Water quality means the chemical, physical, and biological traits of water based on how it is used. These traits are often compared to a set of rules to check if the water meets requirements, which is usually done by treating the water. Common rules used to monitor water quality show how healthy ecosystems are, how safe the water is for people, how much pollution is present, and the condition of drinking water. Water quality greatly affects the availability of water and often decides the best ways to supply it.

Impacts on public health

As time passes, more people understand how important clean drinking water is for public health. This has caused more efforts to protect and manage water quality.

As scientists learn more about how water quality affects health, they are finding new health risks. For example, long-term effects of diseases that spread through water can harm children's growth and development. Also, research shows that certain harmful substances, like manganese, can damage children's brains.

Additionally, new water quality problems are being discovered, such as microplastics, perfluorinated compounds, and antimicrobial resistance.

Categories

Water quality is measured based on how the water will be used. Water quality work often focuses on water that is treated for drinking, industrial or household use, or for restoring ecosystems to protect human or aquatic life.

Untreated water may contain harmful substances, such as microorganisms like viruses, protozoa, and bacteria; inorganic materials like salts and metals; organic chemicals from industrial or petroleum activities; pesticides and herbicides; and radioactive materials. Water quality is influenced by local geology and ecosystems, as well as human activities like sewage disposal, industrial pollution, using water bodies to release heat, and overuse, which can lower water levels.

In the United States, the Environmental Protection Agency (EPA) sets limits for certain harmful substances in tap water provided by public water systems. The Safe Drinking Water Act allows the EPA to create two types of standards:

• Primary standards control substances that could harm human health.
• Secondary standards set guidelines for water’s taste, smell, or appearance.

The U.S. Food and Drug Administration (FDA) also sets limits for harmful substances in bottled water. Drinking water, including bottled water, may contain small amounts of some contaminants. The presence of these substances does not always mean the water is unsafe to drink.

In cities worldwide, water purification technology is used in public water systems to remove harmful substances from source water, such as surface water or groundwater, before it is delivered to homes, businesses, schools, and other users. Water taken directly from streams, lakes, or aquifers without treatment may not be safe to drink.

Polluted drinking water most affects underrepresented and vulnerable communities. People in low-income areas often lack access to clean water services and are more likely to suffer from illnesses caused by water pollution, such as cholera, diarrhea, dysentery, hepatitis A, typhoid, and polio. In these areas, untreated human waste may be released into drainage systems or used in agriculture without proper treatment.

Dissolved minerals, such as calcium and magnesium, can affect how water is used for industrial or household purposes. These minerals can reduce the effectiveness of soap and cause deposits in water heaters or boilers. Hard water can be softened to remove these minerals, often by replacing them with sodium. For some people, hard water may be better than soft water because health issues have been linked to low calcium levels or high sodium intake. Whether additional calcium or magnesium is needed in water depends on the population, as these minerals are often obtained from food.

Environmental water quality, also called ambient water quality, refers to the condition of water in lakes, rivers, and oceans. Standards for surface water vary because of differences in ecosystems, environmental conditions, and human uses. Toxic substances or high levels of certain microorganisms can pose health risks for activities like irrigation, swimming, fishing, or industrial use. These conditions can also harm wildlife that depends on water for drinking or habitat. The EPA states that water laws generally aim to protect fish populations and recreational uses, ensuring that current water quality standards are maintained. In some areas, ideal water conditions include high oxygen levels, low algae levels, clear water, and cool temperatures.

Some people want water bodies to return to their natural, pre-industrial state. However, current environmental laws usually focus on defining specific uses for water, such as fishing or recreation. In some countries, these rules allow limited contamination as long as it does not harm the intended use. Due to changes in landscapes, such as land development, urbanization, or deforestation, restoring water to its original condition is difficult. In these cases, scientists work to maintain healthy ecosystems and protect endangered species and human health.

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

Sampling and measurement

Water quality is a complicated topic because there are many different ways to measure it. Some measurements are best done at the location where the water is found because water interacts with its environment. Measurements that are often taken directly at the water source include temperature, pH, dissolved oxygen, conductivity, oxygen reduction potential (ORP), turbidity, and Secchi disk depth.

Water samples for testing can be collected in several ways, depending on how accurate the results need to be and the type of contaminant being studied. Methods include simple random sampling, stratified sampling, systematic and grid sampling, adaptive cluster sampling, grab samples, semi-continuous monitoring, continuous monitoring, passive sampling, remote surveillance, remote sensing, and biomonitoring. Using passive samplers helps reduce costs and the need for equipment at the sampling site.

Many contamination events happen only for a short time, often during rainstorms. For this reason, "grab" samples, which are collected at a single moment, may not fully show how much contamination is present. Scientists often use auto-sampler devices that collect water at regular time intervals or based on water flow to better track contamination levels.

More complex measurements require a water sample to be collected, preserved, transported, and tested in a laboratory.

The process of collecting water samples introduces two main problems:

• The first problem is whether the sample accurately represents the water source being studied. Water sources change over time and location, and the measurements of interest may vary due to seasons, times of day, human activity, or natural changes in aquatic life. The sample must be collected in a way that ensures it reflects the water source properly, whether through single measurements, averaged values over time and location, or specific measurements during critical events.

• The second problem is that when a sample is removed from the water source, it begins to interact with its new surroundings, such as the container holding it. Sample containers must be made of non-reactive materials and must be cleaned before use. Chemicals in the water may dissolve parts of the container or stick to it, and changes in temperature or pressure can affect the sample. Microorganisms in the water may also change the chemical composition of the sample over time.

Keeping samples cool can slow chemical changes, but it does not stop them completely. A method to check if sample containers affect the results involves using two special samples: one with no detectable chemical of interest (called a "blank") and one with a known amount of the chemical added (called a "spiked sample"). Both are tested alongside the real sample to see if the container or handling methods change the results.

After events like earthquakes or tsunamis, aid groups quickly work to restore basic services and provide essential supplies for survival. Poor sanitation and crowded living conditions increase the risk of disease.

After natural disasters, water quality testing focuses on key parameters such as bacteria from fecal contamination, free chlorine levels, pH, turbidity, and possibly conductivity or total dissolved solids. Many methods are used to clean or treat contaminated water.

After major disasters, it may take a long time for water quality to return to normal. For example, after the 2004 Indian Ocean tsunami, water sources in Colombo took about 1.5 years to recover to pre-disaster quality. Protocols for cleaning saltwater-contaminated wells were developed and later approved by the World Health Organization.

Simple chemical analysis methods measure the total amount of elements in water, regardless of their form. For example, elemental analysis for oxygen would show 890 g/L because oxygen makes up 89% of the mass of water molecules (H₂O). However, measuring dissolved oxygen specifically requires distinguishing between oxygen molecules and oxygen combined with other elements. Elemental analysis has provided much data on heavy metals, but water testing for heavy metals must also consider soil particles suspended in the water, as these may contain measurable amounts of metal.

Climate change impacts

Weather and its related events can affect water quality in many ways. These effects depend on the local climate and conditions. Events linked to weather include water shortages, heavy rain, and extreme temperatures. These events can harm water systems by causing erosion during heavy rain and floods, lead to the loss of water sources during droughts, and make water quality worse.

Climate change can also lower water quality in several ways:

Standards and reports

When setting standards, agencies decide based on how water will be used. For natural water bodies, agencies also estimate what the water would be like in its natural, untouched state. Natural water bodies change depending on a region's environment, such as the types of rocks, soil, land shape, water movement, and weather. Scientists and water chemists study factors that affect water quality, which helps identify where pollution comes from and where it goes. Lawyers and policymakers create laws to ensure water stays clean enough for its intended use.

Water quality is often seen as a simple way to tell if water is polluted. However, water quality is complex because water interacts with a region's environment, geology, and human activities. Activities like manufacturing, mining, farming, and city development often cause water pollution, as do runoff from farms, city streets, and sewage.

• The World Health Organization (WHO) updated its guidelines for drinking water quality in 2017.
• The International Organization for Standardization (ISO) created rules for water quality in the section ICS 13.060, covering topics like water sampling, drinking water, industrial water, sewage, and testing water for chemical, physical, or biological properties. ICS 91.140.60 includes standards for water supply systems.

The European Union's water policies are mainly covered by three laws:
• The Urban Waste Water Treatment Directive (91/271/EEC) from 1991, which controls the release of wastewater from cities and some industries.
• The Drinking Water Directive (98/83/EC) from 1998, which sets rules for safe drinking water.
• The Water Framework Directive (2000/60/EC) from 2000, which guides how water resources are managed.

• Indian Council of Medical Research (ICMR) Standards for Drinking Water.

South Africa's water quality guidelines are grouped by user type (e.g., homes, industries) in the 1996 Water Quality Guidelines. Drinking water quality follows the South African National Standard (SANS) 241 Drinking Water Specification.

In England and Wales, drinking water standards are listed in the "Water Supply (Water Quality) Regulations 2000."

In the United States, water quality standards are set by state agencies based on how water is used (e.g., for fish, drinking, or recreation). The Clean Water Act (CWA) requires states, territories, and tribal areas to report water quality every two years. These reports, called 303(d) and 305(b) reports, are reviewed and approved by the Environmental Protection Agency (EPA). States often combine these reports into one "Integrated Report" that lists polluted waters and the overall condition of water bodies in the state. The National Water Quality Inventory Report to Congress summarizes water quality across the country, including how many miles of rivers and streams are affected. The CWA also requires states to set standards for water uses they assign to their waters. If a water body fails to meet its quality standards, it is added to a list of polluted waters. States must then create plans to reduce pollution, called Total Maximum Daily Loads (TMDLs), to improve water quality.

Drinking water standards for public water systems are set by the EPA under the Safe Drinking Water Act.