In vitro gastrointestinal mobilization and oral bioaccessibility of PAHs in contrasting soils and associated cancer risks: Focus on PAH nonextractable residues

The gastrointestinal mobilization and oral bioaccessibility of polycyclic aromatic hydrocarbon (PAH) nonextractable residues (NERs) from soils remain unexplored, including associated incremental lifetime cancer risks. This study investigated the gastrointestinal mobilization of PAHs and their NERs from contrasting soils, using a physiologically based extraction test that incorporates a silicone-rod (Si-Org-PBET) as PAH sink. Associated cancer risks following soil ingestion were also evaluated. Four solvent-spiked and aged soils, and four long-term contaminated manufactured gas plant (MGP) soils, were utilized. Total-extractable PAH concentrations were measured after exhaustive solvent extractions of soils. We evaluated the PAH sorption efficiency of the silicone rods and associated sorption kinetics, using PAH-spiked silica sand as the contaminated matrix. We then assessed gastrointestinal mobilization of benzo[a]pyrene and benzo[a]pyrene NERs from the solvent-spiked soils, and mobilization of six PAHs and their NERs from the MGP soils. PAH oral bioaccessibility was determined. The incremental lifetime cancer risks (ILCRs), using Si-Org-PBET- and total-extractable PAH concentrations from the MGP soils, were calculated. Sorption kinetics modelling showed that 95% of mobilized PAHs sorbed to the silicone rods within 2-19 h, depending on PAH physico-chemical properties. Total-extractable and Si-Org-PBET extractable PAH concentrations exceeded health investigation levels (3 mg/kg based on benzo[a]pyrene toxic equivalent quotients) in soils. PAH oral bioaccessibility approached 100% for solvent-spiked soils, but only 24-36% for the MGP soils. Associated ILCRs exceeded target levels (10^-5) for one MGP soil, particularly for 2-3 year olds, despite oral bioaccessibility considerations. In contrast, mobilized PAH NERs did not exceed health investigation and ILCR levels, as the NERs were highly sequestered, especially in the MGP soils. PAH nonextractable residues in long-term contaminated soils are unlikely to be mobilized in concentrations that pose cancer risks to humans following soil ingestion, and do not need to be considered in risk assessments.

Bioaccumulation of benzo[a]pyrene nonextractable residues in soil by Eisenia fetida and associated background-level sublethal genotoxicity (DNA single-strand breaks)

The potential for bioaccumulation and associated genotoxicity of nonextractable residues (NERs) of polycyclic aromatic hydrocarbon (PAHs) in long-term contaminated soils have not been investigated. Here we report research in which earthworms, Eisenia fetida, were exposed to a soil containing readily available benzo[a]pyrene (B[a]P) and highly sequestered B[a]P NERs aged in soil for 350 days. B[a]P bioaccumulation was assessed and DNA damage (as DNA single strand breaks) in earthworm coelomocytes were evaluated by comet assay. The concentrations of B[a]P in earthworm tissues were generally low, particularly when the soil contained highly sequestered B[a]P NERs, with biota-soil accumulation factors ranging from 0.6 to 0.8 kg_OC/kg_lipid. The measurements related to genotoxicity, that is percentage (%) of DNA in the tails and olive tail moments, were significantly greater (p < 0.05) in the spiked soil containing readily available B[a]P than in soil that did not have added B[a]P. For example, for the soil initially spiked at 10 mg/kg, the percentage of DNA in the tails (29.2%) of coelomocytes after exposure of earthworms to B[a]P-contaminated soils and olive tail moments (17.6) were significantly greater (p < 0.05) than those of unspiked soils (19.6% and 7.0, for percentage of DNA in tail and olive tail moment, respectively). There were no significant (p > 0.05) differences in effects over the range of B[a]P concentrations (10 and 50 mg/kg soil) investigated. In contrast, DNA damage after exposure of earthworms to B[a]P NERs in soil did not differ from background DNA damage in the unspiked soil. These findings are useful in risk assessments as they can be applied to minimise uncertainties associated with the ecological health risks from exposure to highly sequestered PAH residues in long-term contaminated soils.