Stormwater pollution from urban areas
Natural areas generally have the capacity to absorb large amounts of rainfall before runoff occurs. The absorbed water is retained as groundwater or drains into breeks and rivers over time, maintaining stream flow. When such areas area developed for residential and industrial uses, large areas previously able to absorb water are covered by impervious footpaths, roads and roofs. As runoff (stormwater) drains into the river system, it picks up a variety of pollutants and litter from urban areas.
Sewerage system overflows
Most water that enters our homes leaves as sewage, including effluent from flushing toilets and the water that goes down the drain from washing clothes, showering, preparing food, etc. Sewerage can include food scraps, oils, grease, paints, pesticides, solvents and preservatives. Sewerage is managed by Sydney Water. Sometimes sewers can overflow, and affect a number of freshwater creeks, particularly in the upper and central Georges River. The overflows can result in algal blooms, weed infestations, and fish kills.
Impacts of developments
Development along the shore of the river can cause a number of problems. Wave action on the foreshores of Botany Bay; and the threat to remnant vegetation along foreshore areas are significant. Sediment and erosion due to runoff from urban subdivisions are also a major problem.
Marine vessels discharge into the river and Botany Bay (including ballast water) and cause erosion on the banks. Jet skis and speed boats increase rates of erosion of river banks.
There is inadequate management of leachate from existing and former landfill areas (rubbish dumps).
Rural and agricultural use
Agricultural runoff contains effluent from cattle and sheep, as well as any fertiliser or pesticides that have been used on the property. Traces of effluent contains high levels of nutrients such as phosphorous, and this can cause excessive growth of some aquatic weeds. Many native plants are not able to cope with the excess nutrients and are also killed. Pesticides can enter the food chain through bioaccumulation. The pesticides enter the waterway as runoff, are then absorbed by fish which are then eaten by other organisms. The poisons build up in larger animals causing death.
Land clearing destroys the habitat of native fauna and flora. The removal of vegetation exposes the soils to the actions of wind and water and makes it susceptible to erosion. Land clearing also reduces biodiversity. There are fewer species of plants, and as a result few species of animals will be attracted to the area. This can result in large scale pest infestations, and greater susceptibility to disease.
Eutrophication can occur as a result of fertilisers in runoff. Blue-green algae (the common name given to several species of algae) can create scums on the surface of rivers. The toxins produced by the algae can be poisonous to humans and other animals. As it decomposes the algae uses oxygen which results in fish kills.
Mining and Industrial Land use
In Botany Bay, the historic industrial discharges into the bay resulted in contamination of sediment on the bay floor. Industrial discharges can also result in fish kills, bioaccumulation or contamination of water and sediment.
Mining induced subsidence
Mining can result in the cracking of river beds, partial loss of surface flows, decline in water quality, release of gas reserves from underlying strata and increased rates of cliff-top collapse.
Removing sand or other sediments from the floor of a bay or river can alter the river morphology (flow), and disturb floor dwelling aquatic species.
Deputy Principal at a Sydney high school. Coordinating author of the Geoactive text book series.