Water quality refers to the chemical, physical, biological, and radiological characteristics of water.  It is a measure of the condition of water relative to the requirements of one or more biotic species and or to any human need or purpose.  It is most frequently used by reference to a set of standards against which compliance, generally achieved through treatment of the water, can be assessed. The most common standards used to assess water quality relate to health of ecosystems, safety of human contact, and drinking water.
- 1 Standards
- 2 Categories
- 3 Sampling and measurement
- 4 Standards and reports
- 5 See also
- 6 References
- 7 External links
In the setting of standards, agencies make political and technical/scientific decisions about how the water will be used.  In the case of natural water bodies, they also make some reasonable estimate of pristine conditions. Natural water bodies will vary in response to environmental conditions. Environmental scientists work to understand how these systems function, which in turn helps to identify the sources and fates of contaminants. Environmental lawyers and policymakers work to define legislation with the intention that water is maintained at an appropriate quality for its identified use.
The vast majority of surface water on the Earth is neither potable nor toxic. This remains true when seawater in the oceans (which is too salty to drink) is not counted. Another general perception of water quality is that of a simple property that tells whether water is polluted or not. In fact, water quality is a complex subject, in part because water is a complex medium intrinsically tied to the ecology of the Earth. Industrial and commercial activities (e.g. manufacturing, mining, construction, transport) are a major cause of water pollution as are runoff from agricultural areas, urban runoff and discharge of treated and untreated sewage.
The parameters for water quality are determined by the intended use. Work in the area of water quality tends to be focused on water that is treated for human consumption, industrial use, or in the environment.
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Contaminants that may be in untreated water include microorganisms such as viruses, protozoa and bacteria; inorganic contaminants such as salts and metals; organic chemical contaminants from industrial processes and petroleum use; pesticides and herbicides; and radioactive contaminants. Water quality depends on the local geology and ecosystem, as well as human uses such as sewage dispersion, industrial pollution, use of water bodies as a heat sink, and overuse (which may lower the level of the water).
The United States Environmental Protection Agency (EPA) limits the amounts of certain contaminants in tap water provided by US public water systems. The Safe Drinking Water Act authorizes EPA to issue two types of standards:
- primary standards regulate substances that potentially affect human health;  
- secondary standards prescribe aesthetic qualities, those that affect taste, odor, or appearance. 
The U.S. Food and Drug Administration (FDA) regulations establish limits for contaminants in bottled water that must provide the same protection for public health.  Drinking water, including bottled water, may reasonably be expected to contain at least small amounts of some contaminants. The presence of these contaminants does not necessarily indicate that the water poses a health risk.
In urbanized areas around the world, water purification technology is used in municipal water systems to remove contaminants from the source water (surface water or groundwater) before it is distributed to homes, businesses, schools and other recipients. Water drawn directly from a stream, lake, or aquifer and that has no treatment will be of uncertain quality.
Dissolved minerals may affect suitability of water for a range of industrial and domestic purposes. The most familiar of these is probably the presence of ions of calcium (Ca2+) and magnesium (Mg2+) which interfere with the cleaning action of soap, and can form hard sulfate and soft carbonate deposits in water heaters or boilers.  Hard water may be softened to remove these ions. The softening process often substitutes sodium cations.  Hard water may be preferable to soft water for human consumption, since health problems have been associated with excess sodium and with calcium and magnesium deficiencies. Softening decreases nutrition and may increase cleaning effectiveness.  Various industries' wastes and effluents can also pollute the water quality in receiving bodies of water. 
Environmental water quality, also called ambient water quality, relates to water bodies such as lakes, rivers, and oceans. Water quality standards for surface waters vary significantly due to different environmental conditions, ecosystems, and intended human uses. Toxic substances and high populations of certain microorganisms can present a health hazard for non-drinking purposes such as irrigation, swimming, fishing, rafting, boating, and industrial uses. These conditions may also affect wildlife, which use the water for drinking or as a habitat. Modern water quality laws generally specify protection of fisheries and recreational use and require, as a minimum, retention of current quality standards.
There is some desire among the public to return water bodies to pristine, or pre-industrial conditions. Most current environmental laws focus on the designation of particular uses of a water body. In some countries these designations allow for some water contamination as long as the particular type of contamination is not harmful to the designated uses. Given the landscape changes (e.g., land development, urbanization, clearcutting in forested areas) in the watersheds of many freshwater bodies, returning to pristine conditions would be a significant challenge. In these cases, environmental scientists focus on achieving goals for maintaining healthy ecosystems and may concentrate on the protection of populations of endangered species and protecting human health.
The complexity of water quality as a subject is reflected in the many types of measurements of water quality indicators. The most accurate measurements of water quality are made on-site, because water exists in equilibrium with its surroundings. Measurements commonly made on-site and in direct contact with the water source in question include temperature, pH, dissolved oxygen, conductivity, oxygen reduction potential (ORP), turbidity, and Secchi disk depth.
More complex measurements are often made in a laboratory requiring a water sample to be collected, preserved, transported, and analyzed at another location. The process of water sampling introduces two significant problems:
- The first problem is the extent to which the sample may be representative of the water source of interest. Many water sources vary with time and with location. The measurement of interest may vary seasonally or from day to night or in response to some activity of man or natural populations of aquatic plants and animals.  The measurement of interest may vary with distances from the water boundary with overlying atmosphere and underlying or confining soil. The sampler must determine if a single time and location meets the needs of the investigation, or if the water use of interest can be satisfactorily assessed by averaged values with time and location, or if critical maxima and minima require individual measurements over a range of times, locations or events. The sample collection procedure must assure correct weighting of individual sampling times and locations where averaging is appropriate. :39–40 Where critical maximum or minimum values exist, statistical methods must be applied to observed variation to determine an adequate number of samples to assess probability of exceeding those critical values. 
- The second problem occurs as the sample is removed from the water source and begins to establish chemical equilibrium with its new surroundings – the sample container. Sample containers must be made of materials with minimal reactivity with substances to be measured; and pre-cleaning of sample containers is important. The water sample may dissolve part of the sample container and any residue on that container, or chemicals dissolved in the water sample may sorb onto the sample container and remain there when the water is poured out for analysis. :4 Similar physical and chemical interactions may take place with any pumps, piping, or intermediate devices used to transfer the water sample into the sample container. Water collected from depths below the surface will normally be held at the reduced pressure of the atmosphere; so gas dissolved in the water may escape into unfilled space at the top of the container. Atmospheric gas present in that air space may also dissolve into the water sample. Other chemical reaction equilibria may change if the water sample changes temperature. Finely divided solid particles formerly suspended by water turbulence may settle to the bottom of the sample container, or a solid phase may form from biological growth or chemical precipitation. Microorganisms within the water sample may biochemically alter concentrations of oxygen, carbon dioxide, and organic compounds. Changing carbon dioxide concentrations may alter pH and change solubility of chemicals of interest. These problems are of special concern during measurement of chemicals assumed to be significant at very low concentrations. 
Sample preservation may partially resolve the second problem. A common procedure is keeping samples cold to slow the rate of chemical reactions and phase change, and analyzing the sample as soon as possible; but this merely minimizes the changes rather than preventing them. :43–45 A useful procedure for determining influence of sample containers during delay between sample collection and analysis involves preparation for two artificial samples in advance of the sampling event. One sample container is filled with water known from previous analysis to contain no detectable amount of the chemical of interest. This sample, called a "blank", is opened for exposure to the atmosphere when the sample of interest is collected, then resealed and transported to the laboratory with the sample for analysis to determine if sample holding procedures introduced any measurable amount of the chemical of interest. The second artificial sample is collected with the sample of interest, but then "spiked" with a measured additional amount of the chemical of interest at the time of collection. The blank and spiked samples are carried with the sample of interest and analyzed by the same methods at the same times to determine any changes indicating gains or losses during the elapsed time between collection and analysis. 
Inevitably after events such as earthquakes and tsunamis, there is an immediate response by the aid agencies as relief operations get underway to try and restore basic infrastructure and provide the basic fundamental items that are necessary for survival and subsequent recovery.  Access to clean drinking water and adequate sanitation is a priority at times like this. The threat of disease increases hugely due to the large numbers of people living close together, often in squalid conditions, and without proper sanitation. 
After a natural disaster, as far as water quality testing is concerned there are widespread views on the best course of action to take and a variety of methods can be employed. The key basic water quality parameters that need to be addressed in an emergency are bacteriological indicators of fecal contamination, free chlorine residual, pH, turbidity and possibly conductivity/ total dissolved solids. There are a number of portable water test kits on the market widely used by aid and relief agencies for carrying out such testing.  
After major natural disasters, a considerable length of time might pass before water quality returns to pre-disaster levels. For example, following the 2004 Indian Ocean tsunami the Colombo-based International Water Management Institute (IWMI) monitored the effects of saltwater and concluded that the wells recovered to pre-tsunami drinking water quality one and a half years after the event.  IWMI developed protocols for cleaning wells contaminated by saltwater; these were subsequently officially endorsed by the World Health Organization as part of its series of Emergency Guidelines. 
The simplest methods of chemical analysis are those measuring chemical elements without respect to their form. Elemental analysis for oxygen, as an example, would indicate a concentration of 890 g/L ( grams per litre) of water sample because oxygen (O) has 89% mass of the water molecule (H2O). The method selected to measure dissolved oxygen should differentiate between diatomic oxygen and oxygen combined with other elements. The comparative simplicity of elemental analysis has produced a large amount of sample data and water quality criteria for elements sometimes identified as heavy metals. Water analysis for heavy metals must consider soil particles suspended in the water sample. These suspended soil particles may contain measurable amounts of metal. Although the particles are not dissolved in the water, they may be consumed by people drinking the water. Adding acid to a water sample to prevent loss of dissolved metals onto the sample container may dissolve more metals from suspended soil particles. Filtration of soil particles from the water sample before acid addition, however, may cause loss of dissolved metals onto the filter.  The complexities of differentiating similar organic molecules are even more challenging.
Making these complex measurements can be expensive. Because direct measurements of water quality can be expensive, ongoing monitoring programs are typically conducted by government agencies. However, there are local volunteer programs and resources available for some general assessment.  Tools available to the general public include on-site test kits, commonly used for home fish tanks, and biological assessment procedures.
Although water quality is usually sampled and analyzed at laboratories, since the late 20th century there has been increasing public interest in the quality of drinking water provided by municipal systems. Many water utilities have developed systems to collect real-time data about source water quality. In the early 21st century, a variety of sensors and remote monitoring systems have been deployed for measuring water pH, turbidity, dissolved oxygen and other parameters.  Some remote sensing systems have also been developed for monitoring ambient water quality in riverine, estuarine and coastal water bodies.  
The following is a list of indicators often measured by situational category:
- Color of water
- Taste and odor ( geosmin, 2-Methylisoborneol (MIB), etc.)
- Dissolved metals and salts ( sodium, chloride, potassium, calcium, manganese, magnesium)
- Microorganisms such as fecal coliform bacteria (Escherichia coli), Cryptosporidium, and Giardia lamblia; see Bacteriological water analysis
- Dissolved metals and metalloids ( lead, mercury, arsenic, etc.)
- Dissolved organics: colored dissolved organic matter (CDOM), dissolved organic carbon (DOC)
- Heavy metals
- Hormone analogs
Biological monitoring metrics have been developed in many places, and one widely used measure is the presence and abundance of members of the insect orders Ephemeroptera, Plecoptera and Trichoptera (common names are, respectively, mayfly, stonefly and caddisfly). EPT indexes will naturally vary from region to region, but generally, within a region, the greater the number of taxa from these orders, the better the water quality. Organisations in the United States, such as EPA. offer guidance on developing a monitoring program and identifying members of these and other aquatic insect orders. Many US wastewater dischargers (e.g., factories, power plants, refineries, mines, municipal sewage treatment plants) are required to conduct periodic whole effluent toxicity (WET) tests.  
Individuals interested in monitoring water quality who cannot afford or manage lab scale analysis can also use biological indicators to get a general reading of water quality. One example is the IOWATER volunteer water monitoring program of Iowa, which includes a benthic macroinvertebrate indicator key. 
Bivalve molluscs are largely used as bioindicators to monitor the health of aquatic environments in both fresh water and the marine environments. Their population status or structure, physiology, behaviour or the level of contamination with elements or compounds can indicate the state of contamination status of the ecosystem. They are particularly useful since they are sessile so that they are representative of the environment where they are sampled or placed. A typical project is the U.S. Mussel Watch Programme,  but today they are used worldwide.
The Southern African Scoring System (SASS) method is a biological water quality monitoring system based on the presence of benthic macroinvertebrates. The SASS aquatic biomonitoring tool has been refined over the past 30 years and is now on the fifth version (SASS5) which has been specifically modified in accordance with international standards, namely the ISO/IEC 17025 protocol.  The SASS5 method is used by the South African Department of Water Affairs as a standard method for River Health Assessment, which feeds the national River Health Programme and the national Rivers Database.
- The World Health Organization (WHO) has published guidelines for drinking-water quality (GDWQ) in 2011. 
- The International Organization for Standardization (ISO) published[ when?] regulation of water quality in the section of ICS 13.060,  ranging from water sampling, drinking water, industrial class water, sewage, and examination of water for chemical, physical or biological properties. ICS 91.140.60 covers the standards of water supply systems. 
- Directive on Urban Waste Water Treatment (91/271/EEC) of 21 May 1991 concerning discharges of municipal and some industrial wastewaters;
- The Drinking Water Directive (98/83/EC) of 3 November 1998 concerning potable water quality;
- Water Framework Directive (2000/60/EC) of 23 October 2000 concerning water resources management.
- Indian Council of Medical Research (ICMR) Standards for Drinking Water.
Water quality guidelines for South Africa are grouped according to potential user types (e.g. domestic, industrial) in the 1996 Water Quality Guidelines.  Drinking water quality is subject to the South African National Standard (SANS) 241 Drinking Water Specification. 
In England and Wales acceptable levels for drinking water supply are listed in the "Water Supply (Water Quality) Regulations 2000." 
In the United States, Water Quality Standards are defined by state agencies for various water bodies, guided by the desired uses for the water body (e.g., fish habitat, drinking water supply, recreational use).  The Clean Water Act (CWA) requires each governing jurisdiction (states, territories, and covered tribal entities) to submit a set of biennial reports on the quality of water in their area. These reports are known as the 303(d) and 305(b) reports, named for their respective CWA provisions, and are submitted to, and approved by, EPA.  These reports are completed by the governing jurisdiction, typically a state environmental agency. EPA recommends that each state submit a single "Integrated Report" comprising its list of impaired waters and the status of all water bodies in the state.  The National Water Quality Inventory Report to Congress is a general report on water quality, providing overall information about the number of miles of streams and rivers and their aggregate condition.  The CWA requires states to adopt standards for each of the possible designated uses that they assign to their waters. Should evidence suggest or document that a stream, river or lake has failed to meet the water quality criteria for one or more of its designated uses, it is placed on a list of impaired waters. Once a state has placed a water body on this list, it must develop a management plan establishing Total Maximum Daily Loads (TMDLs) for the pollutant(s) impairing the use of the water. These TMDLs establish the reductions needed to fully support the designated uses. 
- Diersing, Nancy (2009). "Water Quality: Frequently Asked Questions." Florida Brooks National Marine Sanctuary, Key West, FL.
- Johnson, D.L., S.H. Ambrose, T.J. Bassett, M.L. Bowen, D.E. Crummey, J.S. Isaacson, D.N. Johnson, P. Lamb, M. Saul, and A.E. Winter-Nelson (1997). "Meanings of environmental terms." Journal of Environmental Quality. 26: 581–589. doi: 10.2134/jeq1997.00472425002600030002x
- "What are Water Quality Standards?". Washington, D.C.: U.S. Environmental Protection Agency (EPA). 2016-03-17.
- EPA. "National Primary Drinking Water Regulations." Code of Federal Regulations, 40 C.F.R. 141.
- "Drinking Water Regulations". Drinking Water Requirements for States and Public Water Systems. EPA. 2017-09-01.
- "Secondary Drinking Water Standards: Guidance for Nuisance Chemicals". EPA. 2017-03-08.
- "FDA Regulates the Safety of Bottled Water Beverages Including Flavored Water and Nutrient-Added Water Beverages". Food Facts for Consumers. Silver Spring, Maryland: U.S. Food and Drug Administration. 2018-09-22.
- Babbitt, Harold E. & Doland, James J. Water Supply Engineering (1949) ASIN: B000OORYE2; McGraw-Hill p.388
- Linsley, Ray K. & Franzini, Joseph B. Water-Resources Engineering (1972) McGraw-Hill ISBN 0-07-037959-9 pp.454–456
- World Health Organization (2004). "Consensus of the Meeting: Nutrient minerals in drinking-water and the potential health consequences of long-term consumption of demineralized and remineralized and altered mineral content drinking-waters." Rolling Revision of the WHO Guidelines for Drinking-Water Quality (draft). From November 11–13, 2003 meeting in Rome, Italy at the WHO European Centre for Environment and Health.
- Canencia, Oliva P; Dalugdug, Marlou D; Emano, Athena Marie; Mendoza, Richard; Walag, Angelo Mark P. (2016-08-31). "Slaughter waste effluents and river catchment watershed contamination in Cagayan de Oro City, Philippines". ResearchGate. 9 (2). ISSN 2220-6663.
- Goldman, Charles R. & Horne, Alexander J. Limnology (1983) McGraw-Hill ISBN 0-07-023651-8 chapter 6
- Franson, Mary Ann (1975). Standard Methods for the Examination of Water and Wastewater 14th ed. Washington, DC: American Public Health Association, American Water Works Association & Water Pollution Control Federation. ISBN 0-87553-078-8
- "Chapter 8. Data Analysis". Handbook for Monitoring Industrial Wastewater (Report). EPA. August 1973. EPA 625/6-73/002.
- United States Geological Survey (USGS), Denver, CO (2009). "Definitions of Quality-Assurance Data." Prepared by USGS Branch of Quality Systems, Office of Water Quality.
- Natural Disasters and Severe Weather. "Water Quality After a Tsunami". Centers for Disease Control and Prevention. Retrieved 2017-04-27.
- Furusawa, Takuro; Maki, Norio; Suzuki, Shingo (2008-01-01). "Bacterial contamination of drinking water and nutritional quality of diet in the areas of the western Solomon Islands devastated by the April 2, 2007 earthquake⁄tsunami". Tropical Medicine and Health. 36 (2): 65–74. doi: 10.2149/tmh.2007-63.
- Hanaor, Dorian A. H.; Sorrell, Charles C. (2014). "Sand Supported Mixed-Phase TiO2 Photocatalysts for Water Decontamination Applications". Advanced Engineering Materials. 16 (2): 248–254. arXiv: 1404.2652. doi: 10.1002/adem.201300259.
- Method 1680: Fecal Coliforms in Sewage Sludge (Biosolids) by Multiple-Tube Fermentation using Lauryl Tryptose Broth (LTB) and EC Medium (Report). EPA. April 2010. EPA 821-R-10-003.
- International Water Management Institute, Colombo, Sri Lanka (2010). "Helping restore the quality of drinking water after the tsunami." Success Stories. Issue 7. doi: 10.5337/2011.0030
- World Health Organization (2011). "WHO technical notes for emergencies." Archived 12 February 2016 at the Wayback Machine. Water Engineering Development Centre, Loughborough University, Leicestershire, UK.
- State of California Environmental Protection Agency Representative Sampling of Ground Water for Hazardous Substances (1994) pp.23–24
- An example of a local government-sponsored volunteer monitoring program: "Monitoring Our Waters". Watershed Restoration. Rockville, Maryland: Montgomery County Department of Environmental Protection. Retrieved 2018-11-11..
- Distribution System Water Quality Monitoring: Sensor Technology Evaluation Methodology and Results (Report). EPA. October 2009. EPA 600/R-09/076.
- "Water Quality Monitoring". Lyndhurst, New Jersey: Meadowlands Environmental Research Institute. 2018-08-06.
- "Eyes on the Bay". Annapolis, MD: Maryland Department of Natural Resources. Chesapeake Bay. Retrieved 2018-12-05.
- "Whole Effluent Toxicity Methods". Clean Water Act Analytical Methods. EPA. 2018-04-19.
- Methods for Measuring the Acute Toxicity of Effluents and Receiving Waters to Freshwater and Marine Organisms (Report). EPA. October 2002. EPA-821-R-02-012.
- IOWATER (Iowa Department of Natural Resources). Iowa City, IA (2005). "Benthic Macroinvertebrate Key."
- "Center for Coastal Monitoring and Assessment: Mussel Watch Contaminant Monitoring". Ccma.nos.noaa.gov. 14 January 2014. Archived from the original on 7 September 2015. Retrieved 4 September 2015.
- Dickens CWS and Graham PM. 2002. The Southern Africa Scoring System (SASS) version 5 rapid bioassessment for rivers “African Journal of Aquatic Science”, 27:1–10.
- "Guidelines for drinking-water quality, fourth edition". World Health Organization. Retrieved 2 April 2013.
- International Organization for Standardization (ISO). "13.060: Water quality". Geneva, Switzerland. Retrieved 2011-07-04.
- International Organization for Standardization (ISO). "91.140.60: Water supply systems". Retrieved 2011-07-04.
- Republic of South Africa, Department of Water Affairs, Pretoria (1996). "Water quality guidelines for South Africa: First Edition 1996."
- Hodgson K, Manus L. A drinking water quality framework for South Africa. Water SA. 2006;32(5):673–678 .
- National Archives, London, UK. "The Water Supply (Water Quality) Regulations 2000." 2000 No. 3184. 2000-12-08.
- U.S. Clean Water Act, Section 303, 33 U.S.C. § 1313.
- U.S. Clean Water Act, Section 303(d), 33 U.S.C. § 1313; Section 305(b), 33 U.S.C. § 1315(b).
- "Program Overview: 303(d) Listing". Impaired Waters and TMDLs. EPA. 2016-10-24.
- "National Water Quality Inventory Report to Congress". Water Data and Tools. EPA. 2016-08-18.
- More information about water quality in the United States is available on EPA's "Surf Your Watershed" website.
- International organizations
- Drinking water quality guidelines – World Health Organization
- Global Water Quality online database – United Nations Global Environment Monitoring System
- The National River Health Programme – South Africa
- United States
- U.S. Centers for Disease Control and Prevention (CDC) – Drinking water quality and testing
- U.S. National Water Quality Monitoring Council (NWQMC) – Partnership of federal and state agencies
- U.S. Geological Survey – National Water Quality Assessment Program
- U.S. Environmental Protection Agency – Water Data and Tools
- U.S. Dept. of Agriculture – information on water quality and agriculture
- American Water Resources Association – professional association
- E. Coli and Indiana Lakes and Streams – Purdue University
- Other organizations
- [ NutrientNet], an online nutrient trading tool developed by the World Resources Institute, designed to address nutrient-related water quality issues. See also the PA NutrientNet website designed for Pennsylvania's nutrient trading program.
- eWater Cooperative Research Centre ( eWater Ltd) – Australian Government funded initiative supporting water management decision support tools
- MolluSCAN eye – CNRS and the University of Bordeaux, France. Online biomonitoring of water quality by a 24/7 record of various bivalve molluscs' behavior and physiology worldwide (biological rhythms, growth rate, spawning, daily behavior)