rate-limited sorption. These analyses reveal that Triton X-100 sorption to the aquifer solids is slow relative to advective and dispersive transport and that an equilibrium sorption model cannot simulate accurately the observed soil column and field data. Comparison of kinetic sorption parameters from batch, column, and field transport data indicate that both physical heterogeneities and Triton X-100 mass transfer between water and soil contribute to the kinetic transport effects.

Field studies were conducted under an induced gradient in a trichloroethene (TCE)-contaminated aquifer at Picatinny Arsenal, NJ, to study (a) the rate-limited desorption of TCE from aquifer sediments to water and (b) the effect of a surfactant (Triton X-100) on the desorption and transport of TCE. Clean water was injected into the contaminated aquifer for 206 day. Triton X-100 was added for a 36-day period (days 36-71 from the start of clean water injection). The effect of Triton X-100 on the desorption and transport of TCE in the field was examined by observing the concentrations of these two solutes in four monitoring wells 3-9 m from the injection wells. These data show a small but discernible increase in the TCE concentration in two of the wells corresponding approximately to the time when surfactant reaches the wells; in the other two monitoring wells, the increase in TCE concentration is negligible. A solute transport model that assumes local sorption equilibrium and used a laboratory-derived distribution coefficient could not adequately describe TCE desorption and transport observed in the aquifer. Two model formulations that accounted for rate-limited sorptionstwosite and multisite modelssfit the data well. TCE concentrations after surfactant injection were underpredicted by the models unless mass transfer rate was increased to account for the effect of surfactant on the rate of TCE desorption. The concentration data from the two wells and the model analysis suggest that the rate of TCE desorption is increased (by approximately 30%) as a result of Triton X-100 injection.