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action level A specified concentration of a contaminant in water. If this concentration is reached or exceeded,
certain actions (e.g., further treatment and monitoring) must be taken to comply with a drinking
water regulation.
aquifer An underground bed or layer of earth, gravel, or porous stone that yields water.
boil-water advisory A statement to the public advising that tap water must be boiled before drinking it.
class Waterborne-disease outbreaks are classified according to the strength of the epidemiologic and
water-quality data implicating water as the source of the outbreak (see Table 2).
coliforms All aerobic and facultative anaerobic, gram-negative, nonspore-forming, rod-shaped bacteria
that ferment lactose with gas formation <48 hours at 95F (35C).
community water system A public water system that serves year-round residents of a community, subdivision, or mobile
home park that has >15 service connections or an average of >25 residents for >60 days/year.
cross-connection Any actual or potential connection between a drinking water supply and a possible source of
contamination or pollution (e.g., a wastewater line).
disinfection by-products Chemicals formed in water through reactions between organic matter and disinfectants.
distribution system Water pipes, storage reservoirs, tanks, and other means used to deliver drinking water to consumers
or store it before delivery.
equalization basin A holding basin in which variations in flow and composition of a liquid stream are averaged,
allowing storage and controlled release of wastewater to treatment processes.
fecal coliforms Coliforms that grow and produce gas at 112.1F (44.5C) in <24 hours.
filter backwash Water containing the material obtained by reversing the flow of water through a filter to
dislodge the particles that have been retained on it.
filtration The process of removing suspended particles from water by passing it through one or more
permeable membranes or media of limited diameter (e.g., sand, anthracite, or diatomaceous earth).
finished water The water (e.g., drinking water) delivered to the distribution system after treatment, if any.
free, residual chlorine level The concentration of chlorine in water that is not combined with other constituents, thus
serving as an effective disinfectant.
groundwater system A system that uses water extracted from the ground (i.e., a well or spring).
groundwater under the
     influence of surface water
Any water beneath the surface of the ground with substantial occurrence of insects or direct
other macrooganisms, algae, or large-diameter pathogens (e.g., Giardia intestinalis or
Cryptosporidium) or substantial and relatively rapid shifts in water characteristics (e.g., turbidity,
temperature, conductivity, or pH) that closely correlate with climatologic or surface water
conditions. Direct influence must be determined for individual sources in accordance with
criteria established by the state.
hydrogeology The branch of geology that deals with the occurrence, distribution, and effect of ground water.
Individual (or private)
     water system
A water system that is not owned or operated by a water utility and that serves <15
residences or farms not having access to a public water system.
interactive fountain A fountain intended for (or accessible to) recreational use, often located at waterparks. In
contrast, noninteractive (ornamental) fountains are intended for public display rather than
recreational use and are often located in front of buildings and monuments.
maximum contaminant level The maximum permissible concentration (i.e., level) of a contaminant in water supplied to any
user of a public water system.
noncommunity water system A public water system that 1) serves an institution, industry, camp, park, hotel, or business that
is used by the public for >60 days/year but not year-round; 2) has >15 service connections or
serves an average of >25 persons; and 3) is not a community water system.
nontransient noncommunity
     water system
Public water system that serves >25 of the same persons for >6 months/year (e.g., a factory or
school) but not year-round.
norovirus A group of related, single-stranded RNA, nonenveloped viruses (genus Norovirus, family
Caliciviridae) that cause acute gastroenteritis in humans. Norovirus was recently approved as
the official genus name for the group of viruses provisionally described as Norwalk-like viruses.
public water system A system, classified as either a community water system or a noncommunity water system, that
provides piped water to the public for human consumption and is regulated under the Safe
Drinking Water Act.
raw water Surface water or groundwater that has not been treated in any way.
reverse osmosis A filtration process that removes dissolved salts and metallic ions from water by forcing it
through a semipermeable membrane. This process is also highly effective in removing
microbes from water.
source water Untreated water (i.e., raw water) used to produce drinking water.
surface water The water in lakes, rivers, reservoirs, and oceans.
total coliforms Nonfecal and fecal coliforms that are detected by using a standard test.
transient noncommunity
     water system
Public water system that provides water to places where persons do not remain for long periods
of time (e.g., restaurants, highway rest stations, or parks with their own public water systems).
turbidity The quality (e.g., of water) of having suspended matter (e.g., clay, silt, or plankton) that results
in loss of clarity or transparency.
untreated water Surface water or groundwater that has not been treated in any way (also called raw water).
water-quality indicator A microbial, chemical, or physical parameter that indicates the potential risk for infectious
diseases associated with using the water for drinking, bathing, or recreational purposes. The
best indicator is one whose density or concentration correlates best with health hazards associated
with a type of hazard or pollution.
watershed An area from which water drains to a single point; in a natural basin, the area contributing flow
(i.e., water) to a place or point on a stream.
watershed-control program A program to protect a watershed from contamination or pollution.

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