Pure water contains nothing but the essential chemical elements of water (i.e. two parts hydrogen and one part oxygen). Drinking water usually carries a certain amount of minerals which it acquires from its source, treatment, storage, distribution, and household plumbing conditions. These minerals and elements generally occur at very low levels and do not pose a significant risk to health.
A wide variety of chemicals and compounds can become ground water contaminants if discharged to the subsurface environment. They range from inorganic compounds, to organic compounds, to synthetic compounds, such as pesticides, and other contaminants. Because drinking water systems get their water from ground water (and surface water) sources, once the source becomes contaminated, the drinking water can also become contaminated. Shallower drinking water wells, which are most likely to be owned and operated by private and small public water systems, are usually more susceptible to contamination than deep wells, which are usually operated by large public water utilities.
In 1974, the United States Congress recognized the importance of protecting our nation's public water supplies by adopting the Safe Drinking Water Act (SDWA). The SDWA authorized USEPA to develop regulations, to set nationwide uniform drinking water quality standards for all States, and to implement programs to ensure the safety of public water supplies and protect public health. Congress re-emphasized its support for safe drinking water by adopting updated versions of the SDWA in 1986 and 1996.
USEPA has defined drinking water standards according to the latest available human health risk studies, and to provide water systems with a basis for determining drinking water safety. While the standards should be applied to all water systems, the SDWA requires that only public water systems be regulated to ensure that these standards are maintained. A public water system is a water system which serves 25 or more people or 15 or more service connections for at least 60 days out of the year. Systems which do not meet this definition are classified as private water systems and are not subject to federal regulation.
Drinking water standards for contaminants that could affect health are called Primary Drinking Water Standards and are enforceable by law for public water systems. Primary standards are usually established through Maximum Contaminant Levels (MCLs) but may also be established through a mandatory treatment technique requirement.
Secondary Drinking Water Standards, also known as Secondary Maximum Contaminant Levels (SMCL), are concentration limits for nuisance contaminants, which may have aesthetic effects such as taste, odor, or staining.
A laboratory analysis of water identifies the concentrations of contaminants. Concentrations are usually reported in milligrams per liter, (mg/l). One mg/l is equal to one part per million. Advanced laboratory analysis techniques, in some instances, will allow contamination to be reported in micrograms per liter or parts per billion, (ppb).
Inorganic substances are those that do not contain carbon. These include most elements and simple compounds, such as nitrogen and heavy metals.
Organic substances are those that contain carbon. These include many complex, and groups of complex, substances along with living things.
The Other category includes some of the physical properties that can occur in ground water.
Maximum Contaminant Level (MCL) is the maximum permissible level of a contaminant in water which is delivered to any user of a public water system. It is based on scientific research which concludes that greater concentrations could cause health problems in humans.
The Maximum Contaminant Level Goal (MCLG) is the level at which no known or anticipated health effects will occur and must include a margin of safety. The MCLG is set at 0 for known human carcinogens.
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