Water pollution is the contamination of rivers, lakes, groundwater and oceans by substances that make the water harmful to people, animals or plants. The main causes of water pollution are sewage, fertilizer runoff from farms, industrial chemicals, oil and plastics. Its effects range from disease outbreaks and toxic algal blooms to oil soaked coastlines and huge oxygen starved dead zones. This guide explains the types, causes and effects of water pollution, including oil spills and toxic algal blooms, with a case study on the Gulf of Mexico dead zone.
Point Source and Nonpoint Source Pollution
Scientists sort water pollution by where it enters the water:
- Point source pollution comes from a single, identifiable place, such as a sewage treatment outflow, a factory discharge pipe or a leaking oil well. It is easier to measure and regulate.
- Nonpoint source pollution comes from many diffuse sources at once: fertilizer washing off thousands of farms, oil and tire particles running off roads, or animal waste from pastures. It is now the largest source of water pollution in many countries and much harder to control, because no single pipe can be closed.
Main Causes of Water Pollution
- Nutrients (nitrogen and phosphorus) from fertilizers, manure and sewage. They are not toxic in small amounts, but in excess they fuel explosive algae growth.
- Sewage and wastewater carry bacteria, viruses and parasites that cause diarrheal disease, cholera and typhoid where treatment is lacking, plus organic matter that uses up oxygen as it decays.
- Industrial chemicals such as heavy metals (mercury, lead, cadmium), solvents and persistent compounds like PFAS, which do not break down and build up in living things.
- Agricultural chemicals: pesticides and herbicides washed into streams and groundwater.
- Oil from tanker accidents, offshore drilling, pipelines and, above all, everyday runoff from roads and engines.
- Plastics, from large debris to microplastics small enough to be eaten by plankton.
- Sediment from deforestation, construction and eroding farmland, which clouds water, smothers riverbeds and coral reefs, and carries attached pollutants.
- Heat from power plant cooling water, a form of thermal pollution explained in our guide to the types of pollution.
Eutrophication and Toxic Algal Blooms
Eutrophication is what happens when a lake, river or coastal sea receives too many nutrients. It follows a predictable chain:
- Excess nitrogen and phosphorus fertilize the water.
- Algae and cyanobacteria multiply into a dense bloom that turns the water green and blocks light.
- Plants on the bottom die without light.
- When the bloom dies, bacteria decompose it and use up the dissolved oxygen.
- Fish, crabs and other animals that cannot escape suffocate, creating a hypoxic dead zone.
Some blooms are also directly poisonous. These harmful algal blooms are made by cyanobacteria in fresh water and by certain dinoflagellates and diatoms at sea (red tides). Their toxins can kill fish, birds, marine mammals, pets and livestock, contaminate shellfish and make tap water unsafe to drink. In 2014, a cyanobacteria bloom in Lake Erie led the city of Toledo, Ohio to warn hundreds of thousands of residents not to drink tap water for several days. Warmer water and longer summers make these blooms more frequent.
Oil Spills
Oil spills draw the most media attention, although most oil entering the sea comes from many small leaks and runoff rather than single disasters. When a large spill happens, oil floats as a slick, coats the feathers of seabirds and the fur of sea otters so they lose insulation and buoyancy, poisons animals that swallow it while preening, and smothers shoreline life such as marshes and mangroves. Some oil sinks and persists in sediments for years.
Cleanup uses booms to contain floating oil, skimmers to remove it, dispersant chemicals to break it into droplets, controlled burning and, more slowly, bacteria that naturally break oil down. No method removes all of it, which is why prevention matters far more than cleanup. Plants and microbes can also help clean contaminated water and soil over time, as explained in our guide to phytoremediation techniques.
Effects of Water Pollution
- Human health: the Lancet Commission on Pollution and Health estimates that unsafe water caused about 1.4 million premature deaths in 2019, mostly from diarrheal disease in places without sanitation.
- Biodiversity: freshwater species are declining faster than land species in many regions, with pollution among the leading causes alongside dams and habitat loss.
- Food chains: mercury and other persistent pollutants concentrate up the food chain, so large predatory fish carry the highest levels.
- Economy: fisheries close, beaches shut, drinking water costs more to treat and tourism suffers.
Case Study: The Gulf of Mexico Dead Zone
Every summer, a large area of low oxygen water forms off the coast of Louisiana and Texas, fed by nutrients carried down the Mississippi and Atchafalaya Rivers from farmland across a huge part of the United States. NOAA, Louisiana State University and LUMCON measure it each summer by ship. I charted the measured area for ten years, from 2014 to 2023.
- The record was 8,776 square miles in 2017, the largest since measurements began in 1985, larger than the state of New Jersey.
- The size swings hugely from year to year: from 2,116 square miles in 2020 to 6,334 in 2021. River flow and storms explain much of this. Wet springs flush more fertilizer into the Gulf; hurricanes mix oxygen back into the water.
- The goal is far away. The Hypoxia Task Force aims for a five year average of 1,900 square miles or less by 2035. My calculation of the five year average for 2019 to 2023 is 4,347 square miles, more than twice the target. Every year in the chart was above it.
My take as a biologist: the dead zone is a textbook case of nonpoint source pollution. No single farm or city causes it; fertilizer from millions of fields adds up in one river. That is why it has resisted decades of effort. The small dead zones of 2022 and 2023 were welcome, but they followed dry springs with low river flow, so they say more about weather than about reduced fertilizer use. Real progress will show only when wet years also produce smaller dead zones, which needs changes on the farms: precise fertilizer application, cover crops, and restored wetlands and buffer strips that catch nutrients before they reach streams.
Frequently Asked Questions
What are the main causes of water pollution?
Fertilizer and manure runoff, sewage, industrial chemicals, pesticides, oil, plastics and sediment. Nutrient runoff from agriculture is the biggest cause in many regions.
What is a dead zone?
An area of water with so little dissolved oxygen that most marine animals cannot survive, usually caused by algae blooms fed by nutrient pollution.
What causes toxic algal blooms?
Excess nitrogen and phosphorus combined with warm, calm, sunny water allow toxin producing cyanobacteria or algae to multiply rapidly.
How do oil spills affect marine life?
Oil coats birds and mammals so they lose insulation and buoyancy, poisons animals that swallow it, smothers shoreline habitats and can persist in sediments for years.
How can water pollution be reduced?
By treating sewage, using fertilizer more precisely, keeping vegetated buffers along streams, restoring wetlands, regulating industrial discharges and reducing plastic waste.
Sources
- US EPA (2021 archive), Northern Gulf of Mexico Hypoxic Zone.
- Ecomagazine (2023), NOAA and partners announce below average dead zone.
- Fuller, R. et al. (2022). Pollution and health: a progress update. The Lancet Planetary Health. doi.org/10.1016/S2542-5196(22)00090-0
Erzsebet (Eli) Frey is a biologist with a BSc in Biology and an MSc in Ecology from the University of Belgrade. She has field and lab experience across Asia, the Middle East, Europe and South America and uses Python and machine learning on biological data. Based in Serbia, she writes BATAECO and also runs Simply Ecologist and Eco&Stylish.