Water pollution is when water becomes dirty or polluted. This can happen in lakes, rivers, oceans, and other water sources. It is usually caused by human actions. Water pollution occurs when harmful substances mix with these water sources. These harmful substances come from four main causes: sewage, industrial activities, farming, and rainwater runoff from cities. Water pollution can affect both surface water, like lakes and rivers, and groundwater, which is water found underground. This type of pollution can cause many problems. One problem is harm to water environments where plants and animals live. Another is the spread of diseases when people drink or use polluted water for farming. Water pollution also reduces the usefulness of water, such as its ability to provide clean drinking water.

Sources of water pollution are either point sources or non-point sources. Point sources are specific places, like a factory drain, that cause pollution. Non-point sources are harder to trace, such as pollution from farms when rain washes chemicals into water. Pollution happens over time because of many small causes adding up. Pollution can take many forms. One form includes harmful materials like oil, metals, plastics, pesticides, certain chemicals, and industrial waste. Another form includes changes in water conditions, such as changes in acidity, low oxygen levels, high temperatures, too much dirt in the water, or changes in salt levels. Another form is the presence of disease-causing germs. Pollutants can be both natural and man-made. A common cause of water becoming too hot is when industries use water to cool machines.

Managing water pollution needs proper systems, plans, and laws. Solutions can include improving sanitation, treating sewage and industrial waste, controlling pollution from farms, stopping soil erosion, managing soil movement, and reducing runoff from cities (including managing stormwater).

Definition

Water pollution occurs when substances or energy are added to water, changing it in a way that harms its normal uses. This usually happens due to human-made substances. Polluted water may not be safe for drinking, and it can greatly affect the living things in the water, such as fish.

Contaminants

The following compounds can enter water bodies through raw sewage or treated sewage discharges:

• Chemicals from personal hygiene and cosmetic products.
• Disinfection by-products in treated drinking water. These chemicals may pollute water systems but are volatile and usually not found in natural water sources.
• Hormones from animal farming and human contraception, as well as synthetic materials like phthalates that act like hormones. These substances can harm plants, animals, and humans even in small amounts if the water is used for drinking.
• Insecticides and herbicides, often from agricultural runoff.
• Pathogens such as Hepatovirus A. This virus may be present in treated wastewater or receiving water bodies but is usually removed during drinking water treatment.

Untreated wastewater can carry nutrients, pathogens, solid particles, and organic waste from feces.

Common pathogens in wastewater include bacteria, viruses, protozoans, and parasitic worms. Scientists often use indicator organisms, like total coliforms or fecal coliforms (such as Escherichia coli), to test for contamination because detecting pathogens directly is difficult and costly.

Pathogens in water can cause diseases in humans and animals. Examples of harmful microorganisms found in contaminated water include Burkholderia pseudomallei, Cryptosporidium parvum, Giardia lamblia, Salmonella, norovirus, and parasitic worms like Schistosoma.

High levels of pathogens in water can come from human feces (due to open defecation), sewage, blackwater, or manure. This often happens when sanitation systems are inadequate, sewage treatment lacks disinfection steps, or stormwater overflows occur.

Organic substances in water can be toxic. Examples include:
• Petroleum hydrocarbons, such as gasoline, diesel, and motor oil, from oil spills or stormwater runoff.
• Volatile organic compounds, like improperly stored industrial solvents, including harmful chemicals such as polychlorinated biphenyls (PCBs) and trichloroethylene.

Per- and polyfluoroalkyl substances (PFAS) are long-lasting chemicals that do not break down easily.

Inorganic pollutants in water include:
• Ammonia from food waste.
• Heavy metals from vehicles and acid mine drainage.
• Nitrates and phosphates from sewage and agriculture (see nutrient pollution).
• Sediment from construction sites, sewage, logging, or land clearing.
• Salt, which can pollute freshwater through runoff from road de-icing or other human activities.

Pharmaceuticals and personal care products (PPCPs) have been studied since the 1990s. These include medicines used by people and products used in farming to improve livestock health. Over 20 million tons of PPCPs are produced yearly. The European Union has labeled certain pharmaceutical residues as "priority substances" due to their potential to pollute water and soil.

PPCPs have been found in water worldwide. More research is needed to understand their risks, but current studies show they harm the environment and species like fish and coral. These substances are not removed by standard sewage treatment and require advanced steps that few plants have.

A 2022 study found pharmaceutical pollution threatens health in over a quarter of tested river locations. It examined 1,052 sites across 258 rivers in 104 countries, covering 470 million people. The most polluted areas were in low- to middle-income countries with poor waste management and pharmaceutical manufacturing. Common pollutants included antidepressants, antibiotics, contraceptive drugs, and metabolites of illegal drugs like methamphetamine and ecstasy.

Solid waste can enter water through untreated sewage, stormwater runoff, or people throwing garbage into the environment. Wind can carry waste from landfills, causing visible pollution (like plastic debris) and invisible microplastic pollution.

Microplastics are widespread in water, especially in oceans. About 35% of ocean microplastics come from clothing made of synthetic materials like polyester, acrylic, or nylon, which shed tiny plastic particles during washing.

The main ways microplastics reach water are through stormwater, untreated sewage, and wind. The most common sources are synthetic fabrics, tires, and urban dust, which together cause over 80% of microplastic pollution.

Types of surface water pollution

Surface water pollution includes pollution of rivers, lakes, and oceans. A type of surface water pollution is marine pollution, which affects oceans. Nutrient pollution refers to contamination caused by too many nutrients entering water.

In 2017, about 4.5 billion people worldwide did not have access to safe sanitation systems, according to the Joint Monitoring Programme for Water Supply and Sanitation. Lack of sanitation often leads to water pollution. For example, open defecation can cause human waste to enter surface waters during rain or floods. Simple pit latrines may also flood during heavy rain.

In 2022, Europe and Central Asia contributed about 16% of the world's microplastic discharge into the seas. Although plastic waste management and recycling are improving globally, the total amount of plastic pollution continues to increase because of the large amount of plastic already in use. Even if ocean plastic pollution stopped completely, microplastic contamination in the ocean would likely keep growing.

Marine pollution occurs when human-made materials, such as industrial, agricultural, and residential waste; particles; noise; excess carbon dioxide; or invasive organisms, enter the ocean and cause harm. Most of this waste (80%) comes from land-based activities, though marine transportation also contributes. This pollution includes chemicals and trash, much of which comes from land and is washed or blown into the ocean. It harms the environment, the health of all living things, and economies worldwide. Since most pollution comes from land via rivers, sewage, or the atmosphere, continental shelves are more vulnerable to pollution. Air pollution also contributes by carrying iron, carbonic acid, nitrogen, silicon, sulfur, pesticides, and dust into the ocean. Pollution often comes from nonpoint sources, such as agricultural runoff, wind-blown debris, and dust. These sources include runoff from land that enters the ocean through rivers, but wind-blown debris and dust can also reach waterways and oceans. Pollution pathways include direct discharge, land runoff, ship pollution, bilge pollution, dredging (which creates dredge plumes), atmospheric pollution, and possibly deep-sea mining.

Nutrient pollution is a type of water pollution caused by too many nutrients entering water. It is a main cause of eutrophication, which happens when excess nutrients, usually nitrogen or phosphorus, cause too much algae to grow in lakes, rivers, and coastal waters. Sources of nutrient pollution include runoff from farms, waste from septic tanks and feedlots, emissions from burning fuels, and raw sewage, which is rich in nutrients and harms water when dumped. Excess nitrogen causes problems like harmful algal blooms, low oxygen levels (hypoxia), acid rain, nitrogen saturation in forests, and climate change.

Thermal pollution, sometimes called "thermal enrichment," changes the temperature of natural water bodies, harming water quality. It occurs when human activities raise or lower water temperatures. Unlike chemical pollution, thermal pollution changes the physical properties of water. A common cause is the use of water as a coolant by power plants and industries. Urban runoff—stormwater from rooftops, roads, and parking lots—and reservoirs can also cause thermal pollution. Releasing very cold water from reservoirs into warmer rivers is another cause.

Higher water temperatures reduce oxygen levels because gases are less soluble in warmer water. This can kill fish, which may then rot and change the food chain, reducing biodiversity and allowing heat-loving species to move in.

Introducing aquatic invasive organisms is also a form of water pollution, known as biological pollution.

Groundwater pollution

Groundwater pollution happens when harmful substances enter the ground and move into groundwater. This type of water pollution can also occur naturally if small amounts of unwanted substances, such as chemicals or minerals, are already present in the groundwater. In such cases, it is often called contamination instead of pollution. Groundwater pollution can come from human activities like faulty septic tanks, liquid from landfills, wastewater from treatment plants, broken sewer pipes, gas stations, hydraulic fracturing (fracking), or overuse of fertilizers in farming. It can also occur naturally when substances like arsenic or fluoride are found in groundwater. Using polluted groundwater can harm people's health by causing poisoning or spreading diseases that are carried in water.

In many areas around the world, groundwater pollution threatens the health of people and ecosystems. One out of every four people globally depends on groundwater for drinking water. However, when water flows quickly into certain underground water sources made of carbonate rock, it can carry short-term pollutants into these aquifers, making the water less safe to use.

Pollution from point sources

Point source water pollution happens when harmful substances enter a water body from one clear and specific place, such as a pipe or ditch. Examples of these sources include wastewater from sewage treatment plants, factories, and city storm drains.

The U.S. Clean Water Act (CWA) provides a definition of point source for legal purposes (see United States regulation of point source water pollution). This definition was updated in 1987 to include municipal storm sewer systems and industrial stormwater, such as runoff from construction sites.

Sewage is mostly water, with about 0.1% being solid material. It contains many nutrients that can cause excessive growth of plants and algae, a process called eutrophication. Sewage is a major source of phosphate, for example. It may also contain chemicals from personal care products, cosmetics, medicines, and their breakdown products. Water pollution caused by medicines that remain in the environment can harm ecosystems and humans. For instance, when sewers overflow during heavy rain, untreated sewage can pollute water. These events are called sanitary sewer overflows or combined sewer overflows.

Industries that use water also produce wastewater, known as industrial wastewater. In the United States, power plants, oil refineries, steel mills, paper mills, and food processing companies are the largest users of water, consuming more than 60% of the total. Some industries release harmful substances, such as toxic solvents, heavy metals, and other dangerous pollutants.

If industrial wastewater is not treated or managed properly, it can add the following pollutants to water bodies:

• Heavy metals, such as mercury, lead, and chromium
• Organic matter and nutrients, including food waste, which can increase levels of Biochemical Oxygen Demand (BOD), ammonia nitrogen, and oil and grease
• Inorganic particles, such as sand, metal pieces, rubber from tires, and ceramic materials
• Toxins, such as pesticides, poisons, and herbicides
• Medicines, hormone-like substances, and other harmful chemicals
• Tiny plastic pieces, such as beads made from polyethylene or polypropylene, and fibers from polyester or polyamide
• Heat pollution from power plants and factories
• Radioactive materials from uranium mining, nuclear fuel processing, or nuclear waste disposal
• Long-lasting chemicals, such as per- and polyfluoroalkyl substances (PFAS)

An oil spill occurs when liquid petroleum hydrocarbon is released into the environment, often in the ocean or coastal waters, due to human activity. These spills can come from oil tankers, drilling rigs, or wells, and may involve gasoline, diesel fuel, or heavy fuels used by ships. Oil spills can cause serious harm to the environment and the economy.

Pollution from nonpoint sources

Nonpoint source (NPS) pollution is a type of pollution that spreads out over a large area and does not come from one specific place. It happens when small amounts of pollution from many different places collect together. This is different from point source pollution, which comes from one clear place, like a factory or a pipe. NPS pollution often comes from things like rainwater running over land, snow melting, or water moving through soil. It is hard to trace this pollution back to one single place. For example, polluted water from farms running into a river or trash blown by the wind into the ocean are examples of NPS water pollution. For air pollution, smoke from cars or smokestacks can spread pollution over long distances, making it hard to link the pollution to one source. Even though these pollutants started at one place, their ability to travel far and come from many sources makes them nonpoint sources of pollution. If the same pollution happened in one place, it would be a point source.

Agriculture is a major cause of water pollution from nonpoint sources. Using too much fertilizer and runoff from fields, pastures, and animal farms can pollute water with too many nutrients. Farming fish, like in fish farms, also adds to pollution. Runoff from farms often has high levels of pesticides, which can harm water quality.

Air deposition is when pollution from the air, such as from factories or natural events, lands in water. This can pollute water near the source or even far away, sometimes thousands of miles away. Common pollutants from industrial air deposition include sulfur, nitrogen, mercury, heavy metals, and some pesticides or industrial waste. Natural causes of air deposition include wildfires and activities by tiny living things, like microbes.

Acid rain happens when sulfur dioxide and nitrogen oxide are released into the air. These gases react with water in the atmosphere to form acids. Since the 1970s, some governments have tried to reduce the release of these gases. Most of the sulfur and nitrogen that cause acid rain come from human activities, like burning fossil fuels. However, natural events, such as lightning or volcanic eruptions, can also create these gases. Acid rain can harm plants, animals in water, and buildings or structures.

Since the 1850s, the amount of carbon dioxide in the air has increased because of human activities, like burning fossil fuels. This leads to ocean acidification, which is a type of water pollution caused by pollution in the air.

Sampling, measurements, analysis

Water pollution can be studied using three main types of methods: physical, chemical, and biological. Some methods are done in the environment without taking samples, such as measuring temperature. Other methods require collecting water samples and testing them in a laboratory. Standardized and tested methods for analyzing water and wastewater samples are available in published resources.

Common physical tests include measuring temperature, electrical conductivity, the amount of solid particles in water (such as total suspended solids), and turbidity. Water samples can also be tested using chemical methods. Many published methods exist for testing both organic and inorganic compounds. Common chemical tests measure parameters like pH, biochemical oxygen demand (BOD), chemical oxygen demand (COD), dissolved oxygen (DO), total hardness, nutrients (such as nitrogen and phosphorus compounds like nitrate and orthophosphates), metals (such as copper, zinc, cadmium, lead, and mercury), oil and grease, total petroleum hydrocarbons (TPH), surfactants, and pesticides.

Biological monitoring uses living organisms to check the health of water ecosystems. This involves measuring changes in plants, animals, or microbes to learn about the environment around them. Bioindicators are species or groups of species that show how healthy an ecosystem is. For example, copepods and other small crustaceans in water can indicate problems in their environment through changes in their biology, behavior, or health.

Water quality is studied using many different measurements. Some measurements are best done directly at the water source because water interacts with its surroundings. These on-site measurements include temperature, pH, dissolved oxygen, electrical conductivity, oxygen reduction potential (ORP), turbidity, and Secchi disk depth.

Impacts

Water pollution is a serious environmental issue because it harms all water environments, such as rivers, lakes, and oceans. Contaminants that cause water pollution include chemicals, germs, and changes like higher water temperatures. Some chemicals found in water, like calcium, sodium, and iron, are naturally present. However, whether these substances are considered natural or harmful depends on their amounts. Too much of these natural substances can hurt plants and animals in water. Materials that use up oxygen in water can be natural, like leaves, or man-made chemicals. Other substances, both natural and from human activities, can make water cloudy, which blocks sunlight needed for plants to grow and can block fish gills.

A study from 2017 reported that polluted water spreads diseases like stomach infections and parasites and caused 1.8 million deaths. Long-term contact with polluted water can lead to health risks, such as a higher chance of developing cancer or other illnesses.

Nitrogen pollution can lead to eutrophication, which often affects lakes. Eutrophication happens when too many nutrients, like nitrogen, enter water, causing excessive growth of plants and algae. This can reduce oxygen levels in the water and lower water quality, harming fish and other animals.

Eutrophication is a process where nutrients build up in water, leading to rapid growth of plants and algae. This often occurs when chemicals used to help crops grow, such as fertilizers, are washed into water by rain. Eutrophication can happen naturally or because of human activities, such as sewage, industrial waste, or fertilizer runoff. These activities often cause algal blooms and bacteria growth, which use up oxygen in water and damage the environment. Many rules and programs, including goals from the United Nations Development Program, have been created to reduce eutrophication.

Ocean acidification is another effect of water pollution. It happens when the ocean absorbs carbon dioxide from the air, which lowers the ocean’s pH level over time.

Prevalence

Water pollution is a problem in both less developed and more developed countries. For example, water pollution is common in India and China. In China, about 90 percent of the water in cities is polluted.

Control and reduction

Mandatory rules are one way to protect the environment, but they are not the only solution. Other important methods for controlling pollution include teaching people about the environment, using economic tools, relying on market forces, and enforcing rules more strictly. Standards can be "clear" (setting a specific minimum or maximum amount of a pollutant) or "less clear," which would require using the best available technology (BAT) or best practicable environmental option (BPEO). Economic tools that use market forces can include taxes, financial support, deposit or refund systems, creating a market for pollution credits, and offering rewards for following rules.

A complete approach to controlling chemical pollution includes using several methods together: combining different control measures, considering pollution that crosses borders, adding extra steps to control pollution, looking at the full life cycle of chemicals, and understanding how mixtures of chemicals can affect the environment.

Controlling water pollution needs proper infrastructure and management plans. This may include wastewater treatment plants, such as those for sewage and industrial waste. Treating agricultural wastewater from farms and controlling erosion at construction sites can also help prevent water pollution. Managing urban runoff effectively involves slowing down and reducing the amount of water flowing.

Water pollution requires regular checks and updates to water resource policies at all levels, from international agreements to local wells.

Municipal wastewater can be treated in large sewage treatment plants, smaller local systems, natural solutions like wetlands, or on-site systems like septic tanks. For example, waste stabilization ponds can be a low-cost way to treat sewage. Sunlight can help break down some pollutants in these ponds. Providing safe sanitation services can stop water pollution caused by lack of access to toilets.

Well-designed systems, such as those with secondary or advanced tertiary treatment, can remove 90% or more of pollutants in sewage. Some plants also remove nutrients and harmful germs. While these advanced methods reduce pollution, they can also be expensive and increase energy use and greenhouse gas emissions.

Overflowing sewers during storms can be fixed by repairing and upgrading the sewer system. In the United States, cities with large combined sewer systems have not fully separated their systems because of high costs, but they have done partial projects and used green infrastructure. Some cities have added extra storage for overflow or expanded treatment capacity.

Industrial wastewater treatment involves cleaning water produced by industries as waste. After treatment, this water can be reused or released into sewers or natural water sources. Some industries send their wastewater to sewage treatment plants, while others have their own special systems to clean their waste so it meets regulations. This is especially important for industries that produce wastewater with high levels of organic matter, toxic chemicals, or nutrients. Some industries use pre-treatment systems to remove harmful substances before sending the water to the city sewer system.

Agricultural wastewater treatment is part of managing pollution from farms, especially from large animal operations and runoff from fertilizers, pesticides, or irrigation. This is required for farms that keep animals in confined spaces, like dairy or egg farms. Treatment can happen in plants with equipment similar to industrial systems. If land is available, ponds or lagoons may be cheaper for seasonal use. Animal waste is often stored in anaerobic lagoons before being spread on fields. Sometimes, wetlands are built to help treat waste.

Sediment from construction sites can be controlled with erosion prevention methods, like mulching or planting grass, and sediment control measures, like basins or fences. Spills of harmful chemicals, such as fuel or concrete, can be prevented with plans to stop accidents and special containers or structures.

Erosion from cutting down trees or changing water flow can lead to loss of soil and water pollution.

Managing urban runoff involves slowing down stormwater and reducing pollution. Local governments use various techniques, called best management practices, to control runoff. Some focus on reducing water quantity, others on improving water quality, and some do both.

Pollution prevention includes using low-impact development or green infrastructure, like green roofs or better chemical handling. Systems to reduce runoff include basins, wetlands, and retention areas.

In the Philippines, Republic Act 9275, also known as the Philippine Clean Water Act of 2004, is the main law for managing wastewater. It aims to protect and improve the quality of freshwater, brackish water, and seawater, with wastewater management playing a key role.

In 2024, the Royal Academy of Engineering studied the effects of wastewater on public health in the United Kingdom. The study received media attention, with comments from UK health experts, including Sir Chris Whitty. The study suggested 15 steps for UK organizations to reduce health risks by improving water quality in rivers and lakes.

After the study, The Guardian newspaper interviewed Whitty, who said improving water quality and sewage treatment should be a top public health priority. He compared it to ending cholera in the 19th century through better sewage systems. The study also found that low river flow increases sewage concentration, and heavy rain causes sewage to overflow into waterways. UK media warned parents about the risks of children playing in shallow rivers during warm weather.