Probabilistic Contamination Risk Assessment with Application to PFOS in Aquifers under Single-lined Landfills

Abstract

Per- and polyfluoroalkyl substances (PFAS) are contaminants of concern due to their persistence, mobility, and toxicity, with perfluorooctane sulfonic acid (PFOS) being a prominent PFAS compound detected in landfill leachates worldwide. Municipal solid waste landfills (MSWLs) commonly rely on single composite liner systems (SCLS) to protect underlying aquifers; however, these systems were not designed to contain PFAS, and the associated groundwater contamination risk remains poorly understood.

This thesis presents a probabilistic assessment of PFOS transport through typical SCLS into aquifers underlying MSWLs, accounting for uncertainties in material properties, environmental conditions, and construction quality. A coupled water flow and reactive advection-diffusion model is employed. To overcome the prohibitive computational cost of conventional Monte Carlo simulations (MCS), a Polynomial Chaos Expansion (PCE) surrogate model is developed, validated, and adopted for up to 12 uncertain inputs.

Results indicate that the PCE model accurately reproduces MCS predictions while reducing computational cost significantly. Probabilistic analyses reveal a high likelihood of PFOS concentrations in underlying aquifers exceeding drinking water guidelines, even under excellent construction quality. Depending on liner configuration and regulatory standards, the probability of exceedance ranges from 49% to 99%. Peak PFOS impact may occur more than 150 years after landfill operation begins, highlighting risks beyond landfill lifespans. Key factors influencing aquifer concentrations include PFOS load in the waste, hydraulic conductivity of clay liners, and construction quality.

These findings suggest that SCLS may be inadequate for containing PFOS, underscoring the need for improved liner designs, enhanced construction quality, and upstream source control. This framework provides a basis for risk-informed decision-making and assessments of PFAS contamination in landfill contexts.