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Water pollution (or aquatic pollution) is the contamination of water bodies, usually as a result of human activities, in such a manner that negatively affects its legitimate uses. Water pollution reduces the ability of the body of water to provide the ecosystem services that it would otherwise provide. Water bodies include for example lakes, rivers, oceans, aquifers, reservoirs and groundwater. Water pollution results when contaminants are introduced into these water bodies. Water pollution can usually be attributed to one of four sources: sewage, industry, agriculture, and urban runoff including stormwater. For example, releasing inadequately treated wastewater into natural waters can lead to degradation of these aquatic ecosystems. Water pollution can also lead to water-borne diseases for people using polluted water for drinking, bathing, washing or irrigation. Supplying clean drinking water is an important ecosystem service provided by some freshwater systems, but approximately 785 million people in the world do not have access to clean drinking water because of pollution.

Water pollution can be classified as surface water pollution (for example lakes, streams, estuaries, and parts of the ocean in marine pollution) or groundwater pollution. Sources of water pollution are either point sources or non-point sources. Point sources have one identifiable cause, such as a storm drain, a wastewater treatment plant or an oil spill. Non-point sources are more diffuse, such as agricultural runoff. Pollution is the result of the cumulative effect over time.

Pollution may take the form of toxic substances (e.g., oil, metals, plastics, pesticides, persistent organic pollutants, industrial waste products), stressful conditions (e.g., changes of pH, hypoxia or anoxia, stressful temperatures, excessive turbidity, unpleasant taste or odor, and changes of salinity), or pathogenic organisms. Contaminants may include organic and inorganic substances. Heat can also be a pollutant, and this is called thermal pollution. A common cause of thermal pollution is the use of water as a coolant by power plants and industrial manufacturers.

Control of water pollution requires appropriate infrastructure and management plans as well as legislation. Technology solutions can include improving sanitation, sewage treatment, industrial wastewater treatment, agricultural wastewater treatment, erosion control, sediment control and control of urban runoff (including stormwater management). Effective control of urban runoff includes reducing speed and quantity of flow.

Definition
A practical definition of water pollution is: "Water pollution is the addition of substances or energy forms that directly or indirectly alter the nature of the water body in such a manner that negatively affects its legitimate uses". Therefore, pollution is associated with concepts attributed to humans, namely the negative alterations and the uses of the water body. Water is typically referred to as polluted when it is impaired by anthropogenic contaminants. Due to these contaminants it either does not support a human use, such as drinking water, or undergoes a marked shift in its ability to support its biotic communities, such as fish.

Thermal pollution


Elevated water temperatures decrease oxygen levels (due to lower levels of dissolved oxygen, as gases are less soluble in warmer liquids), which can kill fish (which may then rot) and alter food chain composition, reduce species biodiversity, and foster invasion by new thermophilic species.

Biological pollution
The introduction of aquatic invasive organisms is a form of water pollution as well. It causes biological pollution.

Pathogens
Disease-causing microorganisms are referred to as pathogens. The major groups of pathogenic organisms are: (a) bacteria, (b) viruses, (c) protozoans and (d) helminths. In practice, indicator organisms are used to investigate pathogenic pollution of water because the detection of pathogenic organisms in water sample is difficult and costly, because of their low concentrations. The indicators (bacterial indicator) of fecal contamination of water samples most commonly used are: total coliforms (TC), fecal coliforms (FC) or thermotolerant coliforms, escherichia coli (EC).

Pathogens can produce waterborne diseases in either human or animal hosts. Some microorganisms sometimes found in contaminated surface waters that have caused human health problems include: Burkholderia pseudomallei, Cryptosporidium parvum, Giardia lamblia, Salmonella, norovirus and other viruses, parasitic worms including the Schistosoma type. 

The source of high levels of pathogens in water bodies can be from human feces (due to open defecation), sewage, blackwater, manure that has found its way into the water body. The cause for this can be lack of sanitation or poorly functioning on-site sanitation systems (septic tanks, pit latrines), sewage treatment plants without disinfection steps, sanitary sewer overflows and combined sewer overflows (CSOs) during storm events and intensive agriculture (poorly managed livestock operations).

Non-biodegradable organic compounds
Non-biodegradable organic substances can enter water bodies from a variety of sources, for example industrial wastewater. Many of these chemical substances are toxic.
 * Chemicals from insecticides and herbicides.
 * Petroleum hydrocarbons, including fuels (gasoline, diesel fuel, jet fuels, and fuel oil) and lubricants (motor oil), and fuel combustion byproducts, from oil spills or storm water runoff
 * Volatile organic compounds, such as industrial solvents, from improper storage.
 * Persistent organic pollutants, for example per- and polyfluoroalkyl substances (PFAS),
 * Organochlorides, polychlorinated biphenyl (PCBs), trichloroethylene, perchlorate (these are currently or were in the past used as pesticides, solvents, pharmaceuticals, and industrial chemicals).

Inorganic contaminants
Inorganic water pollutants include for example:
 * Acidity caused by industrial discharges (especially sulfur dioxide from power plants) or by increased carbon dioxide concentrations in the atmosphere (see also ocean acidification). In industrialized areas, acid rain has in the past resulted in pollution of lakes and rivers due to air pollution with dissolved oxides of sulfur and nitrogen.
 * Ammonia from food processing waste
 * Heavy metals from motor vehicles (via urban storm water runoff) and acid mine drainage
 * Nitrates and phosphates, from sewage and agriculture (see nutrient pollution)
 * Silt (sediment) in runoff from construction sites or sewage, logging, slash and burn practices or land clearing sites.

Solid waste and plastics
Solid waste can enter water bodies through untreated sewage, combined sewer overflows, urban runoff, people discarding garbage into the environment, wind carrying municipal solid waste from landfills and so forth. This results in macroscopic pollution– large visible items polluting the water– but also microplastics pollution that is not directly visible. The terms marine debris and marine plastic pollution are used in the context of pollution of oceans.

Microplastics persist in the environment at high levels, particularly in aquatic and marine ecosystems, where they cause water pollution. 35% of all ocean microplastics come from textiles/clothing, primarily due to the erosion of polyester, acrylic, or nylon-based clothing, often during the washing process.

Sewage
The following compounds can all reach water bodies via raw sewage or even treated sewage discharges:
 * Various chemical compounds found in personal hygiene and cosmetic products.
 * Environmental persistent pharmaceutical pollutants, which can include various pharmaceutical drugs and their metabolites (see also drug pollution), such as antidepressant drugs, antibiotics or the contraceptive pill.
 * Metabolites of illicit drugs (see also wastewater epidemiology), for example methamphetamine and ecstasy.
 * Disinfection by-products found in chemically disinfected drinking water (whilst these chemicals can be a pollutant in the water distribution network, they are fairly volatile and therefore not usually found in environmental waters).
 * Hormones (from animal husbandry and residue from human hormonal contraception methods) and synthetic materials such as phthalates that mimic hormones in their action. These can have adverse impacts even at very low concentrations on the natural biota and potentially on humans if the water is treated and utilized for drinking water.

Industrial wastewater
If the pollution stems from industrial wastewater, then pollutants of concern may include:
 * Heavy metals, including mercury, lead, and chromium
 * Organic matter such as food waste, slaughterhouse waste, paper fibers, plant material, etc.;
 * Inorganic particles such as sand, grit, metal particles, rubber residues from tires, ceramics, etc.;
 * Toxins such as pesticides, poisons, herbicides, etc.
 * Pharmaceuticals, endocrine disrupting compounds, hormones, perfluorinated compounds, siloxanes, drugs of abuse and other hazardous substances
 * Microplastics such as polyethylene and polypropylene beads, polyester and polyamide
 * Thermal pollution from power stations and industrial manufacturers
 * Radionuclides from uranium mining, processing nuclear fuel, operating nuclear reactors, or disposal of radioactive waste.

Rivers and lakes
Surface water pollution includes pollution of rivers, lakes and oceans. A subset of surface water pollution is marine pollution which affects the oceans. Nutrient pollution refers to contamination by excessive inputs of nutrients.

Globally, about 4.5 billion people do not have safely managed sanitation as of 2017, according to an estimate by the Joint Monitoring Programme for Water Supply and Sanitation. Lack of access to sanitation is concerning and often leads to water pollution, e.g. via the practice of open defecation: during rain events or floods, the human feces are moved from the ground where they were deposited into surface waters. Simple pit latrines may also get flooded during rain events.

Point sources
Point source water pollution refers to contaminants that enter a waterway from a single, identifiable source, such as a pipe or ditch. Examples of sources in this category include discharges from a sewage treatment plant, a factory, or a city storm drain.

The U.S. Clean Water Act (CWA) defines point source for regulatory enforcement purposes (see United States regulation of point source water pollution). The CWA definition of point source was amended in 1987 to include municipal storm sewer systems, as well as industrial storm water, such as from construction sites.

Sewage discharges
Sewage typically consists of 99.9% water and 0.1% solids. Sewage contributes many classes of nutrients that lead to eutrophication. It is a major source of phosphate for example. Sewage is often contaminated with diverse compounds found in personal hygiene, cosmetics, pharmaceutical drugs] (see also drug pollution), and their metabolites Water pollution due to environmental persistent pharmaceutical pollutants can have wide-ranging consequences. When sewers overflow during storm events this can lead to water pollution from untreated sewage. Such events are called sanitary sewer overflows or combined sewer overflows.

Industrial wastewaters


Industrial processes that use water also produce wastewater. Using the US as an example, the main industrial consumers of water (using over 60% of the total consumption) are power plants, petroleum refineries, iron and steel mills, pulp and paper mills, and food processing industries. Some industries discharge chemical wastes, including solvents and heavy metals (which are toxic) and other harmful pollutants such as nutrients. Certain industries (e.g. food processing) discharge high concentrations of biochemical oxygen demand (BOD) and oil and grease. Some industrial discharges include persistent organic pollutants such as per- and polyfluoroalkyl substances (PFAS).

Agriculture
Agriculture is a major contributor to water pollution from nonpoint sources. The use of fertilizers as well as surface runoff from farm fields, pastures and feedlots leads to nutrient pollution. In addition to plant-focused agriculture, fish-farming is also a source of pollution. Additionally, agricultural runoff often contains high levels of pesticides.

Measurement


Water pollution may be analyzed through several broad categories of methods: physical, chemical and biological. Some methods may be conducted in situ, without sampling, such as temperature. Others involve collection of samples, followed by specialized analytical tests in the laboratory. Standardized, validated analytical test methods, for water and wastewater samples have been published.

Common physical tests of water include temperature, Specific conductance or electrical conductance (EC) or conductivity, solids concentrations (e.g., total suspended solids (TSS)) and turbidity. Water samples may be examined using analytical chemistry methods. Many published test methods are available for both organic and inorganic compounds. Frequently used parameters that are quantified are pH, biochemical oxygen demand (BOD), chemical oxygen demand (COD),  dissolved oxygen (DO), total hardness, nutrients (nitrogen and phosphorus compounds, e.g. nitrate and orthophosphates), metals (including copper, zinc, cadmium, lead and mercury), oil and grease, total petroleum hydrocarbons (TPH), surfactants and pesticides.

Biological testing
The use of a biomonitor is described as biological monitoring. This refers to the measurement of specific properties of an organism to obtain information on the surrounding physical and chemical environment. Biological testing involves the use of plant, animal or microbial indicators to monitor the health of an aquatic ecosystem. They are any biological species or group of species whose function, population, or status can reveal what degree of ecosystem or environmental integrity is present. One example of a group of bio-indicators are the copepods and other small water crustaceans that are present in many water bodies. Such organisms can be monitored for changes (biochemical, physiological, or behavioral) that may indicate a problem within their ecosystem.

Ecosystems
Water pollution is a major global environmental problem because it can result in the degradation of aquatic ecosystems. The specific contaminants leading to pollution in water include a wide spectrum of chemicals, pathogens, and physical changes such as elevated temperature. While many of the chemicals and substances that are regulated may be naturally occurring (calcium, sodium, iron, manganese, etc.) the concentration usually determines what is a natural component of water and what is a contaminant. High concentrations of naturally occurring substances can have negative impacts on aquatic flora and fauna. Oxygen-depleting substances may be natural materials such as plant matter (e.g. leaves and grass) as well as man-made chemicals. Other natural and anthropogenic substances may cause turbidity (cloudiness) which blocks light and disrupts plant growth, and clogs the gills of some fish species.

There is concern that water pollution can damage phytoplankton in the oceans who produce 70% of oxygen and remove a large part of carbon dioxide from the atmosphere.

Public health and waterborne diseases
A study published in 2017 stated that "polluted water spread gastrointestinal diseases and parasitic infections and killed 1.8 million people" (these are also referred to as waterborne diseases).

Eutrophication from nitrogen pollution
Nitrogen pollution (a form of water pollution where excessive amounts of nutrients are added to a water body), can cause eutrophication, especially in lakes. Eutrophication is an increase in the concentration of chemical nutrients in an ecosystem to an extent that increases the primary productivity of the ecosystem. Depending on the degree of eutrophication, subsequent negative environmental effects such as anoxia (oxygen depletion) and severe reductions in water quality may occur, affecting fish and other animal populations.

Ocean acidification
Ocean acidification is another impact of water pollution. Ocean acidification is the ongoing decrease in the pH value of the Earth's oceans, caused by the uptake of carbon dioxide from the atmosphere.

Prevalence
Water pollution is a problem in developing countries as well as in developed countries.

By country
For example, water pollution in India and China is wide spread. About 90 percent of the water in the cities of China is polluted.

Pollution control philosophy
One aspect of environmental protection are mandatory regulations but they are only part of the solution. Other important tools in pollution control include environmental education, economic instruments, market forces and stricter enforcements. Standards can be "precise" (for a defined quantifiable minimum or maximum value for a pollutant), or "imprecise" which would require the use of Best Available Technology (BAT) or Best Practicable Environmental Option (BPEO). Market-based economic instruments for pollution control can include: charges, subsidies, deposit or refund schemes, the creation of a market in pollution credits, and enforcement incentives.

Moving towards a holistic approach in chemical pollution control combines the following approaches: Integrated control measures, trans-boundary considerations, complementary and supplementary control measures, life-cycle considerations, the impacts of chemical mixtures.

Control of water pollution requires appropriate infrastructure and management plans. The infrastructure may include wastewater treatment plants, for example sewage treatment plants and industrial wastewater treatment plants. Agricultural wastewater treatment for farms, and erosion control at construction sites can also help prevent water pollution. Effective control of urban runoff includes reducing speed and quantity of flow.

Water pollution requires ongoing evaluation and revision of water resource policy at all levels (international down to individual aquifers and wells).

Sanitation and sewage treatment


Municipal wastewater (or sewage) can be treated by centralized sewage treatment plants, decentralized wastewater systems, nature-based solutions or in onsite sewage facilities and septic tanks. For example, waste stabilization ponds are a low cost treatment option for sewage, particularly for regions with warm climates. UV light (sunlight) can be used to degrade some pollutants in waste stabilization ponds (sewage lagoons). The use of safely managed sanitation services would prevent water pollution caused by lack of access to sanitation.

Well-designed and operated systems (i.e., with secondary treatment stages or more advanced tertiary treatment) can remove 90 percent or more of the pollutant load in sewage. Some plants have additional systems to remove nutrients and pathogens. While such advanced treatment techniques will undoubtedly reduce the discharges of micropollutants, they can also result in large financial costs, as well as environmentally undesirable increases in energy consumption and greenhouse gas emissions.

Sewer overflows during storm events can be addressed by timely maintenance and upgrades of the sewerage system. In the US, cities with large combined systems have not pursued system-wide separation projects due to the high cost, but have implemented partial separation projects and green infrastructure approaches. In some cases municipalities have installed additional CSO storage facilities or expanded sewage treatment capacity.

Management of erosion and sediment control
Sediment from construction sites can be managed by installation of erosion controls, such as mulching and hydroseeding, and sediment controls, such as sediment basins and silt fences. Discharge of toxic chemicals such as motor fuels and concrete washout can be prevented by use of spill prevention and control plans, and specially designed containers (e.g. for concrete washout) and structures such as overflow controls and diversion berms.

Erosion caused by deforestation and changes in hydrology (soil loss due to water runoff) also results in loss of sediment and, potentially, water pollution.

Overview
If the water pollution stems from sewage (municipal wastewater), the main pollutants are: suspended solids, biodegradable organic matter, nutrients and pathogenic organisms.

Legislation
Some examples for legislation to control water pollution are listed below:


 * In the Philippines, Republic Act 9275, otherwise known as the Philippine Clean Water Act of 2004, is the governing law on wastewater management. It states that it is the country's policy to protect, preserve and revive the quality of its fresh, brackish and marine waters, for which wastewater management plays a particular role.
 * The Clean Water Act is the primary federal law in the United States governing water pollution in surface waters. It is implemented by the U.S. Environmental Protection Agency in collaboration with states, territories, and tribes. Groundwater protection provisions are included in the Safe Drinking Water Act, Resource Conservation and Recovery Act, and the Superfund act.