Storage, transport, and fate of perfluoroalkyl acids (PFAAs) in a wastewater re-use 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 vadose zone solute retention properties and loading history. The relative importance of PFAA retention factors under natural 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 transport under near-natural conditions, and estimated historical PFAA concentrations in the irrigated effluent. Total perfluorooctanesulfonic acid (PFOS) and perfluorooctanoic acid (PFOA) masses stored in soils in 2014 were more than 450 times greater than the masses applied during the 2020 effluent irrigation. Equilibrium piston-flow transport models reproduced the observed PFOS and PFOA profiles, allowing us to estimate historical effluent PFOS and PFOA concentrations: 70-170 ng L-1 and 1000-1300 ng L-1, respectively. Estimated concentrations were comparable to concentrations measured in other wastewater effluents in the 1990s and 2000s, indicating that when interpreted with transport modeling, wastewater-irrigated soils function as integrated records of historical PFAA loading. Simulated PFOS breakthrough to groundwater occurred 50 years after the start of wastewater irrigation, while simulated PFOA breakthrough occurred after only 10 years of irrigation. Thus, while wastewater irrigation of soils facilitates retention and reduces effluent PFAA loading to surface waters, the resulting increased PFAA storage in soils potentially creates long-term sources of PFAAs to groundwater.