OCCURRENCE, FATE AND TRANSPORT OF PER- AND POLYFLUOROALKYL SUBSTANCES (PFAS) IN AGRICULTURAL SYSTEMS

Beneficial reuse of biosolids is a sustainable approach to improve soil fertility and address over reliance on inorganic fertilizers. However, active research within the past two decades have revealed that land application of biosolids introduces per- and polyfluoroalkyl substances (PFAS) into agricultural systems which have potential adverse effects on human health and ecosystem function. These chemicals could migrate from surface soils to underlying groundwater aquifers via leaching and/or release to surface water via runoff. Continuous application of biosolids could result in substantial contamination that permanently renders agricultural lands unsuitable for food production which poses challenges to food security and local economies.

This dissertation research focused on PFAS occurrence, fate and transport in agricultural systems relative to biosolids application. PFAS presence in rural surface water of an agricultural-dominated watershed in northern Indiana was driven by hydrological connections to tile-drained agricultural fields and wastewater treatment plant and industry discharges. The groundwater in the watershed was protected from contamination due to the presence of restrictive till layer and the prevalence of subsurface drainage. In a site designed to mimic land reclamation of degraded soils, PFAS leached from biosolids-amended soils at magnitudes that were proportional to biosolids application rates. A 43% biosolids blend with mulch resulted in 21% lower PFAS leachate concentrations even with the application rate of the blended biosolids being 1.5 times higher than biosolids only due to the blend’s lower N-content. The dilution effect from blending biosolids with mulch was more pronounced for long-chain perfluoroalkyl sulfonic acids which have a greater retention in soils. At another site with three unique soil types that periodically received biosolids for over two decades, soil texture-driven hydraulic processes and soil organic carbon content were identified as the major drivers of PFAS retention in the vadose zone. ∑PFAS concentrations between 24.3 ng/g to 133.4 ng/g were detected within the upper vadose zone soil collected nine years after the last biosolids application which demonstrates PFAS persistence. Additionally, PFAS concentrations up to 3478 ng/L were measured in surface runoff of biosolids-applied fields which was dominated by long-chain perfluoroalkyl acids reflecting their abundance in the upper vadose zone. Groundwater within the fields were also impacted by biosolids application. However, the most impacted groundwater sites at several hundred ppt were two shallow wells in a sandy-loam soil and bordering a shallow freshwater lake. This suggest that in addition to geological factors, hydrodynamic interaction between surface and subsurface water influences PFAS presence in groundwater.

Overall results of the studies show that PFAS fate and transport following biosolids application is largely determined by site-specific source and transport factors. Coupled biogeochemical processes within prevailing climatic conditions govern long-term transport outcomes, therefore the risk of water sources contamination from biosolids application can only be accurately assessed within a local context. However, PFAS persistence and longevity in biosolids-applied soils is of great concern given the role of soils in environmental cycling of PFAS. Meanwhile, blending biosolids with non-biowaste organic materials prior to land-application has shown promise as a pragmatic strategy to dilute PFAS loads and leachability in biosolids-applied soils.