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Dynamics of Pesticides in the Environment

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The passage below is taken from Ewing, R.D. 1999. Diminishing Returns: Salmon Decline and Pesticides. Funded by the Oregon Pesticide Education Network, Biotech Research and Consulting, Inc., Corvallis, OR. 55 p. 

Pesticides include a broad class of chemical and biological agents that are purposefully introduced into the environment to kill or damage organisms, including insecticides, herbicides, and fungicides. Once applied, pesticides move into streams and rivers throughout watersheds and may pose problems far from the site of application. Movement often occurs through the medium of water, thereby exposing all aquatic organisms during this transport. Where water quality monitoring has been done, a great variety of pesticides are typically found in salmon habitat. Federal and state agencies have established few criteria or standards for the protection of aquatic life from short-term (acute) and long-term (chronic) exposure to pesticides.

Pesticides do not necessarily disappear with time. They transform into other com-pounds that may be less toxic, of equal toxicity, or of greater toxicity than the original compound. The toxicity of these breakdown products is not well understood, and in general how they affect aquatic life has not been studied. All the while, fish and other aquatic organisms must continue to cope daily with pesticides (and their breakdown products), some of which are no longer used but remain in watersheds.

Although pesticides are diluted by transport in rivers and streams, a number of mechanisms concentrate the chemicals, often to toxic levels. In a process known as bioaccumulation, pesticides absorbed into plant and animal tissues may become concentrated and reach levels many times higher than those in surrounding water.