Water is essential for life on Earth. It supports biological functions and is vital to the health of humans and ecosystems alike. For some organisms, water makes up 90% of their body weight. Up to 60% of an adult human’s mass is water. Water also covers 71% of Earth’s surface. It is so essential for life as we know it that it is the first thing astrobiologists look for when seeking life on other planets. However, pollution can make water dangerous if it’s not cleaned up.

Water’s chemistry makes it vital to life. Its chemistry also makes water vulnerable to pollution. Water is an excellent solvent, meaning that it readily dissolves many types of substances. This property is one of the reasons water is so necessary for life—it can readily dissolve nutrients that help maintain the health of humans, wildlife and ecosystems. Water can also dissolve harmful materials and transport pollutants to new, sometimes far away locations.

All water on Earth is connected. Through the water cycle, water on Earth’s surface evaporates into water vapor to form clouds. Clouds produce rain and snow, which return water to the Earth’s surface. There, water collects to form groundwater. Rain can wash pollutants into streams and lakes. As rain runs from the land into bodies of water, it can pick up substances like animal waste from cattle ranches and pesticides from croplands. Cars drip oil onto the street, and rain carries it into storm drains, which lead to waterways, such as rivers and seas. Freshwater from rivers empty into the ocean which can bring pollution from inland areas into the sea. Once pollutants enter the ocean, currents can move them vast distances.

Sources of Water Pollution

Where does pollution come from? Sometimes pollution can originate from a single, defined source. This is known as point-source pollution. For example, wastewater treatment plants can produce point-source pollution. Around 80% of wastewater enters the environment untreated. This number jumps to over 95% for some low-income countries. Wastewater encourages the growth of germs, like bacteria, viruses and parasites, that can cause illness in humans. Pharmaceuticals can also end up in wastewater and threaten aquatic life and human health. Factories can also produce point-source pollution, discharging waste like oil into waterways. Oil spills in the ocean can have devastating, long-term impacts to ecosystems and human health as well as significant economic consequences.

In many cases, pollution does not originate from a single source. Instead, pollution from numerous sources combine in a way that makes it difficult to determine the source. This is called nonpoint-source pollution, and it can come from urban, agricultural and industrial sources. Pollution from rain washing waste into waterways, or runoff, is typically considered nonpoint-source pollution.

Another type of water pollution is garbage. The Citarum River in Indonesia, for example, has so much garbage floating in it that you cannot see the water. This floating trash makes it difficult to fish in the Citarum River. Aquatic animals, including fish and turtles, in garbage-infested waters can mistake trash for food. Plastic bags and other trash can kill many aquatic creatures. Chemical pollutants in trash can also make water toxic for both fish and the people who use the river as a drinking water source. Since 2008, around 60% of the fish species that used to inhabit the Citarum River are no longer there. The fish that remain often have high levels of chemical toxins in their flesh. People who eat fish contaminated with chemicals can absorb these toxins as they eat.

Garbage also contaminates the ocean. Ocean currents carry plastics and other floating trash all over the globe. The largest of these areas, called the Great Pacific Garbage Patch, is in a remote part of the Pacific Ocean. According to some estimates, this region of garbage-polluted water is twice the size of Texas. The trash is a threat to fish and seabirds, which mistake the plastic for food. Many of the plastics also contain or carry chemical pollutants.

Water can become polluted with radioactive waste, which can persist in the environment for thousands of years and cause extensive health consequences. In 2011, the Great Tohoku Earthquake caused a tsunami that led to the release of radioactive materials from the Fukushima Tokyo Electric Power Company (TEPCO) Daiichi nuclear reactor site. Radioactive materials from Fukushima entered the groundwater and the Pacific Ocean, and became part of rainwater. These radioactive materials were even found in tuna on the west coast of North America.

Clean-up efforts from Fukushima may take up to 40 years. One million tons of contaminated radioactive water is stored on site. The Japanese government has implemented a controversial plan to release this water into the Pacific Ocean. Japanese and international authorities maintain that these releases will not have significant impacts on the Pacific Ocean, but the plan has been criticized by some of its citizens and authorities from other countries, including neighboring China and South Korea.

Mining and drilling also contribute to water pollution. Acid mine drainage is a major contributor to pollution of rivers and streams near coal mines. Mining exposes sulfur-containing minerals in rocks and sand that can react with water and oxygen to form sulfuric acid. Sulfuric acid is toxic to plants, fish, and other aquatic organisms. Sulfuric acid is also toxic to people, making rivers polluted by acid mine drainage dangerous sources of water for drinking and hygiene.

A major source of water pollution is fertilizer used in agriculture. Fertilizer is a substance that is added to soil to make plants grow larger and faster. Fertilizers usually contain large amounts of the elements nitrogen and phosphorus, which help plants grow. Rainwater washes fertilizer into streams and lakes. This creates an excess of nutrients in the water, called eutrophication, and it can have major impacts to ecosystems by disrupting oxygen levels in the water.

Excess nitrogen and phosphorus in the aquatic environment can cause cyanobacteria to form harmful algal blooms. Harmful algal blooms prevent organisms, such as plants and fish, from living in the ocean, by producing toxins that poison them, blocking out light and reducing the amount of oxygen available. Harmful algal blooms are associated with “dead zones” in the world’s lakes and rivers, creating places where little life exists below surface water.

Heat can also pollute water. Power plants, for example, produce a huge amount of heat. Power plants are often located on rivers so they can use the water as a coolant. Cool water circulates through the plant, absorbing heat. The heated water is then returned to the river, causing thermal pollution. Aquatic creatures are sensitive to changes in temperature. Some fish, for example, can only live in cold water. Warmer river temperatures prevent some fish eggs from developing properly. Warmer river water also contributes to harmful algal blooms.

Climate change, caused by the emission of large amounts of greenhouse gases from industrial activities, contributes to ocean acidification. Ocean acidification is the process of ocean waters absorbing more carbon dioxide from the atmosphere, which makes the water more acidic. Fewer organisms can survive in warmer, more acidic waters. This threatens plants and animals, such as coral, unless they can quickly adapt to more acidic oceans.

Noise is another damaging source of pollution in waterways. Marine mammals use echolocation, meaning they use sound to locate objects and prey and to communicate. Loud noises from shipping, drilling and other human activities in the ocean disrupt these abilities.

Impacts of Pollution

Polluted water impacts both human health and the health of the planet. Pollution affects populations and ecosystems as a whole. Oil spills provide an example of the wide-ranging and lasting impacts that pollution can have on aquatic ecosystems and the people who depend on them. In April 2010, the Deepwater Horizon oil rig, owned and operated by Transocean and leased by oil company BP, exploded in the Gulf of Mexico, causing oil to gush from the ocean floor. In the following months, hundreds of millions of gallons of oil spewed into the gulf waters. The spill produced large plumes of oil under the sea and an oil slick on the surface as large as 57,000 square kilometers (149,000 square miles). It polluted at least 1,770 kilometers (1,100 miles) of shoreline.

Many animals died from the Deepwater Horizon oil spill, including thousands of sea turtles, marine mammals, birds and fish. The oil spill killed an estimated 8.3 billion oysters, which provide significant environmental benefits, including filtering water and forming reefs that provide critical habitats for numerous species. The oil slick coated wetlands in the U.S. states bordering the Gulf of Mexico, killing marsh plants and aquatic organisms, such as oysters and fish. Birds, such as pelicans, became coated in oil and were unable to fly or regulate their temperature.

Pollution can decrease biodiversity. Biodiversity is an indicator of ecosystem health. It refers to the number of different species present in an area. Eutrophication from agricultural runoff can cause some species, including algae, to proliferate, use the oxygen other organisms need and reduce the total biodiversity in an area. Pollution from desalination plants can also reduce biodiversity. These plants remove salt from seawater to produce usable freshwater, especially in areas experiencing significant water stress. However, these plants also reduce the amount of oxygen in the marine environment by releasing brine—the salt that remains once the freshwater is extracted—into the ocean. Marine animals need oxygen and often consume it when water dissolves oxygen. Adding more salt to the ocean prevents oxygen from dissolving. Salt molecules bind with water molecules, which leaves less free space between water molecules to dissolve oxygen.

Polluted water is unsafe for drinking and swimming. Drinking polluted water can expose people to hazardous chemicals that may make them sick years later. People may also consume bacteria and other tiny organisms that cause disease. According to the United Nations International Children's Emergency Fund (UNICEF), 1,000 children under the age of five die every day from drinking dirty water. Sometimes, polluted water harms people indirectly. For example, they may get sick by eating fish caught from polluted water.

Improperly treated sewage is a common source of water pollution. Many cities around the world have poor sewage systems and sewage treatment plants. Delhi, the capital of India, is home to more than 16 million people. The Yamuna River supplies drinking water to millions in Delhi. Approximately two-thirds of wastewater is treated, but millions of gallons of untreated water are dumped into the Yamuna River.

Delhi’s growing population and the establishment of unauthorized settlements has contributed to the river’s pollution. The water may enter Delhi with minimal pollution, but the city contributes to 76% of the river’s pollution. Waste from factories and agricultural runoff also inundate the river. The variety of pollution sources mean that there are a variety of substances entering the river, including ammonia and heavy metals. These substances can cause health issues, such as difficulty breathing or skin irritation, which makes the river dangerous to use as a source of water for drinking or hygiene. It also reduces the river’s fishery by poisoning and killing fish, resulting in less food for the local community.

Recently, dumping in the Yamuna River has created large plumes of foam, mainly caused by detergents, that blanket the river. The river is a holy site, where many visit for prayer and religious festivals, such as the Hindu festivals Chhath Puja and Diwali. The Delhi government has attempted to clean up the foam ahead of mass gatherings. Although the government has spent millions of dollars on fixing the pollution, many issues, including roadblocks to regulations within agencies and lack of enforcement of antipollution laws, have halted progress.

Controlling Pollution

There are a number of methods for controlling water pollution. Some include pollution prevention and others focus on cleaning up pollution. A commonly deployed method of preventing pollution is water treatment. Cities and municipalities treat wastewater to prevent pollution and to protect public health. Treating wastewater consists of removing pollutants before discharging water back into the environment.

Other methods of water pollution prevention include implementing more sustainable practices. More sustainable agricultural practices can prevent pollution by reducing harmful runoff. Cities can implement policies that discourage litter, such as bans on single-use plastic. Individuals can ensure their cars don’t leak oil. Clean ups, including community beach clean ups, can also help reduce the amount of litter that ends up in waterways.

Governments can help prevent pollution by creating and enforcing regulations to limit and clean up pollution. Many cities have implemented bans on single-use plastics, such as bags and straws, and fines for littering. It is vital, however, that these efforts to limit pollution do not place undue burden on already marginalized communities. Instead, these efforts should ensure that those creating the most pollution are the ones contributing the most to the clean up.

Nature-based solutions can help clean up pollution too. For example, an oyster can filter up to 190 liters (50 gallons) of water per day. However, climate change, natural disasters and overharvesting have led to the loss of 85% of oyster reefs worldwide. Restoring natural oyster populations could provide a natural way of filtering water pollutants without causing environmental harm. Communities and nonprofits around the world—in North America, Asia, Europe and Australia—have begun to restore oyster reefs. Oyster reefs also have the added benefit of providing habitats. Oyster restoration does have challenges, however. Ocean acidification may make it difficult for newly restored oyster reefs to thrive. Additionally, it may take decades to fully realize the positive ecosystem impacts, yet many efforts lack funding or ability to monitor for this amount of time.

International agreements can also provide a critical tool to combat pollution. This is especially true of areas where water bodies transcend national boundaries and are shared by countries. The high seas, which lie in international waters are an example of waters that need international cooperation to protect against pollution. In 2023, the United Nations adopted an agreement, called the High Seas Treaty, to do just that. More than 130 countries have signed the agreement.

Many gains have been made. In 1969, the Cuyahoga River, in the U.S. state of Ohio, was so clogged with oil and trash that it caught on fire. The fire helped spur the Clean Water Act of 1972. This law limited what pollutants could be released into water and set standards for how clean water should be. Today, the Cuyahoga River is much cleaner. Fish have returned to regions of the river where they once could not survive.

While pollution can present a daunting task to prevent and clean up, it is certainly possible and necessary. Governments and communities can learn from successful measures in preventing and managing pollution to take on the challenge of protecting one of Earth’s most precious resources.