Questions About Pesticide Environmental Fate

 

A pesticide’s fate is described by how and where it enters the environment, how long it lasts, and where it goes. Please see Movement of Pesticides in the Environment.

How a pesticide enters the environment is the first step in determining its fate. Initial distribution is determined by the method of application, the amount, timing, frequency and placement. Weather conditions during application can also affect initial distribution. Land form (topography), vegetation type and density, soil conditions, and the proximity of water bodies also are important. Together, these factors help determine how much pesticide is distributed to the air, soil, water, plants, and animals. The fate of pesticides applied to enclosed areas, such as greenhouse space, and public and private buildings, will differ from outdoor applications.

With time, the pesticide may (1) break down (2) be redistributed within the application site or (3) move off site. Off site movement includes movement to groundwater, surface water, and the atmosphere. Off site movement also includes pesticide on a crop or livestock when it is removed from the site. Break down and movement occur simultaneously. In many cases, the two processes together determine pesticide dissipation at the point of measurement.

How long a pesticide lasts or persists in the environment is determined by its resistance to break down. All pesticides react in the environment to form new chemicals, it is the rate at which they react and products formed that are important. There are many ways that pesticides can react, but most often they react with oxygen (oxidation) or water (hydrolysis). In addition, all pesticides are subject to breakdown in the presence of sunlight. In soil and sediments, microorganisms (bacteria, fungi, etc.) are primarily responsible for pesticide breakdown. Some pesticides may enter plant roots or foliage and break down through plant metabolism. Pesticides applied directly to animals are also subject to uptake and metabolism.

The rate at which pesticides breakdown depends on their reactivity in each media (air, soil, water, plants, animals). Each pesticide has unique properties that determine reactivity. Some pesticides are sensitive to acidic and/or basic conditions (pH), others are sensitive to sunlight, microbial attack, or plant and animal metabolism.

The environmental media also determines how fast pesticides break down. In the atmosphere, most pesticides breakdown rapidly by reaction with oxygen or free radicals, catalyzed by sunlight (indirect photolysis). Some pesticides break down by directly absorbing sunlight (photolysis). Those that persist can travel long distances in the atmosphere. In water, breakdown is usually by hydrolysis, often mediated by pH. In aquatic systems, pesticide break down by microorganisms in sediments may also be important. The predominant pathway in soil is microbial degradation, although for some pesticides chemical degradation is important. Pesticides are also susceptible to photolysis on soil and foliar surfaces. Pesticides break down in plants or animals (including microorganisms) by metabolism. Metabolic reactions are catalyzed by enzymes. Environmental conditions can influence reaction rate and therefore how fast pesticides break down. For air, these conditions include temperature, moisture, sunlight intensity, and free radicals. For water, conditions include temperature, pH, sunlight intensity, and sediment microbial activity. For soil, conditions include temperature, soil type, organic matter, moisture, pH, aeration, and microbial activity. For plants and animals conditions include rates of uptake, metabolism and elimination. Metabolism may be temperature dependent.

How long a pesticide lasts in the environment is determined by a number of factors including (1) how much is introduced and how it is distributed (2) its reactivity in the environmental media, and (3) the conditions of the media. Pesticide persistence is often expressed in terms of half-life. This is the time required for one-half the original quantity to break down. Pesticides can be divided into 3 categories based on half-lives: non persistent -- less than 30 days; moderately persistent -- 30 to 100 days; and persistent -- greater than 100 days. Because half-life values can vary considerably depending on environmental conditions, they are often reported as a range for each media.

The complete breakdown of pesticides and other organic substances is called mineralization. The products of mineralization are carbon dioxide, water, and minerals containing elements which commonly include sulfur, phosphorus, nitrogen, and the halogens: chlorine, fluorine, and bromine. Pesticides usually form many break down products. These products break down to other products. There can be many steps before mineralization. Rarely is it known if and when a pesticide has mineralized. Some pesticide break down products are incorporated into soil organic matter. Those taken up by plants or animals may be used by the organism or the metabolites excreted. At some point in a pesticide’s break down the products are no longer of concern, as they are not biologically active (toxic). Usually the initial break down products are much less toxic than the pesticide, but sometimes they are of similar or greater toxicity.

Pesticides can move from their initial distribution by a number of processes. In air and water (including air and water in soil) pesticides move only short distances by diffusion. To travel longer distances pesticides move by mass transfer, usually in moving water or air. A pesticide’s tendency to move in air or water is determined by how much it is retained by the surfaces to which it was deposited. Pesticides may attach (sorb) to soil, vegetation, or other surfaces. The strength of the sorption often determines a pesticide’s availability to mass transfer.

Transfer from water, soil, or plant surfaces to air is called volatilization. Volatilization occurs when pesticide surface residues change from solid or liquid to a gas. The pesticide vapors diffuse a very short distance and then are swept away with the air current. Under ideal conditions, each pesticide has a characteristic tendency to become a gas, which is called its vapor pressure. The media also influences vapor pressure: the more tightly adsorbed or absorbed, the lower the vapor pressure. Temperature also affects vapor pressure: higher temperature results in a greater tendency to volatilize, resulting in a higher vapor pressure. Pesticides can also move into the air as particles, adsorbed onto dust, or as droplets or aerosols during application. Other pesticide properties that are important to movement are solubility in water, soil adsorption tendency, and tendency for plant uptake. In determining the tendency to move from water to air, for pesticides with similar vapor pressure, those with higher water solubility will have lower volatility.

Water solubility and adsorption to soil are important in determining a pesticide’s tendency to move through the soil profile with infiltrating water, and over the soil with runoff. Most pesticides that have low water solubility also tend to sorb strongly to soil, but there are exceptions. The more strongly a pesticide sorbs to soil, the lower the tendency to move with infiltrating water. Soil properties are also important, as each soil has a characteristic ability to adsorb pesticides. Soils high in clay and organic matter sorb pesticides better than sandy soils low in organic matter. Soil structure also is important as it determines the infiltration rate. Rapidly infiltrating water may move pesticides on the surface deeper into the soil as they have less time for sorption. Soils that weakly adsorb pesticides and have a rapid infiltration rate are more sensitive to groundwater pollution than soils that strongly adsorb pesticides and have a slow infiltration rate. Soil sorption and infiltration rate also determine pesticide loss in runoff. Soils with slow infiltration rate may be more prone to runoff, as more water will remain on the surface. Pesticides sorbed to soil will not be lost to runoff. However, if runoff results in soil erosion, pesticides sorbed to surface soil will also move with runoff. To understand the potential for pesticide movement toward groundwater or in runoff, pesticide properties, application factors, soil and site conditions must be evaluated. Rainfall, irrigation practices, and evapotranspiration will also significantly influence the potential for pesticide movement in water.

Plant uptake can be important to pesticide movement. Pesticides that are taken up by plants are not available for movement into the atmosphere, or movement into ground or surface water. However, if the plants are harvested some pesticide may move from the site with the crop.

Pesticides applied to soil, water, vegetation, or other surfaces indoors usually breakdown at a slower rate than pesticides applied outdoors. This is due primarily to the lack of sunlight indoors. This includes glass greenhouses, as the UV light necessary for pesticide breakdown is filtered out by glass. Pesticides applied indoors are not affected by wind or rain, and are less likely to move by mass transfer from the point of application. Vapor loss may also be less, as surfaces are not exposed to the heat of the sun.

Here are some very informative external links that pertain to pesticide environmental fate:

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This Page prepared by the EXTOXNET FAQ Team. , January 1998