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Summary

During the last decades the Danish counties have put a lot of effort into characterisation of contaminated sites and developing methods for clean up activities. The list of contaminated sites are, however, increasing faster than the economic resources allocated to the activities and thus the need for prioritisation of the effort are intensified. An important challenge in this context is to evaluate the relative importance of the different parameters used to characterise the contaminated sites.

Most priority systems for contaminated sites are based on scoring systems where the influence of the parameters are quantified by developing e.g. parameter score points and weighing factors and aggregate these into a risk index. The strength of a scoring based system is that it is possible to include restrictions into the risk index, a priori, by setting parameter score values. The weakness is that a linear relation among the parameters is anticipated even though the parameters are fundamentally different in origin. Furthermore, the final aggregation functions used to obtain an aggregated risk index manipulate the relative parameter influence complicating the interpretation of the risk index. It is, however, possible to localize unintended influence by independent methods and calibrate scoring systems in order to optimise the system with respect to evaluation of the risk potential.

In this project a scoring system, developed by the County of Copenhagen, "Compound specific priority of point sources" is evaluated in order to optimise the parameter quantification. The general influence on the priority index of the intended and unintended priority restrictions is analysed including a mapping of the influence of the characterisation parameters. Suggestions on elements in the priority system that can be optimised are given.

An independent evaluation method is developed by the Danish National Environmental Research Institute based on partial order theory. This approach is, unlike scoring systems, not based on the use of a unified quantification scale for evaluation of the parameter influence. Partial order theory is used to develop a methodology where the contaminated sites are compared two by two on each parameter. By determining technical assumptions the prioritisation results in a network - opposite to a linear range - but are "projected" by a probability function to a linear scale.

In general the evaluation shows that the parameter quantification and structure of the scoring system has both intended and unintended influence on the risk index. E.g. the parameter weighing factors are not expressed proportionally in the risk index and the influence of the individual parameters strongly depends on the structure of the scoring system.

Two compound specific parameters, i.e. mobility and degradation and an administrative parameter expressing site classification with respect to importance as drinking water supply was found to influence the ranking index the most. If the parameter influence was not weighted a priori the most important ones would be two hydrogeological parameters and an administrative parameter, i.e. the oxidative potential of the groundwater, infiltration rate of rainwater and site classification with respect to importance as drinking water supply.

If the parameter influence is not intended, i.e. may be explained by environmental knowledge, the scoring system must be calibrated to an unbiased index. This can be done by replacing the weighing factors by a calibration factor.

The parameters are grouped into three different risk indexes in the scoring system: one expresses the risk potential of the geological and hydrogeological parameters, one the administrative restrictions and one the compound specific risk potential. For contaminated sites where characterisation is based solely on historical data a parameter expressing the probability of detecting any contamination is further included in the compound risk index. The three indexes are finally aggregated into a risk index. The compound index has proven to influence the risk index the most due to both the classification and the score values of these parameters. The hydrogeological index, however, that are much more differentiated, is not being expressed fully in the risk index.

A comparison of a top 10 list with the highest ranked localities determined with each of the two ranking methods showed a match in 4 out of 10 localities. A further examination of these localities shows a tendency that the scoring system upgrades heavy metal pollution.

The evaluation has shown that there are both intended as well as unintended effects of the structure of the scoring system on the ranking order result. Furthermore unintended effects increase with the complexity of the system. This means that despite the scoring system conceptually is easy to understand the results can be difficult to interpret. Thus it becomes clear that is not quite simple to include specific priority restrictions into a scoring system. Due to the resulting uncertainty in the priority results an evaluation of the system is of interest and contributes to strengthen the reliance in the use of the results of the scoring system.

Full report in pdf. format (1860 KB)
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Helle Thomsen

01.11.2007


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 CVR: 10859387

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