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).^[6]

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;^[7][8]
• secondary standards prescribe aesthetic qualities, those that affect taste, odor, or appearance.^[9]

The U.S. Food and Drug Administration (FDA) regulations establish limits for contaminants in bottled water.^[10] 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.^[3]

The burden of polluted drinking water disproportionally effects under-represented and vulnerable populations.^[11] Communities that lack these clean drinking-water services are at risk of contracting water-borne and pollution-related illnesses like Cholera, diarrhea, dysentery, hepatitis A, typhoid, and polio.^[12] 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.

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.^[13] Hard water may be softened to remove these ions. The softening process often substitutes sodium cations.^[14] For certain populations, hard water may be preferable to soft water because health problems have been associated with calcium deficiencies and with excess sodium.^[15] The necessity for additional calcium and magnesium in water depends on the population in question because people generally satisfy their recommended amounts through food.^{[3]: 99, 115, 377}

Sign in Sandymount, Ireland, describing water quality, giving levels of faecal coliform E. coli and Enterococcus faecalis

 

Urban runoff discharging to coastal waters

Environmental water quality, also called ambient water quality, relates to water bodies such as lakes, rivers, and oceans.^[16] 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^[17] 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.^[18] In some locations, desired water quality conditions include high dissolved oxygen concentrations, low chlorophyll-a concentrations, high water clarity,^[19] and low water temperature.^[20]

There is some desire among the public to return water bodies to pristine, or pre-industrial conditions.^[21] 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.

Regulatory or grading systems related to water quality for bathing include the British Bathing Water Regulations 2013, the European Bathing Waters Directive 2006, and the international Blue Flag Beach scheme.

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.^[22] Scientists gathering this type of data often employ auto-sampler devices that pump increments of water at either time or discharge intervals.

More complex measurements are often made in a laboratory requiring a water sample to be collected, preserved, transported, and analyzed at another location.

Issues

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.^[23] 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.^{[24]: 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.^[25]
• 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.^[24]: 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.^[23]

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.^{[24]: 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.^[26]

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.^[27] The threat of disease increases hugely due to the large numbers of people living close together, often in squalid conditions, and without proper sanitation.^[28]

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.^[29][30]

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.^[31] 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.^[32]

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.^[33] 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.^[34] 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”.^[35] For instance, bionanotechnologists reported the development of ROSALIND 2.0, that can detect levels of diverse water pollutants.^[36][37]

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.^[38] Some remote sensing systems have also been developed for monitoring ambient water quality in riverine, estuarine and coastal water bodies.^[39][40]

The following is a list of indicators often measured by situational category:

• Alkalinity
• Color of water
• pH
• 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)
• Radon
• Heavy metals
• Pharmaceuticals
• Hormone analogs

• The World Health Organization (WHO) published updated guidelines for drinking-water quality (GDWQ) in 2017.^[3]
• The International Organization for Standardization (ISO) published ^[when?] regulation of water quality in the section of ICS 13.060,^[58] 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.^[59]