Threats to Water Quality

Threats to Water Quality

Pollutants can threaten drinking water quality and public health. These pollutants can come from the many different land uses in drinking water catchments. Potential pollution in the catchments needs to be predicted, prevented or reduced. Quick responses are required during emergencies.

Potential pollutants

Drinking water supplies need to be protected from a number of potential pollutants.

  • Pathogens are disease-causing microorganisms including bacteria, protozoa and viruses. High numbers of pathogens are found in human, animal and bird faeces and are the cause of most waterborne disease throughout the world.
  • Sediment can run off the land and be carried into the water by rainfall, particularly after long dry periods or bushfire. It can increase the turbidity (cloudiness) of water.
  • Algae can alter the taste and smell of water and clog up water treatment systems. Some algae, including blue-green algae or cyanobacteria, can produce toxins that cause illness.
  • Nutrients such as phosphorus and nitrogen from fertilisers and detergents can encourage algae to grow and in certain conditions can cause algal blooms.
  • Metals such as aluminium, calcium, iron, magnesium, potassium, sodium or other heavy metals can dissolve in water and affect its taste.
  • Pesticides and chemicals from agriculture and industry can compromise water quality.

Sources of pollutants

Pollutants can come from either point sources or diffuse sources. Point sources include pipes and channels in sewerage systems, stormwater systems, and industrial waste discharge. Diffuse sources include roads and railways, agricultural and horticultural activities, forestry and bushland, feral and native animals and recreational activities. Diffuse sources are harder to predict, identify and control.

Pollutants on the move

Heavy rain and flooding in the catchment can transport pollutants over the land and into creeks and rivers. Creeks and rivers can then carry potential pollutants into our drinking water storages.

  • Heavy rain can cause overflows of sewage and industrial waste.
  • Pathogens from animal droppings and carcasses can wash into creeks.
  • Rain after bushfires can wash soil, nutrients and ash into nearby waterways.
  • Leaks and spills from heavy vehicles and industry can flow directly into watercourses or be washed there by rain.
  • Droughts can lead to a build-up of pollutants that are washed into the water supply in a concentrated dose when it eventually rains.


There are always microorganisms in water. Most are harmless and many help to keep water healthy. Only a few microorganisms are harmful to human health including the pathogens Cryptosporidium and Giardia - found in the gut and faeces of infected animals including humans. We are exposed to these two organisms in our everyday lives. A multi-barrier approach of catchment management, water treatment and disinfection removes or reduces the levels of harmful microorganisms in drinking water.

The presence of Cryptosporidium and Giardia in water may not involve a health risk. The number, whether they are dead or alive and whether they are a species that are infective to humans influences the threat to drinking water quality. All raw water that flows into treatment plants is tested regularly for the presence and number of these two pathogens and other microorganisms.

Water quality monitoring and standards

WaterNSW monitors water quality at many locations throughout the catchment, at the inflows to water storages and in water storages themselves. Water quality standards are set at about 1 percent of the amount that would be potentially harmful to humans. If any of these standards are exceeded, they trigger prompt responses ranging from investigations of the source to increased treatment or isolation of the affected water. Such prompt action helps keep Sydney's water supply among the safest in the world.


Sunday 20 September
2,442,065 ML
2,581,850 ML
10,042 ML
1,435 ML
-19,816 ML
Sunday 20 September