A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z 


action level
A specified concentration of a contaminant in drinking water. If this concentration is reached or exceeded, certain actions (such as further treatment and monitoring) must be taken to comply with a drinking water regulation.

alluvial aquifer
An aquifer formed as a result of sediments deposited in river channels or on floodplains. Because they are shallow and unconfined, alluvial aquifers experience fluctuating water levels and are susceptible to contamination.

A geologic formation or part of a formation (such as gravel, sand, or porous stone) that supplies water to wells or springs.


A hydraulic condition caused by a difference in water pressure that causes water from one piped system to enter another piped system (such as water from an irrigation system that enters a drinking water system by either backpressure or backsiphonage.) See cross-connection.

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

backwash (recreational water)
Flow of water through filter element(s) or media in a reverse direction to dislodge and remove accumulated dirt, debris, or filter aid from the filter tank.

bather load
The number of bathers using a swimming pool or spa at any one time. The maximum bather load is usually determined by a state or local pool code and is based on surface area and depth of the pool or spa.

A biofilm is a group of microorganisms—often a mix of bacteria, fungi, and amebas—that live together and release a slimy, glue-like substance, which allows them to stick to surfaces. This slimy “home” acts as a barrier to water treatment chemicals, like chlorine, helping the germs survive and multiply. A biofilm is more likely to grow abundantly in places where water does not move, such as the inner surfaces of water pipes, water storage tanks, or water heaters. See Preventing Waterborne Germs at Home.

boil water advisory
A statement to the public advising that tap water should be boiled before drinking because of suspected or known microbial contamination. (Other names for boil water advisory are boil water order or boil water notice.)

bottled water
Commercially produced bottled water.


cercarial dermatitis
Dermatitis caused by skin contact with contaminated fresh or marine water. Rash-like symptoms follow direct invasion through the skin or a break in the skin by the cercariae (larval stage) of certain species of schistosomes. The normal hosts of these species are birds and non-human mammals. Dermatitis is an allergic response to contact with cercariae and does not lead to parasitic infestation in humans and produces no long-term disease.

A group of disinfection byproducts or weak disinfectants formed when free chlorine combines with nitrogen-containing compounds. The chloramine family includes monochloramine, dichloramine, and trichloramine (nitrogen trichloride).

chloramines (drinking water)
Chloramines are disinfectants used to treat drinking water and they are most commonly formed when ammonia is added to chlorine to treat drinking water (in a process called “chloramination” or “secondary disinfection”). Chloramines (e.g., monochloramine) are generally long lasting in drinking water as it moves through pipes to consumers.

chloramines (recreational water)
Chloramines are a type of combined chlorine that form in water and then release gas into the air above the water. Most city, county, and state health departments limit the amount of combined chlorine in the water to 0.4 ppm or less.

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

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, but can indicate a problem if found in treated water. See fecal coliforms and total coliforms.

combined chlorine (recreational water)
Chlorine that has combined with organic or inorganic compounds in the water and is no longer an effective disinfectant for recreational water. The combined chlorine level is derived by subtracting the water’s free chlorine test level from its total chlorine test level. This level is likely to include combined compounds in addition to chloramines (see chloramines).

community water system
A public drinking 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.

community-wide outbreak (drinking water)
An outbreak that affects the entire community and is due to exposure to contaminated water from a public drinking water system.

community-wide outbreak (recreational water)
This outbreak typically starts as a focal outbreak associated with one recreational water venue and evolves into an outbreak associated with multiple recreational water venues or other settings, such as childcare centers in a community.

contact time
The length of time that water and any pathogens in the water are exposed to a disinfectant, usually measured in minutes. The effectiveness of the disinfection process is dependent on the disinfectant residual in the water and the amount of time that the water is exposed to the disinfectant. Certain pathogens may require a longer contact times with a given disinfectant residual to be inactivated.

Any actual or potential connection between a drinking water supply and a possible source of contamination or pollution (for example, non-drinkable water system). When a cross-connection is present, contaminated water might flow directly back into the drinking water system via this connection. See backflow and backsiphonage.


An antecedent event or situation contributing to the occurrence of a waterborne disease or outbreak. Outbreaks associated with drinking water and other, non-recreational water (such as drinking from a wilderness stream) are assigned deficiency codes.

Inflammation of the skin. Dermatitis denotes a broad category of skin-related symptoms (such as folliculitis, cellulitis, chemical burns, or rash).

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

disinfection byproducts
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 drinking 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 drinking water systems and nonpublic individual drinking 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. See coliforms and total coliforms.

fill-and-drain pools
Small pools or slides that often are constructed of plastic and that might be inflatable. These pools and slides are filled with tap water without any ongoing chemical disinfection or filtration.

In water treatment, the process of passing water through one or more permeable membranes or media of small diameter (for example, 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 (such as drinking water) delivered to the distribution system after treatment (if any treatment occurred).

free chlorine
A measure of the free chlorine level is a common water quality test for drinking water and treated recreational water.

fresh water
Untreated, nonmarine surface water (such as recreational water from lakes, rivers, or ponds).


Water that is underground and contained in interconnected pores in an aquifer.

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

groundwater 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 duodenalis 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 particular drinking water 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: icon.


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


An inclusive term for the U.S. states, District of Columbia, territories, and Freely Associated States (FAS) (such as the Republic of the Marshall Islands, the Federated States of Micronesia, and the Republic of Palau) that report waterborne disease outbreaks to the Waterborne Disease and Outbreak Surveillance System. Waterborne disease outbreaks investigated by local public health agencies are reported to the state, territorial, or FAS public health agency. Jurisdictions also report single cases of waterborne disease and other water-associated health events to CDC.


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.


marine water
Untreated recreational water at an ocean or estuarine setting.

mixed water source outbreak
More than one type of source water is implicated in the drinking water outbreak (for example, a combination of ground water and surface water).


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


other, non-recreational water
Outbreaks with an environmental exposure to water are not associated with a recreational water venue or drinking water system, but rather are linked to other water types. These types include water from cooling towers, industrial processes, agricultural processes, occupational settings, decorative or display settings (for example, decorative fountains), and water consumed from natural sources such as backcountry streams. Outbreaks involving an undetermined exposure to water could not be definitively linked to a single type of water exposure because of association with multiple suspected or confirmed water types (for example, both spa and drinking water systems) or because insufficient epidemiologic, laboratory, or environmental evidence was available to identify the exposure.


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 (individual) water systems, the plumbing begins at the point where water enters the building or house. See distribution system.

pool chemical-associated health events
Injuries or illnesses resulting from exposure to pool chemicals (such as halogens or disinfection byproducts) used to maintain quality of treated recreational water. These events might not meet the criteria for a waterborne disease outbreak depending on whether they involve exposure to recreational water or two or more people. These events must be associated with treated recreational water venues.

predominant illness
The category of illness reported by >50% of ill respondents (such as 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
A classification used for the source of contaminated water in waterborne disease outbreaks involving other non-recreational water (for example, drinking water from a wilderness stream), water not intended for drinking, or water of unknown intent.

public water system
A drinking water 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 people 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 to make it safer for consumption. See untreated water.
recreational water venue
A body of water used for recreation (such as swimming, soaking, or athletics), including any structure that encloses this water. It can include a lake, pond, river, spring, ocean, or a man-made venue (for example, swimming pool and spa). Some recreational water venues do not include standing water (for example, interactive fountains).

reservoir, impoundment
An artificially maintained lake or other body of water used to collect and store 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 also might 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 (for example, restaurant, hospital, hotel, or water park).

source water
Untreated ground or surface water (such as raw water) used to produce drinking water. Source water may or may not be treated prior to human consumption.

Any structure, basin, chamber, or tank (located indoors or outdoors) containing water that is intended to be used for recreational or therapeutic purposes and that usually contains a waterjet or aeration system. It is operated at high temperatures and usually is not drained, cleaned, or refilled after each use. It also is referred to as a hot tub or whirlpool.

spray park
A recreational water venue consisting of multiple interactive fountains.

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


total chlorine
A common water quality test for drinking water and treated recreational water that quantifies the level of chlorine in water that is free for disinfection (see free chlorine), plus the chlorine that is not available for disinfection because it has reacted with organic or inorganic materials (see combined chlorine level). The water’s combined chlorine level is derived by subtracting the free chlorine test level from the total chlorine test level.

total coliforms
A group containing fecal and nonfecal coliforms that are detected in water using a standard test (see coliforms). 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 drinking water system that provides water in a place such as a gas station or campground where people do not remain for long periods.

treated water (drinking water)
Water that has undergone a disinfection or treatment process to make it safer for to drink.

treated water (recreational water)
Water that has undergone a systematic disinfection process (such as chlorination and filtration) to maintain good microbiologic quality for recreation. Typically, this refers to any recreational water in an enclosed, manufactured structure. This includes water in swimming or wading pools, fountains, and spas but might also include water in fill-and-drain pools filled with treated tap water or untreated water (for example, mineral spring water).


untreated water (drinking water)
Water that has not been disinfected or treated to make it safer to drink (i.e., raw water).

untreated water (recreational water)
Water that has not been disinfected or treated to maintain good microbiologic quality for recreation (lakes, rivers, oceans, and reservoirs).

Any person or bather entering recreational water. Might also be referred to as a patron at some membership clubs or recreational water venues.


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. Standards might vary based on type and degree of water exposure associated with different water uses. Ideally, density or concentration correlates with health effects. Examples include turbidity, coliform counts, fecal coliforms, Escherichia coli, enterococci, and free chlorine level.