Pesticides and the Environment

By Mongi Zekri

Editor’s note: This article grants one continuing education unit (CEU) in the Core category toward the renewal of a Florida Department of Agriculture and Consumer Services restricted-use pesticide license when the accompanying test is submitted and approved.

The fate processes for pesticides fall into three major types: adsorption, transfer and degradation.

PESTICIDE ADSORPTION

The adsorption process binds pesticides to soil particles, similar to paper clips sticking to a magnet. Adsorption often occurs because of the attraction between a chemical and soil particles. Positively charged pesticide molecules, for example, are attracted to and can bind to negatively charged clay particles.

Many soil factors influence pesticide adsorption. Soils high in organic matter or clay are more adsorptive than coarse, sandy soils, in part because a clay or organic soil has more particle surface area, or more sites onto which pesticides can bind.

Pesticides vary in their adsorption to soil particles. Some pesticides such as paraquat and glyphosate bind very tightly. Others bind only weakly and are readily released back into the soil solution.

One problem resulting from pesticide adsorption is reduced pest control. For example, weeds may not be controlled if a herbicide is held tightly to soil particles and cannot be taken up by the roots of the target weeds.

PESTICIDE TRANSFER

Five ways that pesticides can be transferred are through volatilization, runoff, leaching, absorption and crop removal.

Volatilization

The conversion of a solid or liquid into a gas is volatilization. Once volatilized, a pesticide can move in air currents away from the treated surface. The higher the vapor pressure, the more volatile the pesticide.

To reduce pesticide volatilization, avoid applying volatile pesticides when conditions are unfavorable, such as on very hot, dry days or when soils are wet.

Runoff

Movement of water over a sloping surface is runoff. Runoff occurs when water is applied faster than it can enter the soil. Pesticides can be carried in the water itself or bound to eroding soil particles.

The severity of pesticide runoff depends on the slope or grade of an area. Over-irrigation can lead to excess surface water. It also can lead to pesticide runoff, especially when an irrigation system is used to apply a pesticide. Vegetation or crop residue tends to slow the movement of runoff water.

Leaching

The movement of pesticides through the soil rather than over the surface is leaching. Leaching depends, in part, on the pesticide’s chemical and physical properties. For example, a pesticide held strongly to soil particles by adsorption is less likely to leach. Another factor is solubility. A pesticide that dissolves in water can move with water in the soil.

Soil factors that influence leaching include texture and organic matter, in part because of their effect on pesticide adsorption. Soil permeability (how readily water moves through the soil) is also important. The more permeable a soil, the greater potential for pesticide leaching. A sandy soil is much more permeable than a clay soil.

A certain amount of pesticide leaching may be essential for control of a target pest. Too much leaching, however, can lead to reduced pest control, injury of nontarget species and groundwater contamination.

Absorption

The movement of pesticides into plants and animals is referred to as absorption or uptake. Absorption of pesticides by target and nontarget organisms is influenced by environmental conditions and by the chemical and physical properties of the pesticide and the soil. Once absorbed by plants, pesticides may be broken down or they may remain in the plant until tissue decay or harvest.

Crop Removal

Crop removal transfers pesticides and their breakdown products from the treatment site. Most harvested food commodities are subjected to washing and processing procedures that remove or degrade much of the remaining pesticide residue. While we typically associate harvesting with food and feed products, it is easy to forget that pesticides potentially can be transferred during such operations as tree and shrub pruning and turfgrass mowing.

PESTICIDE DEGRADATION

Pesticide degradation, or the breakdown of pesticides, usually is beneficial. Pesticide-destroying reactions change most pesticide residues in the environment to nontoxic or harmless compounds. However, degradation is detrimental when a pesticide is destroyed before the target pest has been controlled. Three types of pesticide degradation are microbial, chemical and photodegradation.

Microbial Degradation

The breakdown of pesticides by fungi, bacteria and other microorganisms that use pesticides as a food source is known as microbial degradation. Most microbial degradation of pesticides occurs in the soil. Soil conditions such as moisture, temperature, aeration, pH and the amount of organic matter affect the rate of microbial degradation because of their direct influence on microbial growth and activity.

The frequency of pesticide application is also a factor that can influence microbial degradation. Rapid microbial degradation is more likely when the same pesticide is used repeatedly in a field. Repeated applications can actually stimulate the buildup of organisms that are effective in degrading the chemical. The possibility of very rapid pesticide breakdown is reduced by using pesticides only when necessary and by avoiding repeated applications of the same chemical. Alternating between different classes, groups or formulations of pesticides can minimize the potential for microbial degradation problems as well as pest resistance.

Chemical Degradation

The breakdown of pesticides by processes that do not involve living organisms is referred to as chemical degradation. Temperature, moisture, pH and adsorption, in addition to the chemical and physical properties of the pesticide, determine which chemical reactions take place and how quickly they occur.

One of the most common pesticide degradation reactions is hydrolysis, a breakdown process in which the pesticide reacts with water. Many organophosphate and carbamate insecticides are particularly susceptible to hydrolysis under alkaline conditions. Some are actually broken down within a matter of hours when mixed with alkaline water.

Product labels may warn against mixing a pesticide with certain fertilizers, other pesticides or water with specific characteristics. Following these precautions can help prevent pesticide degradation and potential incompatibility problems. In some situations, buffers or other additives may be available to modify spray mix conditions and prevent or reduce degradation.

Photodegradation

The breakdown of pesticides by light, particularly sunlight, is called photodegradation. Photodegradation can destroy pesticides on foliage, on the surface of the soil and even in the air.

Factors that influence pesticide photodegradation include the intensity of the sunlight, properties of the application site, the application method and the properties of the pesticide. Pesticide losses from photodegradation can be reduced by adding the pesticide to the soil during or immediately after application.

PESTICIDES AND WATER QUALITY

Groundwater System

Groundwater lies below the soil surface and fills the pore spaces in and around rock, gravel sand and other materials. Contrary to popular belief, groundwater does not move through vast underground rivers and lakes, but through water-saturated zones called aquifers. The upper level of an aquifer is called the water table. The water table level fluctuates throughout the year, lowering as water is removed from wells or discharged at streams and springs. The water table rises through recharge from rain and melting snow that seeps through soil into the aquifer.

For years, it was believed that the natural filtering of water during its slow movement through the soil, sand, gravel and rock formations was adequate to cleanse it of contaminants before it reached groundwater. Today, many chemicals, including some pesticides, have been detected in groundwater. Studies have shown that recharge can carry pollutants down to aquifers. Furthermore, it seems clear that human activities can lead to contamination of the recharge water.

Surface Water System

Surface water is water stored or flowing at the earth’s surface. It includes natural bodies of water such as rivers, lakes and wetlands, as well as constructed (artificial) water reservoirs such as canals, man-made lakes and drainage ditches. The quantity and quality of surface water is important for many activities: consumption, recreation, transportation, waste assimilation, agricultural production and industrial use.

Surface water is linked to both groundwater and atmospheric water through the hydrologic cycle. Surface water moves into groundwater by infiltrating the soil and percolating downward. It also enters the atmosphere through evaporation and transpiration. Likewise, water from the atmosphere and groundwater can recharge surface waters. Atmospheric water falls as precipitation: rain, sleet, hail and snow. Groundwater that moves to the earth’s surface contributes to the base flow of streams, lakes, wetlands and other waterways.

Precipitation initially infiltrates the top layers of the soil. Continuing precipitation may saturate the upper few inches of the soil, temporarily exceeding its capacity to hold water. Water accumulates on the land surface and moves to lower elevations through surface runoff and may occur across a small or large area.

A surface water system is characterized by its watershed or drainage basin. A watershed is the area of land draining to a specific river; the boundary is defined by the region’s topography. Watersheds vary in size and can be nested within other larger watersheds. Land use within a watershed largely determines the quality of the local surface water.

PROTECTING WATER SOURCES

It is difficult to clean water once it has become contaminated. Treatment is complicated, time-consuming, expensive and often not feasible. The best solution to water contamination is to prevent it in the first place. The following pest management and pesticide handling practices can reduce the potential for contamination.

Practice Integrated Pest Management

Pesticide application should be timed carefully and combined with other pest management practices. Pests should be identified accurately. Pesticide applications should be made only when necessary, using the least amount needed for adequate pest control. Minimizing pesticide use cuts expenses and reduces potential for environmental problems.

Dispose of Waste Carefully

Follow all label instructions and restrictions when disposing of pesticides. Triple rinse or pressure rinse containers as soon as they are emptied and pour the rinsate into the spray tank.

Never dispose of pesticides or pesticide containers near a water source, over shallow water tables, in sinkholes or in abandoned wells.

Excess pesticide concentrates can be given to another qualified user, safely stored until there is a hazardous waste collection day or disposed of through a hazardous waste transporter.

Prevent Spills

If a spill does occur, it should be contained and cleaned up immediately. Repeated pesticide spills in the same area can exceed the capacity of the soil to adsorb or degrade the chemical and can increase the likelihood of groundwater contamination.

Leave Buffer Zones Around Sensitive Areas

When mixing, applying, storing or disposing (including cleanup) of pesticides, be aware of sensitive areas. These include springs, streams, ponds, wetlands and other surface waters; wells and groundwater recharge areas; and sinkholes. Establishing vegetation and leaving an untreated border are two ways to provide a buffer zone between sensitive areas and pesticide-use or handling sites.

Checklist for Protecting Water From Pesticides

• Store pesticides in their original containers in a cool, well-ventilated building with a concrete floor.
• Clean your pesticide application equipment in a way that makes it easy to collect rinsates.
• Install a check-valve on your water hose to prevent back-siphoning.
• Grade the area around your well to divert surface runoff.
• Ensure that any abandoned well near a pesticide handling or application site is properly closed.
• Build dikes around your bulk tanks to prevent off-site movement of pesticides.
• Know which pesticides you use have a potential for leaching.
• Delay pesticide applications if rain is forecast.
• Always check pesticide labels to learn irrigation practices, rates and application methods.
• Leave a border of untreated vegetation between treated and sensitive areas.
• Use pesticides only when necessary and then at the lowest rate needed to control a pest.

Acknowledgment: This publication is adapted from Applying Pesticides Correctly, A Study Guide for the General Certifications Standards Exam, By Fred Fishel, University of Florida Institute of Food and Agricultural Sciences (UF/IFAS) Extension.

Mongi Zekri is a UF/IFAS multi-county citrus Extension agent in LaBelle.

To request a hard copy of the article and test, or if you have questions regarding this article, test or CEUs, contact the author at maz@ufl.edu or 863-674-4092. Please allow two weeks to process your CEU request.

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