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Water quality

Picture of the Warragamba catchment area

 Lake Burragorang, formed behind Warragamba Dam

In Australia, water quality standards are set according to national water quality guidelines.

 

Many complex factors affect water quality on its way from rain clouds to your tap.

 

Natural bushland yields relatively clean water with small amounts of nutrients that can affect water quality in reservoirs. However, heavy rainfall after a long dry period can bring sediment, leaves, branches and animal droppings into the rivers and lakes.

 

Natural wetlands hold water for some time. They can form habitats for many aquatic and semiaquatic animals - invertebrates, fish, frogs, reptiles, water birds and mammals. Wetlands affect, and often improve, water quality by filtering sediments and transforming nutrients as water passes through them.

 

Not all of the drinking water supply catchments are unspoilt bushland.

 

Runoff from farmland can carry sediment, animal droppings (which may include pathogens), and traces of chemicals, such as fertilisers, pesticides and herbicides.

Picture of road passing through the catchment area An urban road passing through the catchment

Urban areas contribute many undesirable impurities in stormwater, including oil and grease washed off roads, household and garden chemicals, rubbish and animal droppings. Some urban areas also release treated or untreated sewage effluent into rivers and streams.

 

Once rainfall reaches streams and rivers instream processes begin to affect water quality. These include:

  • nutrient assimilation by algae, sediments and biochemical transformations
  • oxygenation by flowing water, algae and streamside vegetation
  • dilution from mixing
  • settlement of matter in slower flowing areas
  • disinfection by ultraviolet rays in natural sunlight
  • riverbank erosion and streambed scouring which contribute sediment to water during heavy flow periods.

 

Water quality in reservoirs is determined by the quality of water flowing into the reservoir and in-lake processes. Water flowing in can transport materials such as nutrients, sediments and other contaminants. These can then be affected by in-lake processes such as settling and biochemical action.

 

Diagram of reservoir showing temperature profile

Diagram of a reservoir showing temperature profile 

 

The water in lakes often separates into two distinct layers - a warm upper layer (heated by the sun) called the epilimnion, and a colder deeper layer called the hypolimnion. This layering is called thermal stratification and can cause increased concentrations of metals, nutrients, algae and cyanobacteria, as well as turbidity and pathogens during wet weather. The Sydney Catchment Authority artificially mixes water in some of its reservoirs to minimise these problems.

 

Throughout this system, the Sydney Catchment Authority (SCA) monitors raw water quality to detect anything that may affect drinking water quality, and any necessary actions are taken. The SCA and others use a multi-barrier approach to ensure that the water that goes into the drinking water supply system is clean and safe.