Water pollution is the introduction or accumulation of substances, organisms, energy or other agents in rivers, lakes, groundwater, estuaries, coastal waters or oceans in ways that degrade water quality or harm organisms, ecosystems or human uses. Polluted water may contain pathogens, nutrients, sediments, toxic metals, petroleum compounds, pesticides, industrial chemicals, plastics, pharmaceuticals or other contaminants.
Water pollution can originate from point sources, such as a wastewater discharge pipe or industrial outlet, or nonpoint sources, such as agricultural runoff and urban stormwater distributed across large areas. Pollution can also enter water through atmospheric deposition, accidental spills, mining, landfill leakage, sewage, erosion and natural processes intensified by human activity.
Water quality is multidimensional and includes physical properties such as temperature and turbidity, chemical properties such as acidity, salinity, nutrient concentrations and dissolved oxygen, and biological properties such as microbial communities and ecological integrity. A water body can therefore be polluted without displaying obvious visual changes.
Nutrient pollution is one of the most important forms of water pollution. Nitrogen and phosphorus are essential nutrients for aquatic organisms, but excessive inputs can stimulate rapid growth of algae and aquatic plants. When this biomass dies, microorganisms decompose it and consume dissolved oxygen. If oxygen consumption exceeds replenishment, hypoxic conditions can develop, while severe oxygen depletion can produce anoxia. Fish and other oxygen dependent organisms may then become stressed or die, and species composition can change.
This sequence is commonly associated with eutrophication. Some algal blooms also produce toxins or other harmful compounds, creating additional risks to aquatic organisms and people. Nutrient sources include sewage, fertilisers, animal waste, atmospheric deposition and erosion. The effects depend on nutrient ratios, water movement, temperature, light, depth and the ecological characteristics of the receiving water body. Controlling nutrient pollution therefore often requires reducing inputs throughout an entire catchment rather than treating only the visibly affected location.
Pathogens are another major category of water pollutant. Human and animal waste can introduce bacteria, viruses, protozoa and other infectious agents into water. Examples of waterborne pathogens include organisms responsible for diseases such as cholera, dysentery and giardiasis. Sewage contamination is particularly important because wastewater can contain both pathogens and nutrients as well as chemical contaminants.
Drinking water treatment uses combinations of physical removal, chemical processes and disinfection to reduce risks, while wastewater treatment reduces organic matter, nutrients, suspended solids and pathogens before discharge or reuse. Failing sanitation systems can allow contaminants to enter groundwater, rivers and coastal waters. Safe water management therefore requires protection of source waters, effective treatment, distribution system integrity and appropriate monitoring. Water pollution is not solely an environmental issue because contaminated water can directly affect public health, agriculture, food production and economic activity.
Toxic chemical pollution includes heavy metals, pesticides, petroleum hydrocarbons, solvents and numerous industrial compounds. Metals such as mercury, lead, cadmium and arsenic can be toxic at sufficiently high exposure levels, although their behaviour depends on chemical form and environmental conditions. Mercury is particularly important because certain forms can be transformed into methylmercury, which can accumulate in aquatic food webs and reach higher concentrations in predatory organisms.
Petroleum spills can coat shorelines, interfere with aquatic organisms and alter habitats, while dissolved hydrocarbons can produce toxic effects. Pesticides entering water can affect organisms other than their intended targets. Pollutants may attach to suspended sediments, settle into sediments, dissolve in water or accumulate in organisms. Sediments can therefore act both as sinks and later sources of contaminants when environmental conditions change. Understanding chemical speciation and partitioning is essential because total concentration does not always equal biological availability or toxicity.
Physical changes can also constitute water pollution. Excessive sediment from soil erosion can increase turbidity, reduce light penetration, smother aquatic habitats and interfere with fish spawning or feeding. Thermal pollution occurs when human activities substantially alter water temperature, often through heated industrial or power plant discharges. Warmer water generally holds less dissolved oxygen and can alter metabolism, reproduction and species distributions. Salinity changes caused by irrigation, industrial discharges, road deicing materials or seawater intrusion can affect freshwater organisms.
Plastic debris can cause entanglement, ingestion and habitat impacts, while floating materials can transport organisms to new locations. Light and noise can also alter aquatic environments in particular circumstances. These examples demonstrate that water pollution is broader than visible rubbish or toxic chemicals. Any human induced alteration that substantially degrades ecological or human uses of water can be environmentally important.
Water pollution is managed through monitoring, prevention, treatment, regulation and catchment based planning. Wastewater treatment commonly includes preliminary removal of large materials, primary processes that settle suspended solids, secondary biological treatment that removes biodegradable organic matter and, where required, advanced treatment for nutrients, pathogens or specific contaminants. Drinking water treatment can involve coagulation, flocculation, sedimentation, filtration, activated carbon, membrane processes and disinfection depending on source quality and treatment objectives.
Industrial facilities may need specialised treatment for metals, solvents, oils or other compounds. Agricultural pollution can be reduced using nutrient management, buffer strips, erosion control, improved manure handling and careful irrigation. Urban runoff can be managed through drainage design, retention systems, permeable surfaces and other forms of green infrastructure. Effective control generally focuses on preventing contaminants from reaching water in the first place because remediation of contaminated aquifers, sediments and ecosystems can be difficult and expensive.
Water pollution must be understood in relation to the hydrological cycle and watershed connectivity. Rain falling on land can mobilise pollutants and transport them through streams into rivers, lakes and oceans. Groundwater can receive contaminants through infiltration and can later discharge into surface waters, sometimes after long delays. Rivers transport dissolved substances and sediments downstream, linking activities in upstream areas with conditions far away. Estuaries receive mixtures of freshwater and seawater and can accumulate nutrients and contaminants from large catchments.
Oceans can receive pollutants through rivers, atmospheric deposition, coastal activities and direct discharge, and ocean currents can transport floating materials across enormous distances. Because water moves through interconnected systems, a pollution problem cannot always be solved by focusing narrowly on the point where it becomes visible. Catchment management, source control and understanding transport pathways are therefore fundamental components of modern water quality science.
Water pollution has important ecological, social and economic consequences. Healthy freshwater and marine systems support biodiversity, fisheries, agriculture, recreation, navigation and drinking water supplies. Pollution can reduce habitat quality, alter food webs, increase treatment costs and restrict the uses of water bodies. Some pollutants degrade rapidly, while others persist for decades or remain stored in sediments or groundwater.
Pollution can also interact with climate change because changing rainfall patterns, drought, floods and temperature can influence contaminant concentrations, runoff and oxygen conditions. Extreme rainfall can wash large amounts of sediment, nutrients and chemicals into water bodies, while drought can reduce dilution and increase concentrations. Consequently, protecting water quality requires long term management rather than relying only on emergency responses. The fundamental objective is to maintain physical, chemical and biological conditions that allow water bodies to perform their ecological functions and safely support legitimate human uses.
