Surface water quality

Indicator description

The quality of surface water influences not only the health of aquatic ecosystems, but also whether that water can safely be used for drinking, agriculture, or recreation. Surface waters are streams (lakes and rivers), estuaries and the sea. The amounts of selected substances in surface water indicates the impact of human settlements and landuse on the environment, provided any "natural" sources are taken into account.

The suite of substances that can be monitored are:

• concentration of all forms of nitrogen
• concentration of all forms of phosphorus
• concentration of total suspended solids (for streams only)
• concentration of dissolved oxygen
• faecal coliform counts
• water acidity
• water temperature
• water salinity, measured as electrical conductivity (for streams only)
• turbidity in NTUs (for streams and estuaries only)

Exceedances of relevant guidelines should be reported. Any significant trends should also be reported for each substance as these trends reflect a change in water quality, and thus a change in the condition of the environment.

Levels of dissolved oxygen, for example, indicate the stress on aquatic life, with the potential for fish to die when levels of dissolved oxygen fall below 4 mg/L. Erosion and sediment transport are indicted by levels of TSS and turbidity. Phosphorus levels are used to indicate potential algae problems in streams due to nutrient accumulation. Changes in salinity levels can indicate rising groundwater and soil salinity.

Many of these substances have significant natural sources, and these should be taken into account when assessing the data. For example, phosphorus-rich soils contribute much of the phosphorus in surface waters. Many Australian rivers have naturally high amounts of salt, making them marginally saline; and the amount of sediment in streams is also generally high, particularly during floods, resulting in high levels of turbidity and total suspended solids.

Major anthropogenic sources of these substances to Australian streams include soil erosion, agricultural fertilisers and sewage. These factors affect levels of phosphorus, nitrogen, total suspended solids and turbidity in particular. There is also some effect of sewage discharge on dissolved oxygen levels. Concentrations of other substances can also change due to these factors.