3.2 Categories of water pollution

Water pollution is a major part of the UN Sustainable Development Goal for clean water and sanitation, and is also involved in the goals for good health and well being, life in water and sustainable cities and communities.

Sources of water pollution

As with air pollution, water pollution sources can be categorized as point sources or nonpoint sources. Point sources of water pollution are usually pipes or other sources that drain mining, shipping, industrial, agricultural, and municipal waste into bodies of water. Nonpoint water pollution generally enters bodies of water through runoff over the ground or travel through the ground. Nonpoint water pollution can come from pollution on roadways, from yard and agricultural chemicals, mine drainage, faulty septic systems, and pet waste. Air pollution that settles out of the air or is washed out of the air into water or onto land and then into water is also a nonpoint source of water pollution.

Types of water pollution

Water pollution is a subset of water contamination. Contaminants are simply foreign substances, or substances present at higher-than-normal levels, whereas pollutants are contaminants that are harmful to the environment and/or organisms. Thus, all pollutants are contaminants, but not all contaminants are pollutants.

The US EPA classifies drinking-water pollutants broadly into physical, chemical, biological, and radiological categories, and also addresses thermal pollution.

Physical water pollution

Physical contaminants are often referred to as sediments and suspended solids. These are solid materials that do not dissolve in water. Although they may carry adsorbed chemicals (chemicals adhering to the surface of the solids), this category of water pollution is concerned only with their impacts as solid materials. Sediment in water is sand, silt, and other kinds of soil that enter waterways as a result of runoff, often from storm water. Suspended solids is a broader category that can also include any debris carried into bodies of water, from microplastics to automobiles – trash, plant and animal debris, microorganisms, soil, etc. Physical contamination reduces water clarity, blocking light to aquatic plants and animals. It may clog fish gills, reduce stream flow, fill in shallow bodies of water, block drainage pipes and restrict water passage for dams and culverts. It degrades habitat for fish and aquatic invertebrates and can also impede the exchange of water between surface water and groundwater.

Chemical water pollution

Chemical contamination may enter water bodies dissolved in inflowing water (including from point-source discharges such as industrial waste discharge), runoff, or groundwater, or may enter as adsorbed chemicals on sediment and other suspended solids that leaches into receiving waters – the lakes and rivers that receive the contaminated water.

Inorganic chemical pollutants

Inorganic chemical pollutants include strong acids and bases, nutrients from yard and agricultural fertilizers and sewage, salts, heavy metals and radioactive materials (these last are treated by EPA as a separate category).

Acid rain

Acid rain or snow that reaches streams and lakes can lower their pH. Aquatic life forms typically cannot tolerate pH values below 5. In severe cases, “dead” lakes can occur, in which most plankton, fish, amphibians, and aquatic invertebrates are absent. As we learned earlier, acid rain was a greater problem in the past but can still occur in the present where high-sulfur coal is burned, particularly in Asia and parts of Russia.

Nutrient pollution

Nutrient pollution is mostly composed of biologically available forms of N (nitrates, ammonia) and P (phosphates) that enter waterways as runoff or drainage from sewage systems, agricultural areas, livestock operations, and municipal and residential properties where fertilizers are produced or applied. N and P may enter as fairly simple compounds, often from fertilizer or as the urine component (urine is a nitrogenous waste) of livestock and human waste. But organic materials in general are high in bioavailable nutrients, and if they are readily decomposed, they will contribute to nutrient pollution.

The process of human contamination of waters with nutrient pollution is termed anthropogenic eutrophication (sometimes called cultural eutrophication). Where soils are naturally higher in nutrients, nearby waters can be naturally eutrophic or highly productive (the opposite – for low-nutrient waters – is oligotrophic). Anthropogenic eutrophication creates even higher levels of nutrients in the water than occurs naturally on productive soils. The high levels of nutrients support heavy growth of algae; algae become numerous and reduce water clarity. In fact, a standard measure of eutrophication is the level of chlorophyll-a in the water, which is due to the algae. Algae are short-lived – usually only a few days – and when they die, they sink to the bottom, where bacteria decompose them, using oxygen. But  oxygen only diffuses from the atmosphere to the bottom of lakes slowly. As a result, oxygen levels near the bottom of the lake can become too low to support most aquatic life, resulting in the death of less mobile bottom life – mollusks and other invertebrates – and any more mobile life that cannot find more oxygenated waters – often including many fish.

Severely nutrient-polluted water bodies are termed hypertrophic. Often dense algal mats cover the surface, blocking light to aquatic plants, and fish kills are common, as fish run out of oxygen. In estuaries where large rivers carrying heavy nutrient loads meet the ocean, the introduction of such large amounts of nutrients can result in dead zones – areas of ocean that are almost devoid of life.

When large amounts organic waste may enter water, usually from industry or from wastewater treatment plants, biological oxygen demand (BOD) is used to monitor the problem. BOD measures the amount of oxygen needed by oxygen-using decomposers to decompose the organic matter in the body of water. BOD rises with nutrient levels and also with water temperature, because decomposers can decompose more quickly in warmer waters (up to a point).

Both acid rain and nutrient pollution push the planetary boundary in modification of biogeochemical flows by changing the N and P cycles. Both through atmospheric deposition of NOx air pollution (as we saw in Chapter 2) and through additions of N and P to surface waters, humans increase availability of these important nutrients, leading to the present “increasing risk” status for this boundary.

Heavy metals

Familiar, toxic heavy metals include arsenic (As), mercury and lead. Many common metals used in everyday life are also toxic – copper, iron, silver, zinc, nickel. Others include chromium (Cr) and platinum (Pt). Heavy metals vary in the nature of their toxicity, but can inactivate enzymes, cause neurological problems, and trigger cancers, among other forms of toxicity.

Lead is of particular concern in drinking water because many early water-distribution systems in the US and elsewhere were built using lead pipes. Lead is relatively inexpensive and easy to work, and the lead industry pushed hard to continue its use. The risk from lead was recognized by the late 1800s, and cities began moving away from lead pipes by the 1920s. Nevertheless, a nationwide ban on new lead pipes in drinking-water systems was not passed until 1986, when it became part of the Safe Drinking Water Act. That ban did not require replacing existing lead pipes, and older cities and homes still contain some lead pipes.

In some parts of the world, arsenic occurs naturally and contaminates ground water. In Bangladesh and India, many deep wells were built by international efforts, partly to provide reliable water and also to avoid disease organisms in fouled surface water. Many of these wells were later found to contain high levels of arsenic.^[1]

Salts

Salt pollution occurs most often from road salt (mostly NaCl – the same chemical as table salt) but can also result from mining and industrial wastes. Most aquatic organisms have somewhat narrow tolerances for salinity, and organisms that live in freshwater cannot usually tolerate much salinity. Infrastructure – water treatment plants and dams, for example – can be damaged by corrosive salts. If metal drinking-water pipes that are not designed to carry salty water nevertheless receive salty water, metals from the pipes can leach into the water they carry, affecting human health.

Deep, ancient aquifers that hold water from ancient seas are naturally salty. Not all aquifers have been discovered or tested, so we do not know the total proportion of so-called “fossil water” that is salty. An early (1962) state report indicated as much as 75% of aquifer water in New Mexico, in the US, was too salty to use without treatment. Some areas in the world, including some arid areas of the western US and central Australia have salt deposits or saline groundwater close to the surface. In the most saline areas, rainwater is enough to dissolve salt and bring it to the surface (Fig 2); in such areas, only plants that tolerate high salinity can grow. In other areas, under natural conditions, sparse rainwater would be taken up by plant roots before it reaches these saline water bodies. But if the land is cleared, then, in the absence of plant roots, particularly if irrigation water is added, rainwater or irrigation water can penetrate deeply enough to reach the saline water, resulting in salt being wicked up to the surface of the land, sharply reducing the land’s ability to support agriculture or even native vegetation that is not adapted to saline conditions.

Sea-level rise is an important source of salt pollution for coastal areas and islands. Sea-level rise in the present is a result of global warming and has two main components. Melting ice of terrestrial glaciers and ice sheets on Greenland and in the northern extremes of North American and Eurasia and on Antarctica flows into the oceans and, in 2025, contributed about one-third of sea-level rise. Thermal expansion – the increase in the volume of water as it warms – contributes the other two-thirds. As climate change continues, and melting of terrestrial ice accelerates, meltwater will contribute more to sea-level rise, and thermal expansion will contribute less. But water that is warmed will always increase in volume, and thermal expansion will always be an important part of sea-level rise, so long as warming continues.

As sea level rises, ocean waters push inwards against the edges of continents and islands, pushing salt water into aquifers and up rivers that flow into the oceans. The problem is particularly severe for unconfined aquifers – aquifers in sand and gravel beds and in soil – and aquifers in porous or cracked rock layers such as limestones. Coastal swamps fed by freshwater from local water tables and rivers are already experiencing tree death as a result of saltwater coming up the rivers during high tide and contaminating groundwater. Coastal agriculture is also at risk from salinization of coastal surface aquifers.

Organic chemical pollutants

Organic chemical pollutants of water include PFAS, many pesticides, herbicides, artificial hormones (mainly birth-control chemicals that are excreted in urine and enter waterways from water treatment plants that often do not remove such chemicals), biodegradable organic matter (including raw sewage), fire retardants, industrial chemicals, pharmaceuticals, and related chemicals such as caffeine. Many organic pollutants are persistent and remain in the environment for long periods. Some bioaccumulate and biomagnify, causing increasing health impacts as they move higher up food chains. Environmental chemists group a number of problematic organic pollutants into a category called Persistent, Bioaccumulative, and Toxic (PBT) compounds; these include the persistent organic pollutants (POPs) mentioned in Chapter 1.

Biological water pollution

Biological contaminants include disease organisms (also called pathogens), parasites, and invasive plant and animal species. Water polluted with sewage and animal wastes is more likely to support disease organisms that live in such wastes, including Escherichia coli (E. coli), Salmonella, Cryptosporidium, Giardia, and the organisms responsible for cholera, hepatitis, and typhoid. Some parasites and invasive species are more common in eutrophic water. Invasive plant and animal species can outcompete, harm, or eat native species; clog waterways; and damage infrastructure (Fig 3). Some contribute to eutrophication and some are harmful to humans. Once they are widely established, invasive species are often expensive to control and impossible to eradicate completely. Modified genetic material from genetically modified organisms and from organisms that have developed antibiotic resistance is also classified as a biological pollutant. Warming conditions make once-cold waters more hospitable to invasive species, and nutrient pollution also can facilitate their spread.

Several unrelated varieties of algae produce toxins that contribute to water pollution. As a group, their occurrences are termed harmful algal blooms (HABs).^[2] The most common HABs are associated with freshwater algal blooms in eutrophic waters fed by excess fertilizer and sewage. In 2014, a particularly large HAB occurred on Lake Erie, in North America, near Toledo, Ohio, in the US, caused the shutdown of the drinking-water treatment plant there for several days, due to the presence of the toxin microcystin.

So-called “red tides” are examples of coastal marine HABs that produce brevetoxins that build up in shellfish (which are not affected) and sicken or kill vertebrate species that eat shellfish, including humans. Ciguatera poisoning is a result of another algal-produced toxin that builds up in fish and invertebrates and becomes harmful at higher levels, up the food chain. Saxitoxins are similarly harmful to high levels of food chain and were found to be associated with northern fur seal deaths in the southeast Bering Sea in 2025.^[3]

Algal populations reproduce faster in warmer waters, particularly when nutrient pollution is present to fuel their growth. The fur-seal incident in the southeast Bering Sea is attributed to global warming impacts on ocean waters in the polar region.

Radiological water pollution

Radioactive contamination of water can occur naturally when radioactive elements such as radon, radium or uranium occur near wells or groundwater. Mines that target radioactive minerals, or that have radioactive minerals co-occurring with non-radioactive minerals, may release mining wastes into water. Nuclear powerplants may similarly release radioactive materials into waterways, and nuclear accidents can release large volumes of radioactive material. Radioactivity can cause cancers and radioactive elements may accumulate in organs and bones, as heavy metals can, leading to longer exposures and more severe harm.

Thermal pollution of water

Water changes its temperature slowly, providing a stable thermal environment for aquatic life. As a result, many aquatic invertebrates, fish, and aquatic mammals have rather narrow temperature tolerance. In addition, warm water carries less oxygen than cold water; for fish that cannot gulp air at the surface, lack of oxygen in unusually warm water can be lethal.

In industry and in power production, water pumped from rivers or lakes is often used to cool machinery. If it is discharged back to receiving waters while it is still warm or hot, it can cause thermal pollution, killing organisms that cannot tolerate either the temperature or the low oxygen levels. Global warming can exacerbate this problem.

Deforestation and timber harvesting near stream sides can expose small, previously shaded streams to direct sunlight, causing increased water temperatures that inhibit fish movement and reduce reproduction. In contrast, construction of dams on naturally warm rivers can lead to harm from cold water that comes from deep in the resulting reservoirs, at the depth that hydropower dams draw water to produce electricity. Rivers with such dams may lose many of their native fish and invertebrates for considerable distance downstream of the dams.

Off the coast of Florida, a population of West Indian manatees that exists at the northernmost limit of their temperature tolerance, benefits from warm-water discharges from power plants along the coast. In winter, when water temperatures drop below 68°F (20°C), warm water discharges can be vital for their survival, along with naturally warm springs and thermal basins. But most impacts from thermal pollution are less benign.

Knowledge Check

Take a moment to complete the short quiz below to assess your understanding of this section. Read each question carefully and refer to the section content as needed. This quiz is not graded – it’s simply an opportunity for you to reflect on what you’ve learned and reinforce key concepts.