A comparison of different ranking schemes for assessing the effect of large point sources of pollution in the UK

The availability of national maps of critical loads for soils, vegetation and freshwaters helpsenable the assessment of the effects of large point sources of pollution in the UK. The deposition footprint of most major sources has been modelled and combined in a GIS with a national critical loads database. As part of an integrated pollution control strategy (IPC) it may be helpful to rank point sources in order of their effects on the environment. A comparison of the discriminating power and effectiveness of several ranking schemes has been carried out. A variety of ranking schemes were investigated, such as; total area where sulphur deposition exceeds the critical load or average mass deposited on areas where the critical load is exceeded. Their relative merits were compared for several current and future scenarios, such as, actual 1993 emissions or predicted emissions for 2001. Rankings for the unit emissions provided a measure of the pollution potential of each source and were a complex function of the location of sensitive areas and meteorological conditions. Rankings under other scenarios tended to be dominated by the relative magnitude of the emissions. Comparison between the ranking schemes was made using non-parametric statistics. The comparisons reveal complex interactions between different schemes. The approach is providing practical solutions to a pollution control strategy based on maximising environmental benefits.