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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.
A reversal of the normal flow of water or other liquid caused by a negative-pressure gradient (e.g., within a water system).
Boil-water advisory
A statement to the public advising that tap water must be boiled before drinking it.
Cercarial dermatitis Dermatitis caused by contact with the cercariae (larval stage) of certain species of schistosomes whose normal hosts are birds and nonhuman mammals.
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 1).
The process of adding chemicals to water to destabilize charges on naturally occurring particles to facilitate their subsequent aggregation and removal by flocculation or filtration.
All aerobic and facultative anaerobic, gram-negative, nonspore-forming, rod-shaped bacteria that ferment lactose with gas formation within 48 hours at 95
F (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.
Contact time
The length of time water is exposed to a disinfectant (e.g., chlorine contact time).
Any actual or potential connection between a drinking water supply and a possible source of contamination or pollution (e.g., a wastewater line).
The infectious stage of Giardia intestinalis and certain other protozoan parasites that have protective walls that facilitate their survival in water and other environments.
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.
The release of the internal (i.e., encysted) contents (e.g., trophozoites or sporozoites) from cysts or oocysts.
Fecal coliforms
Coliforms that grow and produce gas at 112.1
F (44.5 C) within 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.
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.
The water-treatment process after coagulation that uses gentle stirring to cause suspended particles to form larger, aggregated masses (floc). The aggregates are removed from the water by a separation process (e.g., sedimentation, flotation, or filtration).
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 direct influence of surface water

Any water beneath the surface of the ground with substantial occurrence of insects or other macrooganisms, algae, or large-diameter pathogens (e.g., Giardia intestinalis or for subpart H systems serving >10,0000 persons only Cryptosporidium parvum, or substantial and relatively rapid shifts in water characteristics (e.g., turbidity, temperature, conductivity, or pH that closely correlate to climatological or surface water conditions. Direct influence must be determined for individual sources in accordance with criteria established by the state. The state determination of direct influence might be based on site-specific measurements of water quality or documentation of well construction characteristics and geology with field evaluation.
Heterotrophic microflora
Microorganisms that use organic material for energy and growth.

Infant water
Bottled waters that are marketed for direct consumption by infants or use in mixing with infant formula.
Individual (or private) water system
A water system not owned or operated by a water utility and that serves <15 residences or farms not having access to a public water system.

Maximum-contaminant level
The maximum permissible concentration (i.e., level) of a contaminant in water supplied to any user of a public water system.

Nephelometric turbidity units
The units in which the turbidity of a sample of water is measured when the degree to which light is scattered is assessed with a nephelometric turbidimeter.

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, 2) has >15 service connections or serves an average of >25 persons, and 3) is not a community water system.
Nontransient noncommunity water systems  

Public water systems that serve >25 of the same persons for >6 months/year (e.g., a factory or school).


Oocyst The infectious stage of Cryptosporidium parvum and certain other coccidian parasites with a protective wall that facilitates survival in water and other environments.
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. Total coliforms are a group of closely related bacteria that are usually free-living in the environment, but are also normally present in water contaminated with human and animal feces. With certain exceptions, they do not cause disease. Specifically, coliforms are used as a screen for fecal contamination as well as to determine the efficiency of treatment and the integrity of the water distribution system. The presence of total coliforms in drinking water indicates that the system is either fecally contaminated or vulnerable to fecal contamination.
Transient non-community water systems
Public water systems that regularly serve >25 of the same persons for >6 months/year (e.g., highway rest stations, restaurants, and 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.
Water utility
A water provider that distributes drinking water to a community through a network of pipes.
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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Safer, Healthier People

Morbidity and Mortality Weekly Report
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This page last reviewed 7/2/2002