Several soil characteristics (for example, organic carbon or clay content) may affect PAH bioavailability, although the PAH source and the presence of black carbon (such as soot or char) in the soil will generally be the main factors controlling PAH partitioning (Roberts et al. 2016). Certain PAH sources (for example, soot, chars, coal, coke, and coal tar-based clay targets at skeet range sites) have been shown to strongly sorb PAHs and, therefore, are more likely to result in reduced bioavailability (Xia, Gomez-Eyles, and Ghosh 2016). SERDP recently sponsored research to asses these factors (Lowney et al. 2017). Sites affected by these sources and with high black carbon contents are therefore good candidates for bioavailability investigations. PAH bioavailability also tends to decrease with time as the more bioavailable PAHs are degraded/volatilized, and the remaining PAHs become increasingly sorbed within the soil matrix.

Soot and chars are the remnants of incomplete combustion and are commonly termed “black carbon.” These solid phases offer strong sorption sites for PAHs. Studies indicate that PAHs may have an affinity for black carbon that is several orders of magnitude greater than reported for natural organic matter (Cornelissen et al. 2005). Additionally, solid PAH source materials, such as soot and coal tar from clay target fragments, have a larger influence on the partitioning of PAHs than other soil properties, such as organic carbon or clay content (Xia, Gomez-Eyles, and Ghosh 2016). This increased partitioning by soot and coal tar is reflected in decreased dermal absorption in excised pig skin. Dermal and oral bioavailability studies using soils affected by clay target fragments (USACE 2016) and lampblack soot (Stroo et al. 2005) also report reduced dermal absorption, reduced gastrointestinal (GI) absorption relative to freshly spiked soil samples, or both—further confirming that source material must be considered when evaluating potential bioavailability of PAHs in soil.

Since bioavailability of HOCs in soil is largely related to solubility, most research into the potential effects on bioavailability of HOCs from various soil properties is based on studying partitioning kinetics of HOCs from the stationary phase of the soil matrix into the aqueous phase. After the effects of PAH source material, soil characteristics including black carbon, organic carbon (OC), and clay mineral content are likely to have the most significant effect on PAH bioavailability. Other soil characteristics, such as pH, cation exchange capacity (CEC), and aerobic/anaerobic conditions, may also affect bioavailability of PAHs in soils and sediments to a lesser extent (Chung and Alexander 2002; Luo et al. 2012). Figure 8-1 illustrates PAH availability for oral or dermal absorption as a function of PAH source materials and soil chemistry.

Figure 8‑1. PAH availability for oral or dermal absorption as a function of PAH source materials and soil chemistry.
Source: Reprinted with permission from (Ruby, M.V., Y.W. Lowney, A.L. Bunge, S.M. Roberts, J.L. Gomez-Eyles, U. Ghosh, J. Kissel, P. Tomlinson, and C.A. Menzie. “Oral Bioavailability, Bioaccessibility, and Dermal Absorption of PAHs from Soil – State of the Science.” Environmental Science & Technology 50, no. 5 (2016): 2151-64. Figure 1), Copyright 2016 American Chemical Society.

8.3.2.1 Soil Organic Carbon Read More

8.3.2.2 Clay Mineral Content Read More