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    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.

    FATE OF PESTICIDES IN WATER
    Within a water system, pesticides may partition between a number of compartments (Figure 1), dependent upon their chemical properties. These will include:
    1. bulk water, where they may be in solution, associated with colloids or suspended particulates, or enriched in surface films and in all cases potentially available for uptake by algae, invertebrates, fish or other biota,
    2. bottom sediments where they may be bound to sediments or dissolved in the sediment pore waters, and available to sediment-dwelling biota such as worms, or aquatic plants, or
    3. the surrounding atmosphere, into which they may volatilise if their vapour pressure is high.

    Whilst the presence of pesticides in river water is of fundamental concern, their persistence is ultimately more important. There are a number of possible degradation processes which transform the pesticides into new and potentially less toxic species. The principal route is by reaction with water or hydrolysis (Figure 1). Photolysis, reaction with light, is also possible in surface waters to the depths that light can penetrate. Biological degradation can be an important transformation pathway in sediments. These different processes usually produce different degradation products. The susceptibility of a chemical to degradation is described by its half-life, the time taken for 50% of the compound to degrade.

    Degradation is generally slower in sediments, and since the majority of pesticides have poor water solubility, and ultimately attach to particulates and end up in sediments their ultimate residence is often the bottom sediments, although resuspension can maintain a measurable concentration in the overlying water.

    BIOLOGICAL IMPACTS OF PESTICIDES
    Pesticides can affect aquatic impact at many trophic levels, from algae to fish. The effect on biota is generally reported as the concentration that results in 50% mortality of a species, LC50. Bioaccumulation may also occur producing chronic health effects.

    The ability of a pesticide to bioconcentrate can be predicted from measurements of the extent to which it distributes beween the two layers in a mixture of octanol and water. The octanol-water partition coefficient (Kow), is chosen since octanol has similar solvent properties to the lipids responsible for tissue solubility. The bioconcentration factor (BCF) based on lipid solubility can be estimated by equations such as (1):

    log BCF = 0.124 + A.524log Kow .....(1)

    Typically, pesticides having a strong tendency to bioaccumulate in tissue lipids have log Kow values between 3 and 7.

    Toxicity is assumed to result from dissolved compounds. Even within sediments, the toxicity of sediment-bound pesticides has been estimated on the basis of the equilibrium distribution of the pesticide beween the sediments and water. Within sediments the associated pore or interstitial water may be enriched in dissolved pesticides relative to the overlying water and may represent an important source to sediment-dwelling organisms. The significance of these effects is currently the subject of intensive research both in Australia and abroad.

    ENVIRONMENTAL RISK OF COMMONLY USED PESTICIDES
    Of the twelve most used pesticides in cotton farming (as at 1989) endosulfan is the only major organochlorine pesticide used. The other classes comprise organophosphates (chlorpyrifos, profenofos, parathion, sulprofos and monocrotophos), pyrethroids (deltamethrin, esfenvalerate, lambdacyhalothrin and alphacypermethrin) and carbamates (methomyl and thiodicarb).

    Endosulfan has been the most thoroughty studied pesticide of those used in cotton-growing, primarily because it is extremely toxic to fish (more so than most organophosphates and pyrethroids) and secondly because it is used in the largest quantities, so that the risk of environmental damage is greater. It is frequently detected in sampling programs.

    Endosulfan binds strongly to sediment but is only slowly released into water producing low level concentrations for many months after endosulfan was first introduced (2).

    A wide selection of organophosphate pesticides are registered for use on cotton crops. Solubilities vary from insoluble to around 800 g/L. Octanol-water partition coefficients range from 0.5 to 4.97, suggesting that organophosphates adsorb poorly to moderately to sediment and soil. The phosphate group is vulnerable to hydrolytic attack so that in general organophosphates do not persist in water. Organophosphates are reasonably toxic to fish.

    Pyrethroids are based on pyrethrin, a natural insecticide extracted from plants. New and more effective analogs have been developed over the past few years. Pyrethroids are very hydrophobic with low solubility and vapour pressure, and not surprisingly have large octanol-water partition coefficients (Kow). It is believed that material deposited onto water surfaces from aerial drift will evaporate from the surface film rather than dissolve (3). The tendency to bind to sediment, dissolved organic matter and plant material is also strong. Pyrethroids as a family of chemicals are quite toxic to aquatic organisms. They are however, used in much lower quantities than organochlorine or organophosphate pesticides.

    There are only a few carbamate pesticides registered for use on cotton in New South Wales or Queensland. Carbamates are quite water soluble and have low octanol-water partition coefficients. They are therefore expected to remain in solution and not bind to soils or sediment. They will also not be expected to bioconcentrate.

    Although there has been considerable research both in Australia and abroad which enables prediction of the fate and impacts of many of the pesticides currently used in the cotton industry, there remains a need for additional information on a number of the newer pesticides in order to satisfactorily manage their use. For example, since 1989, cyfluthrin and bifenthrin have been added to the pyrethroid list.

    It would be desirable that the provision of data on toxicity to an Australian fish or invertebrate species and halflife in natural water systems be a prerequisite for registration of any new pesticides and for that information to be made available to researchers.

    Given that research funding is limited, a predictive test to prioritise research would be useful. A common approach is to investigate relationships which link chemical structure to biological impacts, called qualitative structure-activity relationships (QSARs), and to use such relationships to indicate those pesticides that should be looked at more closely (4). The key parameters were determined to be the total amount of pesticide applied per season, the amount distributed in the water compartment (predicted from the octanol-water partition coefficient), the persistence in water (as indicated by half-lives) and the impact in the water compartment (from the toxicity to the most sensitive fish species) (Table 1).

    Risk = (mass applied X t1/2) / (LC50 X log Kow) ....(2)

     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.

    REFERENCES
    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.

    ABOUT THE AUTHORS:
    Sharon Peterson (MRACI) is an environmental chemist with the CSIRO Centre for Advanced Analytical Chemistry. Her research has focused on the sampling, analysis, fate and impacts of organics in natural water systems. Sharon has a B.App.Sc. in chemistry and a M.App.Sc. in Environmental Toxicology from the University of Technology, Sydney.

    Dr Graeme Batley (FRACI) is a Senior Principal Research Scientist and Manager of the CSIRO Centre for Advanced Analytical Chemistry. His research has been directed principally to the analytical and environmental chemistry of pollutants in waters and sediments, with particular interests in trace metals, organometals and pesticides.

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