Ecological and environmental effects
13.1 Movement of pesticides in the environment Introduction
The widespread use and disposal of pesticides by farmers, institutions, and the general public provide many possible sources of pesticides in the environment. After release into the environment, pesticides may have many different fates. Pesticides that 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 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 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 the build-up of pesticides in water, but may also 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 differ-ent 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 may also involve transportation of pes-ticides over long distances. The worldwide distribution of DDT and the presence of pesticides in bodies of water far from their primary use areas are good examples of the vast potential for 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. To under-stand 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 degra-dation 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 a long period of time. The ones that are most rapidly broken down have the shortest time to move through the environment or to produce adverse effects in people or other organisms. The ones that last the longest, the so-called “persistent pesticides,” can move over
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long distances and can build up in the environment, leading to greater potential for adverse effects.
Properties of pesticides
In addition to resistance to degradation, there are a number of other properties of pesticides that determine their behavior and fate. One is how volatile they are or, in other words, how easily they evaporate. The ones that are most volatile have the greatest potential to evaporate 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 to settle to the bottom of bodies of surface water, making it less available to organisms.
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 that vary with respect to the percentage of sand, organic matter, metal content, acidity, etc.All of these soil characteristics inﬂuence the behav-ior of a pesticide so that a pesticide that might be anticipated to contaminate ground-water in one soil may not do so in another.
Similarly, surface waters vary in their properties, such as acidity, depth, temper-ature, clarity (suspended soil particles or biological organisms), ﬂow rate, and general chemistry. These properties and others all can affect pesticide movement and fate. Everyone who is familiar with the difﬁculty of forecasting weather knows it 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. They can also provide some idea as to where the released pesticide will most likely be found at highest levels. Thus, it is possible to gather information that 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
Table 13.1 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 break down. 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
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agents that 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. The measures of soil persistence in the table only describe pesticide behavior at or near the surface.
The downward movement of nonpersistent 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 that limit their potential for movement throughout the environment. Many of the chlorinated hydrocarbon pesticides are very resistant to breakdown but are 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 accu-mulating in remote areas.
Table 13.1 Pesticide Persistence in Soils
Low persistence (half-life < 30 days)
Aldicarb Captan Dalapon Dicamba Malathion
Moderate persistence (half-life 30–100 days)
Aldrin Glyphosate Atrazine Heptachlor Carbaryl Linuron Carbofuran Parathion Diazinon Phorate Endrin Simazine Fonofos Terbacil
High Persistence (half-life > 100 days)
Bromacil Chlordane Lindane Paraquat Picloram TCA Triﬂuralin
Role of living organisms
So far, the discussion has focused on air, soil, and water. However, living organ-isms may also play a signiﬁcant role in pesticide distribution. This is particularly important for pesticides that 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 stores this pesticide, levels can reach much 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 has a speciﬁc name. It 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 that have bioaccumulated pesticides and other organic chemicals. It is not only ﬁsh but also domestic farm animals that can be accumulators of pesticides, and so care must be taken in the use of pesticides in agricultural situations.
The release of pesticides into the environment may be followed by a very com-plex series of events that can transport the pesticide through the air or water, into
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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 pesti-cides that are unnecessarily released, the safer our environment will be.
13.2 Bioaccumulation Deﬁning bioaccumulation
An important process by which chemicals can affect living organisms is through bioaccumulation. Bioaccumulation means an increase in the concentration of a chem-ical over time in a biological organism compared to the chemical’s concentration in the environment. Compounds accumulate in living things any time they are taken up and stored faster than they are broken down (metabolized) or excreted. Under-standing the dynamic process of bioaccumulation is very important in protecting human beings and other organisms from the adverse effects of chemical exposure, and it has become a critical consideration in the regulation of chemicals.
A number of terms are used in conjunction with bioaccumulation. Uptake describes the entrance of a chemical into an organism — such as by breathing, swallowing, or absorbing it through the skin — without regard to its subsequent storage, metabolism, or excretion by that organism.
Storage, a term sometimes confused with bioaccumulation, means the temporary deposit of a chemical in body tissue or in an organ. Storage is just one facet of chemical bioaccumulation. (The term also applies to other natural processes, such as the storage of fat in hibernating animals or the storage of starch in seeds.)
Bioconcentration is the speciﬁc bioaccumulation process by which the concen-tration of a chemical in an organism becomes higher than its concentration in the air or water around the organism. Although the process is the same for both natural and man-made chemicals, the term bioconcentration usually refers to chemicals foreign to the organism. For ﬁsh and other aquatic animals, bioconcentration after uptake through the gills (or sometimes the skin) is usually the most important bioaccumulation process.
Biomagniﬁcation describes a process that results in the accumulation of a chem-ical in an organism at higher levels than are found in its own food. It occurs when a chemical becomes more and more concentrated as it moves up through a food chain — the dietary linkages from single-celled plants to increasingly larger animal species.
A typical food chain includes algae eaten by a waterﬂea, eaten by a minnow, eaten by a trout, and ﬁnally consumed by an osprey (or human being). If each step results in increased bioaccumulation, that is, biomagniﬁcation, then an animal at the top of the food chain, through its regular diet, may accumulate a much greater concentration of chemical than was present in organisms lower in the food chain.
Biomagniﬁcation is illustrated by a study of DDT that showed that where soil levels were 10 parts per million (ppm), DDT reached a concentration of 141 ppm in earthworms and 444 ppm in robins. Through biomagniﬁcation, the concentration of a chemical in the animal at the top of the food chain may be high enough to cause death or adverse effects on behavior, reproduction, or disease resistance and thus
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endanger that species, even though contamination levels in the air, water, or soil are low. Fortunately, bioaccumulation does not always result in biomagniﬁcation.
The bioaccumulation process
Bioaccumulation is a normal and essential process for the growth and nurturing of organisms. All animals, including humans, daily bioaccumulate many vital nutri-ents, such as Vitamins A, D, and K, trace minerals, and essential fats and amino acids. What concerns toxicologists is the bioaccumulation of substances to levels in the body that can cause harm. Because bioaccumulation is the net result of the interaction of uptake, storage, and elimination of a chemical, these parts of the process will be examined further.
Bioaccumulation begins when a chemical passes from the environment into an organism’s cells. Uptake is a complex process that is still not fully understood. Scientists have learned that chemicals tend to move, or diffuse, passively from a place of high concentration to one of low concentration. The force or pressure for diffusion is called the chemical potential, and it works to move a chemical from outside to inside an organism.
A number of factors may increase the chemical potential of certain substances. For example, some chemicals do not mix well with water. They are called lipophilic, meaning “fat loving”, or hydrophobic, meaning “water hating.” In either case, they tend to move out of water and into the cells of an organism, where there are lipophilic microenvironments.
The same factors affecting the uptake of a chemical continue to operate inside an organism, hindering a chemical’s return to the outer environment. Some chemicals are attracted to certain sites, and by binding to proteins or dissolving in fats, they are temporarily stored. If uptake slows or is not continued, or if the chemical is not very tightly bound in the cell, the body can eventually eliminate the chemical.
One factor important in uptake and storage is water solubility, which is the ability of a chemical to dissolve in water. Usually, compounds that are highly water soluble have a low potential to bioaccumulate and do not readily enter the cells of an organism. Once inside the organism, they are easily removed unless the cells have a speciﬁc mechanism for retaining them.
Heavy metals like mercury and certain other water-soluble chemicals are the exceptions because they bind tightly to speciﬁc sites within the body. When binding occurs, even highly water-soluble chemicals can accumulate. This is illustrated by cobalt, which binds very tightly and speciﬁcally to sites in the liver and accumulates there despite its water solubility. Similar accumulation processes occur for mercury, copper, cadmium, and lead.
Many fat-loving (lipophilic) chemicals pass into organism’s cells through the fatty layer of cell membranes more easily than water-soluble chemicals. Once inside the organism, these chemicals may move through numerous membranes until they are stored in fatty tissues and begin to accumulate.
The storage of toxic chemicals in fat reserves serves to detoxify the chemical by removing it from contact with other organs. However, when fat reserves are utilized
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