The total fluorine in the original AFFF was measured as 10 g L -1 by combustion ion chromatography (Table ). Nineteen of the 30 targeted PFAS structures were detected in AFFF by QQQ MS/MS (Table ). The three most abundant targeted PFAS were 6:2 FTS (139 mg L -1 ), 8:2 FTS (7.85 mg L -1 ), and PFOA (3.54 mg L -1 ). However, F elements from all targeted PFAS only accounted for 2% of the total fluorine. F nuclear magnetic resonance (NMR) analysis found the dominant species in AFFF as n=6 FT surfactants (i.e., C6F13-(CH2)m-RO, Fig. ), but the structure of the organic moiety (RO) was unknown. We hypothesized that the surfactants could be defluorinated via similar mechanisms as for individual PFAS with the same RF building blocks (Fig. ). Hence, to effectively monitor the treatment process, we kept tracking the concentrations of FTSs, PFSAs, PFCAs, select surfactant molecules, and F -ion (Fig. ).