4.4 Water pollution

Water bodies become polluted when substances enter them faster than natural processes can neutralise or dilute them. The result is degraded water quality that harms aquatic organisms, disrupts ecosystems, and threatens human health.

Pollution enters water bodies from two broad categories of source:

• Point sources discharge from a specific, identifiable location, such as a factory outfall pipe or a sewage treatment works. These are relatively straightforward to monitor and regulate.
• Non-point sources (also called diffuse sources) are spread across a wide area, such as water draining off farmland during rainfall. Their dispersed nature makes them far harder to control.

The six main categories of water pollution source are outlined below.

Sewage and domestic wastewater. Domestic sewage carries organic matter, pathogens, and nutrients (especially nitrogen and phosphorus). Bacteria decomposing organic waste consume dissolved oxygen, which can trigger fish kills and biodiversity loss. Pathogens from human waste cause diseases including cholera, typhoid, and dysentery. In many low-income countries, up to 80% of sewage enters waterways untreated.

Agricultural runoff. Rain washes fertilisers (rich in nitrates and phosphates), pesticides, and animal waste from farmland into rivers and lakes. Nutrients drive algal growth and eutrophication. Pesticides accumulate in organism tissues (bioaccumulation) and increase in concentration at each trophic level (biomagnification). Soil particles carried by runoff raise turbidity, reducing light penetration and suppressing aquatic plant growth.

Industrial effluent. Factories discharge toxic chemicals, heavy metals, and solvents. Pollutants such as mercury, lead, cadmium, and arsenic disrupt aquatic ecosystems and contaminate drinking water. Many industrial chemicals are carcinogenic and persist in sediments for decades.

Urban runoff. During rainfall, water flowing through cities picks up oil, fuel residues, litter, pet waste, detergents, and metals from road surfaces and rooftops. Storm drains frequently discharge directly to rivers and coastal waters without any treatment. Urban runoff also carries microplastics shed from vehicle tyres and road wear.

Solid waste disposal and leachate. Rainwater percolating through poorly managed landfills or open dumps produces leachate: a liquid containing toxic chemicals, organic matter, and pathogens. This contaminates nearby groundwater and surface water. Properly engineered sanitary landfills use impermeable liners and leachate collection systems to prevent this.

Oil spills. Oil enters the marine environment during extraction, transportation, or pipeline and tanker accidents. A surface oil film blocks sunlight and reduces oxygen exchange between air and water, killing fish and marine plants. Seabirds and marine mammals coated in oil lose insulation and buoyancy.

A specific case study illustrates how multiple factors interact to produce a water pollution crisis and how management strategies are deployed in response.

Location: Flint, Michigan, USA.

Source of pollution: In 2014, the city switched its drinking water supply to the Flint River to cut costs. The river water was more corrosive than the previous supply, and without adequate corrosion-control treatment, it leached lead and other heavy metals from ageing distribution pipes and household plumbing.

Impacts on the environment and public health:

• Lead concentrations in tap water reached levels far above safe limits. Lead has no safe exposure threshold for children; exposure causes irreversible neurological and developmental damage.
• Bacterial contamination, including Legionella, was linked to an outbreak of Legionnaires' disease.
• Residents, disproportionately low-income and from minority communities, lost trust in public institutions and faced long-term health costs.
• The crisis exposed the consequences of inadequate water governance and the inequitable distribution of environmental risk.

Management strategies:

• Bottled water and point-of-use filters were distributed to residents.
• The city undertook a programme to replace all lead service lines (pipes connecting mains to homes) at federal and state expense.
• Federal intervention under the Safe Drinking Water Act provided emergency funding and oversight.
• Long-term health monitoring and support programmes were established for affected children.

Over 14 million tonnes of plastic enter the oceans each year, the majority from land-based sources carried by rivers and wind. Plastics do not biodegrade in the conventional sense; instead, ultraviolet radiation and physical abrasion break them into progressively smaller fragments through a process called photodegradation.

How plastics accumulate in ocean gyres. Ocean surface currents form large, rotating systems called gyres. Floating plastic debris is transported by wind and surface currents and gradually becomes concentrated in the relatively calm central regions of these gyres, where it persists indefinitely.

The Great Pacific Garbage Patch, located within the North Pacific Gyre, is the largest known accumulation zone, containing an estimated 1.8 trillion plastic fragments and covering an area that may exceed 1.5 million km².

Harm to marine life. Macroplastics cause entanglement in fishing gear and plastic rings, and ingestion by animals that mistake plastic for prey. Sea turtles ingest plastic bags resembling jellyfish; seabirds feed plastic fragments to chicks, mistaking them for fish. Entanglement causes drowning, starvation, and injury. Over one million seabirds are estimated to die from plastic ingestion each year.

Microplastics and the food chain. Microplastics are ingested by plankton and filter-feeders at the base of marine food webs. As they pass up the food chain, they undergo bioaccumulation within individual organisms and biomagnification across trophic levels, meaning concentrations increase at each step.

A critical secondary effect is that microplastics adsorb (attract onto their surface) persistent toxic chemicals already present in seawater, including PCBs, DDT, and heavy metals. These plastics therefore act as vectors, concentrating surface toxins and transporting them into food webs where they cause hormonal disruption, reproductive failure, and cancer.

Additional impacts. Plastic debris damages coral reefs, reducing structural complexity and biodiversity. Floating plastics also act as rafts for invasive species, transporting organisms to new habitats where they may outcompete native species.