Pesticides are chemicals that are used in agriculture and residential settings to control a pest. A pest can be an insect, weed, bacteria, fungus, rodent, fish or any other troublesome organism. Pesticides are commonly applied on farms, fruit orchards, golf courses, and residential lawns and gardens. Rain falling on a treated area before the pesticide binds or degrades may carry the pesticide in runoff to surface water sources. Pesticides that bind to soil particles can also move to surface water through soil erosion. Rain can infiltrate with pesticides into the soil and move them to underground water supplies. Pesticides are sometimes applied directly to lakes or wetlands for control of aquatic pests, but are advised not to be used near drinking water systems. When a pesticide is spilled, dumped, or misused, it may reach drinking water at an accelerated rate. Properties of some pesticides may make them last longer in water.
How pesticides affect human health depends on how much pesticide is present, the length and frequency of exposure, and the toxicity of the pesticide. Effects also depend on the health of a person when exposure occurs. The levels of pesticides found in drinking water are usually quite low and unlikely to cause harmful health effects. In the case of a pesticide spill or misapplication near a well, the levels of pesticides in drinking water may be high enough to cause immediate health problems. To prevent or reduce the chances of health effects from drinking water contamination, Maximum Contaminant Levels (MCLs) have been established by the U.S. Environmental Protection Agency (EPA) for many pesticides. MCLs are set at levels well below those known to cause harmful health effects. If your private well exceeded the MCLs for a pesticide, contact your local health department to investigate the source of the contamination and to find out if other local wells are contaminated.
Under EPA regulations, public water supplies are currently monitored for certain pesticides. If you have a private well, you can get your water tested by a state certified laboratory. Your local health department or Cooperative Extension office may help you determine if your well is at risk for contamination from pesticide use in your area. Shallow, poorly constructed wells, wells near pesticide storage or mixing facilities, and wells located on or near farms are most likely to become contaminated with pesticides. There are several different laboratory tests that look for pesticides in drinking water. Each of these tests can detect different kinds of pesticides. Since these tests can be expensive, you should only test your well for those pesticides at highest risk to be in your well.
If you have a private well with pesticide contamination, you should follow the advice of your local health department. For example, they may recommend that you use another source of water, such as bottled water, until the problem is resolved. An alternative may be to drill a deeper well. As a short-term solution, there are some drinking water treatment devices that can be installed in your home. Depending on the pesticides involved, a Granulated Activated Carbon (GAC) system or a reverse osmosis unit may lower pesticide levels to below the MCL.