This project was a multi-disciplinary evaluation to characterize the interactions between PAHs and soils and how these interactions control the oral and dermal bioavailability of PAHs in soil to humans. The study provided results that can inform assessments and risk management considerations for DoD sites where PAH-contaminated soils are driving clean-up decisions.
Several specific tasks were included in the broad research conducted under this effort. The objectives of the different aspects of the project included: 1) Identify which specific PAH sources, exposure pathways, and individual PAHs are driving risk assessments and remedial decisions to focus research where it can be most effective; 2) Develop an understanding of the mechanisms by which PAHs are sequestered in soil, so the magnitude of bioavailability adjustments can be predicted, and elucidate the factors that control the dissolution of PAHs from soil; 3) Develop an animal model that provides quantitative measures of the relative oral bioavailability (RBA) of PAHs in soil and generate a database of information from this animal model to understand bioavailability across a diversity of soil types and contaminant sources; 4) Evaluate potential use of simple in vitro extraction tests to predict in vivo measures of relative bioavailability (as indicated by the in vivo model); and 5) Assess the effect of soil-chemical interactions on the dermal absorption of PAHs.
To allow for a rigorous and controlled evaluation of the effects of PAH concentration, soil compositions, and PAH source materials on the chemistry and bioavailability of PAHs from soil (studied in Tasks 2–5), a series of artificial soils were constructed with a range of PAH concentrations, soil compositions, and different PAH source materials of specific relevance to DoD sites (skeet fragments, soot, and fuel oil). The ability to control factors that might affect soil-chemical interactions was identified as more important to the project goals than a less controlled study design using contaminated field soils. These constructed soils were subjected to several weeks of artificial “weathering” to capture some of the effects of weathering that occur in the natural environment.
Task 1 entailed reviewing publicly available information on relevant exposure pathways for PAHs in soils and combining that with information available in the database of Records of Decision (RODs) for DoD sites. This review identified the specific PAHs driving remedial decisions as well as the emerging regulatory approaches for assessing toxicity of PAH mixtures. Task 2 involved an evaluation of the partitioning behavior of PAHs from the library of soils created for the project. Task 3 involved the development of an in vivo model for measuring bioavailability of oral administration of PAH-contaminated soil using laboratory rats. Task 4 entailed investigation of the bioaccessibility of PAHs from soil under simulated physiological conditions.
The results of each project task are summarized below.
In Task 1, the primary human health risk drivers for PAH-contaminated soils were the larger PAHs (i.e., four- to six-ring) associated with cancer endpoints. Thus, BaP, benz[a]anthracene, benzo[b]fluoranthene, indeno[1,2,3-cd]pyrene, and dibenz[a,h]anthracene would likely be risk drivers for human health risk assessment at DoD sites.
In Task 2, the source of PAH contamination was the primary factor controlling the portioning behavior of PAHs from soil. PAHs that enter soil as part of a matrix that is rich in black carbon (BC), such as within soot or coal tar pitch, were much less bioavailable than PAHs that are spiked to soil in the laboratory or that enter soil within fuel oil. Mineral characteristics of the soil (e.g., type of content of clay, presence of humic acids) had much less influence on the binding of PAHs. Conversely, the addition of charcoal to the soil resulted in higher binding within the soil matrix, possibly pointing to opportunities for in situ remedial opportunities to address PAH-contaminated soils.
In Task 3, the in vivo evaluation of the RBA of PAHs yielded result that supported the finding that PAH sources are the most important factor controlling bioavailability. PAHs introduced to soil in fuel oil demonstrated higher bioavailability than soils contaminated with PAHs in solvent or soot. Over all the soils tested, RBA values ranged from 65% to 100% (for BaP concentrations of 1–100 mg/kg). At the highest concentration tested (100 mg/kg BaP) soot demonstrated lower bioavailability of BaP than soils contaminated with PAHs in solvent or PAHs in fuel oil, with RBA of 24% for soot-spiked soils, and RBAs of 55% and 100% for solvent-spiked soils and fuel oil-spiked soils, respectively. In all cases, adding charcoal to the soil before weathering resulted in a significant (three- to four-fold) decrease in measured RBA. The use of the radiolabel also afforded the ability to understand some of the nature of initial binding of PAHs to soil during the weathering process and the limitations of some analytical methods to capture total PAH content of soils.
In Task 4, the extraction of soils using a PBET method indicated that the partitioning behavior of PAHs in soil observed as part of Task 2 is correlated with PAH dissolution under physiological conditions. While this is promising for possible application of PBET, there remained complexities associated with the PBET system, requiring specific method development to ensure quantitative recovery of PAHs from the PBET solution. Results from bioaccessibility testing of the same soils dosed to rats to identify RBA showed promise for the use of in vitro methods to predict bioavailability as measured in rats: results with a simplified PBET were relatively reproducible for a given soil, and in vitro to in vivo correlation (IVIVC) demonstrated an R2 = 0.57. A simple solvent extraction of soils using n-butanol had a higher IVIVC, with an R2 = 0.74. Extraction with EPA Method 3550C was not a good predictor of RBA as measured in rats (R2 = 0.43), resulting in over predictions for some soils and under predictions for others.
In Task 5, dermal absorption of BaP from soil was examined as flux of the compound into or through the skin over time. Absorption of BaP from soil was significantly lower than absorption of BaP applied to the skin in solvent. The absorption was independent of soil type or concentration of BaP in the soil over the limited range of soil types and concentrations evaluated. Absorption was proportional to the duration of contact between the soil and the skin surface. Additionally, the mass of BaP recovered in the skin after washing to remove soil was proportional to soil concentration and independent of time, possibly suggesting that soil residue remained on the surface of the skin even after washing. Results indicated that the concentration range used for this part of the research may have saturated the binding ability of the study soils. This would result in an overestimate of the dermal bioavailability of BaP in soil, suggesting the need for additional work. Based on these results and other tasks, it was reasonable to expect that the nature of the source of PAHs to soils will be an important factor influencing dermal exposure and absorption.
Overall, the broadest conclusions that can be drawn from the research conducted under this project include:
- The source material in which PAHs are introduced to soil is very important in determining the nature of the soil-chemical interactions between the PAHs and the soil components and controls the dissolution of PAHs from contaminated soils, both in laboratory chemical characterization efforts and within mammalian systems when tested in vivo. PAHs introduced to soil in carbon-rich sources such as soot and coal tar-based skeet are sequestered in the soil in a more stable form and recalcitrant to extraction. For example, in animal testing the RBA of BaP was close to 100% when introduced to soil in fuel oil but as low as 23% when introduced in soot.
- The composition of soils can also affect the dissolution or bioavailability of PAHs from soil, particularly the black carbon content, which enhances the binding of PAHs within the soil. Our evaluations indicate that the presence of charcoal substantially reduces PAH bioavailability; RBA from soils with added charcoal was less than one third of the RBA observed in paired soils without added charcoal.
- Within the concentration range of environmental relevance used in this study (0.1 to 100 ppm BaP), PAH concentration was much less influential on partitioning and bioavailability than source of PAHs.
- Based on paired testing of the RBA of BaP from soils in vivo with a rat model and extraction testing of the same soils, both PBETs and simple solvent extractions show promise as methods for predicting the RBA of BaP.
- EPA Method 3550C, commonly used to characterize the concentration of PAHs in soils from contaminated sites, does not extract the total amount of PAHs from some soils. Our investigations in vivo suggest the gastrointestinal tract of intact animals can be more efficient than EPA Method 3550C at extracting PAHs from some soils but not always.
- Percutaneous absorption of BaP from soil was significantly lower than absorption of BaP applied to the skin in solvent, and absorption was independent of soil type or concentration of BaP in the soil over the limited range of soil types and concentrations evaluated. Absorption was proportional to the duration of contact between the soil and the skin surface. Additionally, the mass of BaP recovered in the skin after washing to remove soil was proportional to soil concentration and independent of time, suggesting that soil residue remained on the surface of the skin even after washing. This preliminary effort was conducted with four field soils that were spiked with BaP in the lab, suggesting studies that incorporate considerations of different sources of PAHs to soil may be important for understanding site-specific exposures.
- This research utilized a series of artificial soils constructed with a range of PAH concentrations, soil compositions, and different PAH source materials of specific relevance to DoD sites (skeet fragments, soot, and fuel oil). Although unable to provide information on the bioavailability of PAHs from any particular site, the ability to control these factors allowed the research to identify key soil-chemical interactions likely to affect bioavailability at any site. The results indicate site-specific factors such as PAH source and some soil characteristics are important to understanding potential exposures to PAH at a contaminated site.