The increased commercial use of pesticides in the United States has allowed us to become one of the largest producers of food in the world, but has also raised concerns about groundwater contamination. While many of the chemical compounds found in pesticides have not been discovered in groundwater sources, several have.

Are pesticides harmful to humans?

Pesticides are hazardous to at least some animals, however they are generally not found in high enough concentrations to harm humans. Toxicity to people is determined based on a variety of factors, such as the concentration of the chemical, exposure length, and the speed of metabolizing. In high enough concentrations, pesticides in water can cause ill effects through skin contact, inhalation, and/or ingestion.

How do pesticides enter groundwater and streams?

Pesticides enter water from point and nonpoint sources. Point sources are those at which the pesticide is directly applied. Nonpoint sources are more commonly the sources of water pollution, and these are more spread out than point sources. Nonpoint sources can be the runoff water from farmland, or areas where pesticides have been blown by wind and deposited.
Several factors can exacerbate water contamination. Heavy rainfall on farmland where pesticides are applied may cause runoff containing pesticides. As the water moves to new areas, it carries the pesticides with it. Each pesticide has a different rate of solubility in water; higher rates mean pesticides can more easily be carried by water. See more factors at safewater.org.

Evaluating risk of pesticide contamination

A distinct chemical taste is often the first indication that something could be in your water. If you are located near an area or areas of heavy pesticide use, there is a higher potential risk for water contamination. You may also wish to have the construction of your well examined and your water tested for contamination. Read more about evaluating the potential for pesticide contamination here.
Once again, pesticides are usually not found in high enough concentrations to cause harm. The new practices and compounds for pest control limit the spread into unwanted areas by focusing the application, limiting the pesticide life span, and decreasing concentrations. That said, if you have concerns about pesticide concentrations in your water, it is best to have your water tested. For more information, contact Brelje and Race Labs.

Further Reading

http://psep.cce.cornell.edu/facts-slides-self/facts/pes-heef-grw85.aspx
http://www.fao.org/3/w2598e/w2598e07.htm
https://www.usgs.gov/special-topic/water-science-school/science/pesticides-groundwater?qt-science_center_objects=0#qt-science_center_objects
https://www.safewater.org/fact-sheets-1/2017/1/23/pesticides