The Total Oxidizable Precursor (TOP) Assay as a Forensic Tool for Per- and Polyfluoroalkyl Substances (PFAS) Source Apportionment

Per- and polyfluoroalkyl substances (PFASs) are a class of synthetic, organic chemicals that contaminate drinking water and natural ecosystems. PFAS source apportionment is challenging because there are many sources, and standard analytical methods quantify fewer than 100 of the thousands of PFASs in commerce. The total oxidizable precursor (TOP) assay augments the number of PFASs that can be quantified and is increasingly incorporated into routine site investigation. Here we examine the ability of the TOP assay to identify PFAS sources, including aqueous film-forming foam (AFFF) impacted sites, municipal wastewater treatment plants (WWTPs), and municipal solid waste landfills in 145 samples from 46 locations and three countries. The bootstrapped mean composition of PFASs from each source was dominated by precursors, particularly in samples from WWTPs where precursors that form short-chain perfluoroalkyl carboxylates during the TOP assay were most common. Compared to when TOP assay data were excluded, inclusion of TOP assay data in dimension-reducing algorithms, such as principal component analysis (PCA), improved separation among sources. We converted the PCA tool into a web application that allows users to initiate PFAS source apportionment efforts on data from sites where contaminant sources are unknown.