Risk-based approaches to managing contaminated land, rather than approaches based on complete contaminant removal, have gained acceptance as they are likely to be more feasible and cost effective. Risk-based approaches aim to minimise risks of exposure of a specified contaminant to humans. However, adopting a risk-based approach over alternative overly-conservative approaches requires that associated uncertainties in decision making are understood and minimised. Irrespective of the nature of contaminants, a critical uncertainty is whether there are potential risks associated with exposure to the residual contaminant fractions in soil to humans and other ecological receptors, and how they should be considered in the risk assessment process. This review focusing on hydrophobic organic contaminants (HOCs), especially polycyclic aromatic hydrocarbons (PAHs), suggests that there is significant uncertainty on the residual fractions of contaminants from risk perspectives. This is because very few studies have focused on understanding the desorption behaviour of HOCs, with few or no studies considering the influence of exposure-specific factors. In particular, it is not clear whether the exposure of soil-associated HOCs to gastrointestinal fluids and enzyme processes release bound residues. Although, in vitro models have been used to predict PAH bioaccessibility, and chemical extractions have been used to determine residual fractions in various soils, there are still doubts about what is actually being measured. Therefore it is not certain which bioaccessibility method currently represents the best choice, or provides the best estimate, of in vivo PAH bioavailability. It is suggested that the fate and behaviour of HOCs in a wide range of soils, and that consider exposure-specific scenarios, be investigated. Exposure-specific scenarios are important for validation purposes, which may be useful for the development of standardised methods and procedures for HOC bioaccessibility determinations. Research is needed to propose the most appropriate testing methods and for assessing potential risks posed by residual fractions of HOCs. Such investigations may be useful for minimising uncertainties associated with a risk-based approach, so that consideration may then be given to its adoption on a global scale. This review critically appraises existing information on the bioavailability of HOC residues in soil to establish whether there may be risks from highly sequestered contaminant residues.
The fate and behavior of polycyclic aromatic hydrocarbons (PAHs) in soil are of interest in the risk assessment of contaminated land and are usually based on determinations of fractions extracted from soil. For decades, either single- or sequential-solvent extractions have been used to determine PAH extractability in soils; however, there is a lack of certainty as to which fractions are being extracted by these techniques. This study is the first report of differences and similarities in the extractability of benzo[ a]pyrene (B[ a]P) in four contrasting soils (sandy loam, loamy sand, clayey loam, and sandy) when determined using both single-solvent (dichloromethane/acetone (DCM/Ace) mixture) and sequential-solvent (butanol followed by DCM/Ace) extraction. Residues after extraction were subjected to methanolic saponification (MeKOH). Butanol (BuOH) extractability and total extractability of B[ a]P following sequential-solvent extraction decreased significantly ( p < 0.05) with time after addition of B[ a]P. The decrease in BuOH extractability was particularly marked in the organic-matter-rich clayey-loam soil, which also had the largest (>40%) amounts of nonextractable residues. The cumulative amounts of B[ a]P extracted in each soil by single- and sequential-solvent extractions were similar ( p > 0.05) at each aging period, which indicated access to similar B[ a]P fractions in soil by both solvent extractions. The similarities in the amounts of B[ a]P nonextractable residues recovered by MeKOH from pre-extracted soils, through either of the extraction methods, confirms that the total extractable B[ a]P fractions from both methods are similar.
12The environmental and health risks associated with 'non-extractable' residues (NERs) of 13 polycyclic aromatic hydrocarbons in soils and their potential for remobilisation remain largely 14 unexplored. In this novel study, sequential solvent extractions were employed to interrogate 15 time-dependent remobilisation of benzo[a]pyrene (B[a]P) NERs and associated kinetics after 16 re-equilibration (REQ) periods lasting 30 d in four artificially-spiked soils aged for up to 200 17 days. Following sequential extractions of the re-equilibrated soils, remobilisation of B[a]P 18 NERs was observed and further confirmed by decreases in the absolute amounts of B[a]P 19 recovered following methanolic saponification after REQ. Remobilisation may occur through 20 slow intercompartmental partitioning of more sequestered into less sequestered B[a]P fractions 21 in soils. The amounts of B[a]P remobilised in soils decreased throughout aging following first-22 order kinetics and the rates of decrease were slow but 2 to 4 times faster than those of 23 extractable B[a]P before re-equilibration. Sandy-clay-loam soils with large amounts of hard 24 organic carbon exhibited less NER remobilisation compared to sandy soils. The amounts of 25 remobilised B[a]P decreased significantly (p < 0.05) with aging. Specifically, butanol-26 remobilised B[a]P in soils spiked at 10 mg/kg and 50 mg/kg B[a]P ranged from 0.15 to 0.39 27 mg/kg and 0.67 to 2.30 mg/kg, respectively, after 200 d of aging. 28 PAHs, with soil serving as a sink, include coking and coal firing plants, vehicular emissions, 37bush fires, fossil fuel burning, and crude oil spillage 3 . The amounts of PAHs in soils can be 38 wide-ranging depending on land use and proximity to industrial activities 4, 5 . Average total 39 concentration of the16 USEPA PAHs in soils collected near gasworks sites can range from 40 300 to over 8000 mg/kg 6 , and many people living close to these sites may be exposed. 41In soil, PAHs may be lost through volatilisation, photolysis, leaching, microbial 42 degradation, and biological uptake 7 . Importantly, PAHs are readily sequestered in soil as 43 they are highly hydrophobic. Sequestration of PAHs in soils occurs through partitioning into 44 soft and hard organic carbon, clay, organo-mineral complexes, as well as diffusion into 45 micropores, or strong sequestration to carbonaceous geosorbents such as black carbon 8, 9 . 46 Detailed mechanisms of HOC sequestration have been described elsewhere 8, 10 . With aging, 47 extractability (percent BuOH + DCM/Ace extractability) among the three batches using one-132 way ANOVA ( Figure S2), as well as variances between batches using Levene's Test 21 . 133Standard error of the mean for each batch (n = 4) was generally 5% or less, and the mean 134 concentration of each batch was not statistically different (p < 0.05) from the mean of original 135 samples (n = 12). Therefore, each batch (n = 4) was statistically indistinguishable from the 136
The influence of soil properties on PFOS sorption are not fully understood, particularly for variable charge soils. PFOS batch sorption isotherms were conducted for 114 temperate and tropical soils from Australia and Fiji, that were well-characterized for their soil properties, including total organic carbon (TOC), anion exchange capacity, and surface charge. In most soils, PFOS sorption isotherms were nonlinear. PFOS sorption distribution coefficients (K d) ranged from 5 to 229 mL/g (median: 28 mL/g), with 63% of the Fijian soils and 35% of the Australian soils showing K d values that exceeded the observed median K d. Multiple linear regression showed that TOC, amorphous aluminum and iron oxides contents, anion exchange capacity, pH, and silt content, jointly explained about 53% of the variance in PFOS K d in soils. Variable charge soils with net positive surface charges, and moderate to elevated TOC content, generally displayed enhanced PFOS sorption than in temperate or tropical soils with TOC as the only sorbent phase, especially at acidic pH ranges. For the first time, two artificial neural networks were developed to predict the measured PFOS K d (R 2 = 0.80) in the soils. Overall, both TOC and surface charge characteristics of soils are important for describing PFOS sorption.
Perfluoroalkyl and polyfluoroalkyl substances (PFAS) are persistent organic contaminants of concern to human and environmental health. Several literature reviews and laboratory column experiments have been conducted to determine the transport parameters and to describe the fate of PFAS as they migrate in subsurface environments. However, there are very few case studies focusing on contaminated sites with high-resolution field data. Such studies are crucial for the validation of transport simulation models that have been developed from experimental studies, prior to their broader applications. The key purpose of this research was to evaluate lithological separations of PFAS fractions as they are transported in the vadose zone of a historically (1979) contaminated site where Aqueous Film Forming Foam (AFFF) formulations (3M Lightwater™ and Ansulite™) have been used for fire training exercises. Surface and subsurface soils, and groundwater samples were collected across the site and a total of 29 PFAS compounds were selected as target analytes. The results indicated a distinct profile of PFAS concentration with depth at most of the test bores, exhibiting separation of PFAS as transported in vadose zone soils. Perfluorooctanoic acid (PFOA), Perfluorooctane sulfonic acid (PFOS), and Perfluorohexane sulfonic acid (PFHxS) were the predominant compounds detected in the site samples and they have been found in near-surface soils (<3 m) with concentrations declining with depth. The concentration of the 6:2 fluorotelomer sulfonate showed little change with depth in most of the test bore wells. The percentage concentration of each compound relative to the sum of PFAS, and the ratio of PFHxS/PFOS with depth, suggested transformation processes. Despite the relatively high solubility of PFAS, and that the application of AFFF has been ceased for some years at the site, there were still significant concentrations of PFAS adsorbed to the vadose zone soils that acted as ongoing sources of contamination to groundwater.
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 were 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 0.6-0.8 kgOC/kglipid. 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.
The fate, impacts and significance of polycyclic aromatic hydrocarbon (PAH) non-extractable residues (NERs) in soils remain largely unexplored in risk-based contaminated land management. In this study, 7 different methanolic and non-methanolic alkaline treatments, and the conventional methanolic saponification, were used to extract benzo [a]pyrene (B[a]P) NERs that had been aged for 180 d from four contrasting soils. Up to 16% and 55% of the amount of B[a]P spiked (50 mg/kg) into soils was non-extractable after 2 d and 180 of aging, respectively; indicating rapid and progressive B[a]P sequestration in soils over time. The recovery of B[a]P from soils after 180 d of aging was increased by up to 48% by the 7 different alkaline extractions, although the extraction efficiencies of the different alkaline treatments did not differ significantly (p > 0.05). Approximately 40% of B[a]P NERs in the sandy-clay-loam organic matter-rich soil was recovered by the exhaustive alkaline extractions after 180 d of aging, compared to only 10% using conventional methanolic saponification. However, the amounts of B[a]P NERs recovered depend on soil properties and the amounts of NERs in soils. A significant correlation (R 2 = 0.69, p < 0.001) was also observed between the amounts of B[a]P recovered by each of the 7 alkaline extractions in the contrasting soils, and corresponding NERs at 180 d of aging, indicating a potential association warranting further investigations. Extraction techniques that estimate the amounts of PAH NERs recoverable in soil can help give a better understanding of the fate of NERs in soil.
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