Quantitative Risk Assessment answers the question:

According to Part IIa of the 1990 Contaminated Land Regulations (CLR), land is defined as contaminated when chemical concentrations pose identifiable risks to human health, controlled waters, buildings or ecosystems. But at what concentrations do chemicals become contaminants? Joint authors at SLR Consulting Ltd, Jonathan Parry, Senior Hydrogeologist, and Dr Luke Wilkinson, Principal, address the question

Soil Guideline Values (SGVs) can be used as an initial risk screening tool. For example, if concentrations of arsenic - a naturally occurring metal - exceed the SGV, then the soils may be contaminated. However, to date, SGVs have only been published for nine contaminants: seven metals, ethylbenzene and toluene. To derive site specific SGVs for all the contaminants that may be found at a site, including the more toxic compounds such as volatile hydrocarbons or polycyclic aromatic hydrocarbons (PAHs), an integrated Quantitative Risk Assessment (QRA) of a wide range of contaminants is required.

Conceptual site model

The QRA approach is based on developing a robust conceptual site model to assess contaminant migration pathways, utilising toxicological, fate and transport data on specific contaminants, published as part of the CLEA programme (Contaminated Land Exposure Assessment). To date, CLEA has published toxicological reports for 23 common contaminants.

Of these, hydrocarbons are of particular importance and SLR Consulting, which has extensive experience in the assessment and remediation of hydrocarbon contaminated land, has been appointed by the Environment Agency (EA) to manage the development of the UK approach to evaluate the human health risk from petroleum hydrocarbons in soil.

QRA uses simplified analytical models to describe the properties of the active pollutant linkages and site investigation data to determine acceptable site concentrations of specific contaminants. A variety of software models have been published to enable QRA. The two regulatory models published in line with the UK guidance are:

  • P20 Excel spreadsheet - Published by the EA for assessment of risks to controlled waters;
  • CLEA 2002 - Published by the EA for human health risk assessment, its use is limited as it is being superseded by the CLEA UK software (due April 2005). It assesses soils risks only and no groundwater assessment is included.
  • Other propriety software generally developed in the US or Europe, but adaptable to the UK guidance, include: RISC Human, BP Risk and RBCA Toolkit. All of these packages vary slightly from each other, but are generally applicable and accepted by the EA and most local authorities when applied correctly.

    SLR Consulting uses the RBCA Toolkit software, amended to incorporate the CLEA toxicological data, exposure scenarios and contaminant specific fate and transport parameters according to the latest EA guidance. This software has the advantage of modelling risks to both controlled waters and human health from soils and groundwater at the same time.

    Key role of QRA

    QRA is a key stage in the development of remedial option appraisals compliant with CLR11. To optimise remediation techniques and design verification strategies, QRA output should be used by consultants in combination with professional judgement to determine site specific guideline values for soil and groundwater quality before remediation or development. A QRA report must present this information in a format that clients and regulators understand in order to meet planning conditions. An example of the benefit of QRA was provided when SLR was retained at a late stage in the redevelopment of a former garage site. Townhouses were constructed but could not be sold due to inadequate evaluation of the risks posed by hydrocarbon impacts identified during earlier investigations.

    The planning consultees - the contaminated land officer and EA - were reluctant to sign off the planning conditions pertaining to contamination. SLR supervised further investigations at the site, which identified weathered historical oil based impact, at depths of greater than 2.5mbgl, within groundwater migrating from an off site source.

    Through the use of a QRA, SLR demonstrated that the levels of impact at the site did not pose a risk to future residents or groundwater at the site's down gradient boundary. The regulators agreed with the assessment and signed off the planning conditions without the requirement for remediation works at the site.

    Assessing health impact

    It should also be recognised that QRA should not be used in isolation. SLR is currently evaluating human health impacts from a historic dissolved petrol plume, which has migrated beneath 15 older houses. Initial QRA results showed that the contamination posed risks to the residents and damage liabilities in excess of £1 million to the polluter.

    However, SLR recognised the limitations of the QRA to describe the soil gas pathway from the groundwater to indoor air. Soil gas sampling around the properties was carried out as recommended by the Environment Agency (Technical report P5-079). Initial monitoring results, combined with further risk based estimates of soil gas dilution to indoor air, indicate that no health risks and therefore no damage liabilities exist - though remediation of the original source zone to protect the groundwater receptor is required.


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