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For many years the chemical testing of soil has used methods intended to extract the maximum amount of various contaminants from the soil – aggressive acidic extractions for metals, or polar solvents such as methanol or dichloromethane for organics are just two examples.
From a human health risk assessment perspective this has been recognised as not necessarily the most meaningful approach, and several physiologically based extraction tests (PBET) have been proposed over the years, to provide an indication of the bioavailable proportion of the contaminant in question.
These methods use extractants designed to mimic the digestive secretions within the body, and so provide more appropriate data. As these values are generally lower than the ‘total’ results, using this information may save significant costs when remediating contaminated sites, either by retaining material on site (no landfill costs), or by reducing the need for soil treatment.
A recent three day conference organised by the British Geological Survey (BGS) and the University of Nottingham, was held at BGS, Keyworth, reviewing some of these methods, and international speakers presented current research and ideas on this subject –the 7th International Workshop on Chemical Bioavailability in the Terrestrial Environment.
Bioaccessible/bioavailable
It is important to understand the difference between bioavailability and bioaccessibility, as this causes some confusion within the industry.
Bioaccessibility is a measure of the contaminant in the soil in a form that can be ingested/solubilised into the gastro-intestinal (GI) tract and is potentially available to the organism.
Bioavailability is a measure of the proportion of the ingested contaminant that passes through the gastro-intestinal lining into the systemic blood and organs and is actually available.
Bioavailability can therefore only be measured by in vivo animal studies – exposing mice/rats/piglets to the contaminant of interest in their food, and then monitoring the levels in their blood or other organs. This route has ethical issues and is very expensive.
The emphasis within the industry has therefore concentrated on defining chemical – in vitro – tests which mimic the uptake by receptors and can provide an indication of risk to exposure. They are sometimes referred to as predictors.
History
Interest in this area of science began in 1990, but only nine papers were published in the subsequent five years, including the first paper in 1993 by Ruby et al, outlining the original PBET method. Research moved fairly slowly, but interest began to gather momentum, and between 2003 and 2007, 135 papers were published. In 2013 so far, more than 100 authors have contributed 72 papers from 40 countries on this topic (data collated by Dr Joanna Wragg, BGS).
Initially, the two contaminants of interest were arsenic and lead, but this has now extended to several other metals including copper, nickel and cadmium, and also organics such as polyaromatic hydrocarbons and total petroleum hydrocarbons. The basis of bioaccessible methods is to mimic the uptake by the human body, and this involves using extraction solutions similar to saliva, gastric (stomach) acid, and intestinal secretions. Of the published methods, these range from a simple one step to a sequential three step method involving the use of a food substrate.
Examples of methods: • PBET, Advances in bioavailability of inorganics in soil, Ruby et al (1996) • IVG (In Vitro Gastrointestinal) method, Rodriguez et al (1999) • Dutch (RIVM) method (2004) • RBALP (Relative Bioacessibility Leaching Procedure) (2007) –only for lead • CE-PBET (2011) Colon Extended Physiologically Based Extraction Test • SHIME Dynamic Bioaccessibility Gastrointestinal Method Simulator System • FOREhST (2010) Fed Organic Estimation human Stimulation Test
The early data was variable, however, with inconsistency between laboratories and methods, and the regulators/end users of data were reluctant to accept the concept of bioavailability as robust enough to use.
A group was set up in 1998 in Europe known as BARGE – the Bioaccessibility Research Group of Europe – and interlaboratory studies conducted in early 2000 demonstrated a wide range of variability in the data due to the different methods in use. BARGE set up trials and reviewed data, eventually producing a Unified BARGE Method (UBM) in 2007 and trials were performed using this, plus some studies correlating the bioaccessibility data to bioavailable data using animal studies.
The BARGE group in the UK is chaired by Dr Mark Cave, with Dr Joanne Wragg as secretariat, both of BGS.
Recent BGS conference
The conference was well attended with more than 100 delegates and ran over the course of three days. International speakers from several countries, including Canada, France, India, Pakistan, Belgium, Germany, Australia, the Netherlands and Slovenia, gave presentations on a variety of topics relating to aspects of bioavailability and human health risks. The conference sponsors were CEM and CRC Care.
Bioaccumulation of metals in plants and biota
Initial sessions discussed the uptake of heavy metals by rice (arsenic), and on mercury in biota (shellfish and bivalves), with the subsequent ingestion by humans. Plants are sometimes referred to as hyperaccumulators, and this may be of benefit when remediating soils, but not when used as food.
In a paper given by David A Polya, University of Manchester, it was proven that detrimental health outcomes such as elevated genotoxic effects, have been observed in West Bengal with people eating cooked rice high in arsenic, which is their staple diet. As little as 2 ug/kgBW/day is enough to give rise to health issues (BW = body weight).
Mousumi Chatterjee, Department of Marine Sciences, Calcutta, presented data on mercury bioaccumulation in the Sudarban wetlands, India, and found that concentrations of mercury in fish are > polychaetes which are > bivalves which are > sediments. The values ranged from <0.06 mg/kg in the sediment to 0.27 mg/kg in some of the fish. The contamination pattern clearly reflects the bioaccumulation and potential health hazards in the food chain.
Two papers discussing the toxicity of zinc to earthworms and plants found that the pH of the soil was critical – acidic soils cause greater toxic effects in earthworms than neutral or alkaline soils.
Taking a different perspective, a paper from Munir Zia, Fauji Fertiliser Company, Pakistan, discussed the lack of iodine uptake and ensuing health effects – evidently more than 1.2 billion people suffer from iodine deficiency diseases (IDD) and the WHO recommend a daily intake of 150ug/g. In many areas of Pakistan, the mean uptake figure is only 3ug/day.
Bioavailability and bioaccessibility comparisons
Sessions on the second day of the conference concentrated both on comparative methods measuring the bioaccessibility of metals and organics, and also provided data from bioavailable studies using juvenile pigs.
Michiel TO Jonker of the University of Utrecht gave a review of various alternative methods ranging from Tenax, XAD resin, cyclodextrin, supercritical CO2, passive sampling and mild solvents. Differing methods may be of use in specific scenarios, but interpretation of data may be complex. The use of Cfree as a measure of the freely dissolved concentration in interstitial soil pore water should be encouraged.
The next speaker, Steven Siciliano, University of Saskatchewan, Canada, discussed in vivo and in vitro studies comparing the co-effect of metals on polyaromatic hydrocarbon bioaccessibility. He used the FOREhST method of bioacessibility testing, and found that the metal content does give a positive correlation with PAH bioaccessibility.
The next two papers looked at the bioavailability/bioaccessibility of organics, chlordecone (CLD) a pesticide, and PCBs. Again, animal studies were used to determine the correlation between the bioaccessible and the bioavailable data. The use of relative bioavailability (RBA) was proposed – this is defined as the response of the pollutant ingestion in soil compared to the response obtained with pollutant ingestion through a reference matrix, often spiked oil. In the case of CLD, the organic content of the soil was not found to modulate the availability. For PCBs, however, the presence of condensed organic matter in the soil strongly reduced the availability.
Martine Bakker, of the National Institute of Public Health and the Environment, the Netherlands, then presented a comparative study of three in vitro methods and one in vivo study using juvenile pigs for the bioavailability of lead:
The results show some degree of correlation between two of the in vitro tests – the IVD and the UBM, and the blood levels from the animals, although there are some variations. The most striking is the low bioaccessibility values for the Maastricht samples, and this was thought to be caused by high levels of calcite found in these soils, which act as a strong absorbent for the lead during the bioaccessibility tests, but not so much for the in vivo absorption. There was a line of evidence that a simple extraction using 0.4N nitric acid gave comparable results.
Other studies
A study from China given by Lena Q Ma compared five in vitro methods, again demonstrating some variation and the effects of pH and iron concentration in the soil on the bioacessibility.
Petra Gunther, IFUA-Projekt-GmbH, Germany gave a presentation on the DIN 19738 method of bioaccessibility testing, which is similar to the Unified Barge Method, but with some differences to the protocol. The standard will be reviewed and updated in 2014.
Sofie de Moor, Ghent University, discussed the effect of soil amendments on the mobility of cadmium and zinc, plus plant uptake. Metal mobility was found to be higher in acidic soils or with higher chloride concentrations, and lower with the addition of calcium carbonate. When a range of extractant solutions was used, the results were as follows:
Aqua regia > ammonium acetate/EDTA > ammonium acetate > calcium chloride > sodium nitrate
Further presentations covered other studies on lead, top soil dust, outdoor playground dust and mining spoil heap dust.
Paul Nathanail, of LQM, gave a summary of the current expectations of the regulator in the UK, which have changed significantly over the last three to four years. In 2009, the Chartered Institute of Environmental Health (CIEH) published a professional practise note (PPN) to assist regulators in reviewing human health risk assessment reports which included bioaccessibility estimates. The advice then was to treat the data with caution and 12 common misuses were listed. The testing has now improved, however, and the legal context across the UK has changed – detailed quantitative risk assessments (DQRA) can now use site specific bioavailability values.
The final speaker, Ravi Naidu, of CRC Care, University of South Australia, spoke of the challenges to be met in the next few years leading up to 2020. His list of targets included: • Developing standard protocols for assessing single and mixed contaminant bioavailability • Working with the regulators with the aim of getting bioaccessibility based guidelines adopted at a national level • Developing bioaccessibility based human health risk assessment tools • Developing a mechanistic model for predicting contaminant bioavailability
Potential for cost savings
In 2005, the UK’s Environment Agency estimated some 300,000 hectares of former industrial land in England and Wales could be classified as ‘contaminated’, when assessed by standard chemical methods.
A well documented study sponsored by the Natural Environment Research Council (NERC) and BGS was conducted and resulted in estimated cost savings of £3.75 million.
An additional example was a joint project performed by BGS, Land Quality Management (LQM), and the University of Nottingham staff, which saved between £7 million and £30 million in remediation expenses on one site. Bioaccessibility testing not only reassured local residents, but also helped to kick start the stalled local housing market.
A paper on the bioaccessibility of polyaromatic hydrocarbons was given by Jamie Cutting of CE Geochem, using the FOREhST method of extraction. This was based on two commercial sites which initially would require extensive remediation (at a cost of > £60K) due to benzo(a)pyrene concentrations above the CLEA site specific assessment criteria (SSAC) of 0.94 mg/kg. Using the FOREhST method, the bioaccesible fraction proved to be less than 25%, thus effectively raising the SSAC to between 2.34 – 4.05 mg/kg.
The use of bioaccessibility testing therefore provides threefold benefits to a remediation site: reduced costs, increased sustainability and reduced waste volumes.
Summary
This is a complex topic with some conflicting opinions on certain aspects of bioavailability/bioaccessibility testing and how best to gain an understanding of quantifying the risks to human health. Much progress has been made over the last ten years, but there is still a profusion of methods, which can lead to varying interpretations.
Most data relates to metals, but several studies have been done on PAHs, and some other organics, but more work is probably needed on these classes of compounds. In Europe, the Unified BARGE Method (UBM) is becoming recognised as a consistent method, and several countries have now run trials using this.
Bioaccessibility is cited as an approved method in some EU documents, and is commonly used in the Netherlands as part of Environmental Risk Assessments (ERA). In the UK, most Local Authorities and the Environment Agency are more favourably inclined to accept data using bioaccessibility protocols, as long as it is robust.
Considering the potential major cost savings which can result from using bioaccessibility testing, this methodology should be considered by any consultant or developer involved in the remediation of contaminated sites.
Published: 27th Nov 2013 in AWE International
