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.

 

 

 

Keep up to date

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

  • Date palm waste biochars alter a soil respiration, microbial biomass carbon, and heavy metal mobility in contaminated mined soil 2017-04-19

    Abstract

    A 30-day incubation experiment was conducted using a heavy metal-contaminated mined soil amended with date palm feedstock (FS) and its derivative biochars (BCs) at three pyrolysis temperatures of 300 (BC-300), 500 (BC-500), and 700 °C (BC-700) with different application rates (0.0, 5, 15, and 30 g kg−1) to investigate their short-term effects on soil respiration (CO2–C efflux), microbial biomass carbon (MBC), soil organic carbon (SOC), mobile fraction of heavy metals (Cd, Cu, Pb, Zn, Mn, and Fe), pH, and electrical conductivity (EC). The results showed that FS and BC-300 with increasing addition rate significantly reduced soil pH, whereas SOC, CO2–C efflux, and soil MBC were increased compared to the control. On the contrary, BC-500 and BC-700 increased soil pH at early stage of incubation and have small or no effects on SOC, CO2–C efflux, and MBC. Based on the results, the date palm biochars exhibited much lower cumulative CO2–C efflux than feedstock, even with low-temperature biochar, indicating that BCs have C sequestration potential. Applying BC-700 at 15 and 30 g kg−1 significantly reduced cumulative CO2–C efflux by 21.8 and 45.4% compared to the control, respectively. The incorporation of FS into contaminated soil significantly increased the mobile content of Cd and Mn, but decreased the mobile content of Cu. However, BC-300 significantly reduced the mobile content of Cd, Cu, Pb, and Zn. It could be concluded that low-temperature biochar could be used as a soil amendment for reducing heavy metal mobility in mining contaminated soil in addition to minimize soil CO2–C efflux.

  • Historical record of anthropogenic polycyclic aromatic hydrocarbons in a lake sediment from the southern Tibetan Plateau 2017-04-17

    Abstract

    High-altitude lake sediments can be used as natural archives to reconstruct the history of pollutants. In this work, the temporal distribution of polycyclic aromatic hydrocarbons (PAHs) was determined in a sediment core collected from the southern Tibetan Plateau (TP), which was dated by using the 210Pb dating method and validated with the 137Cs fallout peak. The concentrations of the anthropogenic PAHs (Σ8PAH) in the sediment core ranged from 0.83 to 12 ng/g dw, and the fluxes of the Σ8PAH were in the range of 2.1–27 g/cm2/year. The temporal variations in the concentration and input flux of anthropogenic PAHs were low with little variability before the 1950s, and then gradually increased from the 1950s to the 1980s, and an accelerated increase was observed after the early 1980s. The content of total organic carbon played an insignificant role in affecting the time trends of PAHs in the sediment core. Diagnostic concentration fractions of PAH components indicate PAHs in the lake sediment of the southern TP which are mainly from biomass burning and/or from long-range atmospheric transport.

  • Determination of the potential implementation impact of 2016 ministry of environmental protection generic assessment criteria for potentially contaminated sites in China 2017-04-12

    Abstract

    The Ministry of Environmental Protection of China issued a 3rd draft edition of risk-based Generic Assessment Criteria (the MEP-GAC) in March 2016. Since these will be the first authoritative GAC in China, their implementation is likely to have a significant impact on China’s growing contaminated land management sector. This study aims to determine the potential implementation impact of the MEP-GAC through an in-depth analysis of the management context, land use scenarios, health criteria values adopted and exposure pathways considered. The MEP-GAC have been proposed for two broad categories of land use scenarios for contaminated land risk assessment, and these two categories of land use scenarios need to be further delved, and a MEP-GAC for Chinese cultivated land scenario ought to be developed, to ensure human health protection of Chinese farmers. The MEP-GAC have adopted 10−6 as the acceptable lifetime cancer risk, given the widespread extent and severe level of land contamination in China, consideration should be given to the decision on excess lifetime cancer risk of 10−5. During risk assessment process in practice, it is better to review the 20% TDI against local circumstances to determine their suitability before adopting it. The MEP-GAC are based on a SOM value of 1%, for regions with particularly high SOM, it might be necessary to develop regional GAC, due to SOM’s significant impact on the GAC developed. An authoritative risk assessment model developed based on HJ25.3-2014 would help facilitate the DQRA process in practice. The MEP-GAC could better reflect the likely exposures of China’s citizens due to vapour inhalation by using characteristics of Chinese exposure scenarios, including China-generic building stock, as inputs into the Johnson and Ettinger model as opposed to adoption of the US EPA parameters. The MEP-GAC once implemented will set the trajectory for the development of the investigation, assessment and remediation of land contamination for years.