What is pollution?

The changes caused by pollution can affect people's safety and how they use the environment. A river, estuary or harbour becomes polluted when a substance, which affects the water quality, plants or animals, enters the waterway and alters its natural functions.

Pollution can reduce the economic and social values of waterways by destroying natural ecosystems, such as killing fisheries, spoiling the scenery and restricting recreation (for example if the water is no longer safe or pleasant for boating or swimming).

Why are waterways prone to pollution?

For more about this see section 4.2

Rivers and estuaries have always been a focus for human settlement and recreation. As well as sometimes supplying drinking water and food, the waters are used for ports, for transport and for recreation. The banks have residential, tourist and industrial development and roads. In the past, waterways were often used as convenient dumping grounds for sewage, rubbish and industrial wastes. Fertile river floodplains are used for intensive agriculture, including market gardens and vineyards. The wider catchment often has a range of land uses including agriculture, mining and forestry.

All these activities can pollute waterways. Waste discharges, accidental spills, urban and agricultural run off, and groundwater flow carry a lot of different pollutants. Some of the pollutants flow to the sea, where they are dispersed. However, estuaries are semi-enclosed waterbodies and some of the pollution stays in the water or in the bottom sediments.

How badly a waterway is affected depends on the type and amount of pollution and also on the characteristics of the waterway itself, especially how well the estuary is flushed by the tides.

How do pollutants enter waterways?

Pollution may be cumulative or direct. An example of cumulative pollution is the damage caused by excessive nutrients. Pollution from nutrients or pesticides entering waterways usually occurs over many years. These substances build up in the water and the sediments. Only when they reach a particular level do they affect the waterway and its plants and animals. In the past, people have not known this was happening until it was too late and there was physical evidence such as algal blooms or fish deaths.

Pollutants may enter water directly. For example, pollutants might get into the waterway in industrial waste discharges or wastes from boats, or in water flowing into the estuary in creeks, rivers, drains or groundwater from the surrounding catchment.

Water from right across the catchment eventually finds its way into the estuary. This means that pollutants from all land uses in the catchment can also end up in the estuary.

Where does pollution come from?

Pollutants enter a river system from a lot of different land uses across its catchment. Often, poor water quality is the result of the combined effects of different activities across the catchment. Some pollution comes from sources which can be pin-pointed (such as a factory or a piggery discharging its wastes into a drain which ends up in the river). These are called `point sources'. However, many of the pollutants which enter estuaries come from a wide area (for example, fertilisers used throughout a farming area, or on a town's parks and gardens can be washed into drains or seep through into the groundwater, and eventually end up in the river or estuary). These `non-point' (also called `diffuse') sources are harder to control.

Pollutants such as nutrients, waste material, toxins and pathogens come from septic tank seepage, sewerage overflows and sewage treatment works; run off from stock yards, abattoirs and vehicle service centres; leachate from landfill sites; drainage and seepage from industrial chemical spills, industrial food-processing plants, sugar mills, dairy factories, market and domestic gardens, dredging activities, sand and gravel extraction, gravel-washing plants, land reclamation, canal estates, waterfront developments, marinas, and illegal dumping of rubbish and other material.

What pollutants can harm waterways?

Pollutants can be divided into four types: nutrients, toxins, pathogens and physical pollutants.

Nutrients

Nutrients are needed by plants and animals for growth. People get their nutrients from the food they eat. Algae in the estuary take their nutrients from the water. When excessive amounts of nutrients, especially nitrogen and phosphorus, enter the estuary it is called `eutrophication' (from the ancient Greek meaning `well-fed'). The nutrients fertilise the growth of algae in the water just like fertiliser on a lawn. Some algae will grow in vast amounts (called a `bloom') and upset the delicate natural balance of plants and animals in the estuary. Large stores of nutrients can build up in the sediment of the estuary floor, forming a nutrient `bank'. Under certain conditions these can be released for use by algae.

Toxins

For more about these see section 6.7

Toxins are substances which are poisonous to living organisms. Chemicals such as pesticides and petroleum products (petrol, oil, diesel) can cause illness or the death of plants and animals. Some toxins, including heavy metals (such as lead) and some chemicals (such as pesticides) build up in the sediments. They can be taken up by plants or animals over a period of time without causing obvious ill-effects. Symptoms such as lesions (skin damage) on fish, thinning of birds egg shells, or birth defects, appear as the level of the toxin builds up. Some animals, like mussels, store toxins in their tissues and can become poisonous to people or other animals that eat them.

Some toxins are of living, rather than chemical, origin. These `biotoxins' can be produced by microscopic plants (such as blue-green algae, diatoms or dino-flagellates) which may occur naturally or be introduced into a waterway from somewhere else. Filter-feeding animals can take up the tiny plants and become poisonous, so care should be taken about eating shellfish or mussels from an estuary where there is a toxic `bloom'.

Oil is a particularly unpleasant pollutant which, as well as being toxic, irritates animals eyes and skin, clogs the gills of fish and smothers plants and small animals.

Pathogens

Pathogens are microscopic organisms (bacteria and viruses) which cause disease in plants or animals. The presence of some bacteria, such as Escherichia coli, in an estuary usually indicates pollution by sewage. It is rare for pathogens to be present in Western Australian estuaries in levels high enough to cause illness in people.

Physical pollution

Physical pollution includes rubbish, litter, industrial discharges, and sediment (soil particles) from dredging activities or erosion. Rubbish, especially plastic and fishing line, can kill birds, dolphins and fish which accidentally eat it or become entangled. Sediments in the water can block fish gills, smother bottom-living plants and animals, reduce the clarity of the water so that birds such as cormorants cannot see their underwater prey, and restrict the light available for the growth of seagrass beds.