Toxicology Reflections

Importance of Soil Background Concentrations


Author –

Decisions on the remediation of metal contaminated soils are based on risk estimates derived from soil guideline values. Soil guideline values for metals are most commonly based on toxicological reference values. However guideline values are not only a reflection of scientific understanding but also reflect political decisions and legislative requirements, implemented by the regulator.

The United Kingdom is one such example of how changes to legislation of influenced the need to understand background soil concentrations. Part 2A of the contaminated land stator Guidance was issued by Department of Environment, Food and Rural Affairs in April 2012 (Defra, 2011). This guidance outlined changes to how contaminated land would be considered in the United Kingdom. Under this new guideline the term “normal” was introduced to the regulatory system with the requirements to remediate soils to normal levels.

Normal background concentration of soil contaminants not only includes geological and natural variation in soil element concentration but that of historical contamination and “normal” concentration of different elements found in soils of different environments (urban vs. rural). This series of post will be considering the important of determining soil background concentration and the different approaches which can be used to derive background concentration of soil contaminates in the environment.

Defra, 2011. Draft Contaminated Land Statutory Guidance


10 thoughts on “Importance of Soil Background Concentrations

  1. Sounds like a pretty big job! I was wondering how they’re going to address historical contamination in their assessment of “normal” concentrations? If historical contamination is included in what is considered “normal” concentrations, will their not be sites that have relatively high concentrations of certain metals at what they would consider “normal’ concentrations.

    I was also wondering if any other countries have attempted this task. Is the UK the first? If that is the case, another point against Mr. Putin’s assessment of Great Britain:)

  2. Author –
    The aim of introducing the concept of normal levels is to deal with the role diffuse pollution. So historically contaminated and point source contaminated sites will not be included in the definition of normal levels. However diffuse pollution and the historic contamination derived from the use of leaded petrol for example will be included. Therefore there will be a high level of expectable lead in the environment. The use of normal levels aims to lower the gap between soil guidance values derived for metals derived from metals to concentration which are more environmentally relevant. In the UK the environmental agency doesn’t actually have soil guidance values for some metals. This is due to the very low concentrations have would be seen as expectable risks to human health and the high environmental concentration of metals which are present in British soils.

    The requirement to consider diffuse pollution and the protection of soils has just been introduced under EU legislation. Britain is however the first to consider the role of diffuse pollution in defining environmental standards and this is not just for metal concentration but all contaminants. Mr Putin may however be correct, in that Britain is a small island and does have the most comprehensive picture of what contaminants are present in the environmental. So Britain is positioned to be the first country to consider the role of diffuse pollution.

  3. I agree, this does seem like a really big (and confusing) job! It will be a very interesting policy development to follow. While I think I follow what is going on with the new policy I’m not sure that I completely understand the underpinnings of it. Could you maybe use a couple more examples to clarify how they are incorporating historical contamination’s into “normal” and therefore acceptable levels? Also when looking at historical diffusing contaminants, such as leaded gas, will higher levels become acceptable in urban areas compared to rural areas? I’m assuming there will be a policy in place to prevent a situation where poorer health conditions in cities are “accepted” vs that in rural areas just because it’s historical contamination. I look forward to your upcoming posts.

  4. Author –
    The redefining of soil guidance limits is meant as a guide for risk assessors. Soil guidance values for most contaminants do not accurately reflect the environment we live in. As a result many soils are labeled as contaminated, where a risk may not actually be present. Additional this means it is hard to differentiate between sites. Canada has a tiered system by which contaminated land is assessed, the united kingdom has a similar system and is aiming to set it’s soil guidance values as values which are more reflective of the environment than toxicological data.

    Historical contamination is seen as a widespread problem which affects all of the UK (Defra, 2012). The aim of defining normal background concentration is to move away from the use of set country wide Soil guidance values as seen in Canada. In Canada, the CCME sets guideline values which are usually followed by the provinces and therefore a uniform single value on acceptable soil concentrations is one value for the whole country. This means that ares which are natural high in a metal maybe defined as a contaminated site under this criteria. This could lead to the requirement to clean up the environment below the standard set by nature itself.

    You are correct in that a decision between rural and urban environments would be made on the basis of land use and this could lead to increased risks. However these are just guideline values and a site specific risk assessment would need to examine if a risk to human health was occurring. However what is therefore recognized is that diffuse pollution such as Pb does not need to be remediated if it at normal levels for that area and land use. This is not saying there isn’t a risk but it is acting as an environmental management policy of defining what is acceptable in urban and rural areas for past activities. The decision between rural and urban environments occurs in Canada too, for example: CCME Pb guideline values are set as 70 for agricultural soils and 300 for residential and 600 for industrial mgkg-1 dry wt. soil. (CCME, 1999)

    Environmental Protection Act 1990: Part 2A Contaminated Land Statutory Guidance

    Department for Environment, Food and Rural Affairs (Defra), UK (2012) [April; 69 pp.

  5. author –
    The main aim of the legislation is to help in targeting unacceptable risks posed by soil, targeting contaminated sites in areas with natural and anthropocentric background variability. All soils pose a risk to human health but some are seen as posing a great unacceptable risk and therefore require remediation.

  6. author –
    The use of “normal” concentrations to define contaminated land status in the United Kingdom and specifically within England, (The countries making up the United Kingdom all regulate the environment independently) is an example of simplification of contaminated land legislation and statuary guidance. This is aimed at improving how contaminated land is defined in the UK and move away from the use of single soil guidance values (SGVs). Single SGVs of acceptable limits often create unnecessary work due to misinterpretation of the Human Health risks posed from a site. A site which exceeds guidance values in the UK or in Canada (difference between tier 1 &2) does not necessary indicate that there is a risk to Human Health It is estimated that the cost of unnecessary remediation to UK business in a single year is £140 million or $230 million Canadian (Defra, 2011). Here we will now consider how the new guidance in England differs from that of Canada’s SGVs and the impact a change to the Canadian legislation might be.

    There can be several problems which arise due to the use of a single SGVs system. The United Kingdom is a diverse geological terrene similar (though not in size) to Canada. Geological various can lead to differences in the range of natural soil contaminant concentrations present in soils. Under a single SGV system the whole county of Cornwall (Place in England) would exceed SGVs for arsenic (As) (Defra,2011). Does this mean that Cornwall is all contaminated and therefore requires remediation? The answer is obviously no, however the question of risk to Human Health may still be present. Another example is that of lead (Pb).Lead was extensively used in industrial and most significantly to the environment in gasoline until the 1970s. However it wasn’t until this time that increased Pb concentrations in the environment were identified by Patterson (1965) and leaded gasoline was subsequently banned in the United Kingdom as well as throughout most of the developed world (Plant et al, 2013). This has left a legacy of pollution across all land uses and most extensity in urban areas (as can be seen: As increased awareness of the health implications of Pb in the environment has come about there has been a steady lowering of SGV lead in all regulatory regimes (Plant et al, 2013). These two factors (1. a legacy of persistence of metals in the environment 2. improved toxicological data on the impact to human health) have led to the majority of all urban environments in the UK being below soil guidance values. These examples highlight the need for an improvement of how SGVs are defined in order to allow the targeting of sites which pose the greatest risk to human health. If all soils are above SGVs which soils do you remediate first?

    What the legislation sets out to define is what risks are seen acceptable and more importantly the unacceptable risks. All soils pose a risk to human health but those soils posing the greatest and most unacceptable risks to health are the soils which need to be remediated. Figure 1 highlight how the use of normal values (the red line) should improve how contaminated sites are screened in the UK. As an example of how the new system would work compared to the old (Canadian system) let us consider Pb levels in soils. CCME guidelines range from 70 for agricultural soils and 300 for residential and 600 for industrial mgkg-1 dry wt. soil. (CCME, 2012). In St Johns Newfoundland, not the most urbanized of population centres 50% of soils identified in one study exceed this CCME guideline (Bell et al, 2010). This again highlights the question of how applicable SGV derived from toxicological data are for environmental management and remediation standards.

    Fig. 1: Image of new designation of land in England based on normal soil contaminant concentrations. (Defra, 2011)

  7. I found a really interesting article from 1992 outlining the implementation of site-specific guidelines in Australia1. Many cities in Australia are plagued by the same problem of high levels of background soil contamination, particularly in urban areas, that have been present for years. The background contamination often falls above SQG but it is impractical to remediate whole cities on this basis. Although this paper is quite old I thought it provided a really good discussion on the varying viewpoints present on the assessment of soil quality in risk assessment.

    The one point I wanted to focus on are the disadvantages associated with using site-specific data which were outlined in the article. It points out that several assumptions have to be made when using a site-specific framework. These assumptions are often subjective and may not accurately define a site. Some of the assumptions that have to be made include exposure pathways and receptors, the environmental fate and transport of the chemical, the uptake and distribution of the chemical in plants and animals and what the dose-effect relationship is. As mentioned in class it is preferable to model and then measure but depending on the extent of historically contaminated sites in a country this may not be economically viable and human resources may be limited.

    Tiller K.G. (1992). Urban Soil Contamination in Australia. Soil, Land, Care and Environmental Research. 30: 937 – 957

  8. Author –
    Soil Guidance Values- Week 4

    In this week’s post I will consider if the use of “normal” soil values could be implemented in Canada and secondly how this would affect targeting of sites for remediation within Canada.

    Firstly I will clarify what exactly the implementation of “normal” soil values is in United Kingdom (U.K.). The system being implemented in the U.K. should be viewed as a cleaning up of statutory guidance and is not a direct change of legislation or environmental protection levels in the United Kingdom. Statutory guidance is influenced by government policy. However it serves only as a guide in interpreting laws and is not the law itself. This means currently only limited changes to Part 2a have actually been made.

    The U.K has had a long history of geochemical exploration and surveying of the environment. The British Geological Survey’s Geochemical Baseline Survey of the Environment (G-BASE) is a long established high resolution geochemical mapping project which has provided much of the data used in the calculation of normal contaminant concentration in the UK. The history of the project is discussed in detail by Johnson et al (2005) and there is extensive information provided on the British geological survey’s website: This project is not only based on the collection of soils but other geochemical media from across the UK and has very recently finished, with the final sample collected on Tuesday of this week.

    The collection of high resolution survey data for metals means the UK can calculated normal guidance levels. There are however gaps and limits. Trace metals are not the only contaminant present in soils. They are however the most persistent. There are very few examples of systematic collection of organic contaminants background concentrations in soils. I myself have taken part in a small pilot study (30km2) which may represent the first and only systematic sampling for organic contaminants in the world (Katherine Knights, peers coms 2010). This means that the calculations of background values of normal concentration are limited even in the United Kingdom.

    So what data is available in Canada in order to calculate background soil exposures and normal soil concentrations and could normal values even be calculated for Canada. Canada has had a similar history of geochemical survey conducted by the Geological Survey of Canada to that of the British Geological survey. Where these surveys differ is that Canada is still exploring for minerals but the United Kingdom has switched focus to Human Health. More details on Canada’s geochemical exploration history:

    The scale of Canada relative to that of the United Kingdom can be identified as one challenge to identifying normal concentration in Canadian soils. Ignoring this challenge the second problem identified for Canada is glacial till (Rencz et al, 2006). The compositions of almost all Canada’s soils have been affected by glacial deposition and therefore a disconnection between underlying geology and soils occurs. This gives rise to greater natural variability in the composition of Canadian soils, due to the nature of till itself.

    The variation of Pb in Canadian soils is between 2-152 mgkg-1 naturally (Rencz et al, 2006). Comparing this to CCME Soil quality guideline value for lead which is 70 (CCME, 1999) we can see how natural soils could be considered as contaminated. So how does Canada deal with this discrepancy? The adoption of CCME Interim Canada environmental quality criteria (CCME, 1991 which raises the value to 375 mgkg-1 is how Canada has dealt with natural background variation. However as we have seen soils in Canada have Pb levels ranging from as little as 2 mgkg-1, this means that a soil could be contaminated with as much as 373 mgkg-1 Pb and not be identified as a contaminated site. Therefore this brings into question whether Canada should try to adopt as similar system to the United Kingdom for the identification of contaminated sites.

    Next week I will consider further how background hydrocarbon concentration could be determined and the guidelines in Canada.


    Johnson et al (2005):

    Rencz et al, 2006:

    CCME, 1999:

    CCME, 1991:

  9. I like the background you provided regarding the concentration of metals in Canadian soil due to glacial deposition. It makes me think about metals and groundwater quality in Saskatchewan and how difficult it is to predict the concentration of metals without an accurate well depth and location. Even then, many people have told me it would be extremely difficult. It would be great to hear your opinion on it.

    We’ll have to chat more when I see you about!

  10. As i have been doing lately, i find it quite related to my project as well. Maybe not as much from the geological part of it, as they tend to have drinking water quite well characterized. However, part of the problem with understanding the actual effect of fluoride in water is due to the multiple exposure from different sources and how to distinguish the effect of one against the effect from others. This makes it really hard to understand epidemiological studies. On this note, i was wondering is you had encounter any literature relating to alternative routes of exposure to metals, such as food, or other products, grown in contaminated soils outside the areas of study.

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