Storage, Transport, and Fate of Perfluoroalkyl Acids (PFAAs) in a Wastewater Reuse and Groundwater Recharge System

Perfluoroalkyl acids (PFAAs), a group of synthetic compounds associated with adverse human health impacts, are commonly found in effluent discharged from wastewater treatment facilities. When that effluent is used for irrigation, the fate of PFAAs depends strongly on soil PFAA retention properties and effluent PFAA loading history. The relative importance of PFAA retention factors under natural field-scale conditions remains uncertain, and the historical record of effluent PFAA concentrations is limited. Using soil cores collected from the Penn State Living Filter (irrigated with treated wastewater effluent for nearly 60 years), we evaluated PFAA mass balance in the irrigated soils and estimated historical effluent PFAA concentrations. Total accumulated perfluorooctanesulfonic acid (PFOS) and perfluorooctanoic acid (PFOA) masses measured in soils in 2014 were more than 450 times greater than annual mass loads applied during the 2020 effluent irrigation. Column mass balance models reproduced the observed PFOS and PFOA profiles, providing an estimate of historical effluent PFOS and PFOA concentrations: 70-170 ng L-1 and 1,000-1,300 ng L-1, respectively. Estimated concentrations were comparable to measurements in other wastewater effluents in the 1990s and 2000s, indicating that wastewater-irrigated soils can serve as integrated records of historical PFAA loading to the environment. Mass balance calculations suggest that PFOS breakthrough to groundwater occurred 50 years after the start of wastewater irrigation, whereas simulated PFOA breakthrough occurred after only 10 years of irrigation. While wastewater irrigation of soils facilitates retention and reduces effluent PFAA loading to surface waters, resulting PFAA storage in soils potentially creates long-term sources of PFAAs to groundwater. Irrigated soils function as records of perfluoroalkyl acid (PFAA) loading, and estimated historical effluent PFAA loads were much higher than current loadsTiming of PFAA leaching differed by four decades depending on PFAA sorption affinityDespite production cessation two decades ago, legacy perfluorooctanesulfonic acid stored in vadose zones could remain a future source of groundwater contamination