Sources and pathways

Water pollution occurs when substances or energy enter water and impair its ecological condition or usefulness. Rivers, lakes, groundwater, estuaries and oceans can all be affected. The effects often spread far beyond the point where the pollutant first entered the system.

The major sources are:

• Sewage introduces organic matter, mineral nutrients, pathogens and household chemicals. As decomposers consume the organic matter, they can reduce dissolved oxygen.
• Agricultural run-off carries fertilizers, manure, sediment and pesticides away from fields. It is often diffuse, or non-point, pollution, so tracing it to a single discharge is difficult.
• Industrial effluent may contain metals, acids, alkalis, synthetic chemicals, organic waste or heated water. A discharge pipe is a point source, which is usually easier to monitor.
• Urban run-off washes oil, tyre particles, road salt, litter, sediment and chemicals from impermeable surfaces into drains and waterways.
• Solid-waste disposal includes waste dumped directly into water, as well as contaminated leachate escaping from poorly managed landfill.
• Oil spills may come from tankers, pipelines, drilling operations and routine vessel discharges.

These inputs may poison organisms or transmit disease. They can also raise turbidity, alter pH or temperature, reduce photosynthesis, lower dissolved oxygen, damage habitats and simplify food webs. Biodiversity may decline, while supplies of safe water, food and recreation are reduced.

A specific example: the Deepwater Horizon spill

The Deepwater Horizon drilling-platform disaster took place in 2010 at the Macondo well in the Gulf of Mexico. After an explosion and well failure, crude oil escaped into deep marine water for almost three months. It reached open water, the seabed, beaches, salt marshes and coastal wetlands.

Oil and its chemical components harmed plankton, corals, fish, seabirds, sea turtles and marine mammals. Direct coating reduced birds' insulation and mobility. Ingestion and exposure to dissolved hydrocarbons also caused toxic effects. Coastal habitats suffered damage, fisheries temporarily closed, and communities that depended on fishing and tourism lost income. Long-term effects were hard to separate from natural population variation, a point to remember when judging any pollution case study.

Management involved capping the well and collecting oil at the surface. Booms and skimmers were used, some surface oil was burned, dispersants were applied, and affected shorelines were cleaned. Fishing closures reduced human exposure. Restoration programmes and legal compensation followed. None of these methods was harmless. For example, dispersants broke slicks into smaller droplets but increased exposure within the water column, while aggressive marsh cleaning could cause further habitat damage.

Pollution and sustainability

Pollution undermines the sustainability of environmental systems when pollutant inputs exceed the system's capacity to dilute, decompose, store or safely export them. Ecological losses then affect society through poorer health, lower fish catches, treatment costs and unequal access to clean water. Management must consider the whole source–pathway–impact chain instead of simply moving pollution somewhere less visible.

Accumulation and harm

Plastic is light, durable and used widely. When waste is poorly managed, rivers, drains or wind can carry it into the sea. Most plastic fragments instead of fully biodegrading. Large pieces can entangle, drown or injure animals. They also cause ghost fishing, in which abandoned fishing equipment continues to catch organisms. Some animals mistake plastic for prey and ingest it, which can obstruct the gut, create false satiation and reduce feeding or growth.

Over time, sunlight and wave action break plastic down, while abrasion wears it into microplastics—plastic particles smaller than 5 mm. Some are fragments of larger objects; others are fibres and particles released directly from products and industrial materials. Because they are so small, plankton, filter feeders and other organisms near the base of food webs can take them up.

Why plastics gather in gyres

An oceanic gyre is a large, rotating system of surface currents. Prevailing winds and Earth's rotation drive these currents, while the positions of continents shape them. Where surface currents converge, they carry buoyant debris towards relatively calm accumulation zones. These zones aren't solid islands of rubbish. Much of the material consists of small particles dispersed through a large volume of water.

Movement through food chains

Bioaccumulation is the build-up of a substance within an organism because uptake exceeds removal. Biomagnification is the increase in a persistent substance's concentration at successively higher trophic levels. Organisms may retain ingested microplastics and their associated chemicals. Predators can then acquire them by consuming contaminated prey, although the degree of magnification varies with particle size and chemical properties, as well as between species.

Hydrophobic pollutants can adsorb onto plastic surfaces. The particles may then carry these surface contaminants through seawater and into organisms. Plastics can also release additives used during manufacture. Concern therefore comes from the particle and from the chemical mixture travelling with it.

Management across the plastic system

It is generally more effective to prevent plastic from entering the environment than to recover particles once they are widely dispersed. Upstream measures include eliminating avoidable single-use items and designing products that are reusable and genuinely recyclable. Other approaches include deposit-return schemes, extended producer responsibility and better waste collection. Restrictions on intentionally added microplastics can reduce releases, while filters can capture fibres or tyre particles.

Interception measures include litter traps in drains and river barriers, along with improved wastewater treatment. Beach clean-ups can reduce existing pollution. So can retrieving lost fishing gear and using carefully designed collection systems in accumulation zones. However, clean-up cannot capture most microscopic or deep-water plastic and may disturb wildlife. It must support—not replace—the removal of plastic from the supply chain.

What water quality means

Water quality is the measurement of chemical, physical and biological characteristics of water. It isn’t a single fixed property. Water quality varies with the characteristics being measured, the water’s intended use, location, season and recent weather.

A water quality index combines several water-quality results, usually with weighting, to produce one overall score. This makes it easier to compare sites or dates, but the score may hide which pollutant caused the deterioration. Keep the individual measurements alongside the index.

Monitoring sets baselines and reveals pollution hotspots. It also shows whether management is working. High nutrient concentrations, for example, may support tighter fertilizer controls. A sudden metal peak could prompt an investigation into an industrial discharge.

Indicators and field methods

Important measurements include:

• Dissolved oxygen: use a calibrated oxygen probe or test kit and avoid trapping air bubbles. Low values may suggest decomposition of organic pollution, though temperature and water movement also affect oxygen.
• pH: measure with a calibrated pH probe or indicator test. Rinse the probe between samples, then take the measurement promptly.
• Temperature: use a waterproof thermometer or probe at a consistent depth. Temperature influences reaction rates, organism metabolism and oxygen solubility.
• Turbidity: turbidity is a physical characteristic representing the cloudiness caused by suspended particles. Lower a Secchi disc until its markings disappear, then record the depth. A turbidity tube or electronic meter can also be used. A smaller Secchi depth indicates greater turbidity.
• Nitrate and phosphate concentrations: measure these with colorimetric test kits, strips or calibrated sensors. After the specified reaction time, match the developed colour against a standard.
• Specific metals: collect uncontaminated samples for instrumental analysis, or use a suitable portable analyser. Name the metal because toxicity and guideline values differ.
• Total suspended solids: pass a known volume of water through a clean, pre-weighed filter. Dry the filter to constant mass and reweigh it. Divide the increase in filter mass by the sample volume to find the concentration of suspended solids.

Producing dependable data

Take samples at comparable depths and times. Use clean, labelled containers, calibrate instruments and repeat measurements. Record recent rainfall or unusual discharges, and don’t disturb bottom sediment before sampling. Measurements should come from several locations, such as upstream and downstream of a suspected source, since one isolated reading cannot establish a spatial pattern.

Abiotic results provide snapshots. Repeated monitoring separates persistent pollution from short-lived variation and gives managers evidence to choose, adjust and evaluate pollution controls.

Meaning and interpretation

Biochemical oxygen demand (BOD) is a measure of the amount of dissolved oxygen required by microorganisms to decompose organic material in water. It provides an indirect measure of how much biodegradable organic matter a sample contains.

The standard result records the mass of oxygen consumed per litre over five days at $20\ ^\circ\text{C}$ and is usually reported in $\text{mg L}^{-1}$. A high BOD shows that microorganisms are using a large amount of oxygen, normally because abundant organic matter allows rapid aerobic decomposition. Low BOD usually points to less biodegradable organic material.

BOD itself is not a toxic chemical. Instead, the risk comes from microbial respiration removing oxygen faster than photosynthesis, diffusion and mixing can replace it. Oxygen-dependent organisms may then become stressed or die.

Five-day method

1. Collect a representative water sample, taking care not to aerate it unnecessarily.
2. Measure the initial dissolved-oxygen concentration.
3. Fill and seal an airtight BOD bottle so that oxygen cannot enter from the atmosphere.
4. Incubate the sample in darkness for five days at $20\ ^\circ\text{C}$. Keeping it dark prevents photosynthesis from adding oxygen; controlling the temperature allows samples to be compared.
5. Measure the final dissolved-oxygen concentration. The drop in concentration is the five-day BOD. If the oxygen could be completely exhausted, dilution is required.

Suitable controls and replicate bottles are needed, along with calibrated oxygen equipment. Toxic substances may suppress decomposers, making BOD appear deceptively low even when the water is polluted. This is another reason not to rely on a single indicator.

Nutrient enrichment

Eutrophication occurs when lakes, estuaries and coastal waters receive inputs of mineral nutrients, especially nitrates and phosphates, often causing excessive growth of phytoplankton. These nutrients supply elements needed for growth, so they stimulate primary production.

An algal bloom is a rapid increase in the abundance of phytoplankton or other bloom-forming aquatic microorganisms. However, nutrient input causes a bloom only if growth was previously limited by a low concentration of nitrate, phosphate or both. Extra nutrients won't necessarily trigger excessive growth if light, temperature or another resource is still limiting. This condition is easily overlooked.

Human sources include phosphate-containing detergents, untreated or partly treated sewage, and fertilizers or manure washed or leached from farmland. Nutrients can enter through identifiable pipes or as diffuse run-off across an entire catchment. Lakes and slowly flushed coastal waters are particularly vulnerable because the nutrients remain available for longer.

From nutrient input to oxygen depletion

The sequence usually runs like this:

1. Inputs of nitrate and phosphate remove nutrient limitation.
2. Phytoplankton grow rapidly and form a dense bloom.
3. Less light penetrates the water, so submerged plants photosynthesize less and may die.
4. The phytoplankton also die when nutrients become depleted or conditions change.
5. As decomposers break down the dead organic material, decomposer populations increase and aerobic respiration rises.
6. This rapidly consumes dissolved oxygen.
7. Hypoxia is a water condition in which dissolved oxygen is too low to support many oxygen-dependent organisms; anoxia is a water condition in which dissolved oxygen is absent.
8. Fish and other aerobic organisms die or migrate. Tolerant or anaerobic organisms then become more important.

Systems model and positive feedback

A systems model should include nutrient and organic-matter storages, the flows between them and the direction of causal links. It should also show the reinforcing loop. More nutrients increase biological production and, later, organism death. The extra dead material increases decomposition, which releases mineral nutrients. These nutrients support further production and death.

This is positive feedback, a feedback process that amplifies an initial change. Oxygen depletion may reinforce the loop: low oxygen kills more organisms, which adds material for decomposition. The process has limits. Nutrient loss, flushing, colder conditions or exhaustion of available organic matter can weaken it, though the loop may still keep a degraded state in place.

Consequences for people and ecosystems

Ecosystem services are the benefits people obtain from ecosystems. By altering ecological processes, eutrophication reduces several connected services:

• Fisheries: hypoxia kills fish and benthic organisms and closes nursery areas. Valuable species may be replaced by pollution-tolerant ones, causing catches and fishers' income to fall.
• Recreation: foul odours and surface scums discourage swimming and boating. Dead fish and health warnings also deter anglers.
• Aesthetics: cloudy water, dense plant growth and decaying material lower the visual and cultural value of rivers, lakes and coasts.
• Health: contaminated recreational water can irritate the skin or cause illness, and some blooms produce toxins. Water suppliers may need extra treatment to make the water safe.

These effects don't occur separately. A bloom that reduces oxygen may damage fish stocks while also closing beaches, increasing treatment expenditure and harming tourism. The costs may be distributed unfairly too: nutrient-producing activities can take place upstream, while downstream communities lose drinking water or livelihoods.

Three levels of intervention

Pollution management can tackle eutrophication at three different levels.

1. Reduce the activities that produce pollutants. This might involve using phosphate-free detergents or applying fertilizer only when crops need it. Other measures include matching application rates to soil tests, keeping winter crop cover and choosing less nutrient-intensive production. These approaches deal with the source, though they may depend on changes to behaviour, regulation and farming practice.
2. Reduce pollutant release into the environment. Before wastewater is discharged, treatment can remove nitrate and phosphate. Vegetated buffer strips and restored wetlands reduce the movement of nutrients into water, as do covered manure stores and carefully timed fertilizer application. The activity can often continue because these methods interrupt the pollution pathway.
3. Remove pollutants and restore the ecosystem. Once water quality improves, managers may reintroduce native plants and animals. They can also remove nutrient-rich sediment, harvest excessive vegetation or oxygenate the water. These actions may bring visible local recovery, but they are expensive and disruptive. Any improvement will also be temporary if nutrient inputs continue.

This hierarchy works for many pollutants. Avoid producing them where possible, intercept releases that can’t be avoided, then clean up the receiving environment when needed. Prevention usually offers the more sustainable option because it addresses the cause. Still, no single level works in every situation.

Perspectives and management choices

Environmental value systems differ in the responses they emphasize. An ecocentric perspective may support lower consumption, less dependence on fertilizer and wetland restoration. A technocentric perspective may instead favour nutrient-removal technology, precision fertilizer application or mechanical aeration. From an anthropocentric perspective, strategies may be judged by their effects on health, employment, food production and recreation.

These perspectives also shape which trade-offs people find acceptable. Source restrictions can cut pollution, but they may increase costs for farmers or manufacturers. Advanced treatment allows production to continue, although it needs infrastructure, energy and reliable funding. Effective policy usually draws on all three levels and includes both those who create the pollution and the communities affected by it. Pollution management is therefore never a purely technical decision.

Pollutants differ in form and behaviour

Water pollution isn't always visible. It occurs in several important forms:

• Organic matter, such as sewage, supports decomposers. It can raise BOD, reduce oxygen and introduce pathogens.
• Dissolved substances include tributyltin. Tributyltin is an organotin compound formerly used widely as a biocide in antifouling coatings. It acts as an endocrine disruptor and can impair reproduction in aquatic organisms, even at low concentrations.
• Persistent chemicals include polychlorinated biphenyls (PCBs). They resist degradation, remain in ecosystems and may bioaccumulate and biomagnify through food webs.
• Plastics range from large debris to microscopic particles. They cause physical harm and transport associated chemicals.
• Heat energy is released when warm cooling water enters a river, lake or coastal area. This thermal pollution alters temperature-dependent processes, lowers oxygen solubility and may push conditions beyond organisms' tolerance ranges.

Physical form affects how a pollutant behaves. Dissolved chemicals can pass through simple filters, while suspended material increases turbidity. Persistent pollutants stay in ecosystems long after a discharge stops, and heat can't be skimmed from the surface like oil. Monitoring and management must match the pollutant rather than rely on one standard solution.

Harmful algal blooms

A harmful algal bloom (HAB) occurs when aquatic microorganisms increase rapidly and damage organisms, human health or ecosystem use. The harm may come from toxin production, oxygen depletion or physical effects. HABs can involve cyanobacteria, algae, protists and dinoflagellates, though only a minority of bloom-forming organisms produce potentially fatal toxins.

Cyanotoxins from cyanobacteria are the most common bloom toxins in freshwater. People may be exposed by drinking contaminated water, swallowing water during recreation or coming into contact with bloom material. Depending on the toxin, effects include skin irritation and damage to the liver or nervous system. Livestock, pets and wildlife may also be poisoned.

Some dinoflagellates in coastal waters produce neurotoxins. Their blooms may look red or brown, but colour alone does not show whether a bloom is toxic. Filter-feeding shellfish can concentrate the toxins without being killed, allowing them to enter human food.

Freshwater example: western Lake Erie

In western Lake Erie, warm, nutrient-rich conditions have supported cyanobacterial blooms dominated by Microcystis. Microcystin can contaminate raw water and threatens the liver. During a major bloom, toxin entered the water supply, so authorities warned residents not to drink tap water until treatment and monitoring showed that it was safe. Management targets phosphorus entering from the catchment and aims to improve water-treatment preparedness.

Marine example: Gulf Coast red tides

The dinoflagellate Karenia brevis forms blooms along the Gulf Coast of Florida and produces brevetoxins. These neurotoxins can kill fish and affect marine mammals. When aerosolized, they irritate human airways, and they can also accumulate in shellfish. Monitoring, temporary shellfish-harvesting closures and public health warnings reduce exposure, but don’t remove the bloom itself.

Interacting causes

An aquatic dead zone is an area of a water body where dissolved oxygen levels are too low for most oxygen-dependent aquatic life to survive. Several processes can interact to produce a dead zone:

• Sewage disposal adds biodegradable organic matter and nutrients, so microbial respiration increases.
• Eutrophication boosts phytoplankton production. When this material dies, it provides a greater food supply for decomposers.
• Global warming raises water temperature. Warm water holds less dissolved oxygen, while many organisms and decomposers respire faster.
• Freshwater input can cause stratification, a layered water condition in which density differences restrict vertical mixing. A warmer or less saline surface layer may sit above denser bottom water, stopping atmospheric oxygen from replenishing the deeper water.

Climate warming can make stratification stronger and longer-lasting. More intense rainfall may also carry extra nutrients and low-density freshwater into the water body. If decomposition continues beneath the surface layer, oxygen levels in the bottom water fall towards hypoxia or anoxia. Sheltered bays, estuaries and deep basins with weak circulation are particularly vulnerable.

The pressures work together. Warming reduces the oxygen supply, and stratification prevents it from being replenished. At the same time, sewage or eutrophication increases oxygen demand. As a result, dead zones become more frequent, extensive or persistent than they would under any one pressure alone.

Purpose of treatment

Sewage is wastewater that contains human waste along with other domestic or commercial contaminants. Effluent is the liquid waste released into the environment after a treatment process. Sewage treatment removes solids, biodegradable matter, nutrients and pathogens, allowing the effluent to be discharged with less risk to ecosystems and human health.

Primary treatment

Primary treatment relies mainly on physical processes. Screens catch large objects, while grit chambers allow sand and gravel to settle. In sedimentation tanks, suspended organic solids sink and form primary sludge. Fats and other floating material are skimmed from the surface. Although this stage reduces suspended solids, much of the dissolved organic matter, nutrients and microorganisms remain in the water.

Secondary treatment

Secondary treatment uses biological processes. In an activated-sludge system, air is added so aerobic microorganisms can consume dissolved and fine organic material. Another method passes wastewater over biofilms in trickling filters. The treated liquid then enters a settling tank, where the microbial biomass is separated. Some sludge may be returned to keep the decomposer population going. This stage substantially lowers BOD.

Tertiary treatment

Tertiary treatment produces higher-quality effluent for reuse or discharge into sensitive receiving waters. Nitrate can be removed biologically by nitrification followed by denitrification. Phosphate may also be removed biologically. Alternatively, it can be precipitated chemically with suitable metal salts, then separated as a solid. Fine filtration catches the particles that remain, while chlorine, ozone or ultraviolet radiation disinfects the effluent. Chlorination works well and is relatively inexpensive, but it needs careful control because residual chlorine and chlorinated by-products may cause harm.

Sludge produced during the earlier stages must also be treated, for example through digestion, dewatering and safe disposal or use. Without this step, treatment simply moves pollution from the water into unmanaged solid waste.

Equity and implementation

Access to treatment is uneven. Plants and sewer networks are expensive, as are skilled staff, electricity, monitoring and maintenance. Infrastructure may not keep pace with rapid urban growth and informal settlements. Rural communities can also be difficult to connect to centralized systems. Ability to pay doesn’t match need: the populations most exposed to untreated sewage often have the fewest resources available for treatment.

Providing treatment equitably may require public funding, affordable tariffs and decentralized or nature-based systems. It may also depend on locally maintainable technology and support for operators. Sophisticated equipment won’t provide reliable treatment if there’s no money for chemicals, power and repairs. Appropriate technology is the system that a society can operate safely over the long term.

Biological evidence of water quality

An indicator species is a species whose presence, absence or abundance provides evidence about environmental conditions. Some aquatic species are sensitive to particular pollutants or the oxygen depletion they cause. Others have adaptations that let them survive in polluted water with low oxygen levels.

When conditions exceed their tolerance, sensitive organisms disappear. Brook trout, for example, need cool, well-oxygenated water. A sustained decline may therefore indicate warming, sedimentation or oxygen stress. Pollution-tolerant aquatic worms may remain abundant in places where organic inputs support decomposers and oxygen levels are low. The presence of a tolerant species alone doesn't prove pollution. Unusually high numbers, together with the loss of sensitive species, provide stronger evidence.

A chemical measurement records conditions at the moment the sample is taken. Indicator species, by contrast, integrate conditions over time, so they may reveal intermittent or chronic pollution that a single water sample misses. There are limitations: habitat type, flow, temperature, season and sampling effort can all affect abundance. For a reliable assessment, researchers compare similar habitats, use standardized sampling and combine biological evidence with abiotic measurements.