Spatial Variation and Temporal Effects of Precipitation on Per- and Polyfluoroalkyl Substances (PFAS) Concentrations in Landfill Leachate from an Unlined Landfill in Norman, Oklahoma

Abstract

The Norman Landfill Environmental Research Site (NLERS) is a closed municipal solid waste (MSW) legacy landfill capped with clay but lacking a liner and leachate collection system. As precipitation and groundwater percolate through the buried waste, contaminants are mobilized and form a leachate plume in the subsurface alluvial aquifer that contains per- and polyfluoroalkyl substances (PFAS). This study conducted event-based time series sampling of monitoring wells within the plume following multiple precipitation events. The objectives were to identify dominant PFAS classes, evaluate spatial variation in PFAS concentration and profile, and assess temporal responses to rainfall. Differences were assessed using Kruskal-Wallis tests followed by Dunn’s post-hoc tests with Bonferroni corrections. Pearson’s correlation analyses were conducted to examine relationships among precipitation depth, leachate volume, sampling day, and PFAS concentrations. Binary logistic regression was used to evaluate significant differences in PFAS detection and Akritas-Theil-Sen (ATS) regression assessed relationships between PFAS concentrations and leachate volume. Results indicated a dominance of short-chain perfluoroalkyl carboxylic acids (PFCAs), short-chain perfluoroalkane sulfonic acids (PFSAs), perfluorooctanoic acid (PFOA), and perfluorooctane sulfonic acid (PFOS). The monitoring well nearest the landfill exhibited the greatest PFAS concentrations, while the intermediate-distanced well showed the most complex PFAS profile. PFAS concentrations increased as leachate volume decreased, suggesting an inverse relationship consistent with dilution effects, though statistically significant declines were only observed for PFOA, short-chain PFCAs, short-chain PFSAs, and fluorotelomer carboxylic acids (FTCAs). These findings contribute to the understanding of how precipitation influences the mobilization and transport of PFAS from legacy landfills and provide insight for long-term groundwater protection.