Water pollution is the introduction of substances into water bodies that interfere with the beneficial use of the water or disrupt natural ecosystems. This contamination affects lakes, rivers, oceans, and groundwater, and typically results from human activities. The sources of pollution are diverse, ranging from direct, easily traceable discharges to widespread, diffuse flows, and even airborne particles. Categorizing these origins helps develop targeted strategies to protect water resources from pollutants like toxic chemicals, organic waste, and heat.
Specific Point Source Discharges
Point source pollution is the release of contaminants from a single, identifiable location, such as a pipe, ditch, or channel. This distinct origin makes these sources easier to monitor, regulate, and manage compared to other forms of pollution. The two primary categories of point sources are industrial and municipal discharges.
Industrial facilities, such as chemical manufacturers, power plants, and food processing operations, discharge wastewater called effluent. This effluent can contain harmful substances, including heavy metals like mercury and lead, chemical solvents, and toxic organic compounds. Manufacturing and power generation also contribute thermal pollution, where heated cooling water is released, raising the water body’s temperature and stressing aquatic life.
Municipal sources are primarily publicly owned treatment works (POTWs), which process domestic sewage. Even after treatment, the final effluent still contains residual pollutants. These include nutrients like nitrogen and phosphorus, which fuel excessive algal growth, and pathogens that pose a health risk. In older cities, combined sewer systems can overflow during heavy rainfall, releasing a mixture of stormwater and untreated sewage directly into waterways.
Diffuse Non-Point Source Runoff
Non-point source (NPS) pollution is diffuse contamination that cannot be traced to a single pipe or discharge location. This pollution is primarily caused by rainfall or snowmelt moving over the ground, picking up and carrying pollutants into water bodies. Because the sources are scattered across broad geographical areas, controlling non-point pollution is more challenging than regulating point sources.
Agricultural runoff is a major contributor to NPS pollution, as water flowing over farm fields transports substances like excess fertilizers. These fertilizers, specifically nitrogen and phosphorus, cause eutrophication and harmful algal blooms in receiving waters. Pesticides, herbicides, and animal waste from livestock operations are also carried into streams and rivers, introducing chemicals and bacteria.
Urban stormwater runoff is another significant source, especially in areas with extensive impervious surfaces like roads and parking lots. Rainwater washes over these surfaces, collecting contaminants such as oil, grease, and toxic chemicals from vehicles, along with litter and sediment from construction sites. Road salt applied for de-icing in winter also contributes high concentrations of chloride to urban waterways.
Mining activities, particularly from abandoned sites, contribute to water pollution through acid mine drainage. When water interacts with exposed sulfide minerals, it creates sulfuric acid that leaches heavy metals into the environment. Sediment washing from disturbed land at construction and mining sites is also a common pollutant, increasing water turbidity and disrupting aquatic habitats.
Atmospheric Deposition and Emerging Contaminants
Some sources of water contamination involve indirect pathways, including pollutants that travel through the air. Atmospheric deposition is the process where airborne contaminants settle onto land and water surfaces, directly linking air quality to water quality. Emissions from industrial facilities and vehicle exhaust release substances like nitrogen oxides and sulfur dioxide into the atmosphere.
These airborne pollutants return to Earth as “wet deposition,” such as acid rain, or “dry deposition,” like dust and particles. Once settled, these contaminants, which include heavy metals and nutrients, are washed into water bodies by subsequent rainfall or runoff. This process introduces distant pollution sources into local aquatic ecosystems.
A modern concern involves emerging contaminants (ECs), which are pollutants recently recognized as harmful but not yet routinely monitored or regulated. These chemicals enter water systems through various routes, including sewage effluent and surface runoff. Examples include pharmaceuticals, such as antibiotics and anti-inflammatory drugs, and personal care products like UV filters and antimicrobials.
Other emerging contaminants include microplastics, which are tiny plastic fragments, and endocrine disruptors, which interfere with hormonal systems. These substances often bypass standard wastewater treatment processes, leading to their persistence in the aquatic environment. The complex nature and variety of these contaminants challenge environmental scientists seeking to understand and mitigate their long-term effects on ecosystems and human health.