Water Quality Terms and Definitions


See "A Guide to Water Terms" web page for additional words and phrases.

Action Level
The concentration of a contaminant which, if exceeded, triggers treatment or other requirements which a water system must follow.
American Water Works Association (AWWA)
The American Water Works Association is an international nonprofit scientific and educational society dedicated to the improvement of drinking water quality and supply. Founded in 1881, AWWA is the largest organization of water supply professionals in the world. Membership includes more than 4,000 utilities that supply water to roughly 180 million people in North America.
Arizona Department of Environmental Quality (ADEQ)
The Arizona Department of Environmental Quality functions as the local management and enforcement arm of the United States Environmental Protection Agency (EPA) and is responsible for ensuring the State of Arizona complies with all federal water, air, and environmental regulations. Tucson Water works with ADEQ to ensure the water we deliver meets all state and federal standards for health and safety.
Arizona Department of Health Services (ADHS)
The Arizona Department of Health Services State Laboratory helps protect the health of Arizonans by providing a full range of public health laboratory services, including identifying and investigating infectious and communicable diseases. The Laboratory monitors both groundwater and surface water for the presence of chemical and microbiological pollutants. The department also maintains a laboratory licensure and consultation program to assure the quality of analytical testing being done by the clinical and environmental laboratory communities of Arizona.
Arizona Department of Water Resources (ADWR)
The Arizona Department of Water Resources works to secure long-term water supplies for Arizona's communities. ADWR administers state water laws (except those related to water quality), explores methods of augmenting water supplies to meet future demands, and develops policies to promote conservation and equitable distribution of water. ADWR also oversees the use of surface and groundwater resources under state jurisdiction and negotiates with external political entities to protect Arizona's Colorado River water supply. ADWR is not a municipal water provider.
See Coliform Bacteria.
Chloramine is a combination of chlorine and ammonia and is a widely used drinking water disinfectant in North America, often used as an alternative to chlorine because it is longer lasting and has less of an odor than chlorine. Adding a small amount of chloramine to drinking water protects the water from bacteria and other microorganisms.
Chlorine is the most widely used drinking water disinfectant in North America. Adding a small amount of chlorine to drinking water protects the water from bacteria and other microorganisms. Chlorine is added to drinking water as either a gas or after having been already dissolved in water.
Chlorine is measured in the field using portable analytical instrumentation. In order to meet Federal and State microbiological drinking water regulations, it is Tucson Water policy to ensure that there is a detectable amount of chlorine in 95 percent of the samples taken from the water distribution system. To optimize microbiological protection and minimize taste and odor problems, Tucson Water works to maintain a chlorine level between 0.6 parts per million and 1.0 parts per million throughout the distribution system.
When the level of chlorine is above 0.5 parts per million, many people can smell chlorine in the water.
Coliform Bacteria
Coliforms are a type of bacteria which occur widely in the environment including surface water, soil, and decaying organic matter. This group of bacteria includes the fecal coliform group which grows in the intestines of warm blooded animals. While this group of coliform bacteria are generally not harmful in themselves, they have long been used as an indication of water quality.
Coliform samples are carefully collected in sterilized bottles at more than 240 sampling sites in the Tucson Water distribution system. A certain portion of each sample is mixed with a nutrient medium and incubated for twenty-four hours. A color change or the production of gas indicates the presence of coliform bacteria.
Corrosion occurs when metal is exposed to conditions which cause the breakdown of the metal through an exchange of ions. In water systems, this term generally refers to the rusting of metalic pipelines. Over time, corrosion will weaken the structure of the metallic pipes, resulting in leaks or breaks. Corrosion also can build up on the inside surface of the pipes, reducing water flow and contributing to water taste, odor, or color problems.
Distribution System
Tucson Water moves groundwater from wells to customer homes and businesses through an interconnected grid of water mains, valves, storage reservoirs, and pressure boosting or reducing facilities. This is generally referred to as the water distribution system.
Entry Point to the Distribution System (EPDS)
The point at which water is discharged into the distribution system from a well, storage tank, pressure tank, or water treatment plant.
Fluoride is a compound which occurs naturally in both groundwater and surface water. At low levels, fluoride can provide protection against tooth decay by increasing the durability of tooth enamel. The USEPA has set a maximum drinking water standard for fluoride of 4 parts per million (ppm). EPA has also set a secondary fluoride standard of 2 ppm to protect against dental fluorosis (tooth staining or pitting in developing teeth of children under 9 years old). In the drinking water delivered by Tucson Water, naturally occurring fluoride is found at levels between less than 0.1 to 1.2 ppm, with an average value of 0.4 ppm. The optimum fluoride level for drinking water is 0.8 ppm. Where the fluoride level of drinking water is less than 0.6 ppm, supplements may be worthwhile depending on the individual situation. Please see your medical provider. Currently, Tucson Water does not add fluoride to the drinking water supply.
Hardness measures the ease with which soap can be lathered. It is easier to produce a lather using soft water. Water hardness is generally divided into the following categories.
Soft 0 to 60 parts per million
Moderately Hard 60 to 120 parts per million
Hard 120 to 180 parts per million
Very Hard more than 180 parts per million
To convert hardness levels in parts per million to hardness in grains per gallon, divide by 17.1.
Less Than Symbol ( < )
Values preceded with "<" indicate that the concentration of the substance being tested for was not detected at that level.
Maximum Contaminant Level (MCL)
The MCL is the maximum amount of a regulated contaminant allowable in drinking water. The EPA sets standards for drinking water that all water providers must meet. Tucson Water routinely monitors the drinking water we deliver for each regulated contaminant and for many other substances.
Maximum Residual Disinfectant Level (MRDL)
Maximum Residual Disinfectant Level (MRDL) means a level of a disinfectant added for water treatment that may not be exceeded without an unacceptable possibility of adverse health effects. The chlorine MRDL is set at 4 mg/L. A public water system is in compliance with the chlorine MRDL when the running annual average is less than or equal to the MRDL. MRDLs are enforceable in the same manner as the MCLs. There is convincing evidence that addition of a disinfectant is necessary for control of microbial contaminants. In certain instances, operators may increase the disinfectant levels of chlorine in the distribution system to a level and for a time necessary to protect public health to address microbiological contamination problems.
Microbiological Quality
Control of microorganisms in water served to customers may be the most important water quality protection that a water utility can perform. Microbiological quality in drinking water distribution systems is measured by the presence or absence of a type of bacteria called coliforms. State and Federal standards limit the number of positive total coliform samples to no more than 5 percent of the total number of samples collected in Tucson Water's main system. This is accomplished by adding a low level of disinfectant, usually chlorine, to the water distribution system and monitoring the water on an on-going basis. Tucson Water collects over 240 samples every month from its distribution system to test for microbiological quality. The levels in Tucson's distribution system are far below this level.
For Tucson Water's isolated systems (WQ Zones 11 - 17), no more than one total coliform positive sample can occur in any month.
Mineral Content
Mineral content measures the amount of total dissolved solids, or TDS, in the water. The federal secondary drinking water standard for TDS is 500 parts per million. There are no health based limits for TDS in water.
90th Percentile Level
This is the value that is used to determine compliance with the action levels. If 90% of the samples are below the action levels (9 out of 10 samples), then the system is in compliance. In other words, no more than 10 percent of the samples may exceed the lead action level of 15 ppb. For copper, the action level is exceeded if more than 10 percent of the samples are above 1.3 ppm. Also note that a 90th percentile result above the action level does not mean all customers are exposed to water above the action level. The number of samples collected is dependent on the population of the public water system. In situations where only 5 samples are collected, the 90th percentile value is the average of the 2 highest values.
See also Action Level, Percentiles.
Nitrate as Nitrogen
Nitrate as nitrogen is a compound that can occur in both groundwater and surface water. In the Tucson area, nitrates as nitrogen are often related to agricultural activities, such as fertilizer use, or animal waste. Over time, rainfall can cause nitrates as nitrogen to percolate down through the ground and eventually reach the groundwater. The USEPA has set a maximum drinking water standard for nitrate as nitrogen at 10 parts per million. At higher levels, nitrate as nitrogen in water can cause serious illness in infants under 6 months of age, but is not considered harmful to older children or adults. Symptoms include shortness of breath and blueness of the skin.
Parts Per Billion (ppb)
Constituents in water are measured in small units. Many of the trace metals such as mercury and organic compounds such as trihalomethanes monitored by Tucson Water are measured in terms of parts per billion (or micrograms per liter). The following may help you visualize how very small this unit is.
One part per billion equals:
  • 2 drops of water in a typical 15,000-gallon backyard swimming pool
  • one second of time in 31.7 years
  • the first 16 inches of a trip to the moon
Parts Per Million (ppm)
Constituents in water are often measured in very small units. Many of the dissolved minerals such as sodium and calcium monitored by Tucson Water are measured in terms of parts per million (or milligrams per liter). The following may help you visualize how very small this unit is.
One part per million equals:
  • 1/4 cup of water in a typical 15,000-gallon backyard swimming pool
  • one second of time in 11.6 days
Percentage of the samples or homes tested for a particular substance which fall above or below the action levels.
See also 90th Percentile Level.
pH is a measurement of a water's acidity. Water with a pH below 7.0 is considered acidic; water with a pH above 7.0 is considered basic. pH is measured in the laboratory on the same day water samples are collected from the system.
Potable and Non-Potable Water
Potable (pronounced "pote-able") water is another term for drinking water. It refers to water which is safe to consume. Tucson Water's potable water must meet very stringent State and Federal standards to be certified as safe for delivery to customers. Non-potable water is any water which does not meet those standards due to pollution, lack of proper treatment, or exposure to environmental contamination.
Public Water System
A public water system refers to a system that provides water for human consumption through pipes or other constructed conveyances, if such a system has at least 15 service connections or regularly serves an average of at least 25 individuals daily at least 60 days out of the year.
Radon is not regulated in drinking water by the EPA. The EPA has proposed a level no higher than 4,000 picoCuries per liter (pCi/L), which contributes about 0.4 pCi/L of radon to the air in your home. When water that contains radon is used in the home for showering, washing dishes, and cooking, radon gas escapes from the water and goes into the air, although some stays in the water.
Residual Sodium Carbonate
The residual sodium carbonate (RSC) equals the sum of the bicarbonate and carbonate ion concentrations minus the sum of the calcium and magnesium ion concentrations, where the ions are expressed in meq/L. A negative RSC indicates that sodium buildup is unlikely since sufficient calcium and magnesium are in excess of what can be precipitated as carbonates. A positive RSC indicates that sodium buildup in the soil is possible. The RSC in Tucson’s reclaimed water indicates the hazard to plants is low.
Sodium is a naturally-occurring mineral in all drinking water in Tucson. While sodium is not regulated as a contaminant in drinking water, it may have an effect on the consumer's health. In general, the sodium contributed to an individual's diet from drinking water is a small part of overall dietary intake. The American Heart Association recommended standard for daily sodium intake is 3,000 milligrams. Persons on severely restricted sodium diets may want to consult their health professional regarding sodium levels in water.
Statistical Terms
The following statistical terms may help in understanding the data presented on this website:
  • Coefficient of Variation is a measure of the relative variation around the mean. It is calculated by dividing the standard deviation by the mean.
  • Count is the number of data values in a data set.
  • Maximum is the highest value of a group of numbers. The listing of the minimum and maximum values describes the range of a data set.
  • Mean is the average value of a group of data calculated by summing up the values and dividing by the number of data points (the count).
  • Median is the mid point of a set of data points; 50 percent of the values will be above the median value and 50 percent will be below the median value.
  • Minimum is the lowest value of a group of numbers.
  • Standard Deviation is a measure of the variation around the mean of a group of numbers.
The temperature of the water delivered by Tucson Water can vary depending on the time of year and the location where the measurement is taken. Part of the reason for this variability is that our water supply comes from more than 200 different wells, and groundwater temperature is a function of depth below the surface and the geology surrounding the well. The temperature of the water can also change as it flows through our drinking water system, depending on outside temperatures. The water at your tap can be 85 degrees or higher and can be affected by your private plumbing. Water temperature can affect the taste of your drinking water. Storing tap water in a clean container at room temperature or in the refrigerator will give a more refreshing taste.
Total Dissolved Solids (TDS)
See Mineral Content.
Trihalomethanes, or THMs, are produced when water is disinfected with chlorine and the chlorine reacts with naturally occurring organic matter found in all waters. Tucson groundwater contains very low levels of natural organic matter which accounts for the extremely low levels of THMs in the Tucson Water distribution system.
The federal primary drinking water standard for THMs is 80 parts per billion. The levels in Tucson Water's distribution system are far below this level.
United States Environmental Protection Agency (EPA)
The United States Environmental Protection Agency is charged with protecting the environment and the health and safety of people by establishing standards for the use of many naturally-occurring and man-made compounds and resources. As a part of this effort, the agency is charged with regulating drinking water quality in the United States. They accomplish this through investigating the possible health effects of many naturally-occurring and man-made compounds and regulating those which are shown to impact human health or the environment. For water systems, these regulations are developed as part of the Code of Federal Regulations under the Safe Drinking Water Act and its amendments. In Arizona, the Arizona Department of Environmental Quality functions as the local agency responsible for ensuring all water utilities comply with these regulations.
Water Quality Zone
A Water Quality Zone is an area of the water distribution system based on water pressure, geographical, and political boundaries.