Understanding the data

What does it mean?

Have you ever wondered how a rise in phosphate levels or a change in water temperature can effect the health or quality of the water? You often hear people talk about issues of water quality and the science behind it, but what does it all mean.  

The Hawkesbury-Nepean River is a large ecosystem - a biological community of interacting organisms and their physical environment. Small or large changes to water quality and health can affect the survival of the whole river ecosystem from water bugs to people.

Temperature and Dissolved Oxygen

Water temperature is an important physical characteristic of rivers. Under natural conditions most water bodies have seasonal variations, being warmer in summer and cooler in winter. Most species that live in rivers are cold-blooded and can only survive and reproduce within a preferred temperature range.

Fish and other aquatic fauna need oxygen to breath just like us. The level of dissolved oxygen is linked to water temperature. Water at a temperature of 10°C carries two times the amount of dissolved oxygen as water at 25°C. If the water temperature gets to hot and the levels of dissolved oxygen drops to low, it could lead to the death of aquatic fauna.

In addition warm water temperatures can accelerate the growth of algae causing algal blooms. Algal blooms reduce dissolved oxygen levels which can affect fish and invertebrates. Algae are an important natural component of aquatic ecosystems, but they can cause serious water quality problems when blooms occur. Some algal species, like blue green algae, produce toxins which affect drinking water supplies.

The dissolved oxygen saturation level for a fresh water river, such as the Hawkesbury-Nepean River, should fall between 85% and 110% to protect aquatic ecosystems (ANZECC Guidelines, 2000).


The pH balance of a water supply describes how acidic or alkaline it is. The acidity (or alkalinity) of a water supply can affect plant growth, irrigation and drinking water.

Water with an extremely low (acidic) or high (alkaline) pH is deadly to aquatic life. Water with a low or high pH (acidic) may reduce the the number of fish eggs hatching, as well as fish and aquatic macroinvertebrates. Amphibians, like frogs, are particularly vulnerable, probably because their skin is so sensitive to pollutants.

The pH for a fresh water river, such as the Hawkesbury-Nepean River, should fall between 6.5 and 8.5 to protect aquatic ecosystems (ANZECC Guidelines, 2000).

Electrical Conductivity and Total Dissolved Solids

Electrical conductivity is a measurement of the ability for water to carry an electrical current. 

When we measure electrical conductivity we are looking for the presences of ions, such as inorganic dissolved solids like chloride, nitrate, sulfate, and phosphate (negative charge) or sodium, magnesium, calcium, iron, and aluminum (positive charge), as well as organic compounds like oil or alcohol which do not conduct electrical current very well and therefore have a low conductivity when in water.

The more conductive the water is, the more ions or potential pollutants in the water, thus the lower quality of water. Poor sewage treatment would raise the conductivity because of the presence of chloride, phosphate, and nitrate where as an oil spill would lower the conductivity. The natural rocks or the geology of a catchment are a source of natural salts, naturally influencing the conductivity. High levels of conductivity may indicate salinity issues in the soil from over irrigation or land clearing causing the water table to rise and salts to be left on the surface of the soil. Following rain, these salts can be washed into a creek or catchment, thus increasing the conductivity.

Conductivity is also affected by temperature; the warmer the water, the higher the conductivity generally is. Fish and macroinvertebrates can be affected by the level of conductivity. Conductivity can affect the ability for certain species to inhabit a water body. Many Aquatic plants, fish and macroinvertebrates having different tolerances to varying levels of salinity, outside a set range they can no longer live. 

Electrical conductivity for a fresh water river, such as the Hawkesbury-Nepean River, should fall between 200 and 300 microsiemens per centimeter (µs/cm) to protect aquatic ecosystems (ANZECC Guidelines, 2000).


Turbidity is the cloudiness or haziness of a fluid caused by individual particles (suspended solids).

Human activities that disturb the land, such as construction, mining and agriculture and urbanised areas with large paved surfaces such as roads and parking lots can lead to high sediment levels entering waterways during rain due to storm water runoff. Turbidity may be caused naturally by growth of phytoplankton - a microscopic plant. High levels of turbidity reduce the amount of light that penetrates into the water. If the level of light penetrating the water is reduced, photosynthesis of aquatic plants may be affected and visibility for aquatic animals lessened, potentially affecting their ability to source food or escape predators.

Turbidity for a fresh water river, such as the Hawkesbury-Nepean River, be less than 50 nephelometric turbidity units (ntu) to protect aquatic ecosystems (ANZECC Guidelines, 2000).


When we are talking about the impacts of nutrients we are looking for the presence of phosphorous and/or nitrogen in the water.

Elevated nutrient levels, known as eutrophication, have caused a serious impact on the rivers health, leading to algal blooms, excessive weed growth and a reduction in fish populations. These extra nutrients can encourage toxic blue-green algal blooms and increase aquatic plant growth in our waterways. As these plants and algae grow and die, they begin to rot. The bacteria that eat the rotting plant material use dissolved oxygen in the water. If too many plants and algae grow and die, the bacteria eating the dead plant material use all the available dissolved oxygen. This leaves not enough oxygen for aquatic animals such as fish and macroinvertebrates, causing them to die.

Discharged from sewage plants and sources of agricultural runoff and urban storm water add solids and nutrients to the river, increasing the risk of eutrophication and overall degrading the water system.

Phosphates for a fresh water river, such as the Hawkesbury-Nepean River, be less than 0.02 milligrams per litre (mg/L) to protect aquatic ecosystems (ANZECC, 2000). Nitrates or nitrates should be less than 0.04 milligrams per litre (mg/L) to protect aquatic ecosystems (ANZECC Guidelines, 2000).