Drinking water will always contain chemicals other than water, such as minerals and dissolved gases. Whether a particular substance presents a risk to human health depends on the type of chemical, the body's reaction to that chemical, and how much exposure people are likely to have. In the case of drinking water, exposure depends on the concentration of the substance and how much is consumed.
Concentrations of substances in water have three parts - a number, units such as milligrams per Liter (mg/L) or parts per billion (ppb), and the name of the specific molecule or chemical ion being measured. You can think of a concentration like a recipe: 10 mg/L of sodium ions in water could be made by weighing out salts to get 10mg of sodium ions and dissolving that in a Liter of water.
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Primary drinking water standards
Under the Safe Drinking Water Act, the U.S. Environmental Protection Agency is responsible for setting the legal limits on the amount of contaminants in public water supply systems. Primary drinking water standards currently exist for more than 90 contaminants. Primary drinking water standards set a maximum contaminant level, or an upper limit, for a contaminant with potential health effects that is allowed in drinking water. These standards provide reasonable assurance that public drinking water supplies are safe to drink. The standards are based on studies on humans and other animals and are reviewed periodically.
While primary drinking water standards are not enforceable for private wells, the maximum contaminant levels established for public water supplies are good guidelines for homeowners to determine whether their private well water is safe to drink.
Secondary drinking water standards
There are also secondary drinking water standards. These are generally non-enforceable guidelines for contaminants that cause cosmetic effects to humans (skin or tooth discoloration) or aesthetic water quality concerns such as taste, color, and odor.
To assess whether your well water is suitable for drinking, compare the concentration of contaminants found in your well to those levels that are considered harmful to human health. Make sure that you are comparing numbers with the same units. You may want to have your water tested again and explore possible causes and solutions if you have a higher concentration of a substance than is recommended.
Related resources
Refer to U.S. EPA's Drinking Water Contaminants for the most current list of the primary and secondary drinking water standards.
For more information on how drinking water standards are established refer to U.S. EPA's Setting Standards for Safe Drinking Water.
Reference
Adapted from U.S. EPA, Office of Ground Water and Drinking Water. Accessed Online November 9, 2009
