Extracting base flow to substitute for potable water use – how much is too much for urban stream health?

Stormwater harvesting is becoming an accepted strategy to provide water for sustainable urban irrigation. Base flow from either the constructed stormwater system or a stream can provide an important contribution to the water supply of a stormwater harvesting system. This can be beneficial as it can reduce the storage size (and thus cost) and improve reliability of supply. However, removing base flows can impact on other management objectives such as maintaining environmental flows for ecosystem health. The volume and origin of base flows in urban stormwater systems and streams can reflect land use changes and anthropogenic sources such as leaking water mains or sewers. In order to assess the origin of urban base flows, a study was undertaken in 2010 that monitored the concentration of fluoride in the water. This was used to indicate the contribution of potable water (including that contained in sewage). Eleven urban and two reference (bushland) streams were assessed across northern Sydney. Four urban streams reported more than 20% of their base flow as potable in origin with one of these urban streams reporting 89% of base flow as potable. Reference streams reported zero to very low concentrations of fluoride. This paper discusses the impact of urbanization on base flows and the potential impacts of removing these flows from urban streams for stormwater harvesting.