Dry weather sewage spills impacts on river health

Abstract

The Yarra River is considered to be an important environmental and recreational asset by the Melbourne community in Australia. The upper reaches of the Yarra provide water for consumptive use including drinking water for the 4.2 Million people in Melbourne and some agricultural use. The lower Yarra is mainly utilized for recreational purposes and is a focal point for tourism in Melbourne. Water quality of the Yarra River in its lower catchments is relatively poor when compared with the upper reaches due to urbanisation. Faecal coliform levels have been observed to be high in the lower sections of the Yarra River even during dry weather periods. One of the contributors to faecal contamination during dry weather has been identified as dry weather sewer spills due to structural collapses or blockages from tree roots and seepage from septic tanks. The main aim of this study is to investigate the effect of dry weather sewer spills on river water quality and to estimate the survival rate of microbes with time on pervious and impervious surfaces after a spill. Dry weather spills are retained on the ground surface and then mobilized through surface runoff to drains and waterways following storm events. The number of faecal microbes carried to the waterways will depend on the volume of the spill, magnitude of the storm generating runoff, elapsed time after the spill and the antecedent climate conditions before the storm. Actual spill data from 2007 were collected from archived information for two catchments in Melbourne. This research draws conclusions by discussing the potential effects on river water quality due to dry weather spills, the impact of rainfall intensity and its potential to mobilize the microbes towards a stormwater drain to be transported to a nearby water course. Overall, the objectives of this study were achieved and the transport of microbes was estimated at different elapsed times depending on the relative humidity after a dry weather spill event, providing its impact on waterways.