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7 September 1998 |
Ref 98/209 |
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Alarmed at the pollution of their rivers, oceans and lakes, Australian communities are increasingly demanding the disposal of sewage effluent on land - but this can cause equally serious environmental problems, a leading CSIRO soil and water scientist has warned.
"The use of effluent to irrigate agricultural or forest crops is a popular alternative to discharging it into rivers or the sea - but it has to be very carefully managed," says Dr Warren Bond.
Research by CSIRO Land & Water has shown there are situations on land where it may be wiser to let effluent go down river or out through a properly-designed ocean outfall.
Writing in CSIRO's Australian Journal of Soil Research, Dr Bond said that there are three main risks with the use of sewage effluent to irrigate crops: salinity, leaching of excess nitrate into groundwater and the collapse of soil structure.
The good news is that with careful planning and management, using guidelines developed by CSIRO, these problems can largely be avoided.
The first problem is salt. Besides nitrogen and phosphorus, effluent often contains high levels of salt, and this has to be managed very carefully to avoid a saline buildup in the soil that will ultimately poison plants or crops on the surface.
"It is essential that effluent irrigation be designed to allow adequate leaching to remove salts from the root zone. This means the soil must be permeable and have good drainage," he says.
Secondly, effluent contains large amounts of nitrogen and it is essential to match this to the needs of the crop or trees growing on the surface - or else the surplus will start to pollute groundwater. This can take many years to correct.
A second problem is that irrigation can mineralise soil organic matter and so release large amounts of nitrate into the groundwater. Where groundwater is used for drinking, excess nitrate is not good for human health, and has been linked with "blue baby" syndrome.
Dr Bond says these factors mean that the site on which effluent irrigation is to be used must be very carefully chosen to ensure the groundwater is not vulnerable. Groundwater must also be continually monitored for nitrogen buildup.
The third impact of effluent irrigation is caused by the addition of sodium, which is present in large quantities in most effluents in Australia. This may degrade otherwise productive land.
The effect of sodium may not be obvious while irrigation is going on, but once the soil is returned to rainfed agriculture or irrigated with fresh water, sodium can cause the collapse of the soil's structure, sealing off its pores, causing waterlogging and making it difficult for plant roots to breathe.
"The decision to use effluent for irrigation is often driven by community concerns, without a comparison of the potential environmental impacts of all the options," Dr Bond says.
"The community sees algal blooms in their river or pollution of their coastline and they demand the effluent be discharged on land.
"But the reality is the side-effects of land-disposal can be just as serious and long-lasting as putting effluent into a water body."
Dr Bond concludes that sewage effluent should only be used for irrigating crops or trees where:
- it is environmentally sustainable to do so
- it is economical (including both direct and indirect costs)
- it makes best use of scarce water
- disposal does not involve loss of water needed to maintain environmental flows in rivers.
In coastal communities where ocean disposal of effluent is deemed undesirable, use of effluent for irrigation is a practical alternative provided it meets the four criteria above, he says.
More information from:
- Dr Warren Bond, CSIRO Land & Water 02 6246 5948
019 442 516
Warren.Bond@cbr.clw.csiro.au
Margaret Bryant, CSIRO Land & Water 08 9333 6215
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