Chemical measures of bioavailability/bioaccessibility of PAHs in soil: Fundamentals to application
Introduction
Contaminated land is a worldwide legacy of industrialization requiring huge investment to remediate. Across England and Wales alone it is estimated some 300,000 ha fall under the designation of contaminated land, cumulatively covering an area almost 2% of the total respective landmass [1]. Within many of these contaminated sites, organic contaminants such as polycyclic aromatic hydrocarbons (PAHs), a group of environmentally persistent and toxic chemicals, and other hydrophobic organic compounds (HOCs) feature as a major component [1]. Consequentially, these chemicals provide a key focus of soil remedial activities [2] and present substantial challenges for management legislation [3]. Historically, regulatory procedure has centred on the contaminants total concentration, identified through exhaustive chemical extractions, assuming that 100% is bioavailable and, thus, able to exert toxicological effects [4], [5]. A growing body of evidence, however, has led to suggestions that such extraction procedures overestimate the ‘true’ fraction available to biota [6], [7], [8], [9]. Translated into contaminated land legislation, this situation may allow for larger quantities of contamination to be left in soils without further risk, resulting in reduced remediation costs, smaller volumes of soil to remediate and the prospect of adopting less intrusive remedial approaches [10].
In the UK, identification, assessment and management of contaminated land has moved towards a risk-based approach derived from Part IIA of the Environmental Protection Act (EPA) 1990. Under this legislation, land is considered contaminated when its current use results in the establishment of a linkage between the contaminant and the defined receptor, which poses the “significant possibility of significant harm”. Simplistically, the risk-based approach to contaminated land comprises three phases; (i) hazard identification and assessment, (ii) risk identification and assessment and (iii) management and remediation. Although different in their implementation, approaches adopted by both the UK and USA (amongst other countries) for organic contaminants implicitly consider bioavailability processes and therefore bioavailability, within their framework [11], [12], [13].
Following a brief introduction to bioavailability with respect to organic contaminants in soil, this paper critically evaluates the concept of measuring bioavailability using chemical techniques. By appraising the more widely applied methodologies, we elicit the causes of variance between studies and different techniques that prevent the widespread adoption of bioavailability adjustment factors by regulatory bodies and environmental consultants alike. Subsequently, a proposal for the incorporation of bioavailability within contaminated land risk assessment as a decision-support tool in identifying that which poses ‘significant possibility of significant harm’ within ecological risk assessment is given, together with due consideration for the inherent variability.
Section snippets
Bioavailability of organic contaminants in soil
The behaviour and ultimate fate of organic contaminants in soil (and sediment) is controlled by factors such as the characteristics of the soil and contaminant(s); environmental variables (e.g. temperature and precipitation) [6], [14], [15] and cumulative loss processes (e.g. leaching, biodegradation/uptake, photo-oxidation, chemical oxidation, volatilization) [16]. Additionally, intra-soil processing occurs with increasing soil-contaminant contact time resulting in decreasing chemical and
Applying bioavailability to contaminated land risk assessment and remediation
Concerns over experimental accuracy and the perceived demand for a direct relationship between chemical extractability and ‘bioavailability’, which is able to predict biodegradation in a robust and reproducible manner, forces us to ask, “how accurately does a technique need to predict bioavailability, given its proposed use as a ‘level 2’ investigation?” Bioavailability is highly contextual, dependent on the specific nature of the receptor and pathway [149], and given the large number of
Conclusion
From the above definitions, it is clear that many of these studies are determining ‘a’ bioavailable fraction, whilst others are measuring the bioaccessible fraction. If bioaccessibility is being measured rather than bioavailability, is it possible to measure ‘actual’ bioavailability? [155] Additionally, is bioavailability the more relevant quantity? It is likely that remediation scientists are preferentially more concerned about what is bioaccessible at a given site, rather than what is
Acknowledgements
The authors would like to acknowledge the Engineering and Physical Sciences Research Council (EPSRC) and National Grid and the European Commission under FP7–Environmental Technologies (ModelProbe, No. 213161) for funding this work.
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