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Appendix A

Glossary of Definitions

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 geologic formation or part of a formation (e.g., gravel, sand, or porous stone) that yields water to wells or springs.


A hydraulic condition caused by a difference in water pressure that causes nonpotable water or other liquid to enter the potable water system by either backpressure or backsiphonage. See cross-connection.


A hydraulic condition caused by negative or subatmospheric pressure within a water system, resulting in backflow.


Microbial cells that adhere to a moist or water-covered surface through a matrix of primarily polysaccharide materials in which they are encapsulated. Biofilms can grow on piping and surfaces of water systems and can be very difficult to remove. They protect microbes from disinfectants (e.g., chlorine) in the water.

boil water advisory

A statement to the public advising that tap water must be boiled before drinking.

bottled water

Commercially produced bottled water.


Waterborne disease outbreaks are classified according to the strength of the epidemiologic and clinical laboratory data, and environmental data implicating water as the source of the outbreak (see Table 3).


All aerobic and facultative anaerobic, gram-negative, nonspore-forming, rod-shaped bacteria that ferment lactose with gas formation within 48 hours at 95°F (35°C). Coliforms are mostly harmless bacteria that live in soil and water as well as the gut of humans and animals.

community water system

A public water system that has at least 15 service connections used by year-round residents or that regularly serves at least 25 year-round residents. The system might be owned by a private or public entity providing water to a community, subdivision, or mobile home park.


Any actual or potential connection between a drinking water supply and a possible source of contamination or pollution (i.e., nonpotable water). Under this condition, contaminated water might flow back into the drinking water system. See backflow and backsiphonage.


An antecedent event or situation contributing to the occurrence of a waterborne disease or outbreak. Outbreaks associated with water intended for drinking, water not intended for drinking and water of unknown intent are assigned deficiency codes, as categorized in Table 2.


Inflammation of the skin. In this report, dermatitis denotes a broad category of skin-related symptoms (e.g., folliculitis, cellulitis, chemical burns, or rash).


A treatment that kills microorganisms (e.g., bacteria, viruses, and protozoa); in water treatment, a chemical (commonly chlorine, chloramine, or ozone) or physical process (e.g., ultraviolet light) can be used.

disinfection by-products

Chemicals formed in water through reactions between organic or inorganic matter and disinfectants. Examples include chloramines, also known as combined chlorines. These chemicals might have acute or chronic health effects.

distribution system

Water pipes, storage reservoirs, tanks, and other means used to deliver drinking water to consumers or to store finished water before delivery to a customer. In community water systems, the distribution system is under the jurisdiction of a water utility and ends at the water meter or at the customer's property line (if the system is not metered). In noncommunity and nonpublic individual water systems, the distribution system ends at the point where water enters the building or house. See plumbing.


The pathogen, chemical, or toxin causing a waterborne disease or outbreak or other health event. Infectious etiologic agents include bacteria, parasites, and viruses.

fecal coliforms

Coliform bacteria that grow and ferment lactose to produce gas at 112.1°F (44.5°C) within 24 hours. These bacteria are associated with human and animal wastes, and their presence in water might be an indication of recent sewage or animal waste contamination.


In water treatment, the process of passing water through one or more permeable membranes or media of small diameter (e.g., sand, anthracite, and diatomaceous earth) to remove suspended particles from the water. Filters might be effective in removing pathogens, depending on the type and operation.

finished water

The water (e.g., drinking water) delivered to the distribution system after treatment (if any treatment occurred).

free chlorine

Chlorine in water (found as an aqueous mixture of hypochlorous acid and hypochlorite anion) that has not combined with other constituents; therefore, it is able to serve as an effective disinfectant (also referred to as free available chlorine or residual chlorine). Measuring the free chlorine level is a common water quality test.

ground water

Water that is contained in interconnected pores in an aquifer.

ground water system

A drinking water system that uses water extracted from an aquifer (i.e., a well or spring) as its source.

ground water under the direct influence of surface water (GWUDI)

As defined by the U.S. Environmental Protection Agency (EPA): Any water beneath the surface of the ground with: 1) significant occurrence of insects or other macroorganisms such as algae or large-diameter pathogens such as Giardia lamblia or, 2) significant and relatively rapid shifts in water characteristics such as turbidity, temperature, conductivity, or pH which closely correlate to climatologic 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 may be based on site-specific measurements of water quality and/or documentation of well construction characteristics and geology with field evaluation. Source:

individual water system

A water system that does not meet the EPA definition for a public water system. The system might regularly serve as many as 24 persons or 14 connections or as few as a single person or connection (e.g., a single family or farm not having access to a public water system). States are responsible for regulating these water systems.

karst aquifer

An aquifer characterized by water-soluble limestone and similar rocks in which fractures or cracks have been widened by the dissolution of the carbonate rocks by ground water; the aquifer might contain sinkholes, tunnels, or even caves.

mixed water source outbreak

More than one type of source water is implicated in the outbreak (e.g., a combination of ground water and surface water).

noncommunity water system

A public water system that is not a community system; it does not serve year-round residents. There are two types: transient and nontransient noncommunity systems.

nontransient noncommunity water system

A public water system that regularly supplies water to ≥25 of the same persons for ≥6 months per year but not year-round (e.g., schools, factories, office buildings, or hospitals with their own water systems).


Water pipes, storage reservoirs, tanks, and other means used to deliver drinking water to consumers inside buildings or houses or to store drinking water inside buildings or houses before consumption. In community water systems, the plumbing begins after the water utility's water meter or at the property line (if the distribution system is not metered). In noncommunity and nonpublic (i.e., individual) water systems, the plumbing begins at the point where water enters the building or house. See distribution system.

predominant illness

The category of illness reported by >50% of ill respondents (e.g., acute gastroenteritis, dermatitis, or acute respiratory illness). When more than one illness category is reported for a single outbreak, they are listed together as predominant illnesses. These mixed illness outbreaks are analyzed separately from outbreaks with single illnesses.

primary water exposure

For use in this report, a classification used for the source of contaminated water in outbreaks involving water not intended for drinking or water of unknown intent.

public water system

A system that provides piped water to the public for human consumption and is regulated under the Safe Drinking Water Act. Such a system must have at least 15 service connections or regularly serve at least 25 persons daily for at least 60 days per year. Each public water system is further classified as either a community water system or a noncommunity water system.

raw water

Surface water or ground water that has not undergone a disinfection or treatment process for the purpose of making it safer for consumption in any way. See untreated water.

reservoir, impoundment

An artificially maintained lake or other body of water used for the collection and storage of water. This body of water can be available as a source of raw water for drinking purposes or recreational use. In certain instances, a finished water storage facility in the distribution system might also be called a reservoir.


Gelatinous biologic layer on the surface of a slow sand filter, consisting of a complex microbial community (including bacteria, fungi, protozoa, and rotifera and other aquatic organisms) and organic particulate matter.


Location in which exposure to contaminated water occurred (e.g., restaurant, hospital, or hotel).

source water

Untreated ground or surface water (i.e., raw water) used to produce drinking water. Source water may or may not be treated prior to human consumption.

surface water

All water on the surface of the earth (e.g., lakes, rivers, reservoirs, ponds, and oceans) as distinguished from subsurface or ground water.

total coliforms

The combined count of fecal and nonfecal coliforms that are detected in water using a standard test. The extent to which total coliforms are present in water can indicate the general quality of that water and the likelihood that the water is contaminated fecally by animal and/or human sources.

transient noncommunity water system

A public water system that provides water in a place such as a gas station or campground where persons do not remain for long periods.

untreated water

Water that has not undergone a disinfection or treatment process for the purpose of making it safer for consumption (i.e., raw water).

water not intended for drinking (WNID)

Water that has not been treated for human consumption in conformance with EPA drinking water standards and that is provided for uses other than for drinking. This category includes water used in industrial settings; untreated water from lakes, springs, and creeks used as drinking water by campers and boaters; irrigation water; and other nonpotable water sources with or without taps. This category does not include exposure to recreational water or flood water.

water of unknown intent (WUI)

Water for which there is insufficient information to determine for what purpose it is being provided or used and whether it has been treated for human consumption in conformance with EPA drinking water standards.

water system

A system for the provision of water for human consumption through pipes or other constructed conduits. This includes any collection, treatment, storage, and distribution facilities used primarily in connection with such a system.

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