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10.2

Risk assessment process

Risk assessment of potentially contaminated area can be a detailed and lengthy process.

In the UK, it is undertaken in a tiered assessment structure (Defra/Environment Agency 2004 ). The tiers are applied to the circumstances of a given site with an increasing level of detail required by the assessor (Figure 10.1 ). Prior to conducting site investigations, a

Box 10.2 Extent of land contamination in England and Wales (Environment Agency 2009a)

In the UK there has not been any systematic study to ascertain the geographical extent of land affected by contamination. An estimate by the Environment Agency in 2005 indicated that around 300,000 hectares of land may be contaminated in England and Wales due to industrial activity, which is approximately 2 % of the land area of England and Wales.

A total of 781 sites have been determined as ‘contaminated land’ under Part IIA of the Environmental Protection Act (1990) by the end of March 2007. Of 781 sites, 659 are in England and 122 in Wales. Over 90 % of these sites had housing on them when the site was inspected.

Metal, metalloids, and organic compounds were identified as the most common pollutants. The energy and waste industries were the biggest contamination source in England and the deposit of ash was reported as the most common contamination source in Wales in the sites identified as contaminated land.

Fig. 10.1 Tiered risk assessment approach.

Preliminary risk assessment

Generic quantitative risk assessment

Detailed quantitative risk assessment Problem formulation

desk-based study of the land-use history along with its underlying geology is typically con-ducted to identify whether there is a problem due to the presence of hazardous substances.

This phase identifies whether or not there is a need for undertaking a systematic risk assessment.

In preliminary risk assessment phase, site reconnaissance activities are undertaken in addi-tion to desk-based studies to ascertain the likely existence of source-pathway-receptor (pollut-ant linkage) linkages on the site and define the exposure scenario (Figure 10.2 ). Based on the information gathered, a conceptual model of risk for the site is developed. The conceptual model aims to identify the key uncertainties and data gaps, which in turn are used to inform the site investigation design and further stages of risk assessment. If pollutant linkage is identi-fied, the assessment moves into the generic quantitative risk assessment (GQRA) phase.

In the GQRA phase, physical investigation of the ground is conducted which involves collection of environmental samples such as soil, water, and gas. The presence of chemicals and their concentrations is determined by using an accredited laboratory and methods.

The quantitative data are used to verify the existence of pollutant linkage and are com-pared with generic assessment criteria (GACs) such as the soil guideline values (SGVs).

GACs are typically conservative to ensure that they are applicable to the majority of site conditions. However, GACs should only be used for the purpose for which they are intended. For example, the GAC for residential land use should not be used to assess risk to people working in an office environment. The uncertainties and limitations associated with GACs need to be understood to ensure they are applied appropriately. If the contami-nant concentrations fall above the GACs, then the assessor should use appropriate statis-tical techniques to provide a representative soil concentration value (such as a mean). If the mean value falls below the GAC, the land is not considered to be contaminated and Fig. 10.2 Source-pathway-receptor model.

Desired information

Components of conceptual model required for health impact

What are the

further assessment is not necessary. If the values exceed the GACs, the assessor may consider whether or not further data collection is necessary.

In cases where GQRA assessment indicates that further detailed risk assessment is nec-essary, the assessor may decide to undertake detailed quantitative risk assessment (DQRA).

This phase may involve additional sampling, more detailed assessment of pollutant link-age and exposure scenario, and derivation of site-specific assessment criteria (SSAC). The presence of uncertainty in the toxicology and assumption in the exposure scenario are also evaluated and, where possible, replaced with more site-specific observations and data. The contaminated land exposure assessment (CLEA) model available in the UK to derive the GACs (SGVs) can also be used to derive SSAC for assessing risk from a specific chemical (Environment Agency 2009b ). As in the GQRA stage, representative soil con-centrations are compared against SSACs and if the values exceed the SSACs, then a risk evaluation can be carried out. If the values do not exceed thef SSACs, the assessor usually decides not to take any further action.

Risk evaluation could consider the likelihood of significant harm, the extent and type of possible effects on the receptor, the risks posed in the context of wider environmental risks, and the socio-economic cost–benefits of regulatory interventions (Defra 2010 ). If sufficient information is available that suggests that a site fulfils the definition of ‘con-taminated land’, it may be so designated under Part IIA of the Environmental Protection Act (1990). An option appraisal may then be produced which may include a consideration of remedial works that could be undertaken to reduce the risks (Defra/Environment Agency 2004 ). The aim of remedial work is to break the pollution linkages identified in the conceptual model (such as the removal of the source, breaking exposure pathways, or removal of receptors). If remedial works are undertaken, verification should be carried out to ensure that remedial objectives have been met (for example ‘clean’ topsoil imported onto a site should be tested to ensure that it really is ‘clean’).

10.3

Exposure assessment models

Exposure models are necessary to generate assessment criteria such as SGVs, which indicate the concentration of a substance present in soil that may result in a daily intake equal to, or less than, a health criteria value (HCV) (Box 10.3 ). The exposure pathways present on a given site are identified by constructing a conceptual model indicating potential pollution linkages (Figure 10.2 ) with degrees of risk associated with them.

In 2009, the Environment Agency released an updated CLEA model (Environment Agency 2009b ). The model uses generic assumptions about chemical fate and transport in the environment and a generic conceptual model for site conditions and human behaviour to estimate exposure to contaminants for those living, working, and playing on contaminated sites over long periods. The CLEA model has been used to generate SGVs by comparing the estimated exposure with HCVs.

The CLEA model can be applied for three generic land-use scenarios: residential, allotments, and industrial. The model includes a number of potential exposure pathways (Figure 10.3 ) and offers flexibility to change/add the pathways and parameters. The approach used is to

identify a ‘critical receptor’ which for a given exposure is likely to receive the greatest dose and therefore potentially be most at risk. For a typical residential site, the critical receptor is taken to be a young female child. The CLEA model can also be used to generate SSACs, where generic exposure parameters are altered to reflect more closely the conditions present on the site of interest.

A number of exposure assessment models have also been developed by other regulatory authorities, such as the Scotland and Northern Ireland Forum for Environmental Research (SNIFFER), Risk-Based Corrective Action (RBCA), and Integrated Exposure Uptake Biokinetic (IEUBK) models. For information on the CLEA exposure model, the Environment Agency’s