Vetiver in Wetland Research in Australia

Paul Truong, DNR, Queensland.

A study into the use of wetland plants as bio-filters for pesticide contaminated run-off water is to be conducted as part of the BRIA (Burdekin River Irrigation Area) Wetlands Project. Pesticides applied to farmlands are essential to control weeds in crops, unfortunately the externalities that are involved in such practices dictates that the effects are wider reaching. For example, atrazine is recommended to be applied at a rate of 4-6L/ha as a pre-emergence weedicide to control broadleaf weeds. Consequently, the expected atrazine concentration rate in run-off water is 200 ug/L (Rice et al., 1997). This is considerable high as atrazine inhibits algal growth and photosynthesis at concentrations less than 1.0 ug/L (Spawn and Siefried, 1997).

Pesticide residues from weed control will primarily accumulate in sediments but other examples of adversely affected accumulators are algae, zooplankton and fish. The need for wetlands to control the amount of pesticides in run-off is essential for all aquatic life. Wetlands have demonstrated to be effective in reducing BOD, suspended solids, coliforms, nitrogen and sometimes phosphorus in controlled flow events (Raisin and Mitchell, 1996). Therefore it is not unreasonable to expect that a wetlands system will reduce pesticide concentrations in run-off from farmlands as well. The concentration of pesticides found in water bodies that enter wetlands depends on the amount of removal by volatilisation, photo-decomposition, microbial degradation and uptake by aquatic organisms. The ability to reduce the amount of pesticide in waterways by plant removal is of particular importance in this study.

The two pesticides under investigation are atrazine and diuron (both active ingredients) due to their use in sugar cane crops in not only the BRIA but also throughout Queensland and NSW. In particular, atrazine has been detected in the water and sediments of the BRIA Wetlands Project site at Clare Agricultural College, where artificial wetlands have been established since 1995.

Diuron and atrazine are formulated to control grasses and broadleaf weeds in pre-emergent plant and ratoon cane crops. Diuron is noted as being more persistent than atrazine in field situations (Hamilton, 1996) creating concerns for more than one run-off event containing detectable levels of diuron. Atrazine has a higher leaching potential than diuron therefore mobility within the soil profile is at a slower rate. This leads to surface layers containing unevenly larger amounts of diuron susceptible to run-off movement.

The purpose for this study is to investigate the effects of pesticide addition to the growth of 4 emerged macrophytes: Phragmites australis (common reed), Typha domingensis (cumbungi), Vetiveria zizanioides L. (Nash) and Schoenopliectus validus (river clubrush). This study centres on plant growth parameters, for example leaf area index and water usage, as indicators to the viability of certain plant species in tropical wetlands. Further study into the quantification of pesticide uptake into the shoots and roots of Vetiver will also be undertaken.

The factorial pot trial will simulate wetland conditions, as plants will be growing in pots surrounded by a constant free water level (1.5cm). Each pot will be subjected to a single dose of atrazine or diuron at a zero, low (20 ug/L), medium (200 ug/L) or a high (2000 ug/L) rate. The pesticide will be applied to each of the four species, once adequate establishment of plants has occurred (approximately 6 weeks). Comparison within each species to pesticide rate responses is made possible using 3 repetitions.

Desired Outcome

During the course of the experiment it is hoped that plant growth will respond sufficiently under various rates of pesticide This will allow for future research into strategically placing these plant species into wetlands to reduce the effects of pesticide contaminated run-off. Vetiver may not only be suitable for bank stabilisation but also acting as a pesticide filter in the deeper margins of the wetlands.

In a preliminary trial conducted to select suitable species for this trial, it was found that vetiver grew at least 4 times faster than other species in terms of leaf area and dry mater. The same growth rate is being recorded for the current trial where all species are planted in pots where the water level is kept constantly at 20mm above ground level. This is a very impressive growth and if found resistant to weedicide in runoff water as well vetiver would be an ideal plant for wetland application.

In conjunction with the above study an additional experiment is being conducted into atrazine degradation in wetland systems. This will centre on the use of 3 wetland plants: Iris, Shoenoplectus and Vetiver, to determine the rate of atrazine degradation by these species as apposed to a non-plant system. Many studies have shown the ability of the wetlands to remove herbicides such as atrazine (Kadlec, 1994), but very little research has been done to investigate the specific role of wetland species in the degradation process. Redox potential will also be a factor in this experiment as under anaerobic conditions, found in wetland soil environment, the rate of atrazine degradation will be slowed compared to an aerobic system.

References

• Hamilton, D. and Haydon G. (1996). Pesticides and Fertilisers in the Queensland Sugar Industry – Estimates of Usage and Likely Environmental Fate. Department of Primary Industries, Brisbane.
• Kadlec, R.H. and Hey, D.L. (1994). Constructed Wetlands for River Water Quality Improvement. Water Science and Technology, 29(4): 159-168.
• Kruger, E.L., Anhalt, J.C., Anderson, T.A. and Coats, J.R. (1997). Phytoremediation of Herbicide-Contaminated Surface Water with Aquatic Plants. In: Phytoremediation of Soil and Water Contaminants Symposium. American Chemical Society, Washington, DC.
• Raisin, G.W. and Mitchell, D.S. (1996). Diffuse Pollution and the Use of Wetlands for Ameliorating Water Quality in the Australian Context. In: Downstream Effects of Land Use. (Eds. Hunter, H.M., Eyles, A.F., Rayment, G. E.). Department of Natural Resources, Brisbane.
• Spawn, R.L. and Siefried, B.D. (1997). Effects of Alachlor on an algal community from a midwestern agricultural stream. Environmental Toxicology and Chemistry, 16: 785-793.
• Sluggett, R. (1997). Chemical Weed Control in Sugarcane. Bureau of Sugar Experiment Stations, Brisbane.