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Water quality refers to the chemical, physical, and biological characteristics of water based on the standards of its usage. 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 monitor and assess water quality convey the health of ecosystems, safety of human contact, extend of water pollution and condition of drinking water. Water quality has a significant impact on water supply and oftentimes determines supply options.
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 potability, industrial/domestic use, or restoration (of an environment/ecosystem, generally for health of human/aquatic life).
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.  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 in terms of potability.
The burden of polluted drinking water disproportionally effects under-represented and vulnerable populations. Communities that lack these clean drinking-water services are at risk of contracting water-borne and pollution-related illnesses like Cholera, diarrhoea, dysentery, hepatitis A, typhoid, and polio. These communities are often in low-income areas, where human wastewater is discharged into a nearby drainage channel or surface water drain without sufficient treatment, or is used in agricultural irrigation.
Industrial and domestic use
Dissolved ions may affect the suitability of water for a range of industrial and domestic purposes. The most familiar of these is probably the presence of calcium (Ca2+) and magnesium (Mg2+) that 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. For certain populations, hard water may be preferable to soft water because health problems have been associated with calcium deficiencies and with excess sodium. The necessity for additional calcium and magnesium in water depends on the population in question because people generally satisfy their recommended amounts through food.
Environmental water quality
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. According to the EPA, 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.
Sampling and measurement
The complexity of water quality as a subject is reflected in the many types of measurements of water quality indicators. Some measurements of water quality are most accurately 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.
Sampling of water for physical or chemical testing can be done by several methods, depending on the accuracy needed and the characteristics of the contaminant. Sampling methods include for example simple random sampling, stratified sampling, systematic and grid sampling, adaptive cluster sampling, grab samples, semi-continuous monitoring and continuous, passive sampling, remote surveillance, remote sensing, and biomonitoring. The use of passive samplers greatly reduces the cost and the need of infrastructure on the sampling location.
Many contamination events are sharply restricted in time, most commonly in association with rain events. For this reason "grab" samples are often inadequate for fully quantifying contaminant levels. Scientists gathering this type of data often employ auto-sampler devices that pump increments of water at either time or discharge intervals.
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. 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 of sampling over 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 the 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; pre-cleaning of sample containers is important. The water sample may dissolve part of the sample container and any residue on that container, and 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 will collect at the top of the container. Atmospheric gas above the water 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 collection or 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 (negative control) and spiked sample (positive control) 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.
Testing in response to natural disasters and other emergencies
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. 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 many decontamination methods.
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 and results released 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.
Biosensors have the potential for "high sensitivity, selectivity, reliability, simplicity, low-cost and real-time response". For instance, bionanotechnologists reported the development of , that can detect levels of diverse water pollutants.
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.
Drinking water indicators
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 family of measurements for freshwater is the presence and abundance of members of the insect orders Ephemeroptera, Plecoptera and Trichoptera (EPT) (of benthic macroinvertebrates whose 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 an EPT 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 (EPT). 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.
Climate change impacts
The impacts of weather on water quality vary by local climate and context, highlighting the complexity of understanding the impact of climate change on water quality and health. There are a diverse range of mechanisms by which weather and weather-related shocks impact on water quality, and the potential ways in which climate change will affect water quality. Weather-related shocks include water shortages, heavy rain and temperature extremes. They can cause damage to water infrastructure from erosion under heavy rainfall and floods, loss of water sources in droughts, and deterioration of water quality. For this reason, climate change threatens the Sustainable Development Goal 6.1 of achieving universal access to safe drinking water.
The impacts of climate change can result in lower water quality through several mechanisms:: 4–39
- Heavy rainfall can have a rapid impact on water quality in rivers, that is delayed but still significant in reservoirs. It may also be rapid for shallow groundwater, although more limited in deeper, unfractured aquifers. For example, increases in fecal contamination of water sources is often linked to rainfall. Previous dry periods can lead to microbial contamination of drinking water in piped water supplies.
- Floods intensify the mixing of floodwater with wastewater and the redistribution of pollutants: Heavy rainfall and flooding can have an impact on water quality. This is because pollutants can be transported into water bodies by the increased surface runoff.
- Droughts reduce river dilution capacities and groundwater levels, increasing the risk of groundwater contamination.
- Saltwater intrusion from rising sea levels:: 16  In coastal regions, more salt may find its way into water resources, especially groundwater, due to higher sea levels and more intense storms.Increased eutrophication at higher temperatures: Warmer water in lakes, reservoirs and rivers can lead to more frequent harmful algal blooms in those surface water bodies.: 140 Higher temperatures also directly degrade water quality because warm water contains less oxygen.
- Permafrost degradation leads to an increased flux of contaminants.
- Increased meltwater from glaciers releases deposited contaminants and reduces water quality downstream.
Standards and reports
In the setting of standards, agencies make political and technical/scientific decisions based on how the water will be used. In the case of natural water bodies, agencies also make some reasonable estimate of pristine conditions. Natural water bodies will vary in response to a region's environmental conditions, whereby water composition is influenced by the surrounding geological features, sediments, and rock types, topography, hydrology, and climate. Environmental scientists and aqueous geochemists work to interpret the parameters and environmental conditions that impact the water quality of a region, 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.
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, geology, and anthropogenic activities of a region. 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 World Health Organization (WHO) 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.
National specifications for ambient water and drinking water
- 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.
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Plastic is the most common type of marine debris found in oceans, and it is the most widespread problem affecting the marine environment. It also threatens ocean health, food safety and quality, human health, and coastal tourism, and it contributes to climate change
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- "Watershed Restoration Program". www.fs.fed.us. Retrieved 25 March 2020.
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- "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 27 April 2017.
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- 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.
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- 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 11 November 2018..
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- UN-Water (2013) Water Security & the Global Water Agenda - A UN-Water Analytical Brief, ISBN 978-92-808-6038-2, United Nations University
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- "What are Water Quality Standards?". Washington, D.C.: U.S. Environmental Protection Agency (EPA). 17 March 2016.
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- "Guidelines for drinking-water quality, fourth edition". World Health Organization. Archived from the original on 6 July 2011. Retrieved 2 April 2013.
- International Organization for Standardization (ISO). "13.060: Water quality". Geneva, Switzerland. Retrieved 4 July 2011.
- International Organization for Standardization (ISO). "91.140.60: Water supply systems". Retrieved 4 July 2011.
- Republic of South Africa, Department of Water Affairs, Pretoria (1996). "Water quality guidelines for South Africa: First Edition 1996."
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- 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. 24 October 2016.
- "National Water Quality Inventory Report to Congress". Water Data and Tools. EPA. 18 August 2016.
- More information about water quality in the United States is available on EPA's "Surf Your Watershed" website.
- International organizations
- Drinking water quality guidelines[dead link] – World Health Organization
- Global Freshwater Quality Database (GEMStat) – United Nations environment program
- South Africa
- River Eco-status Monitoring Programme - Republic of South Africa
- United States
- U.S. Centers for Disease Control and Prevention (CDC) – Drinking water quality and testing
- U.S. Dept. of Agriculture – information on water quality and agriculture
- U.S. Environmental Protection Agency – Water Data and Tools
- U.S. Geological Survey – National Water Quality Assessment Program
- U.S. National Water Quality Monitoring Council (NWQMC) – Partnership of federal and state agencies
- American Water Resources Association Archived 24 March 2018 at the Wayback Machine – Professional association
- Other organizations