Volatile organic compound transport in composite liners

Composite liners consisting of a geomembrane overlying a clay liner are often required as bottom liners for waste containment systems. Models for volatile organic compound (VOC) transport through composite liners have received limited validation. However, the validity of these models must be tested if reliable evaluations of landfill impacts are to be made.;Therefore, the objectives of this study are: (1) to evaluate the suitability of batch tests and double compartment diffusion tests to estimate partition and diffusion coefficients for VOCs and geomembranes, and (2) to validate existing composite liner transport models using individually measured transport parameters with VOC transport measured from bench-scale composite liner experiments.;Partition and diffusion coefficients for VOCs and geomembranes obtained using batch and double-compartment (DC) tests were compared. DC tests yield larger partition coefficients, and smaller diffusion coefficients, due to losses in the DC apparatus. Thus, the kinetic batch tests are recommended due to its simplicity, expedience, and reliability.;Data from a bench-scale composite liner experiment were compared to predictions made with two models (a finite-difference model and a semi-analytical model) used to predict volatile organic compound (VOC) transport through composite liners. Transport parameters for the geomembrane and the clay for five common VOCs were measured independently and used as input. VOC transport parameters for the geomembrane were measured using kinetic batch tests. Equilibrium batch tests and a diffusion column test were used to measure VOC transport parameters for the compacted clay liner. Predictions from both models for all five VOCs were essentially identical. The bench-scale experimental results indicate that models for predicting diffusive VOC transport through composite liners are valid and could be used in conjunction with carefully measured material properties and appropriate boundary conditions. The natural range of variation in measured transport parameters has a significant effect on predictions.;Comparisons with limited and non site-specific field data indicated that the model predictions generally underestimate the field VOC breakthrough concentrations. The reasons for this are unknown.