Pesticides and Water Pollution


The term "pesticide" is a composite term that includes all chemicals that are used to kill or control pests. In agriculture, this includes herbicides (weeds), insecticides (insects), fungicides (fungi), nematocides (nematodes), and rodenticides (vertebrate poisons). 

Pesticide Facts:

  • Residential gardeners reportedly use up to 10 times more toxic chemicals per acre than farmers.
  • Some pesticides are persistent in the environment: they break down slowly. The longer the time period between pesticide application and the next rain, the less residue washes away to your local creek.
  • Insecticides have adverse effects on humans, fish, birds, and beneficial insects (at varying concentrations).
The Pesticide Cycle;

The Pesticide Cycle;

Factors Effecting Pesticide Pollution of Water

Drainage: Farmland is often well drained and natural drainage is often enhanced by land drains. Water from excessive rainfall and irrigation cannot always be held within the soil structure. Therefore, pesticides and residues (also nitrates and phosphates) can be quickly transported to contaminate ground water and fresh water supplies over a large geographical area.

The pesticide: Individual pesticides have unique properties, and many variable factors determine the specific risk in terms of water pollution

  • Active ingredient in the pesticide formulation
  • Contaminants that exist as impurities in the active ingredient
  • Additives that are mixed with the active ingredient (wetting agents, diluents or solvents, extenders, adhesives, buffers, preservatives and emulsifiers)
  • Degradate that is formed during chemical, microbial or photochemical degradation of the active ingredient
  • Pesticide half-life: The more stable the pesticide, the longer it takes to break down. This can be measured in terms of its half life, the longer it takes to break down, the higher its persistence. The half life is unique to individual products but variable depending on specific environmental and application factors.
  • Mobility in soil: All pesticides have unique mobility properties, both vertically and horizontally through the soil structure. Residual herbicides applied directly to the soil are designed to bond to the soil structure.
  • Solubility in water: Many pesticides are soluble in water out of necessity so that they can be applied with water and be absorbed by the target. The higher the solubility of the pesticide, the higher the risk of leaching. Residual herbicides are generally of lower solubility to aid soil binding but their persistency in the soil can cause other problems.

Microbial activity: Pesticides in the soil are primarily broken down by microbial activity. The greater the microbial activity, the faster the degradation. Loss of pesticide residues can also occur by evaporation and photodecomposition.

Soil temperature: Soil microbial activity and pesticide breakdown is largely linked to soil temperature.

Treatment surface: Pesticides such as residual herbicides applied to hard surfaces such as concrete or tarmac (i.e. garden pathways and driveways) have nothing to be absorbed by and are particularly vulnerable to movement into water courses and non target areas, especially after rainfall. These risks are greatly reduced when the pesticide is applied to soil. Hence, pesticides in water can often be the result of non agricultural usage.

Application rate: The more pesticide that is applied, the longer significant concentrations remain.

Another factor effecting pesticide pollution of water is rainfall as high levels of rainfall increase the risk of pesticides contaminating water. Movement into water courses occurs directly by washing from pest and target areas into drains after rainfall. It can also occur within the soil structure by displacement of pesticide from absorption sites by water and on treated soil which has moved to water through soil erosion.

Pesticide Monitoring in Surface Water

Monitoring data for pesticides are generally poor in much of the world and especially in developing countries. Key pesticides are included in the monitoring schedule of most western countries, however the cost of analysis and the necessity to sample at critical times of the year (linked to periods of pesticide use) often preclude development of an extensive data set. Many developing countries have difficulty carrying out organic chemical analysis due to problems of inadequate facilities, impure reagents, and financial constraints. New techniques using immunoassay procedures for presence/absence of specific pesticides may reduce costs and increase reliability. Another problem is that analytical detection levels in routine monitoring for certain pesticides may be too high to determine presence/absence for protection of human health. ND (not detectible) values, therefore, are not evidence that the chemical is not present in concentrations that may be injurious to aquatic life and to human health. That this analytical problem existed in the United States suggests that the problem of producing water quality data that can be used for human health protection from pesticides in developing countries must be extremely serious. Additionally, detection limits are only one of many analytical problems faced by environmental chemists when analyzing for organic contaminants. Pesticide monitoring requires highly flexible field and laboratory programs that can respond to periods of pesticide application, which can sample the most appropriate medium (water, sediment, biota), are able to apply detection levels that have meaning for human health and ecosystem protection, and which can discriminate between those pesticides which appear as artifacts of historical use versus those that are in current use. For pesticides that are highly soluble in water, monitoring must be closely linked to periods of pesticide use.

The Effects of Pesticides

The health effects of pesticides depend on the type of pesticide. Some, such as the organophosphates and carbamates, affect the nervous system. Others may irritate the skin or eyes. Some pesticides may be carcinogens (they may cause cancer). Others may affect the hormone or endocrine system in the body.

Regulatory agencies have long agreed on the presence of and the effects of pesticides in drinking water sources. However, understanding the potential effects of chemical mixtures on humans and the environment is one of the most complex problems facing scientists and regulatory agencies.

Although guidelines and detailed procedures for evaluating potential effects from exposure to chemical mixtures have been provided by USEPA (USEPA, 1986, 2000b) and other agencies (ATSDR, 2004b), implementation has been difficult because of the complexity of mixtures that occur in the environment and the inadequacy of data on the toxicity of the mixtures. Most toxicological testing is performed on single chemicals-usually at high exposure levels-whereas most human and ecological exposures are to chemical mixtures at relatively low doses.

Humans can be exposed to mixtures of pesticides and their degradates that occur in streams and ground water if such water is used as a source of drinking water and if treatment does not eliminate the pesticide compounds. Aquatic organisms are exposed to mixtures that occur in streams. Pesticide mixtures may be derived from common sources (such as point sources) or from multiple nonpoint sources, and may include several different types of pesticide compounds with different mechanisms of toxicity. The present approaches taken by USEPA and other agencies for regulating and assessing pesticide mixtures provide an indication of present knowledge and information gaps.

Evaluation and management of potential risks to humans of pesticide mixtures that may occur in drinking water are primarily addressed at the Federal level by USEPA and the Agency for Toxic Substances and Disease Registry (ATSDR). Much of the attention to potential effects of chemical mixtures on human health has been associated with risk assessments required for hazardous waste sites as part of implementing the Comprehensive Environmental Recovery, Compensation, and Liability Act (CERCLA), but specific assessment of pesticide mixtures is also now occurring to meet requirements of the Food Quality Protection Act (FQPA) of 1996. The USEPA also studies regional exposures from residential and drinking-water sources due to the considerable variation in potential exposures across the country.

To date, USEPA has determined that within each of four different chemical classes (organophosphates, N-methyl carbamates, triazines, and chloroacetanilides), several specific pesticide compounds have a common mechanism of toxicity and require cumulative risk assessments to better define the potential effects of exposure of humans to multiple pesticides within each class. The potential effects of chemical mixtures on aquatic life have not received as much attention as for human health, although USEPA's Office of Research and Development, National Center for Environmental Assessment, has completed ecological risk-assessment guidelines that support the cumulative risk-assessment approach (USEPA, 2003f).

Potential effects of pesticide mixtures on aquatic life also may be considered as part of assessments for National Pollutant Discharge Elimination System (NPDES) permits or hazardous waste sites. Procedures developed by USEPA for conducting assessments for NPDES permits involve a battery of tests, referred to as "whole effluent toxicity" (WET) tests, for both effluents and receiving waters.

Prenatal exposure to harmful chemicals poses an array of other dangers as well, a study in the May 2004 Environmental Health Perspectives found. The investigators tested several common lawn and garden chemicals—including groundwater contaminants 2,4-dichlorophenoxyacetic acid (2,4-D), atrazine and dicamba—for their ability to harm mouse embryos during a period corresponding to the first five to seven days after human conception. These three chemicals, along with nine other common compounds, caused increased cell death among the embryos.

Atrazine, chlorpyrifos and turbufos reduced the odds that an embryo would progress to the next stage of development, the blastocyst. The experiments were conducted using concentrations that an average person might be exposed to during chemical application or from ingesting contaminated groundwater. Atrazine, in particular, may have affected as many as 3,600 drinking water systems throughout the U.S., mainly in the Midwest. "Pesticide-induced injury can occur at a very early period of embryo development and at pesticide concentrations assumed to be without adverse health consequences for humans," wrote the investigators.

Who regulates pesticides and their uses?

EPA is the primary pesticide regulatory agency. Under the Federal Insecticide, Fungicide and Rodenticide Act (FIFRA), all products that contain pesticides must be registered with EPA before they can be lawfully sold or distributed. EPA registration means that pesticide registrants have submitted required scientific research data concerning the risks associated with the use of the pesticide, that EPA has reviewed the data and that EPA finds the data acceptable. In fact, it is illegal for EPA to grant registration to any pesticide product until the Agency is satisfied, by scientific data, that the product can be used safely.

Before EPA approves the use of a pesticide product, it must make a finding, based on sufficient scientific data, that the product can be used safely according to the proposed use instructions. Any new information that comes to light subsequently on possible or actual adverse effects of the product, whether to health or the environment, must be reported promptly to EPA. Furthermore, EPA must reevaluate the safety finding each time a new use is approved or a change is made to the use instructions for the product.

A major consideration in approving pesticides for use is whether they pose an unreasonable risk to humans. EPA assesses risks associated with individual pesticide active ingredients, as well as with groups of pesticides that have a common toxic effect. This latter assessment is called cumulative risk assessment and is designed to evaluate the risk associated with exposure at one time to multiple pesticides that act the same way in the body.

What Can be Done?

The solution is to minimize the use of pesticides and herbicides and use alternatives if possible and to make sure that all directions and warnings (including those regarding timing of pesticide application to avoid potential runoff due to rainfall or irrigation) are read before use. Mix only as much as you use. Leave a one-foot buffer zone between the edge of sidewalks and where you apply lawn or garden chemicals. This will help prevent sprinklers or rain from washing these chemicals into the gutter and storm drain. Use a drop spreader for applying granular pesticides near impervious surfaces or bodies of water. If a granular product is accidentally applied to or spilled on pavement, sweep the product into the grass or place back into the spreader or container. Allow accidental applications of liquid to dry; do not wash them off the pavement with water. Liquid pesticide spills must be removed as much as possible by soaking excess product with absorbent material such as calcined clay or absorbent pads. Use pesticides only when you have a significant problem and seriously consider using less toxic alternatives. Only use chemicals after the rainy season is over, and never when rain is forecast because the toxins found in pesticides and herbicides can runoff lawns and gardens into storm drains and streams whenever it rains. Properly dispose of all unused pesticides.

Pesticides and herbicides contain toxic materials that pose both environmental and human health risks. Humans, animals, aquatic organisms, and plants can be severely threatened by these chemicals. However, with an aggressive march toward the protection of source waters from pesticide and chemical mixtures, as well as improving technology to better treat suspect waters, there is hope that the flow of pesticides into humans via drinking water can be brought to a tiny trickle for future generations.

The Safe Drinking Water Foundation has educational programs that can supplement the information found in this fact sheet. Operation Water Drop looks at the chemical contaminants that are found in water; it is designed for a science class. Operation Water Flow looks at how water is used, where it comes from and how much it costs; it has lessons that are designed for Social Studies, Math, Biology, Chemistry and Science classes. Operation Water Spirit presents a
First Nations perspective of water and the surrounding issues; it is designed for Native Studies or Social Studies classes. Operation Water Health looks at common health issues surrounding drinking water in Canada and around the world and is designed for a Health, Science and Social Studies collaboration. Operation Water Pollution focuses on how water pollution occurs and how it is cleaned up and has been designed for a Science and Social Studies collaboration. To access more information on these and other educational activities, as well as additional fact sheets, visit the Safe Drinking Water Foundation website at


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The City of San Jose Environmental Services Department. Pesticide Alternatives.

Grounds Maintenance. 2008. Questions & Answers About Pesticides.

Natural Resources Management and Environment Department. 1996. Chapter 4: Pesticides as water pollutants.

Stier, John. 2008. Grounds Maintenance: In the Running.

The University of Reading ECIFM. Pesticides.

U.S. Environmental Protection Agency. Pesticides.