Development of a farmworker soil exposure scenario to support the derivation of soil screening levels for per-and polyfluoroalkyl substances (PFAS)

BACKGROUND: Soil screening levels (SSLs) are an important guidance tool for identifying sites where exposure to soil contaminants may pose health risks to people residing or working nearby; however, to date, no soil screening levels or default exposure scenario have been established specifically for farmworkers who have frequent, direct soil contact through their typical occupational tasks and behaviors. OBJECTIVE: We developed an exposure scenario for a vegetable farmworker's soil exposure. We apply this exposure scenario to derive and compare noncarcinogenic-based soil screening levels for four PFAS commonly detected in soil from farm fields with a history of land application of biosolids in Maine. METHODS: We used existing EPA models for incidental soil ingestion and dermal contact, parameterizing model inputs with distributions drawn from the literature to be more representative for farmworkers. We developed an empirical model of the inhalation pathway using literature-reported exposure and time-activity data. We used Monte Carlo simulation to evaluate these models with toxicity values in the EPA Regional Screening Levels database to derive SSLs for the individual and combined pathways and compared these results to soil PFAS levels from a statewide testing program of biosolids-amended farmland. RESULTS: percentile values from the distribution of combined pathway SSLs were 26, 7.9, 790, and 0.5 ng/g for PFOS, PFOA, PFNA, and PFDA, respectively. Sensitivity analyses identified the incidental soil ingestion rate as the most influential parameter in deriving a SSL for our farmworker scenario. We found overlap for PFOS, PFOA, and PFDA in the distributions of plausible SSLs for a vegetable farmworker scenario and measured soil levels from a statewide testing program. SIGNIFICANCE: Our comparison of these farmworker SSLs to more routinely evaluated adult exposure scenarios (residential, outdoor worker, construction worker) indicated that these other scenarios may not be protective of farmworkers. IMPACT STATEMENT: The farmworker exposure scenario and PFAS soil screening levels we developed are a novel and necessary public health tool for protecting farmworkers from exposure to contaminants in soils and informing risk management decisions for farms and fields amended with biosolids.