Sepetiba Bay is located at 23° S, 44° W in Rio de Janeiro State, Brazil. Water samples were taken at eight locations adjacent to the north shore of the Bay, near to villages and towns without sewage treatment provision. The samples were analysed and total and faecal coliform concentrations determined. A hydrodynamic model of the Bay was used together with a species dispersion model based on an adaptive quadtree mesh to predict faecal concentrations in the Bay. Effluent sources used in the model were defined using population data from census returns with flow and concentration values estimated using standard values recommended by the World Bank (WB) and the World Health Organisation (WHO). Sufficient agreement was obtained between the measured and predicted concentrations to support the use of WB and WHO summary statistics to estimate sources of sewage.