In New Zealand, there are now increasing demands being placed on outfall investigations to accurately predict the fate of microbial contamination from proposed outfall options. In this context, a detailed numerical model study of proposed twin outfalls for the cities of Wellington and Lower Hutt focused on realistically simulating the dispersion and decay of faecal indicator bacteria. This meant including the effects of treatment level, adverse winds, light attenuation in the water column, time-varying discharge and decay rates, anticipated changes in the choice of faecal indicator bacteria, and the close proximity of two large outfalls. The relative importance of these input parameters in determining outfall lengths are discussed along with suggestions for future microbiological research; in particular the need for further microbial survival field experiments.

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