Poly- and perfluoroalkyl substance (PFAS)-bearing wastes were disposed in unlined landfills in southern Washington County, Minnesota in the 1940s to 1970s. The resulting co-mingled groundwater plumes cover an area of over 100 square miles, affecting four major aquifers, and impacting the municipal water supplies of eight communities and thousands of private drinking water wells. The area affected is much larger and the plumes far more complex than predicted by modeling. Groundwater and surface water sampling and "fingerprinting" of the PFAS chemical signatures of the various sites helped to reveal a complex transport history that explains the genesis of this "megaplume" while also highlighting the environmental fate and transport of differing PFASs. For example, the extreme persistence and mobility of PFASs allowed them to migrate between groundwater and surface water multiple times, with the shorter-chain carboxylate PFASs migrating the furthest due to their higher solubility and mobility. Bedrock structures (such as buried valleys, faults, and karst) and stormwater management projects also allowed the PFASs to migrate much further than expected, complicating the full delineation of the affected area. Meanwhile, areas within the megaplume that have unique PFAS chemical signatures not attributable to the disposal sites may indicate the presence of more recent releases that have been "superimposed" on the older plume(s) or possibly degradation of PFAS precursor chemicals within the aquifers. Partitioning of PFASs also affects drinking water treatment considerations. Rapid breakthrough of short-chain, carboxylate PFASs has been observed in the granular activated carbon (GAC) treatment plant in Oakdale, Minnesota, with effluent concentrations of perfluorobutanoic acid (PFBA) eventually exceeding influent concentrations as it is displaced by more readily adsorbed PFASs. Such treatment difficulties may become an issue of increasing concern as PFAS chemistry is shifted toward shorter-chain PFASs.

Minnesota Poly- and Perfluoroalkyl Substance (PFAS) megaplume: A case study of PFAS fate and transport and implications for site investigation and drinking water treatment

253rd National Meeting (2017)

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