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water quality

What is ‘good’ water? How do we monitor the quality of our water resources? Why do we care? Come on a journey with us through Victoria’s surface and groundwater systems to find out more.

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What is surface water?

Surface water comes from rain falling on a catchment, the natural landscape that collects water. It moves overland or through the layer below the earth’s surface into creeks, streams, rivers, lakes or into the groundwater.

What is groundwater?

Groundwater is water below the earth's surface in the pores and crevices of rocks and soil. Groundwater can move into surface water as base flow, or provide sustained low flow into rivers or streams.

What are our water resources?

Our water resources can be categorised as either surface water or groundwater.

What do we mean by surface water and groundwater?

What is good water quality?

Water quality can change across Victoria and at different times of the year and from year to year. The quality of the water decides what it can be used for. Water is good quality if it meets the standards called ‘beneficial uses’ that apply to how we use water. These are discussed in the next section.

Photo Credit: Tarmo A. Raddik

The same water quality may not be suitable for all uses because water for drinking, farming, industry or recreation has different water quality standards.

And what’s considered ‘good’ quality in one part of Victoria may not be suitable in another part. An example is salinity, or how salty the water is. Salinity is measured in μS/cm, or recognised electrical conductivity units of microseimens per centimetre.

Salinity varies naturally across Victoria. Surface water in the far east is considered good if it’s less than 500μS/cm, while in the west surface water is considered good if it’s less than 1,500μS/cm. Higher salinity in western Victoria is because a large inland sea covered this area millions of years ago and left behind big quantities of salt in soils and groundwater when it retreated.

Just because the quality of some surface and groundwater is not suitable for every use doesn’t mean it’s degraded or polluted.

Protecting the water quality of ecosystems that depend on water is regarded as the best way to protect the water quality of many different beneficial uses. If it’s good for entire ecosystems of aquatic life, it’s probably good for most other uses.

An icon depicting nitrogen and phosphorus in the water
An icon depicting the pH levels in the water
An icon depicting oxygen levels in the water
An icon depicting turbidity in the water
An icon depicting salinity in the water
An icon depicting organic material in the water

Why are chemical indicators important?

Nutrients like total phosphorus and total nitrogen:

Nutrients need to be in the water to support the different plants and organisms that live there. But when the nutrient concentration is too high it can lead to algal blooms like blue-green algae. These can be toxic and choke the river or stream and block light entering the waterway. Aquatic plants can’t photosynthesise and produce food to survive if light can’t penetrate the water column.

Dissolved oxygen:

Dissolved oxygen is critical for life, such as helping fish to breathe. Oxygen levels drop at night when plants don’t photosynthesise and produce oxygen. But fish continue to consume oxygen at night and if oxygen levels drop too low for too long, the fish will die. Big drops in dissolved oxygen can lead to ‘fish kills’ when large numbers of fish die.

Bacteria also use up the oxygen in water. If a large amount of organic material, like trees and leaves, are swept into waterways during floods, bacteria levels can rise dramatically when bacteria have this large source of nutrition. Huge numbers of bacteria can cause the dissolved oxygen levels to drop and fish can die as a result of not having enough oxygen to breathe.


Turbidity is an indicator of the cloudiness of the water. This cloudiness is due to suspended sediment or small dirt particles in the water column. This can have negative effects on aquatic life. Suspended sediments can block light to aquatic plants, smother aquatic organisms and carry contaminants like lead and mercury, bacteria and pathogens.

Electrical conductivity (salinity):

Electrical conductivity is a measure of salinity, or how salty water is. Most aquatic species have a preferred range of salinity and can’t survive if the salinity levels move above this range.


pH is a measure of the acidity of the water. Most aquatic species have a preferred pH range like they do with electrical conductivity. Big changes in pH can cause a chemical reaction that makes toxicants in the water harmful to aquatic life.

How do we know what the water quality is like?

We measure different indicators of water quality that tell us something about the quality of the water. These are often chemical or biological indicators.

Chemical indicators can include nutrients like nitrogen and phosphorus, dissolved oxygen, a measure of cloudiness called turbidity, electrical conductivity that measures salinity and a measure of acidity called pH. Biological indicators include fish, macroinvertebrates and E. coli, a bacterial indicator of faecal pollution.