The Hawkesbury Estuary from 1950 to 2050

This chapter describes the eutrophication of the Hawkesbury-Nepean River estuary that drains Sydney western suburbs. The project started in the 1950s by spot measurements of nutrient concentration and in the 1970s with a hydrodynamic and water quality study of the estuary. This was to gauge the effect of sewage treatment plants already proposed in the 1970s to deal with the growth of Sydney and their projected effects on the flow, chemistry and biology of the system by the year 2000. The study, using a mathematical model of proposed effluents, predicted an increase of nitrogen and phosphorus nutrients that was large enough to significantly degrade the water quality of the estuary. In particular it predicted a large increase in plankton, mainly blue/green algae, if planners did not make careful decisions concerning land use, urbanization and catchment development. A recent detailed study of water quality by Sydney Water has shown that the river system has indeed been degraded with the occasional occurrence of outbreaks of floating flowering plants (macrophytes) in the upper Nepean and blue/green algae in the saline/freshwater interface and “red tides” of toxic diatoms near the mouth of the estuary. This is despite upgrading of sewage treatment plants. More upgrades are needed especially in the area of South Creek. The biggest problem to address is ongoing urbanization and the resulting wet-weather inflow of degraded stormwater, sediment, nutrients and many other contaminants that reach the estuary. A public information program should be started and wet weather runoff should be treated preferably at the source of the runoff, somewhat mimicking the planned new developments at the Gold Coast Broadwater. An estuary, once urbanized will always be degraded, the extent of which however can be managed.