|
Understanding the Fate of Pesticides after Application Several possible fates await a pesticide after it has been applied outdoors. The potential for a pesticide to leach into groundwater or run off into surface waters depends on the characteristics of the pesticide and its interaction with plants, soil, and water. The most common fates are listed below:
Absorption: Some pesticide active ingredients are absorbed, or taken up, into plant tissues. This action removes the pesticide from the environment and prevents the pesticide from becoming a water contaminant.
Adsorption is the physical binding of pesticide molecules to soil particles. The strength of the bonds depends on the interaction of the pesticide's chemical properties, its concentration in the soil water and on the composition of the soil (sand, loam, clay, organic matter). If bound to the soil, the pesticide is unlikely to leach or runoff. Some highly soluble pesticides bind strongly with soil. The more clay particles and organic matter that are in the soil, the more the pesticide is held by the soil and becomes immobile. Strongly adsorbed pesticide molecules do not leach or move unless the soil particle to which they are adsorbed moves through erosion or runoff. The longer the molecules of a pesticide are held, the more likely it is that microbiological degradation will occur, which reduces the risk of leaching.
Erosion:Heavy rains or excess irrigation may erode soil particles from the application site. If the pesticide is adsorbed to the soil particle, the pesticide is also being moved off site.
Movement in or with water: Soluble pesticides may move from the application site with water runoff and cause contamination of surface waters. Pesticides dissolved in runoff water may quickly reach streams and rivers. The water solubility of a pesticide will determine how readily the pesticide will dissolve in water. A highly soluble pesticide has a greater potential of being washed from any surface it has been applied to and will tend to readily leach into groundwater. Solubility is usually expressed as the maximum amount of pesticide that will dissolve in 1 liter (1.06 quarts) of water. Water solubility is measured in milligrams per liter (mg/l), which is equivalent to one part per million (ppm). The larger this number, the more soluble a pesticide is in water.
Degradation: As soon as the pesticide is applied, chemical compounds begin to break down or degrade into simpler compounds which are usually less toxic. Each pesticide has its own speed of degradation which depends on the active ingredient, the formulation, and environmental conditions. There are both benefits and drawbacks to a long degradation time. The longer a pesticide takes to break down, the longer it is present to control the insect, weed, or disease for which it was applied. This is called residual activity. The drawback to long residual activity, or persistence, is that the pesticide may also be available for leaching or runoff over a longer period of time.
Degradation Processes 
Photolysis: The degradation of chemicals by light is called photolysis. Photolysis occurs on the plant, soil, water, or any other surface that sunlight reaches.
Hydrolysis: Water also degrades pesticides by dividing large molecules into smaller ones, breaking them down in the process called hydrolysis. Hydrolysis of pesticides can occur on the soil surface and in the root zone. Hydrolysis may be very active in warm water at or near the soil surface. As the water temperature cools at depths below the root zone, the rate of hydrolysis slows. In groundwater deep in the soil, hydrolysis slows dramatically.
Microbiological Degradation: Microorganisms break down or degrade pesticides after application. Most microorganisms-a category which includes bacteria, viruses, fungi, algae, and protozoa-live in the upper foot of soil where they find warm temperatures, moisture, and organic matter and where they do most of their work degrading pesticides. Microorganisms are most active in soils having high organic matter. As the pesticide moves down below the root zone, microbiological degradation decreases because of less favorable living conditions for the microorganisms.
Volatilization is the conversion of a liquid chemical into a vapor which escapes into the atmosphere. A pesticide that evaporates or volatilizes soon after application has little chance of contaminating water. The faster a pesticide is lost to the atmosphere, the lower the likelihood of leaching. Pesticide vapors that drift through the air may, however, be hazardous to plants, humans, and animals. Applicators should read the label carefully to find warnings that will tell them that the pesticide is volatile. Look for a statement like this in the Environmental Hazards section of the label: "Vapors from this product may injure susceptible plants in the immediate vicinity."
 |