Storage, transport, and fate of perfluoroalkyl acids (PFAAs) in a
wastewater re-use and groundwater recharge system
Abstract
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.