The role of reference conditions in water quality assessment: application of a fuzzy logic-based Decision Support System (DSS) in the Danube and Elbe River Basins

The recent defi nition and submission of WFD-compliant River Basin Management Plans required large efforts of water managers and highlighted a number of data, knowledge and methodological gaps that need to be fulfi lled during the next management cycle. The complexity of the WFD implementation process calls for easy-touse, reliable and scientifi cally sound decision support tools that consider relevant aspects of the basin of concern. For this purpose, a Decision Support System (DSS) for risk-based assessment and management of river basins was developed within the EU MODELKEY project. The MODELKEY DSS allows assessing the chemical and ecological status of water bodies by applying both the One-Out All-Out principle, suggested by the WFD, and a novel Integrated Risk Assessment (IRA) methodology, developed within the project. In this paper, main results obtained by DSS application to two large European rivers, i.e. Elbe and Danube, are presented and discussed, based on a set of biological, chemical and physico-chemical indicators, which have been evaluated in relation to available reference conditions. The WFD ecological and chemical status assessment as well as analysis of intermediate results was strongly affected by the methods used to derive reference conditions, the applied indicators, and the methods used to aggregate indicators results. Minimally disturbed conditions (MDC) are the best choice as reference conditions, being characterized by generally high index values (good status) for all biological indices. Where MDC are not available, historical conditions could be a good substitute; they should be preferred to least disturbed sites (in particular when the river is generally impaired as the Danube River)