Pesticide Transport and Fate
Monitoring in the Environment
Methods for Measuring Human Exposure to the Agent
Strategies for Preventing and Controlling Pesticides
Harmful Effects of Alachlor
Dose Response of Alachlor
Absorption, Distribution and Metabolism
Sites of Toxicity
Biomarkers of Disease and Molecular Mechanisms of Action
Pesticide Transport and Fate
When pesticides are applied to places such as homes, offices, lawns, gardens, fields, and water sources they become mobile in the environment. Several factors affect how a pesticide will move in the environment once it is introduced. Some of the factors that affect pesticide movement can be found under the following headings accompanied by explanations:
How much Pesticide is Applied
Pesticides vary in the amount and rate at which they should be applied. Most pesticides come with recommendations about how much can be safely used and at what intervals. These recommendations are based on experiments that take into account many factors. However, the mentality that more pesticide will take care of the problem faster, better, or for good often causes the use of pesticides beyond recommended quantities. Excess pesticides that do not reach their target organism are free to move in the environment in other ways.
How and Where Pesticide is Applied
Pesticides are applied in many different ways that depend upon the medium (liquid, solid, gas) of the pesticide, the area over which the pesticide will be applied, and what device is used to apply it. How a pesticide is applied will affect how free it will be to move in the environment. For instance, a liquid pesticide applied from a large sprayer over a farm field will have different considerations, in terms of transport and fate, than solid pellets of pesticide applied around the foundation of a house.
When Pesticide is Applied
The time at which a pesticide is applied does affect its possible routes of transport in the environment. Most of the considerations for timing are climatic or temporal. This means that they are based upon conditions such as weather or season. For example, a liquid pesticide sprayed from an aerosol can will fall to the ground or move with the air. The amount that moves with the air will depend upon wind conditions. Seasons can affect pesticide transport, as well. Some of the factors that are seasonal are temperature (both inside and outside), humidity, rain, and snowmelt.
The Nature of the Pesticide
Pesticides vary in the way that they are structured. This is what allows them to target certain organisms such as a particular weed or insect. Variance in chemical structure also helps define how a pesticide will move in the environment. Some pesticides are soluble in water, which means that they can move wherever water moves. Some pesticides volatilize easily, which means that they can change from a liquid to a gas and move more easily with the air. Other factors of transport and fate of a pesticide to consider when looking at the chemical structure are based on how they will degrade or change form in the environment and how long it takes that change to occur. Some pesticides lose their potency when they are degraded and become harmless both to their target organisms and the rest of the environment. Other pesticides may degrade into chemicals that are more toxic then the original chemical. The daughter or degradation product(s) may then be toxic to organisms other then the one it was intended for. Pesticides are also degraded at different rates in the environment depending upon their chemical structure. For example, soil organisms might degrade a pesticide within days, whereas another pesticide might take hundreds to thousands of years to degrade. Degradation or transformation of a pesticide results in a change of structure and will change how it moves in the environment. Transformation may occur in any medium that a pesticide is in.
Pesticide Mobility Occurs in Soil, Water, Air, and Biological Systems
It is important to remember that pesticides have the potential to move in many environmental mediums and that their movement is three-dimensional. Three dimensionality of pesticide transport can be envisioned if you imagine what happens when you drop liquid food coloring into water. The food coloring diffuses through the liquid moving out in a roughly spherical shape from its center of origin. The nature of the pesticide and what kind of medium it is transported in will determine where it will move, where it will collect, how fast this will occur, and how long it will stay in the environment. Pesticides can build up anywhere that soil, water, air, and the tissues of organisms such as birds, fish, and humans can transport them.
Pesticide Transport in Air
Pesticides, regardless of the medium that they are applied in, all have the potential to be transported by air. Airborne pesticides can move very long distances and can occur in several ways. They can be carried in the wind during application. They can be carried on small particulates such as soil or on larger objects like leaves that are caught up by wind, and they can volatilize off of any surface that they are applied to. Deposition is what occurs when the wind carrying a pesticide slows down enough that its velocity can no longer hold it in the air, and it falls on whatever is beneath it. This is called dry deposition. Very small particulates or molecules of pesticide may remain in the atmosphere even when the air is relatively still. These pesticides may be removed from the atmosphere when it rains and the droplets catch them on their way down. This process is called wet deposition.
Pesticide Transport in Water
Water transport of pesticides can occur through wet deposition, run-off from surfaces, infiltration of water through the ground, ditches, storm sewers, tile lines, drains, rivers, and open water currents. Water can behave much like air in terms of transporting pesticides. Water that is moving at a high velocity can carry heavier pesticides, or particles that pesticides may be attached to, then water that is moving slowly. Faster moving water also has the potential to move pesticides farther. Pesticides in open water systems may float on the water, diffuse into the water, or deposit onto the sediments at the bottom of the water body. Pesticides that move from the ground surface through the soil may reach shallow ground water or deeper aquifers.
Pesticide Transport in Soils
Once a pesticide is in the soil it will most likely follow one of three pathways. It will move through the soil with water, attach to soil particles, or be metabolized by organisms in the soil. Soil texture (percent sand, silt, and clay) and structure plays a large role in the transport processes of pesticides. Soils that are very sandy will allow water to move through them quickly, do not attach easily to pesticides, and generally do not contain a large population of soil organisms relative to other soil types. Soils that are high in clays and organic matter will slow the movement of water, attach easily to many pesticides, and generally have a higher diversity and population of soil organisms that can metabolize the pesticide.
Pesticide Transport in Organisms
There are pesticides that can accumulate in the tissues of organisms. This process is called bioaccumulation. Generally, bioaccumulation results in higher concentrations of a chemical as one goes further up the food chain. Pesticides that bioaccumulate in organisms are often very persistant in the environment. They do not break down easily and retain their form even when ingested and stored in the body. Most pesticides that have these characteristics have been banned from use in the United States of America, though such pesticides that were used historically are still present in the environment.
Evangelou, V.P. 1998. Environmental Soil and Water Chemistry: Principles and Applications. John Wiley & Sons, Inc. New York.
Himel, Chester M., Harry Loats, and George Bailey. 1990. Pesticide Sources to the Soil and Principles of Spray Physics. In Pesticides in the Soil Environment. Ed. H.H. Cheng. SSSA Book Series:2. Madison, Wisconsin. p. 7-14.
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