Residual hydrophobic organic contaminants in soil: Are they a barrier to risk-based approaches for managing contaminated land?

Abstract

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.

Extremely small amounts of B[a]P residues remobilised in long-term contaminated soils: A strong case for greater focus on readily available and not total-extractable fractions in risk assessment

“…Risk-based approaches to contaminated land management is gaining ground in a number of countries 22,53,54 . However, not all regulators of contaminated lands agree to adopting risk-based approaches that rely on contaminant bioavailability or bioaccessibility due to associated uncertainties 55 , such as the fate of contaminant residues in soils 4 . In this study, B[a]P remobilisation occurred in soils after 4 years of aging, but the amounts remobilised were extremely small (generally < 5% of initially spiked concentration, i.e.…”

“…The impacts and significance of nonextractable residues (NERs) of hydrophobic organic contaminants (HOCs), such as polycyclic aromatic hydrocarbons (PAHs), to human and environmental health risk assessments of contaminated land are gaining increasing attention within the scientific and regulatory communities [1][2][3][4] . Specifically, whether NERs will be remobilised in soil and pose unacceptable levels of risks to human and environmental health remain uncertain.…”

Abiotic factors controlling bioavailability and bioaccessibility of polycyclic aromatic hydrocarbons in soil: Putting together a bigger picture

“…In published studies PAH source materials are often considered as sorption phases for PAHs due to their high partitioning coefficients. This could result in the categorisation of source material-associated PAHs under residual fraction and brings extra uncertainties to risk assessment (Umeh et al, 2017). Future research should focus on their roles in the release of PAHs as high affinity for PAHs does not necessarily mean irreversible sorption.…”

Prediction of bioavailability and toxicity of complex chemical mixtures through machine learning models

“…Bioavailability provides a basis to make robust decision regarding appropriate risk assessment, predicting contaminants' fate, transport, and potential environmental impact (Lehmann and Joseph, 2015). While greater attention has been given to the role of bioavailability of complex contaminants in regard to remediation end points (Brand et al, 2012;Harmsen and Naidu, 2013;Kördel et al, 2013;Ortega-Calvo et al, 2015) as well as its implication for regulatory frameworks (Umeh et al, 2017), its recognition and effective application by European-based environmental regulators is still limited (Harmsen and Naidu, 2013).…”