Modelling impacts of diffuse and point source nutrients on the water quality of South Creek catchment

Abstract

The Hawkesbury-Nepean river system is one of the most important water resources of the Sydney region. It provides over 90% of Sydney's drinking water and receives effluent from over 40 sewage treatment plants. The river is also targeted as one of the venues for the Olympic 2000 sporting activities. The Hydrological Simulation Program - FORTRAN (HSPF) was chosen to model Hawkesbury River catchments and water quality in the associated stream reaches because it is the only model available that incorporates the wide range of significant processes involved. This paper describes a study modelling the generation of diffuse sources of nutrients on a sub-catchment of the Hawkesbury-Nepean River system. The South Creek catchment is a major diffuse source of nutrients. HSPF model was calibrated successfully and frequency duration analysis was used to identify the relative contribution of diffuse and point sources in the urbanising catchment. Contributions from diffuse sources are significant for a substantial proportion of time. The contribution from St. Marys Sewage Treatment Plant (STP) in its present operating condition is quite high too. The model results show that even the highest level of nutrient removal at the STPs would not reduce nutrient levels in South Creek sufficiently to meet ANZECC nutrient guidelines for a substantial proportion of the time.