There are many causes of water pollution – general environmental conditions like overpopulation, individual incidents like oil spills, specific pathogens in sewage or fertilizer. And there are different categories: point source pollution, such as from a pipe at an industrial plant, and nonpoint source pollution, in which contaminants reach waterways through runoff, soil saturation, groundwater, and rainfall. Because water problems are so local, with so many reasons behind them, there is no single solution. Here are 12 common causes of pollution to consider.

MINING OPERATIONS
King River, Australia

Mining can contribute to water pollution in a variety of ways – and not just when a containment dam holding pools of mine slurry sustains a breach. Rainwater can wash tailings (mine waste) into nearby bodies of water, and leach metals and sulfur compounds from rocks that are exposed during the excavation process. In gold mining, cyanide used to extract gold from ore often leaks into the water table, contaminating groundwater. A classic example of a body of water polluted by mining is the King River, in an area where copper was mined for more than a century. Although mining was curtailed in 1995, its effects continue as a result of discarded tailings and dissolved metals still present in the river. 

OIL SPILLS
Niger Delta, Nigeria

Oil spills contaminate anything on the water’s surface, and kill plants below the surface by blocking sunlight. Fish and wildlife experience increased mortality rates, either from direct exposure or from eating contaminated invertebrates. It is estimated that the Niger Delta, on the west coast of Africa – home to more than 600 oil fields – experiences the equivalent of the Exxon Valdez oil spill every year. The spills can come from leaking terminals, pipes, pumping stations, and oil platforms. Shell, which is reportedly working in partnership with the Nigerian government, says that 98 percent of the spills are caused by vandalism or theft, and a minimal amount is related to infrastructural issues. 

NUTRIENT POLLUTION (EUTROPHICATION)
Chesapeake Bay, North Carolina, USA

Nutrients may sound good, but excessive amounts of those like nitrogen and phosphorous can wreak havoc on coastal ecosystems. By elevating algae levels and causing hypoxia – lack of oxygen – they can result in widespread destruction of fish and other aquatic life. The nutrients reach the water through fertilizer runoff and sewage discharge as well as air pollution, and lead to overgrowths of toxic algae called red tides or brown tides. Zooplankton eat the algae, and the toxins work their way up the food chain, reaching humans through shellfish. When the overgrowths die, the algae decomposes, creating severe hypoxic conditions called dead zones. In 1997, several Chesapeake Bay tidal creeks experienced an outbreak of Pfiesteria, a toxin-producing pathogen that caused a massive fish kill as well as human health problems.

RADIOACTIVE WASTE
Lake Karachay, Russia

It’s reasonable to expect that any conversation linking radioactive waste to a location in the former Soviet Union would focus on the 1986 accident at the Chernobyl nuclear power plant. But Lake Karachay could be called the most polluted place on earth. In the 1950s, it was used as a nuclear waste disposal site by the Mayak military production facility. In 1957, an explosion at Mayak led to further contamination in the area. And during a 1967 drought that caused the lake level to drop dramatically, wind blew nuclear dust over a large area, resulting in contamination as far as 40 miles away. 

GARBAGE
Citarum River, Indonesia 

The relationship of garbage to water pollution is obvious: Trash that is thrown or blown into a body of water creates unsanitary conditions that endanger fish, damage vegetation, and disrupt the ecosystem. The one with the largest concentration of garbage appears to be the Citarum River. The river’s surface is so thoroughly blanketed with garbage in some spots that it is difficult to see the water. Foraging for trash in the Citarum is a more viable livelihood than fishing. 

AGRICULTURAL RUNOFF
Sarno River, Italy

Agricultural runoff is classified as nonpoint source pollution because it comes from diffuse sources rather than from a specific point, such as an industrial plant or waste-treatment facility. Runoff is caused by rainfall or melting snow, which carries contaminants and then deposits them into nearby bodies of water. Agricultural pollutants include pesticides, fertilizers, salts, metals, and other byproducts of farming found in the soil. Improper farming – plowing at the wrong time, overgrazing of livestock, bad irrigation – exacerbates the problem. One body of water that has been notably contaminated by agricultural runoff is the Sarno River, which runs through Naples and is one of the most polluted rivers in Italy.

MERCURY 
Onondaga Lake, Syracuse, New York, USA

Mercury, a heavy metal – others include cadmium, chromium, and lead – accumulates readily in the food chain and can be harmful even in low concentrations. Exposure of a fetus to mercury can cause permanent neurological damage; mercury exposure in adults is associated with neurological diseases and disorders of the central nervous system. Once in water, mercury is converted to methylmercury and moves up the fish food chain. It usually gets into water as an atmospheric byproduct of coal-fired power plants or industrial operations. But in the case of Onondaga Lake, near Syracuse, the pollution resulted from the discharge of about 165,000 pounds of mercury by Allied Signal Inc. (now Honeywell International) from 1946 to 1970. Considered among the most polluted lakes in the United States, Onondaga was put on the priority list for long-term hazardous waste cleanup through the U.S. Environmental Protection Agency’s Superfund program in 1994. Work is underway, but swimming is still banned. 

CARBON DIOXIDE EMISSIONS
Arctic Ocean

A 2009 study by the United Nations Environment Program estimates that the world’s oceans absorb about one-fourth of all carbon dioxide (CO₂) emissions. And as more CO₂ is released into the atmosphere through the burning of fossil fuels and deforestation, the oceans are taking in greater amounts at faster rates. While this absorption helps reduce the impact of emissions , it alters the chemical balance of ocean water by increasing its acidity. The UN report estimates that by 2050, ocean acidity could increase by 150 percent – 100 times faster than any change experienced in the world’s marine environment for 20 million years, leaving little time for evolutionary adaptation by ocean species. It also predicts that by 2032, the Arctic Ocean will experience a decline in essential carbonate minerals that could disrupt the production of crabs, lobsters, mussels, oysters, and shrimp.

SEWAGE
Yamuna River, India

Even in countries with modern sewage treatment facilities, beaches are sometimes closed because of high bacteria levels. That gives us some perspective on the water quality in developing nations, where an estimated 90 percent of wastewater is discharged into rivers, lakes, and streams without any treatment. Human sewage contains bacteria, viruses, and parasites that can cause serious illness and death. The Yamuna River is used for religious rituals, but it also is contaminated by hundreds of millions of gallons of raw sewage that pass through it daily, making the survival of fish or plants impossible. While the Yamuna is an extreme example, unless an intervention occurs, the pollution of India’s waterways will lead to future water scarcity. 

OIL USAGE AND RUNOFF
Gulf of Mexico

Mention the Gulf of Mexico and oil in the same sentence, and the catastrophic 2010 spill at British Petroleum’s Deepwater Horizon installation comes immediately to mind. But even in the absence of a major disaster, that body of water is subject to excessive oil exposure through a combination of factors that can be grouped under the category of consumer use. According to a 2003 “Oil in the Sea” study by the National Research Council, the oceans that border North America take in about 29 million gallons of petroleum a year. About 85 percent of that comes not from major spills, but from the accumulation of oil released by airplanes and recreational boats, as well as runoff from land and polluted rivers. 

INDUSTRIAL POLLUTANTS
Huai River, China

Waste discharged into rivers from factories and industrial facilities contains a cocktail of toxic chemicals that can be lethal to fish and plants, poisoning the water to the point where there is no possibility of its being treated for human consumption. In China, an estimated one-third of all industrial wastewater is released into rivers without treatment. According to one government report, about 70 percent of the country’s rivers, lakes, and waterways are seriously polluted, and 78 percent of the water from its rivers is unfit for human consumption. Along the Huai River and its tributaries, which originate in Henan Province, there are an estimated 100 “cancer villages” – communities with excessively high cancer rates caused by pollution. And that’s despite the shut down of more than 1,000 paper mills and 400 industrial plants.

POLYCHLORINATED BIPHENYLS (PCBs)
Aconcagua Mountain, Argentina

Possibly the most notorious of all industrial toxins, PCBs were banned from manufacture in the United States in 1979 and banned worldwide in 2004, along with 11 other chemicals (the original “dirty dozen”), under the Stockholm Convention on Persistent Organic Pollutants. PCBs are in a class of chemicals called chlorine compounds that can cause cancer, damage the nervous system, and disrupt human reproduction. Used in many industrial and commercial applications because of their chemical stability, PCBs can remain in the environment for long periods because they don’t readily break down. In 2009, a team of international researchers detected low concentrations of PCBs in snow samples taken from Aconcagua Mountain in the Andes, the highest mountain in the Americas.