'":-: ;/" 

                                                                       EPA 810-F-99-Q*f
                                                                       December 1999

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 USEPA 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 Maxi-
mum Contaminant Level.

Coliform: A group of related  bacteria whose
presence in drinking water may indicate
contamination by disease-causing micro-

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, chemi-
cals, radionuclides, etc.)  which may be
harmful to human health.
Cryptosporidium: A microorganism com-
monly 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 se-
verely weakened immune systems are likely
to have more severe and more persistent
symptoms than healthy individuals.

Disinfectant: A chemical (commonly chlo-
rine, 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 chlo-
rine), 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 USEPA 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 USEPA health
standards (Maximum Contaminant Levels or
Treatment Techniques); (2) it was in opera-

tion 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. See Source

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 USEPA document that pro-
vides guidance and information on  contaminants
that can affect human health  and that may occur
in drinking water.

Inorganic Contaminants: Mineral-based com-
pounds 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. USEPA has set legal limits on 15
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 tech-
nology 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 micro-
scope. 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 USEPA's moni-
toring 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. USEPA has set legal
limits on 56 organic contaminants.

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
USEPA's, may apply for, and receive, primary
enforcement authority, or primacy.

Public Notification: An advisory that USEPA or the
state requires a water system to distribute to af-
fected 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 15 service
connections or 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.

Radionuclide: 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 USEPA-regulated  drinking water
contaminants.  Depending on  the regulation,
USEPA requires water systems and states to take
samples from source water, from water leaving
the treatment facility, or from  the taps of selected

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 discolora-
tion) 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 in-
tended 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 USEPA permission not to  meet
a certain drinking water standard. The water
system must prove that: (1) it cannot meet a
Maximum Cpntaminant 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 USEPA 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 drink-
ing 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 pro-
tected to prevent contamination of the well(s).