Water authorities apply a diversity of models and input data to set water quality-based emission limits in discharge permits. To illustrate the consequences of model and data selection, two complete mixing models and four mixing zone models used in Germany, the United Kingdom (UK), the Netherlands and the United States of America (USA) were selected and applied to various discharges of cadmium. The maximum allowable annual cadmium load was calculated for each model and diverging input data for upstream flow, upstream concentration, effluent flow and effluent concentration. Due to model selection, differences in pollutant loads amounted to a factor 3. Harmonisation of the derivation of water quality-based emission limits is necessary to prevent widely divergent pollutant loads under comparable environmental conditions.

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