Water quality in rivers results from the physical, chemical and biological processes that take place due to multiple pollutant loadings along a river stretch. A sound understanding and description of these processes - usually encapsulated into predictive models - is needed so that design engineers and water authorities can develop appropriate discharge solutions and control strategies that meet applicable water quality laws and objectives. Continuous discharges from municipal, industrial or agricultural sources are subject to a permitting process by water authorities. The "combined approach" in the new EC-Water Framework Directive (WFD, 2000) consisting of environmental quality standards in addition to emission limit values promises improvements in the quality characteristics of surface waters. However, the specification of where in the water body the environmental quality standards do apply is missing in the WFD. This omission will limit its administrative implementation. A clear mixing zone regulation is needed so that the quality objectives of the WFD are not jeopardized. Instantaneous or accidental pollutant releases are another source of pollution loadings on rivers. Water authorities rely increasingly on "river alarm models" that predict the downriver propagation and dispersion of the pollutant mass as well as biochemical decay or adsorption processes. Currently, these models are fraught with considerable parameter uncertainties and an improved understanding of the transport mechanics in river systems with strong morphological variability is needed to minimize these. This is demonstrated for the case of rivers with a long sequence of groin fields that alternately retain and release the pollutant mass, greatly increasing the longitudinal dispersion.
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