SEGH Articles

Measuring the Bioaccessibility of Potentially Harmful Elements in Soil

01 May 2013
Mark Cave provides some background for bioaccessibility testing and insight into the contribution it has made to the risk assessment industry.
Dr Mark Cave is a leading scientist who has been a major driving force behind the development and adoption of bioaccessibility testing within the risk assessment and contaminated land community.  He is organising an upcoming International Conference in November 2013 at the British Geological Survey, bringing together many world players in bioavailability and bioaccessibility research http://www.bgs.ac.uk/news/events/bioavailabilityWorkshop/home.html.  Here he provides background for bioaccessibility testing and insight into the contribution it has made to the risk assessment industry.
 
In terms of human health risk assessment there are three main exposure pathways for a given contaminant present in soil. The largest area of concern is the oral/ingestion pathway followed by the dermal and respiratory exposure routes (Paustenbach, 2000). Whether contaminated soils pose a human health risk depends on the potential of the contaminant to leave the soil and enter the human bloodstream. The use of total contaminant concentrations in soils provides a conservative approach as it assumes that all of the metal present in the soil can enter the bloodstream. Results from animal tests e.g. (Denys et al., 2012) suggest that contaminants in a soil matrix maybe absorbed to a lesser extent and show fewer toxic effects compared to the same concentration of soluble salts of the contaminants in a food or liquid matrix.
 
There is, therefore, a clear need for a practical methodology that measures the fraction of the contaminant in the soil that, through oral ingestion, can enter the systemic circulation of the human body and cause toxic effects. This is known as the oral bioavailability and can be formally defined as the fraction of an administered dose that reaches the central (blood) compartment from the gastrointestinal tract (Paustenbach, 2000). This term must not to be confused with the oral bioaccessibility of a substance, which is defined as the fraction that is soluble in the gastrointestinal environment and is available for absorption (Paustenbach, 2000).
 
Since bioavailability data is essentially related to the amount of contaminant in the animal/human bloodstream the data must be produced from the dosing of animals with contaminated soil and the subsequent measurement of the contaminant in the blood or organs of the animal; these are known as in-vivo animal models. Bioaccessibility data, however, is normally determined in a test tube environment (in-vitro) and represents the amount of contaminant dissolved in the gastrointestinal tract prior to crossing the mucosal walls. The amount of pollutant which is actually absorbed by an organism is generally less than or equal to the amount which is mobilised (Paustenbach, 2000). Bioaccessibility extraction tests are generally based around the gastrointestinal parameters of young children (0-3 yr). This age group is thought to be at most risk from accidental ingestion of soil. Also, since children can absorb a higher percentage of contaminant through the digestive system than adults, they are more susceptible to adverse health effects (Hamel et al., 1998).
 
Mammal dosing trials are time consuming and expensive. To supersede the use of animals in determining the bioavailability of potentially harmful elements for human health risk assessment, or to estimate bioavailability where animal studies are not available, a potential alternative is the use of in-vitro tests.
 
A number of in vitro bioaccessibility tests for mimicking human ingestion have been reported in the literature. As a result of research carried out by the Bioaccessibility Research Group of Europe (BARGE) and other research groups it was clear that the different bioaccessibility tests showed similar trends when used on the same soil samples, but the different operating conditions for each test produced widely ranging bioaccessibility values between the methods (Oomen et al., 2002). To overcome this problem, BARGE undertook a joint decision to progress the development of a harmonised in vitro bioaccessibility method (the Unified BARGE Method – UBM).
 
The chosen method was the RIVM method (Versantvoort et al., 2004). A schematic outline of the method is shown in Figure 1.
 
 
 

Figure 1 schematic outline of the BARGE unified method

The UBM has now undergone initial inter-laboratory trials (Wragg et al., 2011) and been validated against an in-vivo model (Denys et al., 2012)and has become widely accepted as the method of choice in European Countries.

In a study of the financial impact of research carried out for the Natural Environment Research Council by the British Geological Survey (Natural Environment Research Council (NERC), 2009) examples of the use of bioaccessibility testing were given that showed that:

i) In one case the assessment enabled the re-use of existing site materials as part of the land remediation process, which subsequently led to reduced costs of approximately £3.75 million. In addition, approximately 3,750 lorry trips to landfill were avoided and 105 tonnes of CO2 equivalent were saved. 

ii) In another example, BGS worked with Land Quality Management and University of Nottingham staff to save between £7-£30 million remediation expenses on one site. The more accurate bioaccessibility testing not only reassured local residents, but also allowed the stalled housing market in the area to restart.

Across England, there are an estimated 15,470 hectares of land in need of remediation. The cost of remediating this land is between £100,000-£325,000 per hectare, giving a potential market of £1.5-£5.0 billion. The research methods developed by BGS have the potential to save between £3.9 million and £12.6 million per year in remediating derelict land for development. Over a 20 year period, these cost savings are estimated to have a Net Present Value of between £55.0 million and £178.6 million.

The method is also being used on a national scale to provide bioaccessibility maps arsenic and Pb (Appleton et al., 2012a, b). Figure 2 shows an example of how a combination of the UBM test and data modelling has produced a map of the bioaccessible lead in soils in the Greater London area.

 

Figure 2 Estimated bioaccessible Pb in topsoils in the Greater London area (solid lines = motorways, major (A, B) and minor roads; Ordnance Survey Strategi data © Crown copyright 2012) (Appleton et al., 2012b)

 

Bioaccessibility testing cuts across a number of disciplines including chemistry, geochemistry, toxicology, human health and risk assessment but recent collaborative work untaken by research consortia such as the BARGE group have enabled the development of standardised testing protocols which have a direct impact on human health risk assessment and demonstrable economic benefits when used on a national and international scale.

Dr Mark Cave, British Geological Survey

mrca@bgs.ac.uk


References

Appleton, J D, Cave, M R, and Wragg, J. 2012a. Anthropogenic and geogenic impacts on arsenic bioaccessibility in UK topsoils. Science of the Total Environment, Vol. in Press.

Appleton, J D, Cave, M R, and Wragg, J. 2012b. Modelling lead bioaccessibility in urban topsoils based on data from Glasgow, London, Northampton and Swansea, UK. Environmental Pollution, Vol. in Press.

BARGE. Bioaccessibility Research Group of Europe. Cave, M. [cited November 27]. http://www.bgs.ac.uk/barge/home.html 

Denys, S, Caboche, J, Tack, K, Rychen, G, Wragg, J, Cave, M, Jondreville, C, and Feidt, C. 2012. In Vivo Validation of the Unified BARGE Method to Assess the Bioaccessibility of Arsenic, Antimony, Cadmium, and Lead in Soils. Environmental Science & Technology, Vol. 46, 6252-6260.

Hamel, S C, Buckley, B, and Lioy, P J. 1998. Bioaccessibility of metals in soils for different liquid to solid ratios in synthetic gastric fluid. Environmental Science & Technology, Vol. 32, 358-362.

Natural Environment Research Council (NERC). 2009. Bioaccessibility Testing of Contaminated Land for Threats to Human Health.

Oomen, A G, Hack, A, Minekus, M, Zeijdner, E, Cornelis, C, Schoeters, G, Verstraete, W, Van de Wiele, T, Wragg, J, Rompelberg, C J M, Sips, A, and Van Wijnen, J H. 2002. Comparison of five in vitro digestion models to study the bioaccessibility of soil contaminants. Environmental Science & Technology, Vol. 36, 3326-3334.

Paustenbach, D J. 2000. The practice of exposure assessment: A state-of-the-art review (Reprinted from Principles and Methods of Toxicology, 4th edition, 2001). Journal of Toxicology and Environmental Health-Part B-Critical Reviews, Vol. 3, 179-291. 

Versantvoort, C H M, Van de Kamp, E, and Rompelberg, C J M. 2004. Development and applicability of an in vitro digestion model in assessing the bioaccessibility of contaminants from food. RIVM, RIVM report 320102002/2004 (Bilthoven).

Wragg, J, Cave, M R, Basta, N, Brandon, E, Casteel, S, Denys, S e b, Gron, C, Oomen, A, Reimer, K, Tack, K, and Van de Wiele, T. 2011. An Inter-laboratory Trial of the Unified BARGE Bioaccessibility Method for Arsenic, Cadmium and Lead in Soil. Science of the Total Environment, Vol. 409, 4016-4030.

 

 

 

Back to archive
Keep up to date
Join Us or Renew

SEGH Events

34th SEGH International Conference: Geochemistry for Sustainable Development

Victoria Falls, Zambia

02 July 2018

Submit Content

Members can keep in touch with their colleagues through short news and events articles of interest to the SEGH community.

Science in the News

Latest on-line papers from the SEGH journal: Environmental Geochemistry and Health

  • Fertilizer usage and cadmium in soils, crops and food 2018-06-23

    Abstract

    Phosphate fertilizers were first implicated by Schroeder and Balassa (Science 140(3568):819–820, 1963) for increasing the Cd concentration in cultivated soils and crops. This suggestion has become a part of the accepted paradigm on soil toxicity. Consequently, stringent fertilizer control programs to monitor Cd have been launched. Attempts to link Cd toxicity and fertilizers to chronic diseases, sometimes with good evidence, but mostly on less certain data are frequent. A re-assessment of this “accepted” paradigm is timely, given the larger body of data available today. The data show that both the input and output of Cd per hectare from fertilizers are negligibly small compared to the total amount of Cd/hectare usually present in the soil itself. Calculations based on current agricultural practices are used to show that it will take centuries to double the ambient soil Cd level, even after neglecting leaching and other removal effects. The concern of long-term agriculture should be the depletion of available phosphate fertilizers, rather than the negligible contamination of the soil by trace metals from fertilizer inputs. This conclusion is confirmed by showing that the claimed correlations between fertilizer input and Cd accumulation in crops are not robust. Alternative scenarios that explain the data are presented. Thus, soil acidulation on fertilizer loading and the effect of Mg, Zn and F ions contained in fertilizers are considered using recent \(\hbox {Cd}^{2+}\) , \(\hbox {Mg}^{2+}\) and \(\hbox {F}^-\) ion-association theories. The protective role of ions like Zn, Se, Fe is emphasized, and the question of Cd toxicity in the presence of other ions is considered. These help to clarify difficulties in the standard point of view. This analysis does not modify the accepted views on Cd contamination by airborne delivery, smoking, and industrial activity, or algal blooms caused by phosphates.

  • Effects of conversion of mangroves into gei wai ponds on accumulation, speciation and risk of heavy metals in intertidal sediments 2018-06-23

    Abstract

    Mangroves are often converted into gei wai ponds for aquaculture, but how such conversion affects the accumulation and behavior of heavy metals in sediments is not clear. The present study aims to quantify the concentration and speciation of heavy metals in sediments in different habitats, including gei wai pond, mangrove marsh dominated by Avicennia marina and bare mudflat, in a mangrove nature reserve in South China. The results showed that gei wai pond acidified the sediment and reduced its electronic conductivity and total organic carbon (TOC) when compared to A. marina marsh and mudflat. The concentrations of Cd, Cu, Zn and Pb at all sediment depths in gei wai pond were lower than the other habitats, indicating gei wai pond reduced the fertility and the ability to retain heavy metals in sediment. Gei wai pond sediment also had a lower heavy metal pollution problem according to multiple evaluation methods, including potential ecological risk coefficient, potential ecological risk index, geo-accumulation index, mean PEL quotients, pollution load index, mean ERM quotients and total toxic unit. Heavy metal speciation analysis showed that gei wai pond increased the transfer of the immobilized fraction of Cd and Cr to the mobilized one. According to the acid-volatile sulfide (AVS) and simultaneously extracted metals (SEM) analysis, the conversion of mangroves into gei wai pond reduced values of ([SEM] − [AVS])/f oc , and the role of TOC in alleviating heavy metal toxicity in sediment. This study demonstrated the conversion of mangrove marsh into gei wai pond not only reduced the ecological purification capacity on heavy metal contamination, but also enhanced the transfer of heavy metals from gei wai pond sediment to nearby habitats.

  • Cytotoxicity induced by the mixture components of nickel and poly aromatic hydrocarbons 2018-06-22

    Abstract

    Although particulate matter (PM) is composed of various chemicals, investigations regarding the toxicity that results from mixing the substances in PM are insufficient. In this study, the effects of low levels of three PAHs (benz[a]anthracene, benzo[a]pyrene, and dibenz[a,h]anthracene) on Ni toxicity were investigated to assess the combined effect of Ni–PAHs on the environment. We compared the difference in cell mortality and total glutathione (tGSH) reduction between single Ni and Ni–PAHs co-exposure using A549 (human alveolar carcinoma). In addition, we measured the change in Ni solubility in chloroform that was triggered by PAHs to confirm the existence of cation–π interactions between Ni and PAHs. In the single Ni exposure, the dose–response curve of cell mortality and tGSH reduction were very similar, indicating that cell death was mediated by the oxidative stress. However, 10 μM PAHs induced a depleted tGSH reduction compared to single Ni without a change in cell mortality. The solubility of Ni in chloroform was greatly enhanced by the addition of benz[a]anthracene, which demonstrates the cation–π interactions between Ni and PAHs. Ni–PAH complexes can change the toxicity mechanisms of Ni from oxidative stress to others due to the reduction of Ni2+ bioavailability and the accumulation of Ni–PAH complexes on cell membranes. The abundant PAHs contained in PM have strong potential to interact with metals, which can affect the toxicity of the metal. Therefore, the mixture toxicity and interactions between diverse metals and PAHs in PM should be investigated in the future.