Environmental fate provides an indication of what happens to a pesticide once it enters the environment, as well as likely exposure levels for non-target organisms. Evaluation of the data makes it possible to determine the behaviour of a pesticide in soil, water and air, the potential for its uptake by plants or animals, and the potential for bioaccumulation in organisms.
Data on the proposed use-pattern are used in determining the fate and expected environmental concentrations of a pesticide in various compartments of the environment. These data include details on:
- rates and methods of application;
- number of applications per season;
- times of application;
- target pests;
- target crops/resources; and
- geographic area.
Evaluation of the fate and behaviour of microbial or other biological control agents is based on a study of the identity, biology, persistence, multiplication and dispersion of the biological agents. Information on fate may not be required if toxicity tests indicate minimal environmental concern. Similarly, with pheromones and other semiochemicals, environmental fate data are required only when tests indicate toxicity to non-target organisms.
The determination of the fate of a chemical pesticide takes into account the following aspects:
Physical and chemical properties
Physical and chemical properties of a pesticide are obtained from laboratory studies and are essential for the prediction of environmental behaviour. These properties include water solubility, which is useful in predicting pesticide mobility in soil and deposition into sediments in an aquatic environment. Vapour pressure is a key indicator of the potential of a compound to volatilize. The octanol/water partitioning coefficient (Kow) indicates the likelihood of a pesticide transfer from soil or water to organisms, as well as the pesticide's potential to bioaccumulate. If the Kow indicates a potential to bioaccumulate, a laboratory bioaccumulation study would be required for confirmation purposes.
Pesticides in the environment may undergo transformations or degradation due to reactions caused by light, chemical reactions, biological reactions, or a combination of these phenomena. Therefore, it is important to review data on phototransformation (reactions caused by light), hydrolysis (chemical reactions) and biotransformation (biological reactions) in order to determine their significance in the transformation of the pesticide in different environmental media. In addition to providing answers on transformation pathways, biotransformation studies are designed to provide information on persistence under both aerobic and anaerobic conditions. These studies aid in the design of field studies, which are necessary to confirm laboratory-based predictions.
Mobility studies provide information concerning the ability of terrestrial-use pesticides and their major transformation products to move through soils, and their potential to contaminate aquatic environments by leaching into groundwater or moving in surface runoff or eroding soil. Investigation of mobility and leaching potential includes laboratory studies on a range of soil types typical of the major areas of proposed use. The potential for leaching is further examined during terrestrial field-dissipation studies.
Field studies are needed to demonstrate fate in the Canadian environment and to substantiate information from laboratory studies on persistence and mobility. Outdoor field studies are carried out under representative soil or aquatic conditions.
Currently, Canadian terrestrial field studies are mandatory and take into consideration the cooler climate, precipitation patterns, and soil types of Canada. Field studies conducted at appropriate sites in the northern United States under similar climatic conditions and with the major types of soil found in proposed Canadian-use regions are acceptable in lieu of some Canadian studies. A map is being developed under the North American Free Trade Agreement Technical Working Group on Pesticides that will identify similar zones in both Canada and the United States where field dissipation studies can be conducted to support the registration of a pesticide in both countries.
Aquatic field studies are required where there is a high potential for the parent compound or the major transformation product to enter aquatic systems and to directly or indirectly affect non-target aquatic organisms. Water and sediment samples are taken at regular intervals for residue analysis.
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