Measuring the Bioaccessibility of Potentially Harmful Elements in Soil

 

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.