Our drinking water comes either from surface-water bodies (lakes, streams, or reservoirs) or from ground-water. In the United States, water must meet the national drinking-water standards proposed by the Environmental Protection Agency (EPA) and mandated by Congress through the Clean-Water Act. As of 1986, these rules define acceptable quantities of eighty three different contaminants, of which there are three kinds: inorganic chemicals like arsenic, lead, and mercury; organic chemicals such as pesticides, benzene, and vinyl chloride; and microbes like coliform bacteria. Many of the chemicals on the list are carcinogens or damage the nervous system, while the bacteria cause gastrointestinal illness. Chemicals mostly enter ground-water by seeping into the ground from surface or near-surface spills. Bacteria may enter from sewage or from septic tanks.

In order to meet water-quality standards, some water must be filtered and mixed with chlorine Many municipalities also ad fluoride to the water, because the fluoride becomes incorporated in people’s teeth, making tooth enamel stronger and less susceptible to decay. In regions where water has not been treated, inhabitants are well advised to drink only purified bottled water.

Even in industrialized countries, people increasingly drink bottled groundwater. Many different companies market this water—each company has its own source, either a well or a spring that taps the groundwater supply. Like all groundwater, bottled water contains slight quantities of many chemicals, listed as "minerals" on the bottle’s label. If you compare different brands, you’ll see that not all bottled waters have the same composition of the bedrock and sediment through which the water has passed, and the age of the water (how long it has had to react with its surroundings).

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