Well water and health

Private well water

Most private well water comes from underground water supplies, called groundwater.  All water, even water from deep underground, contains some impurities.  Impurities are contaminants or pollutants that come from natural as well as anthropogenic (human) and animal sources.1 Contaminants can enter groundwater through the erosion of natural mineral deposits, precipitation (rainfall or snowfall), and run-off.  Contaminants can also be released by agricultural and industrial activities. 

Examples of industrial sources include hazardous waste sites, old or operational factories, petroleum refineries, metal smelters, and mining sites.  Ground water can also be impacted by chemical spills, leaking storage tanks, and the underground injection of wastewater.  Drinking water supplies can be impacted by activities close to the drinking water source, as well as some activities that occur many miles away.1   

Contaminants in drinking water

Private well owners must take special precautions to ensure the protection and maintenance of their drinking water supplies.2    Some contaminants in well water, like bacteria and nitrates, can cause short-term illnesses (such as gastrointestinal upset, diarrhea, nausea) and may be especially dangerous for certain high-risk populations such as small children, pregnant women, elderly, or individuals with a compromised immune systems such as people on chemotherapy or with long-term infection.1    Contact with high levels of some contaminants in drinking water for many years may increase the risk of long-term (chronic) diseases such as anemia, high blood pressure, and in some cases, cancer.  Contact with chemicals that are genotoxic carcinogens will have some degree of risk at any level of exposure. 

The degree of risk from contact with contaminants in drinking water depends upon the length of time that you are in contact with the contaminant and the amount of the contaminant to which you are exposed.  The effects of exposure to any substance also depend upon how you are exposed, your individual susceptibility to a contaminant, and whether other chemicals are present. 

Drinking water standards

National Drinking Water Standards

The Environmental Protection Agency (EPA) has identified approximately 80 contaminants that can impact drinking water quality.  Of these, the contaminants are divided into two classes: primary and secondary contaminants. 

Primary contaminants are substances that can cause changes to your health if you drink them in levels that exceed the regulatory standards for a long period of time.3

Secondary contaminants are substances that are more of a nuisance to homeowners because they alter the taste and smell of the water or cause staining of household fixtures, but are not likely to cause adverse impacts to health and are not enforceable in public drinking water supplies.3 

The EPA regulates contaminants in public drinking water supplies and establishes standards for drinking water quality.  The EPA has identified Maximum Contaminant Levels (MCL), possible health effects, and common sources of contaminants in drinking water. 
The EPA has established Maximum Contaminant Level Goals (MCLG) below which there are no known or expected health risks.  MCLGs allow for a margin of safety and are non-enforceable public health goals.

Possible sources of groundwater contamination

•    Improperly sealed well pipes or nearby abandoned wells4
•    Run-off from roadways, parking lots, and paved areas
•    Gasoline and chemical spills
•    Sewage, untreated wastewater, leaking septic tanks
•    Leaking underground storage tanks
•    Landfills and old dump sites, unlined waste impoundments
•    Hazardous waste sites
•    Airborne emissions from factories and power plants
•    Mine and drilling sites
•    Agricultural run-off (fertilizers, pesticides)
•    Run-off from livestock operations5


How might I be exposed to contaminants?

Explore the National Library of Medicine’s interactive website about sources of contamination and where you might encounter contaminants in your daily activities. 

Visit the National Library of Medicine ToxTown.

  1. US Environmental Protection Agency.  (2009). Water on tap: what you need to know. Water.  Retrieved from http://water.epa.gov/drink/guide/
  2. US Environmental Protection Agency.  (2010, February).  Private drinking water wells.  Water: Private Wells.  Retrieved from http://water.epa.gov/drink/info/well/index.cfm
  3. US Environmental Protection Agency.  (2011c). National primary drinking water regulations.  Water: Drinking Water Contaminants. Retrieved from http://water.epa.gov/drink/contaminants/index.cfm
  4. US Environmental Protection Agency.  (2002). Drinking water from household wells. Water: Private Wells.  Retrieved from http://water.epa.gov/drink/info/well/basicinformation.cfm
  5. The Groundwater Foundation.  (2011).  Sources of groundwater contamination.  The Groundwater Foundation.  Retrieved from http://www.groundwater.org/get-informed/groundwater/contamination.html