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Threats to Sources of Drinking Water and Aquatic Ecosystem Health in Canada

3. Pesticides

Groundwater and surface water contamination by pesticides will likely intensify in the future, necessitating a better understanding of their sources, fate and effects, and degradation products.R.J. Maguire,1 P.K. Sibley,2 K.R. Solomon3 and P. Delorme4

1Environment Canada, National Water Research Institute, Burlington, ON
2University of Guelph, Department of Environmental Biology, Guelph, ON
3University of Guelph, Centre for Toxicology, Guelph, ON
4Health Canada, Pest Management Regulatory Agency, Ottawa, ON


Current Status

Natural pesticides such as certain botanicals, sulfur and arsenic have been used by humans for thousands of years. However, it was not until about twenty years after the introduction and widespread use of the synthetic chlorinated pesticides in the 1940s and 1950s that serious environmental problems began to be noticed. The publication of Rachel Carson's book Silent Spring in 1962 described the effects of some chlorinated pesticides, and arguably marked the beginning of the environmental movement.

Although pesticides form a subset of what are commonly known as toxic chemicals, they are unique in the sense that, unlike other toxic chemicals, they are deliberately applied to the natural or constructed environment. Pesticides are used extensively because of their benefits to different sectors of society and thereby to consumers, and, for example, in protecting public health. From an agricultural perspective, pesticides are intended to increase crop yields and farming efficiency, decrease loss of food during transport and storage, and ensure a stable and predictable food supply. Societal concerns, scientific advances and regulatory pressures have driven and continue to drive some of the more hazardous pesticides from the marketplace. Synthetic organic insecticides traditionally associated with broad non-target effects, with potentially hazardous residues, and with exposure risks to applicators are expected to have decreasing market share (U.S. National Academies of Science 2000). This trend has been promoted by regulatory changes that restrict the use of older chemicals and by technological changes that lead to competitive alternative products. With intensive monoculture, society will always be faced with pest control problems. We have made advances in integrated pest management in some areas, and we are developing less hazardous pesticides, biopesticides, and crops that are genetically modified to resist pests. Nevertheless, for the foreseeable future we will continue to be reliant upon chemical pesticides.

Since the environmental effects of organochlorine pesticides first became apparent, there has been an evolution in pesticide use from organochlorines to organophosphate pesticides to carbamates to pyrethroids and beyond (including natural pesticides)-- in general with decreasing persistence and increased specificity toward target pests. A consequence of this development is that, in general, there are fewer effects to non-target organisms.

Nevertheless, because of their widespread use and the continuing evolution of their chemistries, pesticides still pose a threat to water quality and aquatic ecosystems (e.g., U.K. Environment Agency 2000). One way of ensuring protection of aquatic systems and resources is through scientifically sound risk assessments, in conjunction with good risk communication. However, it is necessary to have sufficient capacity in research and monitoring in order to support the risk assessment process. In addition, in Canada environmental health risk assessment and communication in relation to pesticides needs to be better developed.

It should be noted that there are many connections between pesticide issues and those of other topic areas which are being considered (e.g., groundwater issues, endocrine effects, urban runoff, persistent organic pollutants, climate change). It is important to realize the relative risk of pesticides compared to other stressors. For example, the U.S. Environmental Protection Agency (1990) investigated the impairment of water quality in streams, and found the following declining order of importance of stressors: siltation (42%), nutrients (28%), pathogens (20%), organic enrichment (15%), metals (12%), pesticides (10%), suspended solids (7%) and salinity (4%).

There are about 550 pesticide active ingredients currently registered under the Pest Control Products Act, and the Pest Management Regulatory Agency (PMRA) registers 10 to 15 new actives each year. Perhaps 80% of pesticides used in Canada are used for agricultural purposes. The remaining 20% are used in urban areas or as material preservatives, antisapstains, heavy-duty wood preservatives, slimicides, antifouling agents, aquaculture pesticides, and so forth. PMRA has undertaken the task of reevaluating, over the next few years, about 400 pesticides that were registered before December 31, 1994. PMRA has reviewed comprehensive databases on the remaining 150 pesticides. PMRA expects to have re-evaluated most organophosphorous pesticides by the end of 2001, and has recently announced a re-evaluation of lawn and turf pesticides. Chemical classes like the carbamates and synthetic pyrethroids will follow.

In Canada we currently lack a systematic, coordinated, interjurisdictional system for monitoring pesticides in aquatic systems (both water and sediment). At present our database in this respect is poor. This lack of monitoring data diminishes our ability to identify problematic or potentially problematic chemicals, and/or to identify areas that may be threatened. In part, this lack of data is due to the lack of coordination between provincial and federal authorities. Available data indicate that there are a number of problematic current use pesticides that are persistent, mobile and potentially toxic (e.g., lindane, endosulfan and others).


The use of chemical pesticides will continue to be a key component in pest management. New chemical pesticides will continue to be registered and introduced to the market. These will use existing chemistries and will likely include novel chemistries (e.g., the sulfonylureas, imidazolinones and strobilurins in recent years).

Biocontrol techniques have been effective for imported pests and controlled environments (e.g., beetles used against purple loosestrife) and in inundative release of biocontrol agents in greenhouses, but their long-term prospects and importance are uncertain.

The future market share of genetically modified (GM) crops with insecticide-resistant genes or herbicide-tolerant genes is difficult to estimate at this time because of public concerns about GM crops, especially in Europe. It may become significant. Although the use of GM crops may result in reduced use of traditional chemical pesticides, it may also pose some unique threats (see below), and it may result in alterations in the use of traditional chemical pesticides (e.g., McHughen 2000).

Groundwater and surface water contamination by pesticides will likely continue, and perhaps intensify because of changing land use practices and the discovery of hitherto unknown groundwater contamination that often takes years to be detected. It is difficult to anticipate trends because of a lack of monitoring data.

Emerging Issues

Changing agricultural practices may change pesticide use patterns. For example, the use of GM crops with genes to produce insecticides, or GM crops with herbicide-tolerant characteristics can increase or decrease the amount and even type of pesticides used. One tool to track changes in agricultural practices is the use of geographic information systems (GIS) and remote sensing databases, but further development is needed (an example would be the overlay of patterns of crop production, pesticide use and factors such as climatic events to identify areas most at risk of pesticide runoff to water courses following extreme events). GIS databases could also be used to identify and map vulnerable groundwater.

Climate change may change cropping practices in Canada. It may allow crops to be planted in areas hitherto unsuitable, with a requirement for pest control. Climate change may also result in changes to demands for fresh water through increased use of irrigation, with the attendant risk of contamination of this water by pesticides. Alternatively, in areas receiving more precipitation there can be a need for increased use of fungicides, as has occurred in the last few years in certain parts of the Prairies. Climate change may result in the introduction of new pests to Canada, and may require the increased use of pesticides for control. The spectrum of the pests being controlled and the pesticides used may change.

The introduction of new pests to Canada is always a threat. Such introductions often require pesticide use for control (e.g., potential spraying of mosquitoes to control West Nile virus). "Emergency" spray operations may have unintended consequences. For example, there was a large kill of lobsters in Long Island Sound close to New York City attributed to pesticides used there in the West Nile virus mosquito eradication program in 1999.

There are issues with regard to pesticide use in greenhouses. Pesticide-containing effluents from greenhouses are often untreated. Sound management practices need to be established.

The threat to water quality and human health from pesticides in urban areas (i.e., through urban runoff) has received more attention recently. This is in part fueled by actions in the U.S.A. to ban or severely restrict certain pesticides (e.g., U.S. Environmental Protection Agency 1999). PMRA has developed a "Healthy Lawn Strategy" and announced the re-evaluation of eight of the most frequently used lawn and turf pesticides (chloropyrifos, diazinon, malathion, carbaryl, 2,4-D, MCPA, mecoprop and dicamba).

Knowledge and Program Needs

We need to know the state of the environment with respect to pesticide residues. This can be accomplished by targeted monitoring geared to the assessment of risks by Environment Canada, which would feed back into the regulatory process of re-evaluation or special review by PMRA.

Little is known of the toxicological significance of constant exposure of aquatic organisms to low levels of chemicals. Little is known about the more subtle sub-lethal effects of pesticides, for example, on behaviour or the immune system. We need to know the consequences of these effects at population and community levels. We need to work towards linking effects at the biochemical level to organism health, thence to population health.

Additional research is needed on the level of effects and the potential for recovery at different levels of the ecosystem following episodic exposures to pesticides.

Little is known about the potential effects of mixtures, yet pesticides are often applied as mixtures and certainly are found in the environment as mixtures. Little is known of the cumulative effects of multiple stressors in aquatic systems. There may be combinations of physical, chemical and biological stresses occurring in aquatic ecosystems (e.g., extreme rain events washing both soil and pesticides into aquatic ecosystems).

The move towards probabilistic risk assessment methods will require some research support (e.g., fate, exposure mechanisms, and exposure data), but it is difficult at this time to identify requirements. Mesocosm experiments may be important validation tools here. There is also a great need for targeted monitoring data (in water, sediment, and biota) to validate models and methods used in aquatic risk assessments.

In general, the issue of pesticides in sediments has received little attention compared to pesticides in water. Sediments may serve as both a sink and a source of pesticides, particularly those that are lipophilic (i.e., those that have high octanol-water partition coefficients). There are implications with respect to desorption and resuspension affecting water quality, effects on benthic organisms, and the possibility of bioaccumulation to pelagic organisms.

Groundwater and surface water contamination by pesticides will continue, and perhaps intensify. It is difficult to anticipate trends because of a lack of monitoring data. Various distribution and transport models exist (e.g., the Groundwater Loading Effects of Agricultural Management Systems--"GLEAMS"-- U.S. Department of Agriculture 1996), but many also require validation.

We need to determine hazards to non-target organisms of genetically expressed pesticides in crops and eventually in forestry. For example, what is the biological availability and environmental fate of genetically expressed pesticides in plant parts left on fields after harvest?

In addition to research characterizing effects, research, development and validation of appropriate models is needed to characterize inputs of pesticides (or other chemicals) into aquatic systems. Although a number of models have been developed to address the inputs resulting from traditional agricultural uses of pesticides, models to address inputs from other types of uses (e.g., antifoulants, urban and industrial stormwater discharge, aquaculture) are currently lacking.

  • It is recommended that the federal government invest in the targeted monitoring of pesticide residues in environmental media (water, sediment, biota) to determine trends, assess hazards, and when warranted, provide for regulatory review.
  • It is recommended that the federal, provincial and territorial governments cooperate in the development of a systematic and coordinated interjurisdictional system for the collection of pesticide use data and data on environmental concentrations of pesticides.
  • It is recommended that the federal government invest more in research into risk assessment methodologies used in aquatic risk assessment. Research should include fate characterization methods (e.g., modelling) and effects characterization methods. Environment Canada should develop a program dedicated to such an activity. (It should be noted that the Canadian Network of Toxicology Centres has such a group, the Risk Assessment Methodologies group, but there is a need for similar expertise in government.)
  • It is recommended that ecological risk assessment methodology for pesticides be refined, such as in the use of probabilistic methods and advanced exposure modelling. The federal government should harmonize aquatic risk assessment methodologies across Departments.
  • It is recommended that there be research to improve sustainable decision-making by regulators, i.e., research to help determine what constitutes adequate levels of protection at various levels in aquatic ecosystems.
  • It is recommended that there be research into the effectiveness and viability of risk mitigation and risk management options for protecting aquatic systems (e.g., use of vegetated filter strips, riparian zones). The creation of riparian buffer zones to control non-point source pollution has been widely applied in the U.S.A. but has only recently been given serious consideration in Canada (in Prince Edward Island). This practice involves the loss of arable acreage, for which farmers are compensated.
  • It is recommended that more research be carried out on analytical methods for environmental residues of pesticides applied at low levels (e.g., grams per hectare), and on methods of analysis for chiral pesticides. Newer techniques of molecular biology (e.g., DNA microarrays, PCR amplification, etc.) hold promise for both the analysis of pollutants such as pesticides and the determination of effects at the molecular level; more research should be invested in such areas.
  • It is recommended that more research be carried out on the fate and effects of pesticides in the aquatic environment in order to identify emerging issues.
  • It is recommended that links between environmental research scientists and risk assessment scientists (regulators) be strengthened in order to provide for the timely exchange of information and expert opinion.
  • It is recommended that the communication of risks of pesticides to human and ecological health be improved.
  • It is recommended that Canadians be encouraged and educated to use pesticides in a more sustainable fashion by more widespread adoption of integrated pest management and integrated crop production techniques. Apart from reducing use, this will have the additional benefit of avoiding the promotion of pesticide resistance.


  • McHughen, A. 2000. Pandora's picnic basket--the potential and hazards of genetically modified foods. Oxford University Press, New York, U.S.A., ISBN 0-19-850674-0.
  • U.K. Environment Agency. 2000. Environment 2000 and beyond. Rio House, Waterside Drive, Aztec West, Almondsbury, Bristol BS32 4UD, U.K.
  • U.S. Department of Agriculture. 1996. GLEAMS (Ground Water Loading Effects of Agricultural Management Systems).
  • U.S. Environmental Protection Agency. 1990. The quality of our nation's water. A summary of the 1988 National Water Quality Inventory. U.S. EPA Report 440/4-90-005, Washington, DC, U.S.A.
  • U.S. Environmental Protection Agency. 1999. Office of Pesticides Programs biennial report for FY 1998 and 1999. Prevention, Pesticides and Toxic Substances report EPA 735-R-99-002.
  • U.S. National Academies of Science. 2000. The future role of pesticides in U.S. agriculture. National Academy Press, Washington, DC, U.S.A.
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