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Question of judgement

CONTAMINATED LAND

There is some way to go before the new guidelines on contaminated land assessment will bring about much improvement, says Steve Branch.

Geotechnical and geoenvironmental engineers and consultants involved in the assessment of contaminated land will have been digesting the recently published and longawaited Contaminated Land Exposure Assessment guidelines.

The CLEA model was developed for the Department for the Environment, Food and Rural Affairs and the Environment Agency by the Centre for Research into the Built Environment at the Nottingham Trent University. It is presented as a series of documents, with more to follow, together with a software version of the model on CD-ROM.

The new model relates only to the assessment of risk to human health arising from long-term exposure to soil contamination and therefore needs to be used with, for example, an assessment of risks to groundwater. It has been framed to work with the recently introduced Contaminated Land Regulations (Part IIA of the Environmental Protection Act 1990).

It replaces the much-maligned ICRCL 59/83 tables (Interdepartmental Committee on the Redevelopment of Contaminated Land, 1987), which covered 17 contaminants selected largely on the basis of research into soil contamination associated with former gasworks sites. The new model does, however, concentrate on the ICRCL substances.

The ICRCL threshold concentrations presented practitioners with many problems.

One was that local authorities and other regulators frequently adopted the lower threshold concentration as a trigger for soil remediation rather than a site-specific assessment of contamination risks.

It was generally hoped that the CLEA model would allow consultants and engineers to exercise a far greater degree of judgement in their assessment of risk. Unfortunately this may not be the case.

The CLEA documents present soil guideline values (SGVs) for the relevant contaminants on the basis of four land-use categories:

residential - with and without plant uptake, allotments and commercial/industrial.

In general, a single concentration of each contaminant is provided for each land-use scenario.

Thus, as was the case with the ICRCL guidelines, the geoenvironmental practitioner will be faced with a single value, which in most cases does not differ markedly from the ICRCL lower threshold trigger value. However, the use of a limited range of generic land-use situations is not unreasonable as most sites will readily fit into one of the four categories. The geoenvironmental consultant will have an important role in identifying sites that differ from one of the standard land-use categories and assessing how its particular characteristics affect the overall risk assessment.

The CLEA guidance stresses that SGVs are a screening tool for use in contaminated land risk assessment and that where the concentration of a particular contaminant exceeds the SGV there might be an unacceptable level of risk to users of the site.

However, it is likely, in the short term at least, that local authorities will use the SGVs in the same way as the former ICRCL trigger levels and leave little scope for judgement on the part of the geoenvironmental consultant.

One encouraging aspect is the acknowledgement that in some areas there are naturally occurring substances in the ground as a result of, for example, mineralisation, such as the very high arsenic concentrations found in natural soils in south west England. In such circumstances the local authority will be allowed to take account of the bioavailability of the substance.

While the SGVs and the former ICRCL concentrations are based on total concentrations of contaminants measured in the laboratory, clearly it is the proportion of the contaminant in a form representing a risk to human health that is most significant.

Thus, the bioavailability test essentially attempts to replicate in the laboratory the way in which the particular contaminant is broken down in the human body. In areas where elevated concentrations of contaminants are a result of natural processes it may generally be expected that the bioavailability test will yield concentrations significantly lower than measured in a conventional test procedure and in many cases below the SGV.

Unfortunately this testing is not generally offered by commercial laboratories and until it is more widely available the cost of the testing may be prohibitive on smaller projects.

It is to be hoped that acceptance of bioavailability testing to assess risks associated with naturally occurring contamination will allow its use in every situation where an initial screening of total concentrations that one or more SGV is exceeded, and that this will be extended to sites where the contamination is not from natural sources.

The use of such testing, supported by a proper assessment of the risks, may at last allow geoenvironmental engineers to adopt a more informed approach to the effects of soil contamination.

Steve Branch is managing director of geoenvironmental specialist LBH Wembley.

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