General Drinking Water Glossary
The definitions found in here are designed to be general in nature.
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Action Level: The level of lead or copper which, if exceeded, triggers treatment or other requirements
that a water system must follow.
Acute Health Effect: An immediate (i.e. within hours or days) 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 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 MCL.
Coliform : A group of related bacteria whose presence in drinking water may indicate contamination
by disease-causing microorganisms.
Community Water System : A 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 (that is, severely immuno-compromised) 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.
Distribution System : A network of pipes leading from a treatment plant to customers' plumbing
Exemption : State or 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 a MCL or Treatment
Technique; (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.
Finished Water : Water that has been treated and is ready to be delivered to customers.
Giardia lamblia : A microorganism frequently found in rivers and lakes, which, if not treated
properly, may cause diarrhea, fatigue, and cramps after ingestion.
Ground Water : The water that systems pump and treat from aquifers (natural reservoirs below the
Health Advisory : An EPA document that provides guidance and information on contaminants that
can affect human health and that may occur in drinking water, but which EPA does not currently
regulate 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. EPA has set legal limits on 15 inorganic
Maximum Contaminant Level (MCL) : The highest level of a contaminant that EPA allows in
drinking water. MCLs ensure that drinking water does not pose either a short-term or long-term
health risk. EPA sets MCLs at levels that are economically and technologically feasible. Some states
set MCLs which are more strict than EPA's.
Maximum Contaminant Level Goal (MCLG) : The level of a contaminant at which there would be
no risk to human health. This goal is not always economically or technologically feasible, and the
goal is not legally enforceable.
Microorganisms : Tiny living organisms that can be seen only with the aid of a microscope. Some
microorganisms can cause acute health problems when consumed in drinking water. Also known as
Monitoring : Testing that water systems must perform to detect and measure contaminants. A water
system that does not follow EPA's monitoring methodology or schedule is in violation, and may be
subject to legal action.
Non-Transient, Non-Community Water System : A 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. EPA has set legal limits on 56
Pathogen : A disease-causing organism.
Primacy State : A State that has the responsibility and authority to administer EPA's drinking water
regulations within its borders. The State must have rules at least as stringent as EPA's.
Public Notification : An advisory that EPA requires a water system to distribute to affected consumers
when the system has violated MCLs 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 25 people for at
least 60 days annually. There are more than 170,000 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.
Radionuclides : Any man-made or natural element that emits radiation and that may cause cancer after
many years of exposure through drinking water.
Raw Water : Water in its natural state, prior to any treatment for drinking.
Sample : The water that is analyzed for the presence of EPA-regulated drinking water contaminants.
Depending on the regulation, 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 systems 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
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.
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 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 people drink the water over a long period. They still must test their water for microbes
and several chemicals.
Treatment Technique : A required process intended to reduce the level of a contaminant in drinking
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 EPA permission not to meet a certain drinking water standard. The water system
must prove that: (1) it cannot meet a MCL, 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 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).