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 Publications You Drinking Water: Pesticides

Your Drinking Water: Pesticides

Mary Lou Dixon
Former Extension Clothing & Textiles Specialist  
Tony Tyson
Extension Engineer  

Drinking water supplied by municipal water systems is monitored for many contaminants. As authorized by the 1974 Safe Drinking Water Act and its amendments, the United States Environmental Protection Agency (EPA) has established the concentration of certain drinking water contaminants allowed in public water supplies. However, if your water comes from a private well or a system that serves fewer than 25 people or has fewer than 15 service connections, it is not covered by these standards. The safety of the water from these sources is your responsibility.

Drinking water standards are expressed as Maximum Contaminant Level Goal and Maximum Contaminant Level. Both are written as milligrams per liter (mg/l).

The Maximum Contaminant Level Goal (MCLG) is the concentration of a contaminant that experts believe a person can drink safely over a lifetime. It is based entirely on health considerations and is set at a level where no adverse health effects should occur. These calculations are based on available toxicological information and include a safety margin. The MCLG is not enforced by EPA but is used to set enforceable drinking water standards.

The Maximum Contaminant Level (MCL) is the level enforced by EPA. It is the highest allowable concentration of a contaminant in drinking water supplied by municipal water systems. Regulators consider the health effects, the feasibility and combined cost of analyzing water for a contaminant and for treating water to remove the contaminant. Therefore, the MCL is often less stringent than the MCLG.

Standards for drinking water contaminants do not guarantee that water with a contaminant level below the standard is risk-free. Neither do they mean that water with a higher level is unsafe. The standards also do not take into account the possible presence of other chemicals that may increase or decrease the toxicity of the contaminant.

With contaminants such as pesticides in drinking water, health officials are almost always concerned about chronic effects, such as cancer or damage to the central nervous system. Chronic effects result from exposure to a substance over a period of weeks or years. Acute effects are usually seen within a short time after exposure to the toxic substance.

Reducing Risk

The chemical characteristics of pesticides (insecticides, miticides, nematicides, herbicides, plant growth regulators, fungicides, bactericides and many more) determine the resulting health effects. Prior to 1940, most pesticides were inorganic compounds of arsenic, mercury, copper or lead. These compounds seldom are used today. These compounds change very slowly in the soil and do not actually break down. They seldom leach from the soil and, therefore, rarely appear in groundwater.

Synthetic organic compounds are chemicals synthesized from carbon and other elements such as hydrogen, nitrogen or chlorine. They do not occur naturally but are manufactured to meet hundreds of uses in our daily lives. Pesticides are only a small portion of these synthetic organic compounds. Others include hair sprays, solvents and many cleaning compounds. They have only recently been discovered in groundwater. One reason for this is their greatly increased use over the past 40 years. A second reason is that laboratory equipment to detect these chemicals has been greatly improved in recent years.

The synthetic organic pesticides introduced during World War II were thought to be safer and more effective than inorganic compounds. The chlorinated hydrocarbons (DDT, aldrin, dieldrin, chlordane, heptachlor, lindane, endrin and toxaphene) have low solubility in water and a strong tendency to attach to soil particles. These pesticides are insecticides and have rarely contaminated groundwater. Originally, they were thought to be safe to the environment but later were discovered to accumulate in the environment and build up to toxic concentrations in the food chain. The use of many of these pesticides has been restricted, suspended or canceled.

Organophosphorus compounds such as malathion, diazinon and parathion are one group of pesticides replacing chlorinated hydrocarbons. Although some are highly toxic to humans, they generally break down rapidly in the environment and are rarely found in groundwater.

Carbamates are a fourth group of pesticides. They include aldicarb, carbofuran and oxamyl. These compounds tend to be soluble in water and weakly absorbed to soil. Thus, if they are not degraded in the upper soil layer, they have a tendency to migrate t groundwater. The most significant occurrences of groundwater contamination have been with carbamate pesticides.

There are many other groups of pesticides too numerous to describe in detail. These include synthetic pyrethrins (Pydrin), benzoic acids (Dicamba), thiocarbamates (Eptam), phenoxies (2,4-D, Silvex, MCPP), dinitroanalines (Balan), substituted aromatics (Daconil), triazoles (Bayleton), organotins (Super-Tins), triazianes (atrazine), pathalic acid (Dacthal) and many others. The potential for any of these or other groups of pesticides to contaminate our groundwater is dependent on the chemical properties of the compound and its movement through soil and/or water.

The Clean Water Act (CWA) and the Safe Drinking Water Act (SDWA) are the basic laws protecting our water supply. They are supported by other legislation. The Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA) regulates specific pesticides based on cost and benefits.

Under FIFRA, pesticides are registered for specific uses. Restricted use pesticides are those that are classified as potentially harmful to man or the environment even when used in compliance with the label. Pesticide applicators who use restricted use pesticides must be certified by taking required training or passing an examination administered by the Georgia Department of Agriculture. Pesticide labels and accompanying information give precautions on use to ensure safety to non-target organisms, man and water. It is important that all label precautions be observed.

Pesticides are applied to crops by aerial spraying, topsoil application (granular, dust or liquid formulations), soil injection, soil incorporation or through irrigation. Soil injection and incorporation pose the greatest likelihood for groundwater contamination. The application of pesticides through irrigation (chemigation) can also cause groundwater contamination. An irrigation pump may fail while the pesticide-metering equipment continues to operate. This malfunction can cause a backflow of pesticides into the water source -- well, pond or stream -- or cause highly concentrated pesticide levels to be applied to a field. There are state laws requiring backflow prevention devices on any irrigation system equipped to inject chemicals. These devices are also required by Federal pesticide labels.

Chemicals that like water (hydrophilic chemicals) are water soluble. Water solubility means the pesticide is inclined to dissolve in water. The higher a pesticide's water solubility, the greater the amount of pesticide that can be carried in solution to groundwater. Soil with high organic matter content and silt and clay soils have slow water movement. Coarse, sandy soils with low organic content allow more rapid movement of surface water downward.

As soil water percolates downward, the distance it has to travel can determine whether or not a chemical will reach groundwater. Obviously, the greater the depth to groundwater, the more soil particles the water must flow through, making contamination less likely. If you have a choice, use the pesticide less likely to leach.

Some general guidelines for the protection of groundwater include:

  • Avoid mixing, handling or storing pesticides near a well. Fill spray tanks from a water line at least 100 feet from a well or fill a nurse tank and mix pesticides in the field. Prevent back-siphoning from the spray tank.
  • Triple-rinse or pressure rinse empty containers and dispose of properly.
  • Spray any leftover spray mix or rinsate on a labeled crop or site or store for later use.
  • When injecting chemicals through an irrigation system (chemigation), use anti-backflow equipment as required by the chemical label or state law.
  • Use pesticides in irrigation systems only when the label permits such use. Be sure to follow any label precautions and restrictions. This is an exception to the general rule that you can apply a pesticide by any method not prohibited by the label as long as you use the correct ratio on the labeled crop or site.
  • When applying agricultural chemicals on sandy soil with low organic matter content or to a field where the groundwater is near the soil surface, be aware that a product with higher water solubility, longer persistence and low soil absorption has a greater probability of leaching into groundwater. Use the lowest effective rate recommended for this soil type. Use Integrated Pest Management practices whenever possible.
  • When mixing or applying agricultural chemicals near sinkholes or areas draining directly into rivers or streams, remember that surface runoff or spills can wash directly into the sinkhole or the stream. Leave an adequate untreated barrier immediately surrounding the sinkhole or drainage area. Do not dispose of chemical products or waste materials in drainage ditches, near sinkholes or near streams. Consult the label for container disposal directions.
Most of today's pesticides break down relatively quickly when used according to the label and under the correct environmental conditions. They continue to break down in groundwater but, because of the lack of light, heat and oxygen in the water-saturated layers below the earth's surface, chemical breakdown is generally much slower than in the surface layers of the soil. The relatively constant temperature of groundwater throughout the year probably allows a constant if slow rate of breakdown. Breakdown is slowest in deeper levels of groundwater, where oxygen and bacteria are either totally absent or present only at very low levels.

Groundwater does not remain stationary; it moves vertically and horizontally in response to gravity and hydraulic pressure. Groundwater flow rate is frequently only several feet per year, although, in permeable sand and gravel aquifers, groundwater can move one or two feet per day. Even at this fast rate, groundwater and substances dissolved in it might take several years to move only one mile. Because the movement of groundwater is slow and difficult to predict, substances that enter the groundwater in one location can unexpectedly turn up years later in different locations. However, groundwater flow in limestone solution channels can be extremely rapid as can the flow in crystalline rock fractures. Both of these types of flow occur in Georgia.

Pesticide Current July 30, 1992 Current July 30, 1992
Alachlor 0 0 ---- 0.002
Aldicarb * 0.0090 0.001 ---- 0.003
Aldicarb sulfoxide * ---- 0.001 ---- 0.003
Aldicarb sulfone * ---- 0.002 ---- 0.003
Arsenic 0.05 0.05 0.05 0.05
Atrazine ---- 0.003 ---- 0.003
Carbofuran 0.036 0.04 ---- 0.04
Chlordane 0 0 ---- 0.002
2,4-D 0.07 0.07 0.1 0.007
1,2-D: bromo-3-chloropropane (DBCP) 0 0 ---- 0.0002
1,2 Dichloropropane 0.006 0 ---- 0.005
Endrine ---- ---- 0.0002 0.0002
Ethylene dibromide (EDB) 0 0 ---- 0.00005
Heptachlor 0 0 ---- 0.0004
Heptachlor Epoxide ---- 0 ---- 0.0002
Lindane 0.0002 0.0002 0.004 0.0002
Methaoxychlor 0.34 0.04 0.10 0.04
Pentachlorophenol 0.22 0.22 ---- 0.001
Toxaphene 0 0 0.005 0.003
2,4,5-TP (Silvex) 0.052 0.05 0.01 0.05
*Proposed Rule.

How to Check Your Water for Pesticides

Pesticide contaminated drinking water usually has no detectable odor or taste. Tests done by a certified water laboratory are the only reliable method to detect their presence. The lab will supply the container and instructions for taking the sample. Follow the instructions precisely and supply all information on the forms as well as additional information on why you suspect pesticide contamination. This should include past and current agriculture land use, past and current industry in the area, and household and lawn pesticide application and any pesticide spillage or potential backflow occurrences. Pesticide testing is expensive. Knowing which pesticide(s) to look for can reduce the amount of testing required.

Based on the above information, the laboratory conducts a scan to look for a group of related pesticides to determine if further testing is needed. Final test results will give you the specific contaminant and the level of contamination. Compare this level to the MCL for that pesticide. If it is above the MCL, treatment of drinking water or the use of an alternate water source is recommended.

If Your Water Contains Pesticides

Once a water supply becomes contaminated with a pesticide, it is very difficult and costly to treat. One solution is an alternate water source: either connecting to a public water system or boring or drilling a new well. Bottled water is a short-term solution to meet the needs of drinking water and food preparation. It may be practical to treat household drinking water, but it would be very costly to treat the large volumes of water necessary for general household use and livestock consumption.

Activated charcoal and reverse osmosis are the two treatments most often used for the removal of pesticides. However, they do not remove all pesticides, and treatment should be based on specific recommendations for the pesticide reported in your water.

Your county Extension office can help you secure a pesticide water test by providing a list of certified laboratories. They can also secure advice on treatment. Other resources are the Drinking Water Program, Environmental Division, Georgia Department of Natural Resources; and the Entomology and Pesticide Office of the Georgia Department of Agriculture.

EPA publishes health advisory summaries for pesticides and updates them as new information becomes available. You can get a copy of a specific advisory and answers to your specific questions by contacting EPA's toll-free Safe Drinking Water Hotline, Monday through Friday, 8:30 a.m. to 4:30 p.m. E.S.T. at 1-800-426-4791. Additional information on the health effects of pesticides is available from the National Pesticide Telecommunications Network, toll free, 24 hours a day, at 1-800-858-7378.


Code of the Federal Register. Ch. 1 (7-1-90 Edition) 141.11, 141.12.

Deer, Howard M. Groundwater Quality: Groundwater and Pesticides. EC 425.2, Utah State University, Cooperative Extension Service. 1988.

Federal Register. Vol. 56, No. 20, Wednesday, January 30, 1991. Rules and Regulations.

Federal Register. Vol. 56, No. 20, Wednesday, January 30, 1991. Proposed Rules.

Jackson, Gary, & Bruce Webendofter. How Drinking Water Standards Are Established. G338. University of Wisconsin - Extension, Cooperative Extension Service. 1985.

Stewart, Judith C., Ann T. Lemley, Sharon T. Hogan, & Richard A. Weismiller. Drinking Water Standards. Water Quality Fact Sheet 1. Cooperative Extension System, Cornell University and the University of Maryland. Revised, 1988-89.

Stewart, Judith C., Ann T. Lemley, Sharon T. Hogan, & Richard A. Weismiller. Health Effects of Drinking Water Contaminants. Water Quality Fact Sheet 2. Cooperative Extension System, Cornell University and the University of Maryland. Revised, 1988-89.

Trautmann, Nancy M., Keith S. Porter, & Robert H. Wagenet. Pesticides: Health Effects in Drinking Water. Groundwater Pesticides Fact Sheet, page 400.13. Cornell Cooperative Extension, Cornell University. 1988.

Webendorfer, Bruce, & Gary Jackson. Drinking Water Contamination: Understanding the Risks. G339. University of Wisconsin - Extension. Cooperative Extension Service. 1989.

Will, Loren. Groundwater Contamination Fact Sheet: Human and Animal Health Considerations. Cooperative Extension Service, Iowa State University, Ames, Iowa. 1987.

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Issued in furtherance of Cooperative Extension work, Acts of May 8 and June 30, 1914, The University of Georgia College of Agricultural and Environmental Sciences and the U.S. Department of Agriculture cooperating.

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