Land and Water Link

April 2003

Reaching for a Positive Chemical Equation

Both farmers and the environment stand to benefit from a CSIRO Land and Water research project that is investigating what happens to chemicals in water used on irrigated rice crops.

The project, part-funded by the Cooperative Research Centre for Sustainable Rice Production, aims to track the fate of pesticides used in rice cropping after these substances enter drainage water.

Chemical persistence in drainage water – the water that leaves the rice bay when crops are dried off before harvest, or if there is an overflow from a bay during an extreme rainfall event – is a concern for irrigation users and regulators.

According to CSIRO researcher Dr Wendy Quayle, 'We need to control the quality of surface water drainage from rice farms, and one of the ways of doing this may be through water management in rice bays and using water flows within the regional drainage network.

'Our aim is to find out how long pesticides remain in rice floodwater and drainage water, and how its flow path in regional drainage channels impacts on salt and pesticide levels in the river.'

This entails the complex interaction between water pesticide concentrations, regional dilution through run-off from other crops and high-quality escape flows, the chemical structure of different pesticide residues and ever-changing environmental conditions.

The data indicates that in most cases the concentrations of molinate and thiobencarb in rice fields are below NSW Environmental Protection Authority licensing levels after 28 days of application, while clomazone may persist for longer periods, particularly during field lock-up. Molinate concentrations tend to decrease more quickly in shallow, warmer waters although this is not necessarily the case for the other pesticides due to different chemical properties.

'The results of this research will be used to assist farmers to improve management strategies', says Dr Quayle. 'It will help rice growers in NSW's Murrumbidgee Irrigation Area assess just how and when contaminated waters can be re-used and/or drained – based on specific Australian soil and climatic conditions.'

By giving the rice industry another tool to ensure pesticide levels are below environmental guidelines limits if drainage water is released off-farm, this work will help growers quantitatively demonstrate good environmental stewardship to the wider community.

Dr Quayle explains, 'We are examining pesticide breakdown in drainage water from rice crops under different water management options. In particular, we are looking at the relation of the rate of breakdown to water depth and field top ups.

'Because our primary aim is to protect the river, we are also looking at how regional dilution between the irrigation area network and the river may influence the environmental impacts of pesticides.'

'Our goal is twofold. We want to provide farmers with more information about when it is safe to drain rice bays following applications of old and new chemical combinations. We also hope to produce management options to help remove pesticides from field water escapes more quickly.'

Further work will examine pesticide movement through newly constructed paddy banks under realistic field conditions. Risk assessment models will be utilised to determine the risk of pesticides used in the rice industry having a significant impact on the environment.

The study, which started two rice seasons ago, is now in its final stages. Field sites have encompassed the main field layouts, pesticides and water management used in the Murrumbidgee Irrigation Area in NSW and automated monitoring has provided a tightly constrained chemical and hydrological dataset.

'The project should provide greater flexibility to rice growers by offering more information about the behaviour of chemicals under Australian environmental conditions', says Dr Quayle.

'The environment should also benefit, as the rice industry will have another guide to help them make sure they don't release water with pesticide levels above the environmental guideline limits.'

For further information

Contact

Dr Wendy Quayle
Ph: 02 6960 1500