ASSESSING THE IMPACT OF PESTICIDES ON NATURAL WATER SYSTEMS

S.M. Peterson and G.E. Batley
CSIRO Centre for Advanced Analytical Chemistry
Lucas Heights, NSW

INTRODUCTION

Modern Australian agriculture is highly dependent on chemical control for its economic success. The use of such chemicals has been targeted by environmentalists and the media as a significant contributor to the pollution of our inland waterways. Much of this concern has focused on the cotton industry, which relies more than most on insecticides, herbicides, conditioners and defoliants during the various stages from cultivation to harvesting, and at the same time involves a large irrigation system.

The impact of pesticides is closely monitored by environmental regulatory agencies and, in collaboration with the industry, improved use practices have minimised the extent to which they enter riverine systems. In the cotton industry, one such practice limits water release by requiring on-farm containment. At the same time there has been a move to environmentally friendly pesticides which degrade readily and have minimal impacts on non-target aquatic organisms.

Pesticides are registered for use by state agricultural agencies. Although it is commonly believed that all registered pesticides have been rigorousty studied prior to their introduction, relevant physicochemical information is frequently difficult to obtain and often laboratory-derived data have little relationship to behaviour in the field. Specialist meetings organised by the cotton industry have highlighted a need for information on the fate of commonly used pesticides in the aquatic environment including any toxic effects on Australian aquatic species.

In order to assess the impact of pesticides on the riverine environment, it is necessary to understand the routes by which pesticides are transported from the point of application to the aquatic systems. With the exception of some herbicides which are added to the soil, direct spraying of crops is the major application route. As a consequence of spraying, and particularly with aerial application, overspraying and spray drift can be significant transport paths for pesticides to water bodies. 

 The soil is the next most important source of pesticide contamination in rivers. Washoff from crops and excess runoff during foliar application will contribute pesticides to the soil. Depending on their chemical structure, these pesticides may remain firmly bound to soil particles, being slowly degraded by chemicals and/or biological processes.

Some compounds are relatively volatile and may evaporate from plant foliage and the soil surface, to be atmospherically transported and degraded by sunlight. Other compounds may be leached from the soil by rain or irrigation water, and may percolate down through the soil column to eventually enter groundwater reservoirs. This route requires that the pesticide be water soluble and the soil permeable. Irrigation flows or storm events will transport appreciable quantities of pesticides, some in dissolved forms and some bound to soil particles.

 This simple model predicts the relative impact a pesticide will have in an aquatic system (Equation 2), on the basis of a risk ranking. It is, however, limited by the quality and consistency of data employed. The predictions also assume that the risk is related to the water-soluble concentration. More researchers are now trying to determine the importance of the sediment phase on the impact of pesticides, not only to the benthic organisms within the sediment, but also to the organisms exposed to pesticide remobilised following a disturbance to the sediments.

However, despite the limitations of such an approach, it still is probably a necessary tool in determining which pesticides should be most closely monitored. In our example the high risk associated with endosulfan is based on its wide usage, but the high ranking of less-used cypermethrin, would imply the need for additional studies before its routine usage was extended.

In general there is a worldwide need to better understand the fate of pesticides associated with colloids and particulate matter. Do these represent a source of bioavailable chemicals? There is a general assumption that pesticides are less of a problem once bound to sediments. We need to know more about the equilibrium concentrations of pesticides in porewaters in sediments. Are they accumulated by sediment-dwelling biota? What is the lifetime of sediment bound pesticides? It is likely to be considerably longer than in solution.

As to the fate of pesticides in our rivers, much still needs to be determined. Laboratory work, coupled with modelling, can predict the environmental behaviour of the compounds. Field sampling is required to confirm the model's predictions. Monitoring and research in this area needs to go hand in hand. It is difficult to determine which pesticides are of high priority for research until their presence in the environment is confirmed. A coordinated approach should enable a well-managed program of pesticide use to proceed with minimal damage to the environment.

1. Neely, W.B., Branson, D.R., and Blau, G.E., Partition coefficient to measure bioconcentration potential of organic chemicals in fish. Environ. Sci. Technol. 1974, 8, 1113.
2. Peterson, S.M., and Batley, G.E., "Fate and Transport of Endosulfan and Diuron in Aquatic Ecosystems" Investigation Report CET/LH/IR013, CSIRO Division of Coal and Energy Technology, 1991, 105 pp.
3. Maguire, R.J., Carey, J.H., Hart, J.H., Tkacz, R.J., and Lee, H., Persistance and fate of deltamethrin sprayed on a pond. J. Agric. Food Chem., 1989, 37, 1153.
4. Batley, G.E., and Peterson, S.M., in "Impact of Pesticides on the Riverine Environment", Proceedings of the Workshop, Goondiwindi, 20th-21st May, 1992, Cotton Research and Development Corporation, Narrabri, 1992, p. 38.