Centurial Persistence of Forever Chemicals at Military Fire Training Sites

Slow transport and biotransformationof unregulated polyfluoroalkylprecursors are likely to sustain centurial timescales of PFAS contaminationat AFFF-contaminated sites Drinking water contamination by per- and polyfluoroalkylsubstances(PFAS) is widespread near more than 300 United States (U.S.) militarybases that used aqueous film-forming foams (AFFF) for fire trainingand firefighting activities. Much of the PFAS at these sites consistof precursors that can transform into terminal compounds of knownhealth concern but are omitted from standard analytical methods. Here,we estimate the expected duration and contribution of precursor biotransformationto groundwater PFAS contamination at an AFFF-contaminated militarybase on Cape Cod, Massachusetts, United States, by optimizing a geochemicalbox model using measured PFAS concentrations from a multidecadal timeseries of groundwater and a soil survey in the source zone. A toolboxof analytical techniques used to reconstruct the mass budget of PFASshowed that precursors accounted for 46 +/- 8% of the extractableorganofluorine (a proxy for total PFAS) across years. Terminal PFASstill exceed regulatory limits by 2000-fold decades after AFFF useceased. Measurements and numerical modeling show that sulfonamidoprecursors are retained in the vadose zone and their slow biotransformationinto perfluoroalkyl sulfonates (half-life > 66 yr) sustains groundwaterconcentrations of perfluorobutane sulfonate (PFBS) and perfluorohexanesulfonate (PFHxS). The estimated PFAS reservoir in the vadose zoneand modeled flux into groundwater suggest PFAS contamination aboveregulatory guidelines will persist for centuries without remediation.