Rivers, lakes, streams, reservoirs and groundwater wells serve as drinking water sources for both tap and bottled water. As water travels over the surface of the land or through the ground, it dissolves naturally occurring minerals and, in some cases, radioactive materials. It can also pick up substances resulting from the presence of animals or human activity.
Source water, such as rivers, lakes, streams, reservoirs and groundwater wells can contain the following contaminants:
- Microbial contaminants, such as viruses and bacteria, which may come from wastewater (sewage) treatment plants, septic systems, agricultural live stock operations and wildlife
- Inorganic contaminants, such as salts and metals, which can be naturally occurring or result from urban stormwater runoff, industrial or domestic wastewater (sewer) discharges, oil and gas production, mining, or farming
- Pesticides and herbicides, which may come from a variety of sources such as agriculture, urban stormwater runoff and residential uses
- Organic chemical contaminants, including synthetic and volatile organic chemicals, which are byproducts of industrial processes and petroleum production, and also come from gas stations, urban stormwater runoff and septic systems
- Radioactive contaminants, which can be naturally occurring or be the result of oil and gas production and mining activities
To ensure tap water is safe to drink, the U.S. Environmental Protection Agency (EPA) prescribes regulations which limit the amount of certain contaminants in water provided by public water systems. Food and Drug Administration (FDA) regulations establish limits for contaminants in bottled water which must provide the same protection for public health. Drinking water, including bottled water, may reasonably be expected to contain at least small amounts of some contaminants. The presence of contaminants does not necessarily pose a health risk.
Groundwater treatment plants aerate and filter water to remove dissolved iron and manganese. Chlorine is added to destroy any bacteria present and to maintain a level of disinfectant as the water travels through the distribution system. Fluoride is added to help strengthen resistance to cavities in teeth. A small amount of ammonia is used to minimize byproducts of the disinfection process and to enable chlorine to persist longer in the distribution system.
For a few weeks each year, when the water temperature is cool, no ammonia is added in order to help maintain good water quality in the distribution system. This chlorine residual without ammonia known as "free chlorine" is a more active form of chlorine. The "free chlorine" has a more noticeable bleach or chlorine smell with the same level of chlorine. An annual maintenance procedure, the chlorine change is a standard practice in the drinking water industry. Persons on dialysis should check with their physicians regarding any necessary medical changes.
The surface water treatment process is more complicated. Facilities treating surface water are continually staffed by certified operators. Ammonia and fluoride are used the same as in the groundwater process. On occasion, powdered activated carbon is used to remove herbicides or other organic chemicals that wash into surface water.
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