Performance-based design of landfill liners

The performance-based design of pollutant containment systems, such as landfill bottom liners, requires the impact of pollutant migration on groundwater quality to be assessed. The effectiveness of pollutant containment systems is indeed demonstrated through the verification that the risk for human health and the environment due to pollutant migration is limited to an acceptable level. This risk is quantified through the calculation of the pollutant concentration in the groundwater, which is expected to remain less than some prescribed level at a compliance point. The paper describes analytical and numerical solutions to pollutant transport, which allow the pollutant concentration in the groundwater to be calculated under the restrictive assumptions of steady-state conditions and taking into account both the advective and diffusive transport through the containment liner. The scenarios considered in this paper include vertical pollutant transport through an engineered barrier, which can overlie a foundation or attenuation layer, and horizontal transport in the underlying aquifer. The solutions that are derived in this paper allow the contaminant concentration distribution to be evaluated in the horizontal direction within the aquifer beneath the landfill. In such a way, the progressive increment of the contaminant concentration that occurs along the direction of the groundwater flow can be appreciated. A first analytical solution is derived for the case of aquifers that are sufficiently thin in order to neglect the vertical distribution of the contaminant concentration in the groundwater. When the aquifers are thick, and the vertical dispersion of the contaminants cannot be neglected, a two-dimensional (2D) mass balance in the aquifer has to be considered, and a set of analytical and numerical solutions is developed to evaluate the contaminant concentration in semi-infinite aquifers and aquifers of finite thickness.