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Human Bioaccessibility of Heavy Metals and PAH from Soil

10 Discussion and recommendations

Most quality criteria and cleanup levels (maximum contaminant levels, MCLs) for soil contaminants are based upon oral exposure and effect studies with contaminants as pure chemical substances ingested with water or with food. When ingested with soil, the bioaccessibility of substances such as the metals and PAH reviewed here is likely to be different from that in the studies that the MCLs are based on.

Bioaccessibility of the soil contaminants depends upon the contaminant chemistry, the soil properties and the chemical conditions in the gastrointestinal system.

In inorganic form, the metals cadmium, copper, lead, nickel and zinc occur as divalent cations only, whereas chromium can occur both as a trivalent cation and as an anion, and arsenic occurs as one of two anionic species. All metals can occur in different mineral forms and associations and with soil constituents depending upon the source of contamination and the weathering of the contaminated soil. Similarly, PAH is expected to exhibit reduced availability after aging of a PAH contaminated soil. However, at the same time some studies indicate increased mutagenic activity from metabolites of PAH in contaminated soils.

Uptake of the contaminants predominantly takes place in the small intestine, where conditions range from the acidic, high chloride gastric conditions just after transit from the stomach to subsequent neutral to slightly alkaline, high phosphate intestinal conditions. The importance of uptake from the acidic high chloride conditions of the stomach and of aerobic/anaerobic processes is not clear. The chemical conditions in the gastrointestinal tract are complex and vary between individuals of different physiology, age, health etc and for each individual with parameters such as feeding conditions, activity etc.

A number of different in vitro test methods are available to measure bioaccessibility of soil contaminants, but the results are not generally comparable between methods. The data on quality of the bioaccessibility test methods are limited but it would be expected that methods of required quality are available or can be made available. A suitable bioaccessibility method simulating human physiology must include all important dissolution processes and emphasis all important test details, most important:

• buffered low pH (pH < 2) high chloride gastric compartment
• buffered slightly alkaline (pH > 7) phosphate containing intestinal compartment
• aerobic followed by anaerobic conditions (stomach and intestine, respectively, optional)
• separate assessment of bioaccessibility in the two compartments (gastric and gastric followed by intestinal)
• addition of enzymes, bile and milk powder (or similar food constituent)
• sufficient time in each compartment (3 hour in gastric compartment, 10 hours in intestinal compartment)
• L/S stability (L/S > 100)

No currently available method satisfies all these requirements, but several methods will require only limited adjustment in order to do so: PBET (different versions), Digestive tract model, DIN and RIVM.

It is mandatory and urgent for the future use of bioaccessibility testing of soil contaminants that one single method is agreed upon. Alternatively, a set of methods applicable each to different purposes (e.g.: heavy metals and organic contaminants) should be the aim. To reduce costs and complexity of testing, the lowest number of tests possible should be aimed at.

Data are available on bioaccessibility of soil contaminants, in particular for lead and arsenic, to some degree for cadmium and chromium, but very limited for copper, nickel, zinc and PAH. The overall picture is, that reduced soil bioaccessibility is very likely for cadmium, lead and chromium (III) (uptake in small intestine), likely for arsenic and chromium (VI), and possible for copper, nickel, zinc and PAH. The degree of uptake in humans for cadmium and lead will though depend upon the degree to which uptake takes place in the stomach and in the first part of the small intestine prior to neutralisation of the gastric low pH and precipitation of metals. The bioaccessibility of all the reviewed contaminants is highly variable even within the same soil type, source type and test, as far as can be concluded from the limited the data available.

Bioaccessibility will impact human exposure if dissolution of the soil contaminants is rate limiting compared to absorption or if only one fraction (e.g.: mineral species) of the soil contaminant is readily bioaccessible and another fraction, that might be 100%, is not. Correlation between bioavailability and bioaccessibility has been demonstrated for lead, arsenic and PAH in some test system but not in others. Still, the data material is not sufficient to establish whether, to what degree and for which contaminant bioaccessibility is rate or dissolution limiting.

A large number of bioavailability in vivo studies with experimental animals have been published, a review of these is outside the scope of the present review, but reduced bioavailability has been reported for at the least arsenic, cadmium, lead and PAH. Reduced bioaccessibility and/or bioavailability has been taken into consideration in site specific regulation of cleanup levels for contaminated sites in the US and Canada, in particular for mine waste and ore processing sites.

The general conclusion is that regulation of soil quality criteria and cleanup levels based upon reduced bioavailability/bioaccessibility of the contaminants is recommended after site specific risk assessment.

Conversely, the data available at present does not allow for general regulation of soil quality criteria and cleanup levels for specific contaminants, soil types or sources.

A short term and a long term model for implementation of bioaccessibility in risk assessment of contaminated sites can be suggested.

The short term approach (figure 10.1) can be employed for arsenic, lead and PAH where bioaccessibility has been shown to be rate limiting for bioavailability. For other contaminants, the rate or dissolution limiting role must be established prior to application of the bioaccessibility approach, either on a site specific basis or as a general study.

The long term approach (figure 10.2) requires establishment of “calibration” curves of in vivo bioavailability versus in vitro bioaccessibility but has the advantage of being based upon a direct, proven link between the measured quantity (bioaccessibility) and the toxicological interpretation (bioavailability). A 1:1 correlation can not be expected due to potential interspecies differences between bioavailability studies using experimental animals and bioaccessibility tests simulating the human physiology.

Figure 10.1
Short term approach to implementation of bioaccessibility of soil contaminants in risk assessment

Figure 10.2
Long term approach to implementation of bioaccessibility of soil contaminants in risk assessment

In order to implement the short term approach in Denmark, see also section 9.5, the following is required:

• compilation of in vivo bioavailability data for the contaminants from soil available in the literature
  • purpose: to secure that reduced bioavailability is occurring for all contaminants that are candidates for bioaccessibility testing
• compilation of the contaminant forms, vehicles and bioavailability used in toxicity studies behind current criteria and cleanup levels‘
  • purpose: to produce the conditions required for bioaccessibility testing yielding relative bioaccessibility factors
• selection, implementation and validation of one test method for relative bioaccessibility of soil contaminants
  • purpose: to give access to a reliable method for testing
• testing of a selection of contaminated soils for relative bioaccessibility
  • purpose: to produce a Danish reference set (contaminants, soil types, sources) for deciding if bioaccessibility testing is likely to yield regulation of criteria and cleanup levels

With use of different bioaccessibility test methods in different countries, access to stable and homogeneous subsamples of soils used around the World in bioavailability studies is essential in order to allow for extrapolation of data obtained in these studies to the sites where bioaccessibility tests are used.

In order to implement the long term approach, international collaboration is required in order to accomplish what is needed:

• selection, implementation, validation and interlaboratory comparison of one test method or one set of test methods for bioaccessibility of soil contaminants (European or preferentially transatlantic scale, ISO, CEN and US EPA)
  • purpose: to give access to a reliable method or set of methods for testing as common reference and to ensure compliance of all future data
• production of corresponding high quality in vivo bioavailability and in vitro bioaccessibility data for the important contaminants, soil types, sources and speciations (European or preferentially transatlantic scale)
  • purpose: to produce relative bioavailability versus bioaccessibility “calibration” curves and demonstrate bioaccessibility as rate limiting factor for bioavailability for more contaminants

As research tasks, further refinement of the theory behind implementation of bioaccessibility and bioavailability in risk assessment of soil contaminants should include:

• identification of in vivo compartment of contaminant uptake
  • purpose: to enable precise selection of test compartment (stomach or stomach and intestine) to be used for bioaccessibility testing of different contaminants
• evaluation of gut redox conditions and impact upon bioaccessibility
  • purpose: to enable selection of aerobic/anaerobic conditions for bioaccessibility testing
• description of the mechanisms of uptake, in particular the kinetics of dissolution and absorption in different compartments, with different vehicles etc
  • purpose: to ensure that the conceptual model of human uptake used is correct and that the bioaccessibility is de facto rate or dissolution limiting for bioavailability
• investigation in the use of the pharmaceutical approach with time dissolution profiles in soil contaminant bioaccessibility testing
  • purpose: to develop a more precise tool for bioavailability predictions and to identify the rate limiting process (dissolution or absorption)

 Selection of target contaminants should take into account the significance of each compound as soil contaminant (toxicity and occurrence).

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