E X T O X N E T
Extension Toxicology Network
A Pesticide Information Project of Cooperative Extension Offices of
Cornell University, Michigan State University, Oregon State University, and
University of California at Davis. Major support and funding was provided
by the USDA/Extension Service/National Agricultural Pesticide Impact
Assessment Program.
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Toxicology
Information
Brief
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Movement of Pesticides in the Environment
Publication Date: 9/93
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INTRODUCTION
The widespread use and disposal of pesticides by farmers,
institutions and the general public provide many possible sources
of pesticides in the environment. Following release into the
environment, pesticides may have many different fates.
Pesticides which are sprayed can move through the air and may
eventually end up in other parts of the environment, such as in
soil or water. Pesticides which are applied directly to the soil
may be washed off the soil into nearby bodies of surface water or
may percolate through the soil to lower soil layers and
groundwater. Pesticides which are injected into the soil may
also be subject to the latter two fates. The application of
pesticides directly to bodies of water for weed control, or
indirectly as a result of leaching from boat paint, runoff from
soil or other routes, may lead not only to build up of pesticides
in water, but also may contribute to air levels through
evaporation.
This incomplete list of possibilities suggests that the
movement of pesticides in the environment is very complex with
transfers occurring continually among different environmental
compartments. In some cases, these exchanges occur not only
between areas that are close together (such as a local pond
receiving some of the herbicide application on adjacent land) but
also may involve transportation of pesticides over long
distances. The worldwide distribution of DDT and the presence of
pesticides in bodies of water such as the Great Lakes far from
their primary use areas are good examples of the vast potential
of such movement.
While all of the above possibilities exist, this does not
mean that all pesticides travel long distances or that all
compounds are threats to groundwater. In order to understand
which ones are of most concern, it is necessary to understand how
pesticides move in the environment and what characteristics must
be considered in evaluating contamination potential. Two things
may happen to pesticides once they are released into the
environment. They may be broken down, or degraded, by the action
of sunlight, water or other chemicals, or microorganisms, such as
bacteria. This degradation process usually leads to the
formation of less harmful breakdown products but in some
instances can produce more toxic products.
The second possibility is that the pesticide will be very
resistant to degradation by any means and thus remain unchanged
in the environment for long periods of time. The ones that are
most rapidly broken down have the shortest time to move or to
have adverse effects on people or other organisms. The ones
which last the longest, the so-called persistent pesticides, can
move over long distances and can build up in the environment
leading to greater potential for adverse effects to occur.
PROPERTIES OF PESTICIDES
In addition to resistance to degradation, there are a number
of other properties of pesticides which determine their behavior
and fate. One is how volatile they are; that is, how easily they
evaporate. The ones that are most volatile have the greatest
potential to go into the atmosphere and, if persistent, to move
long distances. Another important property is solubility in
water; or how easily they dissolve in water. If a pesticide is
very soluble in water, it is more easily carried off with
rainwater, as runoff or through the soil as a potential
groundwater contaminant (leaching). In addition, the
water-soluble pesticide is more likely to stay mixed in the
surface water where it can have adverse effects on fish and other
organisms. If the pesticide is very insoluble in water, it
usually tends to stick to soil and also settle to the bottoms of
bodies of surface water, making it less available to organisms.
ENVIRONMENTAL CHARACTERISTICS
From a knowledge of these and other characteristics, it is
possible to predict in a general sense how a pesticide will
behave. Unfortunately, more precise prediction is not possible
because the environment itself is very complex. There are, for
example, huge numbers of soil types varying in the amount of
sand, organic matter, metal content, acidity, etc. All of these
soil characteristics influence the behavior of a pesticide so
that a pesticide which might be anticipated to contaminate
groundwater in one soil may not do so in another.
Similarly, surface waters vary in their properties, such as
acidity, depth, temperature, clarity (suspended soil particles or
biological organisms), flow rate, and general chemistry. These
properties and others can affect pesticide movement and fate.
Everyone is familiar with the difficulties of forecasting
weather, which is partly due to problems in predicting air flow
patterns. As a result, determination of pesticide distribution
in the atmosphere is subject to great uncertainty.
With such great complexity, scientists cannot determine
exactly what will happen to a particular pesticide once it has
entered the environment. However, they can divide pesticides
into general categories with regard to, for example, persistence
and potential for groundwater contamination and they can also
provide some idea as to where the released pesticide will most
likely be found at its highest levels. Thus, it is possible to
gather information which can help make informed decisions about
what pesticides to use in which situations and what possible
risks are being faced due to a particular use.
MOVEMENT OF PESTICIDES IN SOIL
The table below lists some of the more commonly used
pesticides with an estimate of their persistence in soil. In
this table, persistence is measured as the time it takes for half
of the initial amount of a pesticide to breakdown. Thus, if a
pesticide's half-life is 30 days, half will be left after 30
days, one-quarter after 60 days, one-eighth after 90 days and so
on. It might seem that a short half-life would mean a pesticide
would not have a chance to move far in the environment. This is
generally true; however, if it is also very soluble in water and
the conditions are right, it can move rapidly through certain
soils. As it moves away from the surface, it moves away from the
agents which are degrading it such as sunlight and bacteria. As
it gets deeper into the soil, it degrades more slowly and thus
has a chance to get into groundwater. Our measures of soil
persistence only describe pesticide behavior at or near the
surface.
The downward movement of non-persistent pesticides is not an
unlikely scenario and several pesticides with short half-lives,
such as aldicarb, have been widely found in groundwater. In
contrast, very persistent pesticides may have other properties
which limit their potential for movement throughout the
environment. Many of the chlorinated hydrocarbon pesticides are
very persistent and slow to breakdown but also very water
insoluble and tend not to move down through the soil into
groundwater. They can, however, become problems in other ways
since they remain on the surface for a long time where they may
be subject to runoff and possible evaporation. Even if they are
not very volatile, the tremendously long time that they persist
can lead, over time, to measurable concentrations moving through
the atmosphere and accumulating in remote areas.
PESTICIDE PERSISTENCE IN SOILS
Low Persistence
(half-life 30 days) | Moderate Persistence
(half-life 30-100 days) | High Persistence
(half-life >100 days) |
| Aldicarb | Aldrin | TCA |
| Captan | Atrazine | Bromacil |
| Dalapon | Carbaryl | Chlordane |
| Dicamba | Carbofuran | Lindane |
| Malathion | Diazinon | Picloram |
| Methyl Parathion | Endrin | Trifluralin |
| Oxamyl | Fonofos | Paraquat |
| 2,4-D | Glyphosate | |
| 2,4,5-T | Heptachlor | |
| Linuron | |
| Parathion | |
| Phorate | |
| Simazine | |
| Terbacil | |
ROLE OF LIVING ORGANISMS
So far, the discussion has focused on air, soil and water.
However, living organisms may also play a significant role in
pesticide distribution. This is particularly important for
pesticides which can accumulate in living creatures. An example
of accumulation is the uptake of a very water-insoluble
pesticide, such as chlordane, by a creature living in water.
Since this pesticide is stored in the organism, the pesticide
accumulates and levels increase over time. If this organism is
eaten by a higher organism which also can store this pesticide,
levels can reach higher values in the higher organism than is
present in the water in which it lives. Levels in fish, for
example, can be tens to hundreds of thousands of times greater
than ambient water levels of the same pesticide. This type of
accumulation is called bioaccumulation.
In this regard, it should be remembered that humans are at
the top of the food chain and so may be exposed to these high
levels when they eat food animals which have bioaccumulated
pesticides and other organic chemicals. It is not only fish but
also domestic farm animals which can be accumulators of
pesticides and so care must be used in the use of pesticides in
agricultural situations.
SUMMARY
The release of pesticides into the environment may be
followed by a very complex series of events which can transport
the pesticide through the air or water, into the ground or even
into living organisms. The most important route of distribution
and the extent of distribution will be different for each
pesticide. It will depend on the formulation of the pesticide
(what it is combined with) and how and when it is released.
Despite this complexity, it is possible to identify situations
that can pose concern and to try to minimize them. However,
there are significant gaps in the knowledge of pesticide movement
and fate in the environment and so it is best to minimize
unnecessary release of pesticides into the environment. The
fewer pesticides that are unnecessarily released, the safer our
environment will be.
Revised 9/93
Disclaimer: Please read
the pesticide label prior to use. The information contained at this web
site is not a substitute for a pesticide label. Trade names used herein
are for convenience only; no endorsement of products is intended, nor is
criticism of unnamed products implied. Most of this information is historical
in nature and may no longer be applicable.
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