Part IIA of the Environmental Protection Act 1990, provides a new regulatory regime for the identification and remediation of contaminated land. The "suitable for use" approach, the cornerstone in the legislation, necessitates that the risks from potentially contaminated sites are evaluated on a site-by-site basis utilising the "contaminant-pathway-receptor" (CPR) conceptual model. A pollutant linkage is said to exist when all three components of the model can be identified on a site. It is only when the pollutant linkage is deemed to be "significant" (ie indicates that the condition of the site is causing or has the potential to cause significant harm to a receptor) that the site can be determined to be contaminated land. The Environment Agency estimates that the UK has approximately 100,000 contaminated sites occupying a land area of around 300,000 hectares.

At this point it is pertinent to consider the question of how one should define the physical extent of a contaminated site. The statutory guidance (Part 4: chapter B) states that "in practice, the land to be covered by a single determination is likely to be the smallest area which is covered by a single remediation action" and that the land is "likely to be the smaller of: a) the plots which are separately recorded in the Land Register or are in separate ownership or occupation; and b) the area of land in which the presence of significant pollutants has been established." Many industrial sites in previous decades covered large areas of land, much of which was subsequently redeveloped for residential and other uses. Using the definition of a contaminated site given above, it is not difficult to imagine that the numbers of potentially contaminated sites within a typical metropolitan local authority's area are likely to be in the range of thousands rather than hundreds. The resources available for implementing contaminated land strategies are limited in the case of most local authorities. Given the likely numbers of potentially contaminated sites, it is clear that site prioritisation systems will play an important role in ensuring that the available resources are put to the best use. Official technical guidance available on risk management advocates a tiered approach to risk assessment. Tier one level risk assessments (ie hazard identification, risk screening and site prioritisation) help to minimise unnecessary effort and provide a mechanism for targeting resources towards sites that present the greatest risks. They also provide an auditable trail to support or explain the omission of certain risks from further consideration.1 This paper considers some of the current approaches to site prioritisation in the UK with the aim of drawing some conclusions about the important elements of an ideal site prioritisation system. CLR6 The Department of the Environment, in 1995, published CLR6, a method for prioritising and categorising potentially contaminated sites. Based on the CPR model, the method is split into two parts. Part I, which consists of a preliminary assessment of sites based mainly on an assessment of their proximity to receptors, results in the sites being placed into one of three priority classes (A, B and C). Part II of the procedure uses information gathered from desk studies, site walkovers and/or exploratory surveys, to re-categorise priority sites derived from the Part I assessment into four new categories based on the suitability of the site for its current use, the degree of risk posed by contaminants on the site and the urgency of the need for remedial action. T

he main strengths of CLR6 are that it provides a structured framework which to a large extent avoids the use of subjective judgements. However, a number of criticisms have been levelled against it, including: - it is resource intensive requiring a detailed level of knowledge of individual sites; - in some areas, utilisation of Part 1 of the procedure may result in the classification of the majority of sites as Class A; and - it does not provide an explicit method for the relative prioritisation of sites within a particular group. DREAM Some local authorities have devised their own site prioritisation methodologies. Dundee City Council for example, have developed a system known as DREAM (Dundee Risk Evaluator Assessment Model) which assigns scores to contaminant sources, pathways and receptors based on the contaminants polluting potential, the receptors sensitivity and the importance of the pathway.2 A pollutant linkage score (PLS) specific for each receptor type is derived by adding the source score to the product of the receptor and pathway scores. A site index (SIS) or overall score for the site is derived by summing all the PLS scores. Each pollutant linkage can score up to a maximum of 100, resulting in a possible maximum SIS of 500. Pollutant linkages are then reviewed and classified into "priority categories" in accordance with CLR6. The major strength of the DREAM methodology is that it overcomes the problem of relative ranking of sites within a group observed in the CLR6 methodology, by assigning them a PLS. Its main weaknesses are associated with the way in which sources (ie industries) are hazard ranked. First, it appears that sources are allocated pre-defined scores irrespective of the type of receptor under consideration. In addition, the scores assigned to the industries, although based on professional judgements, have been subjectively derived. As these judgements could easily be called into question (for example during a notice appeal), it is important that detailed documentation showing how the scores were derived is made available for public scrutiny and comment.

GEOENVIRON
GeoEnviron is a modular database management system dedicated to handling a wide range of environmental information (figure 1). One of the modules within the system is designed for the purpose of handling the disparate types of information likely to be gathered in the process of implementing a contaminated land strategy. The system has powerful GIS linkable data handling capabilities coupled with an embedded site prioritisation system and extensive decision support facilities. The prioritisation system used in GeoEnviron is derived from a methodology produced by the Danish Environmental Protection Agency (DEPA).3 Regulations requiring Danish local authorities to inspect their areas for contaminated land were first passed in 1993. The early move to implement a strategy can be explained by the fact that the country derives most of its water supply from groundwater sources. DEPA estimates that there are approximately 40,000 contaminated sites in Denmark. By 1999, a total of 4,940 sites had been identified with 1,313 of these being considered a serious threat to groundwater. By the end of 1999, 1,727 of the sites identified had been cleaned up.4 A government-funded technology programme has facilitated much of the clean up. Each year, new priority sites are proposed for clean up by the county and local authorities, of which approximately 70 per cent and 30 per cent respectively use the GeoEnviron system, which has been adapted to suit the UK regulatory stance. Also based on the CPR model, it can prioritise sites in terms of their impacts on land use based receptors (humans, buildings and ecology) as well as surface water and groundwater receptors.

The first stage of the procedure involves identifying the contaminants likely to be present on the site. This process is aided by the built-in decision support facility, which contains information on DoE industry profiles and a detailed chemical inventory. The contaminants are allocated pre-defined hazard scores based on their chemical (ie mobility and degradability) and toxicological (ie acceptable and tolerable daily intakes) properties having regard to the type of receptor under consideration. The contaminant having the highest hazard score is identified as the "significant pollutant". A "site specific hazard score" is then assigned to the site based on the nature and significance of the pathways present or likely to be present, and the sensitivity of the receptor. In the case of land use-based receptors, the scoring is carried out using a simplified exposure assessment where the important factors are the contaminant volatility, its accessibility (ie depth and type of ground cover present on the site) and the receptor's sensitivity (ie high, medium or low). In the case of landfill sites and the associated gas-related risks, the type of waste (ie organic or inorganic), the landfill's capacity, the distance from the site to occupied buildings and the use sensitivity of the buildings use are important exposure factors. With regard to groundwater, the degree to which the aquifer is protected by overlying geology and the groundwater quality classification are the important exposure factors. In the case of a surface water receptor, the considerations are whether there is existing evidence that the contamination is impacting the water body and the water bodies desired quality objective. Generally speaking, the "contaminant hazard score" and the "site specific hazard score" are summed to obtain a final risk score for the site. The possible pre-assigned maximum scores for land use, ground water and surface water-based receptors are 26, 32 and 25 respectively. It is worthwhile noting that the sensitivity of the scoring system can be altered by the user themselves in line with specific requirements or changes in legislation or guidance. The final site risk scores within each receptor category are compared and the sites are prioritised accordingly. GeoEnviron incorporates a simple numerical scoring system which means that prioritisation can be facilitated relatively rapidly. Again, by considering the actual contaminants present or likely to be present on the site, it provides a particularly useful means of distinguishing between low probability, low consequence risks and high probability, high consequence risks. In other words, sites contaminated with potentially carcinogenic substances can be flagged up with a degree of consistency. The system is also integrated within a larger contaminated land information management system, giving the decision-maker the ability to easily access all the relevant site information while carrying out the prioritisation. However, there are weaknesses associated with the system; mainly that it requires a degree of baseline information.

However, information such as the contaminants likely to be present on the site is provided within the system in the form of industry profile information. Information on receptors can be obtained from historical maps and will also sometimes be available from existing local authority records (ie planning records). As the CPR approach is recommended in the statutory guidance, it is important that any risk assessment tool should take it into account. All the methods examined in this paper were found to be firmly based on the CPR model. In the light of the scarce resources available to local authorities, it is important that prioritisation systems are pragmatic in the sense that they can be easily and rapidly applied with a minimum amount of effort. At the same time, they should minimise the chances of serious threats to human health being overlooked. In this respect approaches that consider the specific contaminants present or likely to be present on a site and their chemical and toxicological properties are to be preferred. Very few of the prioritisation systems currently available in the UK have been applied, either in Europe or the USA, over a long period of time. It is important therefore, that whatever system is selected it should be capable of being easily adapted to account for unforeseen circumstances. Although it is unlikely that subjective judgements can be avoided altogether, it is important that they are minimised in order to maintain consistency and transparency in the process. Where subjective judgements are made, they should be well documented. As the process of risk assessment is an iterative one, where the unprescribed re-evaluation of information may occur at any time, it is important that risk assessment tools are integrated within the overall information management framework so that all the information available on a site can be used to benefit the risk assessment. Finally, the fact that the nature of risk assessment means that there are no absolute answers, along with the diversity of local authority areas, means that no one method will be able to satisfy every circumstance. For further information or comments please contact Simon Makoni on telephone: 020 7353 0222 or e-mail: stm@geoenviron.co.uk

REFERENCES
1 Department of the Environment, Transport and the Regions, "Guidelines for environmental risk assessment and management revised departmental guidance", 1999.
2 Dundee City Council, 'DREAM - Dundee Risk Evaluator Assessment Model, 2001, (www.dundeecity.gov.uk/ecp/cont-land/dream.pdf).
3 Danish Ministry of Environment and Energy (Milj¿-og Energiministeriet), Danish Environmental Protection Agency (Milj¿styrelsen), "Project number 19 on soil and groundwater - a system for prioritising point sources", 1995.
4 Fuglsang I A, "The Danish EPA's Technology Programme for Soil and Groundwater Contamination (TUP-program)", CLAIRE View, summer 2001.