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| Glossary
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SAFE DRINKING WATER ACT • 1974-2004 • PROTECT OUR HEALTH FROM SOURCE TO TAP
These definitions are not intended to be complete or to have legal force, but rather to help consumers
quickly understand drinking water-related terms in the context of their daily lives.
Action Level: The level of lead or copper which, if exceeded in over 10% of the homes tested, triggers
treatment or other requirements that a water system must follow.
Acute Health Effect: An immediate (i.e., within hours or days) adverse health effect that may result from
exposure to certain drinking water contaminants (e.g., pathogens).
Aquifer: A natural underground layer, often of sand or gravel, that contains water.
Best Available Technology: The water treatment(s) that US EPA certifies to be the most effective for
removing a contaminant.
Chronic Health Effect: The possible result of exposure over many years to a drinking water contaminant
at levels above its Maximum Contaminant Level.
Coliform: A group of related bacteria whose presence in drinking water may indicate contamination by
disease-causing microorganisms.
Community Water System: A public water system which supplies drinking water to 25 or more of the
same people year-round in their residences.
Compliance: The act of meeting all state and federal drinking water regulations.
Contaminant: Anything found in water (including microorganisms, minerals, chemicals, radionuclides,
etc.) which may be harmful to human health.
Cryptosporidium: A microorganism commonly found in lakes and rivers which is highly resistant to
disinfection. Cryptosporidium has caused several large outbreaks of gastrointestinal illness, with symptoms
that include diarrhea, nausea, and/or stomach cramps. People with severely weakened immune systems are
likely to have more severe and more persistent symptoms than healthy individuals.
Disinfectant: A chemical (commonly chlorine, chloramine, or ozone) or physical process (e.g., ultraviolet
light) that kills microorganisms such as bacteria, viruses, and protozoa.
Disinfectant Byproducts: Chemicals that may form when disinfectants (such as chlorine), react with
plant matter and other naturally occurring materials in the water. These byproducts may pose health risks
in drinking water.
Distribution System: A network of pipes leading from a treatment plant to customers, plumbing systems.
Exemption: State or US EPA permission for a water system not to meet a certain drinking water standard.
An exemption allows a system additional time to obtain financial assistance or make improvements in order
to come into compliance with the standard. The system must prove that: (1) there are compelling reasons
(including economic factors) why it cannot meet US EPA health standards (Maximum Contaminant Levels
or Treatment Techniques); (2) it was in operation on the effective date of the requirement; and (3) the
exemption will not create an unreasonable risk to public health. The state must set a schedule under which
the water system will comply with the standard for which it received an exemption.
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Finished Water: Water that has been treated and is ready to be delivered to customers. See Source
Water.
Giardia lamblia: A microorganism frequently found in rivers and lakes, which, if not treated properly,
may cause diarrhea, fatigue, and cramps after ingestion. People with severely weakened immune systems
are likely to have more severe and more persistent symptoms than healthy individuals.
Ground Water: The water that systems pump and treat from aquifers (natural reservoirs below the earth's
surface).
Health Advisory: A US EPA document that provides guidance and information on contaminants that can
affect human health and that may occur in drinking water.
Inorganic Contaminants: Mineral-based compounds such as metals, nitrates, and asbestos. These
contaminants are naturally-occurring in some water, but can also get into water through farming, chemical
manufacturing, and other human activities. US EPA has set legal limits on 16 inorganic contaminants.
Maximum Contaminant Level (MCL): The highest level of a contaminant that is allowed in drinking
water. MCLs are set as close to the MCLG as feasible using the best available treatment technology and
taking cost into consideration. MCLs are enforceable standards.
Maximum Contaminant Level Goal (MCLG): The level of a contaminant in drinking water below
which there is no known or expected risk to health. MCLGs allow for a margin of safety. MCLGs are non-
enforceable health goals.
Microbes (microorganisms): Tiny living organisms that can only be seen with the aid of a microscope.
Some microbes can cause acute health problems when consumed (see pathogens).
Monitoring: Testing that water systems must perform to detect and measure contaminants. A water system
that does not follow US EPA's monitoring methodology or schedule is in violation, and may be subject to
legal action.
National Primary Drinking Water Regulations: Legally enforceable standards that apply to public
water systems. These standards protect drinking water quality by limiting the levels of specific contaminants
that can adversely affect public health and which are known or anticipated to occur in public water
supplies.
Non-Transient, Non-Community Water System: A public water system which supplies water to 25 or
more of the same people at least six months per year in places other than their residences. Some examples
are schools, factories, office buildings, and hospitals which have their own water systems.
Organic Contaminants: Carbon-based chemicals, such as solvents and pesticides, which can get into
water through runoff from cropland or discharge from factories. US EPA has set legal limits on 56 organic
conta minants.
Pathogens: Disease-causing organisms, such as some bacteria, viruses, or protozoa.
Primacy: Primary enforcement authority for the drinking water program. Under the Safe Drinking Water
Act, states, U.S. territories, and Indian tribes that meet certain requirements, including setting regulations
that are at least as stringent as US EPA's, may apply for, and receive, primary enforcement authority, or
primacy.
Public Notification: An advisory that US EPA or the state requires a water system to distribute to
affected consumers when the system has violated Maximum Contaminant Levels or other regulations. The
notice advises consumers what precautions, if any, they should take to protect their health.
Public Water System (PWS): Any water system which provides water to at least 1 5 service connections
or 25 people for at least 60 days annually. There are more than 1 70,000 PWSs providing water from
wells, rivers, and other sources to about 250 million Americans. The others drink water from private wells.
There are differing standards for PWSs of different sizes and types.
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Rddionuclide: An unstable form of a chemical element that radioactively decays, resulting in the
emission of nuclear radiation. Prolonged exposure to radionuclides increases the risk of cancer. All of the
radionuclides known to occur in drinking water are currently regulated, except for radon and naturally-
occurring uranium, both of which were proposed for regulation in October 1999.
Raw Water: Water in its natural state, prior to any treatment for drinking. See finished water.
Sample: The water that is analyzed for the presence of US EPA-regulated drinking water contaminants.
Depending on the regulation, US EPA requires water systems and states to take samples from source water,
from water leaving the treatment facility, or from the taps of selected consumers.
Sanitary Survey: An on-site review of the water sources, facilities, equipment, operation, and
maintenance of a public water system for the purpose of evaluating the adequacy of the facilities for
producing and distributing safe drinking water.
Secondary Drinking Water Standards: Non-enforceable federal guidelines regarding cosmetic
effects (such as tooth or skin discoloration) or aesthetic effects (such as taste, odor, or color) of drinking
water.
Sole Source Aquifer: An aquifer that supplies 50 percent or more of the drinking water of an area.
Source Water: Water in its natural state, prior to any treatment for drinking. See finished water.
Surface Water: The water that systems pump and treat from sources open to the atmosphere, such as
rivers, lakes, and reservoirs.
Transient, Non-Community Water System: A public water system which provides water in a place
such as a gas station or campground where people do not remain for long periods of time. These systems
do not have to test or treat their water for contaminants which pose long-term health risks because fewer
than 25 of the same people drink the water over a long period. They still must test their water for microbes
and several chemicals posing short-term health risk.
Treatment Technique: A required process intended to reduce the level of a contaminant in drinking water.
Turbidity: The cloudy appearance of water caused by the presence of tiny particles. High levels of
turbidity may interfere with proper water treatment and monitoring.
Variance: State or US EPA permission not to meet a certain drinking water standard. The water system
must prove that: (1) it cannot meet a Maximum Contaminant Level, even while using the best available
treatment method, because of the characteristics of the raw water, and (2) the variance will not create
an unreasonable risk to public health. The state or US EPA must review, and allow public comment on, a
variance every three years. States can also grant variances to water systems that serve small populations
and which prove that they are unable to afford the required treatment, an alternative water source, or
otherwise comply with the standard.
Violation: A failure to meet any state or federal drinking water regulation.
Vulnerability Assessment: An evaluation of drinking water source quality and its vulnerability to
contamination by pathogens and toxic chemicals.
Watershed: The land area from which water drains into a stream, river, or reservoir.
Wellhead Protection Area: The area surrounding a drinking water well or well field which is protected
to prevent contamination of the well(s).
For More Information
To learn more about drinking water topics, call the Safe Drinking Water Hotline at 1-800-426-4791 or visit the
safewater web site at www.epa.gov/safewater.
Office of Water (4606) www.epa.gov/safewater EPA 816-F-04-035 June 2004
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