This paper describes a PC-package for simulation of bathing water quality. The actual case reported is the beach of Rødvig, SE Zealand, Denmark. The package computes time series and frequency distributions for E.coli, the indicator organism for bathing water quality. Point sources of pollution from combined sewer overflows (CSO) are computed from historical rain series and the existing MOUSE-package (Modelling Of Urban Sewers). Pollution from dry weather flow (DWF) (eg. purification plants) is also included. The bathing water model is based on time series for flow, dispersion and decay and is calibrated from measured frequency distributions in 4 stations within the period 1981-1986. A standard deviation factor STDe = 2.4 is achieved inbetween measured and computed distributions. Best fit values are obtained for 1) scaling factors er and es for pollution from CSO and DWF respectively, 2) diffuse (non-point) E.coli pollution Ed and 3) dispersion parameters Cb1 and Cb2 in the direction of coastline and vertical hereto. The calibrated model has revealed that an area of 102 ha along the beach exceeds the statistically acceptable quality line for bathing water. The paper presents 4 proposals for improving the quality by reducing the pollution from CSO and DWF, and/or by using an outfall pipe further into the sea for CSO.

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