WSU Whatcom County Extension

Integrated Pest Management for Blueberries

Pesticides and Water


In most cases, pesticides can be used according to label instructions without harming ground and surface water. However, the unintentional transport of pesticides to surface and ground water does occur. It occurs through a combination of a number of different mechanisms including pesticide application technique, pesticide properties, and site characteristics. However, before discussing these specific mechanisms, it is helpful to review the environmental features related to water resources and how they function.

Description of Environmental Features


A watershed is an area of land that drains into a common body of water. A ridge or other area of elevated land, called a land divide, separates one watershed from another. Streams on one side of the land divide flow in one direction and streams on the other side flow in a different direction. As water flows overland or through soils, it recharges surface and ground water supplies. It continually cycles among the atmosphere, oceans, lakes, streams, plants, soils, and other materials at and below the Earth's surface. This movement and exchange of water among the various components of the environment is referred to as the "hydrologic cycle".

Hydrologic Cycle

Therefore, every activity that occurs on the land or in the air can affect the watershed system. As water flows through the watershed, it picks up manure, sediments, pesticides, pathogens and other contaminants and transports them to other bodies of water such as streams, rivers, ponds, estuaries and, in some cases, ground water.


In Western Washington we have a number of different types of wetlands; marshes, bogs, and swamps are a few examples. All wetlands, however, serve the same basic functions. They act as "nurseries" for juvenile fish and other aquatic life, they help control flood waters by acting as a giant sponge, they recharge ground and surface water resources, and they are important habitat for wildlife. Wetlands may also protect and improve water quality by removing and storing sediments and pollutants transported in runoff.

Ground Water

Ground water supports a number of very important functions. In addition to supplying drinking water to half of the country's population, ground water provides recharge to surface streams and sustains aquatic wetlands and terrestrial ecosystems..

Often times, people think of ground water as underground streams, rivers, or lakes. Although such bodies do occur, ground water generally exists as subsurface water filling spaces between particles of sand, soil or rock beneath the earth's surface.

If you were looking at a cross-section of the land surface, the first zone you would encounter would be the plant root zone. Generally, this zone extends into the first few feet of the surface but in some cases can extend to over 15 feet (i.e., alfalfa). In the plant root zone, a number of biological processes take place some of which may be responsible for the degradation of pesticides (Pye and Kelly). In the "zone of aeration", which is just past the plant root zone, there is some water present (vadose water) along with a considerable amount of air. At the bottom of the zone of aeration is the water table, which is also the top of the "zone of saturation". In the zone of saturation, the soil and rock are completely filled with water.

Zone of Aeration

The amount of water that a rock formation can contain is a result of its porosity (the space between the grains of soil and rock or the cracks in the rock). If the grains are of even size or randomly arranged, the spaces between them account for much of the total available space and can accommodate large volumes of water. If tightly packed, the rock will accommodate much less water.

In order for water to move through rocks, the spaces or cracks must be connected. If the connected spaces are large enough for water to move through, it is described as "permeable". Saturated permeable rock can store and provide large quantities of water. When references are made to ground water sources, the term aquifer is used to describe the saturated area. Aquifers are usually classified as either "confined" or "unconfined". A confined aquifer is separated from the water table above by a layer of relatively impermeable sediment or rock and is sealed at its base by another layer of materials having low permeability.

Confined aquifers are resupplied with new water (referred to as recharge) only at the point where the formation meets the surface or where it ends somewhere underground. In other words, confined aquifers do not receive water from overlying land surfaces. This also makes confined aquifers less vulnerable to ground water contamination.

An unconfined aquifer is one in which the water table is usually the top of the aquifer. There are three types of unconfined aquifers: 1) those that are not connected to other aquifers or surface lakes and streams; 2) those that are interconnected hydrologically with other streams, and 3) perched aquifers. Perched aquifers occur where an impermeable layer exists in the zone of aeration, creating a ground water formation above the water table. Perched aquifers produce wells and are likely sources of springs. (Agricultural Law and Policy). Depending on local geology and ground water flow characteristics, water in any given well may be recharged from the land directly adjacent to the well or from areas miles away. Shallow wells typically are recharged by water originating from adjacent land. The water for recharging the aquifers comes from rainfall, snowmelt and runoff, or it has been trapped in aquifers since geologic time. Because unconfined aquifers are generally recharged from overlying land surfaces, they are much more vulnerable to ground water contamination. Most private wells in western Washington are shallow wells, which draw water from unconfined aquifers.

Pesticides in ground water are an extremely serious problem due to the long turnover rate for ground water. Although the rate may be as short as a few months, it is more commonly years or decades before the water in an aquifer is replaced. In addition, with the exception of perched aquifers, oxygen is generally not present in ground water and the microorganisms that live in an oxygen-free environment are less effective in breaking down pesticides. (Michigan State University).

Surface Water

Water that flows over land is referred to as surface water. This includes streams, lakes, ponds, rivers and even drainage ditches. Ground water and surface water are closely linked and often interconnected. The flow of one to the other depends on the relative altitudes of the surface water and the ground water table. It has been estimated that about 30% of the flows in streams and rivers during an average year is provided by ground water discharge. (U.S. EPA; Agricultural Law and Policy Institute).

The concerns associated with contamination of surface water by pesticides are somewhat different than those associated with ground water contamination. Unlike ground water, most surface waters have a rapid turnover rate, and contain free oxygen and microorganisms; all of which can enhance the rate at which pesticides are broken down.

Transport of pesticides to surface water is a concern with regard to the effect it may have on wildlife. Both aquatic organisms and land-based organisms depend on streams, creeks, ponds and even ditches for habitat and food. The degree of toxicity presented by a pesticide is variable depending on the organism affected. For example, a pesticide with a low mammalian toxicity may be extremely toxic to fish.


Transport of Pesticides into Water

Water flow is an important transport mechanism for pesticides. When water is added to the soil through precipitation or irrigation, the portion that doesn't evaporate may either infiltrate into the soil or runs off the soil surface. The fraction of water that infiltrates compared to the fraction that runs off depends largely on the intensity of precipitation and the infiltration capacity of the soil.

Water that infiltrates into the soil is either stored within the soil profile or percolates downward toward ground water, depending on the soil water conditions. When soil conditions are dry, the added water will increase soil water storage. If the moisture-holding capacity of the soil is exceeded, the excess water percolates downward through the soil to ground water. Pesticides present on vegetation or soils may be transported along with the water depending on the properties of the pesticide and the composition of the soil.

Pesticides applied to land may be transported from the application site to surface water by a number of different mechanisms including: 1) in solution with surface runoff and in association with sediment in surface runoff (adsorption); 2) volatilization into the atmosphere followed by deposition into surface water; 3) deposition into surface water through drift from aerial and ground spraying; 4) in association with inaccurate application rates; 5) movement through soil (leaching), and; 6) improper handling, storage and disposal of pesticides followed by deposition to ground or surface water.

1. Surface Runoff and Adsorption

Surface runoff occurs when water is applied to the soil at a faster rate than it can enter the soil. Runoff water can carry pesticides in the water itself or by adsorption to eroding soil particles. The extent to which runoff occurs depends on several factors including: 1) the slope or grade of an area (topography); 2) the texture and moisture content of the soil; 3) the percent organic matter in the soil; and 4) the amount and timing of rainfall. Runoff containing pesticides can cause direct injury to nontarget species, harm aquatic organisms in streams and ponds, and can lead to ground water contamination. The presence of vegetation or crop residue tends to slow the movement of runoff water thereby reducing the amount of pesticides which may enter surface water.

2. Volatilization

Volatilization occurs when a solid or liquid changes into a gas. When this change of state takes place the possibility of vapor drift occurs which may result in airborne chemical vapors being transported by air currents from a treated area to other locations, where rainfall can deposit them on land surfaces, lakes, streams, and vegetation. This occurrence has been confirmed in a study conducted by the USGS. Rainwater sampled from states in the upper Midwest and Northeast resulted in the detection of low-levels of triazine and acetanilide herbicides.

3. Aerial Drift

Aerial and ground application of pesticides may also transport pesticides into water through pesticide drift, which occurs when air-borne pesticides move beyond the intended target. Factors that may contribute to pesticide drift include weather conditions and equipment configuration and operation.

4. Application Techniques

Transport of pesticides to water may occur as a result of inaccurate application rates. In a 1979 study conducted by the University of Nebraska, applicators missed the intended application rate by over 5% with 40% of the error resulting from under application and 60% from over application. Reasons most often identified were mistakes in calibration calculations, unknown or inaccurately marked tank volumes, worn nozzles, or inaccurate pressure gauges. A study of 184 Missouri farmers found half of them using questionable calibration techniques and half of them "eyeballing" nozzle spacing and mounting height. (Jackson, et al). In other cases, errors resulted from poorly maintained or outdated equipment, especially pumps and sprayer nozzles. Depending on the properties of the pesticide and soil conditions, over application of pesticides may result in leaching to ground water or runoff to surface water.

5. Leaching

Leaching is the movement of pesticides through the soil. Pesticide leaching partially depends on the chemical and physical properties of the pesticide. For example, adsorptivity, which is the ability of a pesticide to bind with soil particles, influences the leaching potential of pesticides. A pesticide that binds tightly to soil particles is less likely to leach than one that does not. Another property of pesticides that influences leaching is the solubility of the pesticide. A pesticide that dissolves in water can move with water through the soil.

Soil properties are an equally important consideration when looking at the leaching potential of pesticides to ground water. Soil factors that influence leaching include soil texture, amount of organic matter, and permeability. For example, a sandy soil which is much more permeable than a clay soil and which has less organic matter has a much greater leaching potential.

6. Improper Storage, Handling, and Disposal

In some respects, improper pesticide handling, storage, and disposal represent a greater threat to ground water than field application because these activities can result in high concentrations of pesticides in small areas. Studies conducted in Iowa have shown that in commercial loading and handling areas and in areas where equipment is rinsed, pesticide concentrations in pools and soils are in the range of formulation concentrations (Jackson, et al). The risk to ground and surface water is increased as a result and, in combination with site characteristics, may result in contamination of a water resource.

Improperly rinsed pesticide containers contain pesticide residues. Therefore, when containers are improperly disposed of they present a potential source for pesticide contamination of water resources for the same reasons as those presented when pesticides are improperly stored and handled.

Source: Excerpt from Puget Sound Pest Management Guidelines: A Guide for Protecting Our Water Quality, WSU Extension, Whatcom County, March 1993.


Other Web Resources

Cornell University - Pesticides and Their Behavior in Soil and Water

Cornell University - Groudwater: What it is and how to protect it.

Oregon State University Extension; How soil properties affect groundwater vulnerability to pesticide contamination. Properties and Groundwater Contamination.pdf

Oregon State University Extension: Site assessment for groundwater vulnerability to pesticide contamination.

Oregon State University Extension: Understanding pesticide persistence and mobility for groundwater and surface water protection.

Washington State Department of Agriculture: Buffers Imposed by the U.S. District Court Order

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WSU Whatcom County Extension • 1000 N. Forest St., Bellingham, WA 98225 • (360) 778-5800 •