Water & Irrigation

Irrigation using blue-green algae contaminated water

Series: Agfact AC.28 Edition: First edition Last updated: 04 Mar 2003

Author: John Gillett

Many landholders use river water and farm dams to irrigate crops. These water sources are vulnerable to blue-green algal (BGA) blooms and may become contaminated.

This Agfact is designed to assist irrigators who may, at times, wish to irrigate with BGA-contaminated water.

What are blue-green algae?

Blue-green algae, also known as cyanobacteria, are the most ancient of all photosynthetic organisms.

Very few species of blue-green algae produce toxins that are harmful to humans, stock, domestic animals and fish, but these species are common in still waters in New South Wales. The toxins can cause liver and nerve damage and may lead to death.

Blue-green algae do not generally affect the suitability of water for irrigation or plant productivity, although they can clog systems. Any produce irrigated with BGA-affected water, however, may well be scrutinised for health impacts on consumers, because of possible toxins.

Does my farm dam or river contain blue-green algae?

If you suspect a water source contains blue-green algae or if you observe any of the following signs, you should not touch or use the water in any way until the algae have been identified by an expert (in the field or in the laboratory):

BGA blooms discolour the water. Surface scums may be present, and the water can appear olive, greenish-yellow, or white. Blooms may look like paint, oil slicks, ropy lines, or tiny green flecks in the water. This appearance may change with the time of day or changing weather conditions. In some cases scums may not be visible at all.
You notice an unpleasant odour: the water smells musty or earthy.
Surface scums can be blown around by the wind, and so may be concentrated at different places at different times.
Some algae contain gas vacuoles to regulate buoyancy. This means the algae may be found at different depths during the day, depending on their light and nutrient needs.

How can BGA-contaminated water affect health?

Algal toxins can affect human and animal health when swallowed, through skin contact (such as handling irrigation equipment), or when contaminated spray mist is inhaled.

People who have high exposures to blue-green algae, such as water works staff, have a greater risk of developing skin and other health problems. Skin irritants in blue-green algae affect approximately 30% of people, and those who have asthma, eczema or hay fever are thought to be more susceptible to blue-green algae. If you have occupational exposure to blue-green algae, seek appropriate advice.

If anyone has any medical conditions, such as gastroenteritis or skin rashes, that might have been caused by body contact or from eating vegetables irrigated with BGA-contaminated water, they should seek medical advice.

How can BGA-contaminated irrigation water affect plants?

Are toxins produced by BGA taken up by plants when they are irrigated with BGA-contaminated water? We are not certain.

Through the roots: Current evidence suggests that toxins can be absorbed through the root system of plants.

On leaves and surfaces: Fruit and vegetables do not appear to take up toxins, but toxins may remain viable on the surface of plants for long periods.

Of most concern are the leafy vegetables, such as lettuces and cabbages, where water may pool on the vegetable surface and leave a dried, concentrated residue. Toxins produced by blue-green algae are heat-stable, and may take many weeks to degrade, especially in a dry state as a residue on vegetables. Research has shown that dried blue-green algae cells may remain toxic for several months.

The general guidelines for edible plants are:

Do not use a BGA-contaminated water source on edible plants if you have an alternative supply.
If you have no other supply, avoid direct overhead spray irrigation on the edible part of the plants.

Irrigating crops with BGA-contaminated water

Where there is no direct water contact with the edible parts of the plant, for example where under-tree sprinklers are used (for example, for stone fruit), or with a drip irrigation set-up where water does not touch the edible parts of the crop, irrigating with BGA-contaminated water is unlikely to be a problem.

Vegetables and fruit

It is recommended that water contaminated with BGA should not come in contact with plants being grown for food. This is particularly important with plants such as grapes, strawberries, cabbages, lettuces, tomatoes and other salad vegetables, as toxins may be concentrated on surfaces.

Consumers should note that, before use, all fruit and vegetables, particularly salad vegetables which are consumed raw, should be thoroughly washed and rinsed with fresh, clean water. Washing the fruit and vegetables in clean water will remove the blue-green algae easily, as the toxins are water-soluble. The fruit and vegetables will then be safe to consume.

Pasture

Keep stock off pasture irrigated with BGA-contaminated water for at least seven days after irrigation.

BGA toxins may remain on dry pasture for a long time following irrigation, until there is a rain or further irrigation with an uncontaminated supply.

Pasture irrigated with water containing blue-green algae is unlikely to cause any problem in livestock after seven days due to the effect of sunlight.

Turf

Algae-contaminated water does not infect turf grasses, but the algae are highly invasive and may out-compete grasses for space in wet or shaded environments.

Algal scum can cause chronic problems on greens, especially those with poor air circulation, compacted soils and wet areas. Algal growth is encouraged by extended periods of rainy, overcast and warm weather. Algal scums slow water infiltration, keep thatch wet for extended periods, and impede oxygen and other gas diffusion into and out of soils.

Cautions for spray and micro systems

Spray drift: To avoid spray drift irrigation affecting neighbouring properties and people, irrigate in the evening when the air is generally still.

Filtration: The use of an activated carbon filter on the irrigation line is recommended to remove the toxins.

The high organic load of algae-contaminated water can block irrigation equipment. The algae can clog pumps, meters, valves, pipes, filters, sprinklers and micro outlets of irrigation systems, reducing irrigation system efficiency.