Validation of Transport of VOCs from Composite Liners

Summary

Composite liners consisting of a geomembrane overlng a clay liner are often required as bottom liners for waste containment systems. Composite liners are effective because the geomembrane component restricts the crosssectional area over which advection occurs and the soil beneath the geomembrane is designed to have low hydraulic conductivity, which limits advection through defects in the geomembrane. Therefore, diffusive transport c becomes the dominant mode of contaminant transport in well-constructed composite liners.

Volatile organic compound (VOC) transport through geomembranes and compacted clay liners individually are well understood, and VOC transport models have been developed and verified. In contrast, VOC transport through composite liners is not completely understood, and models for VOC transport through composite liners have received little verification. Moreover, recent research has raised significant questions regarding the effectiveness of composite liners to mitigate VOC transport. Being able to predict VOC transport is essential when evaluating potential impacts on groundwater.

A two-stage study was undertaken to conduct experiments, analyses, and assessment. The first stage consists of bench-scale laboratory tests simulating intact composite liners to verify existing transport models for composite liners. Transport parameters for the materials used in the bench-scale tests were obtained by separate experiments. In the second stage, sensitivity analyses were performed to assess the effects of diffusion coefficients, partition coefficients, hydraulic conductivity, and porosity on model simulations. The parameters related to geomembrane showed little effect on breakthrough concentrations and the transport parameters related to the clay liner dominantly affect the breakthrough concentration. Even if all the properties are independently measured, measurement error changes the breakthrough concentration up to 2 orders of magnitude.

Fielddata was adopted from Klett (2006) to evaluate the contaminant model simulations for toluene using widely accepted published transport parameters for clay and composite liners in Wisconsin. The simulation results show that predicting relative concentrations with widely accepted transport parameters is difficult because relative concentration is very sensitive to the properties of the clay liner. Thus, the most conservative case must be considered in landfill designs.