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Technical concepts

Technical concepts

Part IIA introduces a number of technical concepts and some key concepts are considered in this section.

A number of methods and models are available to assist in the derivation of site specific assessment criteria, as highlighted in the other 'technical concepts and tools' sections.

Site specific risk assessment

A risk assessment represents the structured gathering of information in order to form a judgment about the risks associated with contaminants at a site, given its environmental setting.

The process of risk assessment is built around an iterative approach, whereby initial information enables a picture of the site to be formulated.

This picture, or conceptual site model, is refined as further information is obtained, for example, through desk studies, walkover surveys and intrusive investigations.

It is important that the actual problem is defined before the risks are considered in detail, to ensure that all sources, pathways and receptors and potential linkages between them are considered.

The first stage of any risk assessment is to identify and assess which hazards, with the potential to cause harm to the receptors, are present. In a quantitative assessment, it is usual to consider a threshold above which harm may be caused. However, it may, in some circumstances, be possible to form a judgment based on a qualitative assessment.

The second stage in a risk assessment is to consider how much, how often and for how long the receptor is exposed to the hazard (exposure assessment), as well as the effect on the receptor of the exposure (consequence assessment). This will enable a concentration to be estimated which can then be related back to the hazard assessment.

The final stage in a risk assessment is to integrate the outcomes of the above stages with risk estimation, in which the consequences and probability of the risk is considered, with risk evaluation in which the significance or seriousness of the risk is considered.

A risk assessment culminates in considering whether the risks are tolerable and the assessment may be reiterated using additional information to obtain a more informed view about the tolerability of the risk.

The Environment Agency’s model procedures for the Management of Land Contamination, CLR 11 contains more information on the risk assessment process.

Assessment criteria

Risk assessment is concerned with deciding whether estimated or observed concentrations of contaminants actually matter, usually through comparison with assessment criteria (guideline values).

When considering risks posed by land with contaminants and applying assessment criteria, it is essential that they are appropriate to the site under consideration.

As paragraph B.48 in the statutory guidanceindicates:

  • any assumptions underlying the value must be fully understood and should not limit application to the site under consideration;
  • the value must be appropriate to the site and receptor under consideration.

A common practice previously was to use guideline values derived for generic situations either within or outside the UK, such as ICRCL (now withdrawn by the Scottish and UK governments) or Dutch values.

Such criteria should not be used if the assumptions used in their derivation are unknown or inappropriate, if the criteria are not applicable to the site being assessed or if they have been derived for receptors other than those under consideration.

Pollution of the water environment

Water pollution is an important concept, as it may be one of the reasons for land being identified as contaminated by a local authority.

The following guidance documents have been produced to provide an outline of SEPA’s approach to the assessment of significant pollution of the water environment. They are the basis for site specific advice that we will provide when consulted by local authorities:

The following supporting guidance documents are referenced in the above documents:

Human health effects

The Part IIA site specific risk-based approach means human health effects need to be assessed on a site by site basis; there are no mandatory standards defining concentrations of contaminants in soil above which there may be an unacceptable risk to human health.

Contaminant toxicology and the extent of exposure to contaminants, which is linked to human behaviour at a site, form an essential part of the risk assessment process.

A practical indicator of harm is intake from the site exceeding health criteria (tolerable daily intake for threshold substances and index dose for non-threshold substances). Intake, expressed as milligram (mg) per kilogram (kg) body weight per day, is typically related to soil concentrations, expressed as mg per kg, through an exposure assessment.

The exposure assessment considers how much of a contaminant the critical receptor will be exposed to, as well as for how long and how often. There are three routes of exposure which may need to be considered depending on the site conditions: ingestion, inhalation and skin contact. Common exposure pathways include: ingestion of soil, consumption of contaminated food and water, inhalation of dust and vapours, and skin contact with contaminated materials.

Various tools have been devised to assist in the risk assessment process and to derive generic and site-specific criteria as indicators of unacceptable risk. Whatever assessment criteria are used, they must be appropriate to the site under consideration, scientifically based and compatible with UK policy.

The Contaminated Land Exposure Assessment (CLEA) model has been developed specifically to derive generic assessment criteria (soil guideline values) for UK conditions. We anticipate that CLEA soil guideline values will be used where they are appropriate to the site under consideration, taking into account the site conditions, site use, the receptors under consideration and the conceptual model for the site.

Where such circumstances do not apply, an alternative risk assessment tool should ideally be used to derive site specific assessment criteria, such as the Scotland and Northern Ireland Forum for Environmental Research (SNIFFER)’s method for deriving site-specific human health assessment criteria for contaminants in soil.

Ecological systems effects

Part IIA only applies to specific ecological systems:

  • notified areas of special scientific interest;
  • declared national nature reserves;
  • designated marine nature reserves;
  • notified areas of special protection for birds;
  • Natura 2000 sites, including designated, potential or candidate special areas of conservation and special protection areas;
  • habitats or sites afforded policy protection such as listed Ramsar sites;
  • designated nature reserves;
  • designated or candidate national parks.

Harm which is occurring to ecosystems not included in the above ecological systems is disregarded for the purposes of Part IIA.

Harm is described in the statutory guidance as:

  • harm which results in an irreversible adverse change, or some other substantial adverse change, in the functioning of the ecosystem;
  • harm which affects any species of special interest which endangers the long-term maintenance of the population of that species;
  • for Natura 2000 sites only, harm which is incompatible with the favourable conservation status of natural habitats or species.

While consideration of ecosystem effects has many similarities to human health effects, in some ways it is more complex, as a multitude of organisms and communities of these organisms need to be considered.

However, in other aspects ecosystem effects may be easier to quantify as testing can often be done on the component of the ecosystem of concern.

As with human health effects:

  • many organisms have varying sensitivities to chemicals;
  • different organisms have different exposure scenarios;
  • some substances may be essential to different components of the ecosystem;
  • some species may have acquired a natural tolerance to some contaminants.

All these factors need to be taken into account when considering harm.

There are two approaches that can be taken when considering harm to ecosystems: a chemical approach, in which assessment criteria are derived from available toxicity data; and a direct toxicity assessment approach, where assessment criteria are derived through actual measurement of harm at the site under consideration.

Property effects

Under Part IIA there are two broad categories of property.

1. Property in the form of animals or crops, such as:

  • crops, including timber;
  • produce grown domestically, or on allotments, for consumption;
  • livestock;
  • other owned or domesticated animals;
  • wild animals which are the subject of shooting or fishing rights.

2. Property in the form of buildings, including any part below ground level:

Chapter A of the statutory guidance provides a description of what harm to the above receptors is to be regarded as significant and these are described as an "animal or crop effect" or a "building effect" respectively.

Although approaches to assessing animal and crop effects arising from contaminated land are not currently well developed in the UK, building effects are more clearly understood.

Harm to buildings can arise from the following:

  • Migration and accumulation of combustible gas, such as landfill gas (although site specific consideration needs to be given as to whether or not this could constitute a human health effect).
  • Subterranean fires including the spontaneous combustion of underlying colliery spoil leading to creation of underground voids and subsidence.
  • Expansive slags leading to ground heave.
  • Chemical attack (the performance of building materials used in contaminated ground may be adversely affected by a wide variety of aggressive chemicals, including alkalis, acids organic solvents and inorganic salts such as sulphates and chlorides).
  • Whilst research and guidance has been developed over the past few years there is, to date, little in the way of Part IIA specific guidance on how to go about determining building effects. The following references may be useful:

Risks of contaminated land to buildings, building materials and services: a literature review
Building Research Establishment (BRE) 1999

Risk assessment for methane and other gasses in the ground
Construction Industry Research and Information Association (CIRIA) Report 152 1995

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