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 Water Quality Glossary

Absorption: The uptake of water or dissolved chemicals by a cell or an organism (as tree roots absorb dissolved nutrients in the soil).

Action Level: The level of lead or copper which, if exceeded, triggers treatment or other requirements that a water system must follow.

Activated Carbon: Adsorptive particles or granules of carbon usually obtained by heating carbon (such as wood). These particles or granules have a high capacity to selectively remove certain trace and soluble materials from water.

Acute Health Effect: An immediate (i.e. within hours or days) effect that may result from exposure to certain drinking water contaminants (e.g., pathogens).

Adsorption: The process by which chemicals are held on the surface of a mineral or soil particle (compare with Absorption).

Aeration: The process of adding air to water. Air can be added to water by either passing air through water or passing water through air. Aquifer: A natural underground layer, often of sand or gravel, that contains water.

Air gap: An open vertical drop, or vertical empty space, that separates a drinking (potable) water supply to be protected from another water system in a water treatment plant or other location. This open gap prevents the contamination of drinking water by backsiphonage or backflow because there is no way raw water or any other water can reach the drinking water.

Air Stripping: A treatment process used to remove dissolved gases and volatile substances from water. Large volumes of air are bubbled through the water being treated to remove (strip out) the dissolved gases and volatile substances. Also see packed tower aeration.

Alkaline: The condition of water or soil which contains a sufficient amount of alkali substances to raise the pH above 7.0.

Alkalinity: The capacity of water to neutralize acids. This capacity is caused by the water's content of carbonate, bicarbonate, hydroxide and occasion- ally borate, silicate, and phosphate. Alkalinity is expressed in milligrams per liter of equivalent calcium carbonate. Alkalinity is not the same as pH because water does not have to be strongly basic (high pH) to have a high alkalin- ity. Alkalinity is a measure of how much acid can be added to a liquid without causing a great change in pH.

Anion: A negatively charged ion in an electrolyte solution, attracted to the anode under the influence of a difference in electrical potential. Chloride (CI-) is an anion.

Aquifer: A natural underground layer of porous, water-bearing materials (sand, gravel) usually capable of yielding a large amount or supply of water.

Backwashing: The process of reversing the flow of water back through the filter media to remove the entrapped solids.

Bacteria: Singular: bacterium. Microscopic living organisms usually consisting of a single cell. Bacteria can aid in pollution control by consuming or breaking down organic matter in sewage, or by similarly acting on oil spills or other water pollutants. Some bacteria in soil, water or air may also cause human, animal and plant health problems.

Best Available Technology: The water treatment(s) that EPA certifies to be the most effective for removing a contaminant.

Black Water: Liquid and solid human body waste and the carriage water generated through toilet usage.

Breakpoint Chlorination: Addition of chlorine to water until the chlorine demand has been satisfied. At this point, further additions of chlorine will result in a free residual chlorine that is directly proportional to the amount of chlorine added beyond the breakpoint. Chronic Health Effect: The possible result of exposure over many years to a drinking water contaminant at levels above its MCL.

Calcium Carbonate Equivalent: An expression of the concentration of specified constituents in water in terms of their equivalent value to calcium carbonate. For example, the hardness in water which is caused by calcium, magnesium and other ions is usually described as calcium carbon- ate equivalent.

Catalyst: A substance that changes the speed or yield of a chemical reaction without being consumed or chemically changed by the chemical reaction.

Cation: A positively charged ion in an electrolyte solution, attracted to the cathode under the influence of a difference in electrical potential. Sodium ion (Na+) is a cation.

Chloramines: Compounds formed by the reaction of hypochlorous acid (or aqueous chlorine) with ammonia.

Chlorination: The application of chlorine to water, generally for the purpose of disinfection, but frequently for accomplishing other biological or chemical results (aiding coagulation and controlling tastes and odors).

Chronic Exposure: Occurring over a long period of time, either continuously or intermittently; used to describe ongoing exposures and effects that develop only after a long exposure.

Coagulation: The clumping together of very fine particles into larger particles caused by the use of chemicals (coagulants). The chemicals neutralize the electrical charges of the fine particles and cause destabilization of the particles. This clumping together makes it easier to separate the solids from the water by settling, skimming, draining, or filtering.

Coliform: A group of related bacteria whose presence in drinking water may indicate contamination by disease-causing microorganisms.

Community Water System: A public water system which serves at least 15 service connections used by year- round residents or regularly serves at least 25 year-round residents.

Compliance: The act of meeting all state and federal drinking water regulations.

Contaminant: Anything found in water (including microorganisms, minerals, chemicals, radionuclides, etc.) which may be harmful to human health.

Cryptosporidium: A microorganism commonly found in lakes and rivers which is highly resistant to disinfection. Cryptosporidium has caused several large outbreaks of gastrointestinal illness, with symptoms that include diarrhea, nausea, and/or stomach cramps. People with severely weakened immune systems (that is, severely immuno-compromised) are likely to have more severe and more persistent symptoms than healthy individuals.  

Dechlorination: The deliberate removal of chlorine from water. The partial or complete reduction of residual chlorine by any chemical or physical process.

Defluoridation: The removal of excess fluoride in drinking water to prevent the mottling (brown stains) of teeth.

Demineralization: A treatment process which removes dissolved minerals (salts) from water.

Desalinization: The removal of dissolved salts (such as sodium chloride, NACI) from water by natural means (leaching) or by specific water treatment processes.

Disinfectant: A chemical (commonly chlorine, chloramine, or ozone) or physical process (e.g., ultraviolet light) that kills microorganisms such as bacteria, viruses, and protozoa.

Disinfection By-Product: A compound formed by the reaction of a disinfectant such as chlorine with organic material in the water supply.

Distillate: In the distillation of a sample, a portion is evaporated; the part that is condensed afterwards is the distillate.

Distribution System: A network of pipes leading from a treatment plant to customers' plumbing systems.

Effluent: Water or some other liquid-raw, partially or completely treated-flowing from a reservoir, basin, treatment process or treatment plant.

Evaporation: The process by which water or other liquid becomes a gas (water vapor or ammonia vapor). Water from land areas, bodies of water, and all other moist surfaces is absorbed into the atmosphere as a vapor.

Evapotranspiration: The combined processes of evaporation and transpiration. It can be defined as the sum of water used by vegetation and water lost by evaporation.

Exemption: State or EPA permission for a water system not to meet a certain drinking water standard. An exemption allows a system additional time to obtain financial assistance or make improvements in order to come into compliance with the standard. The system must prove that: (1) there are compelling reasons (including economic factors) why it cannot meet a MCL or Treatment Technique; (2) it was in operation on the effective date of the requirement, and (3) the exemption will not create an unreasonable risk to public health. The state must set a schedule under which the water system will comply with the standard for which it received an exemption. 

Exposure:Contact between a person and a chemical. Exposures are calculated as the amount of chemical available for absorption by a person.

Fecal Coliform Bacteria: Bacteria found in the intestinal tracts of animals. Their presence in water or sludge is an indicator of pollution and possible contamination by pathogens.

Filtration: A process for removing particulate matter from water by passage through porous media.

Finished Water: Water that has been treated and is ready to be delivered to customers.

First Draw: The water that immediately comes out when a tap is first opened. This water is likely to have the highest level of lead contamination from plumbing materials.

Fluoridation: The addition of a chemical to increase the concentration of fluoride ions in drinking water to a predetermined optimum limit to reduce the incidence (number) of dental caries (tooth decay) in children. Defluoridation is the removal of excess fluoride in drinking water to prevent the mottling (brown stains) of teeth.

Fluorosis: An abnormal condition caused by excessive intake of fluorine, characterized chiefly by mottling of the teeth.

Free Residual Chlorination: The application of chlorine to water to produce a free available chlorine residual equal to at least 80 percent of the total residual chlorine (sum of free and combined available chlorine residual).

Fresh Water: Water that generally contains less than 1,000 milligrams-per-liter of dissolved solids

Giardia Lamblia: A protozoan frequently found in rivers and lakes, which can survive in water for 1 to 3 months, associated with the disease giardiasis. Ingestion of this protozoan in contaminated drinking water, exposure from person-to-person contact, and other exposure routes may cause giardiasis. The symptoms of this gastrointestinal disease may persist for weeks or months and include diarrhea, fatigue, and cramps.

Grey Water: Wastewater other than sewage, such as sink drainage or washing machine discharge.

Ground Water: The supply of fresh water found beneath the Earth's surface. usually in aquifers. which is often used for supplying wells and springs. Because ground water is a major source of drinking water there is growing concern over areas where leaching agricultural or industrial pollutants or substances from leaking underground storage tanks are contaminating ground water.

Hard Water: Alkaline water containing dissolved salts that interfere with some industrial processes and prevent soap from lathering. Water may be considered hard if it has a hardness greater than the typical hardness of water from the region. Some textbooks define hard water as water with a hardness of more than 100 mgAL as calcium carbonate.

Health Advisory: An EPA document that provides guidance and information on contaminants that can affect human health and that may occur in drinking water, but which EPA does not currently regulate in drinking water. 

Heavy Metals: Metallic elements with high atomic weights, e.g., mercury, chromium, cadmium, arsenic, and lead. They can damage living things at low concentrations and tend to accumulate in the food chain.

Heterotrophic Microorganisms: Bacteria and other microorganisms that use organic matter synthesized by other organisms for energy and growth.

Heterotrophic Plate Count (HPC): The number of colonies of heterotrophic bacteria grown on selected solid media at a given temperature and incubation period, usually expressed in number of bacteria per milliliter of sample.

Human Health Risk: The likelihood (or probability) that a given exposure or series of exposures may have or will damage the health of individuals experiencing the exposures.

Hydrologic Cycle: Movement or exchange of water between the atmosphere and the earth.

Infiltration: 1) The gradual flow or movement of water into and through (to percolate or pass through) the pores of the soil. Also see percolation. 2) the penetration of water from the soil into sewer or other pipes through defective joints, connections or manhole walls.

Inorganic Contaminants: Mineral-based compounds such as metals, nitrates, and asbestos. These contaminants are naturally-occurring in some water, but can also get into water through farming, chemical manufacturing, and other human activities. EPA has set legal limits on 15 inorganic contaminants.  

Insecticide: Any substance or chemical formulated to kill or control insects.

Ion: An electrically charged atom, radical (such as SO42-), or molecule formed by the loss or gain of one or more electrons.

Ionization: The splitting or dissociation (separation) of molecules into negatively and positively charged ions.

Leachate: A liquid that results from water collecting contaminants as it trickles through wastes, agricultural pesticides or fertilizers. Leaching may occur in farming areas, feedlots, and landfills, and may result in hazardous substances entering surface water, ground water, or soil.

Lead (Pb): A heavy metal that is hazardous to health if breathed or swallowed. Its use in gasoline. paints, and plumbing compounds has been sharply restricted or eliminated by federal laws and regulations. See heavy metals.

Lead Service Line: A service line made of lead which connects the water main to the building inlet and any lead pigtail, gooseneck or other fitting which is connected to such lead line.

Legionella: A genus of bacteria, some species of which have caused a type of pneumonia called Legionnaires Disease.

Maximum Contaminant Level (MCL): The highest level of a contaminant that EPA allows in drinking water. MCLs ensure that drinking water does not pose either a short-term or long-term health risk. EPA sets MCLs at levels that are economically and technologically feasible. Some states set MCLs which are more strict than EPA's.

Maximum Contaminant Level Goal (MCLG): The level of a contaminant at which there would be no risk to human health. This goal is not always economically or technologically feasible, and the goal is not legally enforceable.

Medium-Size Water System: A water system that serves greater than 3,300 and less than or equal to 50,000 person.

Milligrams Per Liter (mg/L): A measure of concentration of a dissolved substance. A concentration of one mg/L means that one milligram of a substance is dissolved in each liter of water. For practical purposes, this unit is equal to parts per million (ppm) since one liter of water is equal in weight to one million milligrams. Thus a liter of water containing 10 milligrams of calcium has 10 parts of calcium per one million parts of water, or 10 parts per million (10 ppm).

Microorganisms: Tiny living organisms that can be seen only with the aid of a microscope. Some microorganisms can cause acute health problems when consumed in drinking water. Also known as microbes.

Monitoring: Testing that water systems must perform to detect and measure contaminants. A water system that does not follow EPA's monitoring methodology or schedule is in violation, and may be subject to legal action. 

Nitrates:Inorganic compounds that can enter water supplies from fertilizer runoff and sanitary wastewater discharges. Nitrates in drinking water are associated with methemoglobinemia, or blue baby syndrome, which results from interferences in the bloods ability to carry oxygen.

Non-Potable: Water that may contain objectionable pollution, contamination, minerals, or infective agents and is considered unsafe and/or unpalatable for drinking.

Non-Transient, Non-Community Water System: A water system which supplies water to 25 or more of the same people at least six months per year in places other than their residences. Some examples are schools, factories, office buildings, and hospitals which have their own water systems. 

Organic Contaminants: Carbon-based chemicals, such as chlorohydrocarbons, solvents and pesticides, which can get into water through runoff from cropland or discharge from factories. EPA has set legal limits on 56 organic contaminants.

Osmosis: The passage of a liquid from a weak solution to a more concentrated solution across a semipermeable membrane. The membrane allows the passage of the solvent (water) but not the dissolved solids (solutes). This process tends to equalize the conditions on either side of the membrane.

Oxidation: Oxidation is the addition of oxygen, removal of hydrogen, or the removal of electrons from an element or compound. In the environment, organic matter is oxidized to more stable substances. The opposite of reduction

Oxidation-reduction potential: The electrical potential required to transfer electrons from one compound or element (the oxidant) to another compound or element (the reductant); used as a qualitative measure of the state of oxidation in water treatment systems.

Oxidizing agent: Any substance, such as oxygen (O2) or chlorine (Cl2), that will readily add (take on) electrons. The opposite is a reducing agent.

Ozonation: The application of ozone to water for disinfection or for taste and odor control.

Particulate: A very small solid suspended in water which can vary widely in size, shape, density, and electrical charge. Colloidal and dispersed particulates are artificially gathered together by the processes of coagulation and flocculation.

Parts Per Million (PPM): Parts per million parts, a measurement of concentration on a weight or volume basis. This term is equivalent to milligrams per liter (mg/L) which is the preferred term.

Pathogens: Microorganisms that can cause disease in other organisms or in humans, animals and plants. They may be bacteria, viruses, or parasites and are found in sewage in runoff from animal farms or rural areas populated with domestic and/or wild animals, and in water used for swimming. Fish and shellfish contaminated by pathogens, or the contaminated water itself, can cause serious illnesses.

Per capita:Per person; generally used in expressions of water use, gallons per capita per day (gpcd).

Permeate: To penetrate and pass through, as water penetrates and passes through soil and other porous materials.

Pesticide: Any substance or chemical designed or formulated to kill or control weeds or animal pests. Also see algicide, herbicide, insecticide and rodenticide.

pH: pH is an expression of the intensity of the basic or acid condition of a liquid. Mathematically, pH is the logarithm (base 10) of the reciprocal of the hydrogen ion concentration, [H+]. pH= Log (I/[H+]) The pH may range from 0 to 14, where 0 is most acid, 14 most basic, and 7 neutral. Natural waters usually have a pH between 6.5 and 8.5.

Point-of-Entry Water Treatment (POE): Refers to devices used in the home where water pipes enter to provide additional treatment of drinking water used throughout the home.

Point-of-Use Water Treatment (POU):Refers to devices used in the home or office on a specific tap to provide additional drinking water treatment.

Pollutant: Generally, any substance introduced into the environment that adversely affects the usefulness of a resource.

Potable Water: Water that is safe and satisfactory for drinking and cooking.

ppb: Parts per billion. Also pg/L or micrograms per liter.

ppm: Parts per million. Also mg/L or milligrams per liter.

Primacy State: A State that has the responsibility and authority to administer EPA's drinking water regulations within its borders. The State must have rules at least as stringent as EPA's.

Product Water: Water that has passed through a water treatment plant. All the treatment processes are completed or finished. This water is the product from the water treatment plant and is ready to be delivered to the consumers. Also called finished water.

Public Notification: An advisory that EPA requires a water system to distribute to affected consumers when the system has violated MCLs or other regulations. The notice advises consumers what precautions, if any, they should take to protect their health.

Public Water System (PWS): Any water system which provides water to at least 25 people for at least 60 days annually. There are more than 170,000 PWSs providing water from wells, rivers and other sources to about 250 million Americans. The others drink water from private wells. There are differing standards for PWSs of different sizes and types.  

Radionuclides: Elements that undergo a process of natural decay. As radionuclides decay, they emit radiation in the form of alpha or beta particles and gamma photons. Radiation can cause adverse health effects, such as cancer, so limits are placed on radionuclide concentrations in drinking water.

Raw Water: Water in its natural state, prior to any treatment for drinking.

Reservoir: Any natural or artificial holding area used to store; regulate, or control water.

Residual Chlorine: The amount of free and/or available chlorine remaining after a given contact time under speci- fied conditions.

Reverse Osmosis: The application of pressure to a concentrated solution which causes the passage of a liquid from the concentrated solution to a weaker solution across a semipermeable membrane. The membrane allows the passage of the solvent (water) but not the dissolved solids (solutes). The liquid produced is a demineralized water. Also see osmosis.

Risk:The potential for harm to people exposed to chemicals. In order for there to be risk, there must be hazard and there must be exposure.

Run-Off: That part of precipitation, snow melt, or irrigation water that runs off the land into streams or other surface water. It can carry pollutants from the air and land into the receiving waters.

Sacrificial Anode: An easily corroded material deliberately installed in a pipe or tank. The intent of such an installation is to give up (sacrifice) this anode to corrosion while the water supply facilities remain relatively corrosion free.

Safe Drinking Water Act (SDWA): Commonly referred to as SDWA. An Act passed by the U.S. Congress in 1974. The Act establishes a cooperative program among local, state and federal agencies to insure safe drinking water for consumers.

Safe Water: Water that does not contain harmful bacteria, or toxic materials or chemicals. Water may have taste and odor problems, color and certain mineral problems and still be considered safe for drinking.

Salinity: 1)The relative concentration of dissolved salts, usually sodium chloride, in a given water. 2) A measure of the concentration of dissolved mineral substances in water.

Sample: The water that is analyzed for the presence of EPA-regulated drinking water contaminants. Depending on the regulation, EPA requires water systems and states to take samples from source water, from water leaving the treatment facility, or from the taps of selected consumers.

Sand: Soil particles between 0.05 and 2 .0 mm in diameter.

Sanitary Survey: An on-site review of the water sources, facilities, equipment, operation, and maintenance of a public water systems for the purpose of evaluating the adequacy of the facilities for producing and distributing safe drinking water.

Secondary Drinking Water Standards: Non-enforceable federal guidelines regarding cosmetic effects (such as tooth or skin discoloration) or aesthetic effects (such as taste, odor, or color) of drinking water.

Septic System: An onsite system designed to treat and dispose of domestic sewage; a typical septic system consists of a tank that receives wastes from a residence or business And a system of tile lines or a pit for disposal of the liquid effluent that remains after decomposition of the solids by bacteria in the tank.

Sewer: An underground system of conduits (pipes and/or tunnels) that collect and transport wastewaters and/or runoff; gravity sewers carry free-flowing water and wastes; pressurized sewers carry pumped wastewaters under pressure.

Silt: Soil particles between 0.05 and 0.002 millimeter in approximate diameter.

SMCLs: Secondary Maximum Contaminant Levels. Secondary MCLs for various water quality indicators are established to protect public welfare.

Soft Water: Water having a low concentration of calcium and magnesium ions. According to U.S. Geological Survey guidelines, soft water is water having a hardness of 60 milligrams per liter or less

Sole Source Aquifer: An aquifer that supplies 50 percent or more of the drinking water of an area.

Source Water: Water in its natural state, prior to any treatment for drinking.

Surface Runoff: Precipitation, snow melt, or irrigation in excess of what can infiltrate the soil surface and be stored in small surface depressions; runoff is a major transporter of non-point source pollutants.

Surface Water: The water that systems pump and treat from sources open to the atmosphere, such as rivers, lakes, and reservoirs. 

Total Coliform: Bacteria that are used as indicators of fecal contaminants in drinking water.

Total Dissolved Solids (TDS): All of the dissolved solids in a water. TDS is measured on a sample of water that has passed through a very fine mesh filter to remove suspended solids. The water passing through the filter is evaporated and the residue represents the dissolved solids.

Total Residual Chlorine: The amount of available chlorine remaining after a given contact time. The sum of the combined available residual chlorine and the free available residual chlorine. Also see residual chlorine

Toxic Pollutants: Materials contaminating the environment that cause death, disease. birth defects in organisms that ingest or absorb them. The quantities and length of exposure necessary to cause these effects can vary widely.

Toxicity: The property of a chemical to harm people who come into contact with it.

Transient, Non-Community Water System: A water system which provides water in a place such as a gas station or campground where people do not remain for long periods of time. These systems do not have to test or treat their water for contaminants which pose long-term health risks because fewer than 25 people drink the water over a long period. They still must test their water for microbes and several chemicals. 

Treatment Technique: A specific treatment method required by EPA to be used to control the level of a contaminant in drinking water. In specific cases where EPA has determined it is not technically or economically feasible to establish an MCL, EPA can instead specify a treatment technique.

Trihalomethane: One of a family of organic compounds named as derivatives of methane. THMs are generally the by-product from chlorination of drinking water that contains organic material. The resulting compounds (THMs) are suspected of causing cancer.

Turbidity: The cloudy appearance of water caused by the presence of suspended and colloidal matter. In the waterworks field, a turbidity measurement is used to indicate the clarity of water. Technically, turbidity is an optical property of the water based on the amount of light reflected by suspended particles. Turbidity cannot be directly equated to suspended solids because white particles reflect more light than dark-colored particles and many small particles will reflect more light than an equivalent large particle.

Variance: State or EPA permission not to meet a certain drinking water standard. The water system must prove that: (1) it cannot meet a MCL, even while using the best available treatment method, because of the characteristics of the raw water, and (2) the variance will not create an unreasonable risk to public health. The State or EPA must review, and allow public comment on, a variance every three years. States can also grant variances to water systems that serve small populations and which prove that they are unable to afford the required treatment, an alternative water source, or otherwise comply with the standard.

Violation: A failure to meet any state or federal drinking water regulation.

Volatile Organic Chemicals (V.O.C.'s): Chemicals that, as liquid, evaporate into the air at relatively low temperature.

Vulnerability Assessment: An evaluation of drinking water source quality and its vulnerability to contamination by pathogens and toxic chemicals.  

Wastewater: The used water and solids from a community (including used water from industrial processes) that flow to a treatment plant. Storm water, surface water, and ground- water infiltration also may be included in the wastewater that enters a wastewater treatment plant. The term -sewage usually refers to household wastes, but this word is being replaced by the term -wastewater.

Wastewater Treatment Plant: A facility that receives wastewaters ( and sometimes runoff) from domestic and/or industrial sources, and by a combination of physical, chemical, and biological processes reduces (treats) the wastewaters to less harmful byproducts; known by the acronyms , STP (sewage treatment plant), and POTW (publicly owned treatment works).

Waterborne Disease Outbreak: The significant occurrence of acute infectious illness, epidemiologically associated with the ingestion of water from a public water system that is deficient in treatment, as determined by the appropriate local or state agency.

Water Supply System: The collection, treatment, storage, and. distribution of potable water from source to consumer.

Water Table: The level of ground water. The upper surface of the zone of saturation of groundwater above an imperme- able layer of soil or rock (through which water cannot move) as in an unconfined aquifer. This level can be very near the surface of the ground or far below it.

Watershed: The land area from which water drains into a stream, river, or reservoir.

Well: A bored, drilled, or driven shaft, or a dug hole, whose depth is greater than the largest surface dimension and whose purpose is to reach underground water supplies or oil, or to store or bury fluids below ground.

Wellhead Protection Area: The area surrounding a drinking water well or well field which is protected to prevent contamination of the well(s).