Definition
Water pollution
Water pollution is the contamination of water bodies by harmful substances, making them unsafe for humans, wildlife, and ecosystems.
- Water pollution occurs when harmful substances, chemical, physical, or biological, contaminate natural water bodies, degrading their quality and harming organisms.
- It affects both marine and freshwater systems through multiple sources such as sewage, agriculture, industries, and urban runoff.
- Pollutants may come from point sources (e.g., factory discharges, sewage outlets) or non-point sources (e.g., agricultural runoff, urban stormwater).
Water pollution can be chemical (e.g., pesticides, heavy metals) or microbial (e.g., pathogens).
NotePoint-source pollution is easier to monitor and manage, while non-point-source pollution is diffuse and widespread, making it harder to control.
Major Sources of Water Pollution
1. Sewage and Domestic Wastewater
- Domestic sewage contains organic matter, pathogens, and nutrients such as nitrogen and phosphorus.
- When untreated sewage enters rivers or lakes, bacteria decompose organic matter, consuming dissolved oxygen (DO) and leading to oxygen depletion.
- Low oxygen levels cause fish kills and loss of aquatic biodiversity.
- Pathogens from human waste cause diseases such as cholera, typhoid, and dysentery.
In many developing nations, up to 80% of sewage is released untreated, resulting in eutrophication and public health crises.
Common Mistake- Don't assume “sewage” refers only to toilet waste.
- It also includes water from sinks, baths, and washing machines.
2. Agricultural Runoff
- Water running off agricultural land carries fertilizers, pesticides, animal waste, and sediments into nearby water bodies.
- Fertilizers rich in nitrates and phosphates promote algal blooms, which decompose and cause oxygen depletion (eutrophication).
- Pesticides accumulate in the tissues of non-target organisms (bioaccumulation) and increase in concentration up the food chain (biomagnification).
- Soil erosion contributes sediments that increase turbidity, reducing light penetration and photosynthesis.
The Gulf of Mexico Dead Zone forms annually due to fertilizer runoff from the Mississippi River Basin, leading to widespread oxygen depletion.
3. Industrial Effluent
- Industries discharge toxic chemicals, heavy metals, and solvents into nearby water bodies.
- Pollutants like mercury, lead, cadmium, and arsenic disrupt aquatic ecosystems and contaminate drinking water.
- Many industrial pollutants are carcinogenic and persistent, remaining in ecosystems for decades.
Minamata Disease, Japan
- In the 1950s, a chemical factory released methylmercury into Minamata Bay.
- It bioaccumulated in fish and shellfish, leading to neurological disorders, birth defects, and death in local communities.
- This case became a landmark in environmental law, leading to the polluter-pays principle and the Minamata Convention on Mercury (2013).
4. Urban Runoff
- During rainfall, water flowing through cities carries oil, fuel, litter, pet waste, detergents, and metals from roads and rooftops.
- Urban storm drains often lead directly to rivers and oceans without treatment.
- Pollutants in runoff cause oxygen depletion, algal blooms, and contamination of groundwater sources.
Stormwater pollution often contains microplastics and hydrocarbons from vehicle tire abrasion and road wear.
5. Solid Waste Disposal and Leachate
- Open dumps and poorly managed landfills release leachate, a contaminated liquid formed when rainwater percolates through waste.
- Leachate contains toxic chemicals, organic matter, and pathogens, contaminating groundwater.
- Properly engineered sanitary landfills use impermeable liners to collect and treat leachate safely.
In Mumbai (India), leachate from the Deonar landfill contaminates surrounding groundwater and mangrove wetlands, harming biodiversity.
6. Oil Spills
- Occur during oil extraction, transport, or accidents involving pipelines and tankers.
- Oil forms a surface layer that blocks sunlight and oxygen exchange, killing fish and marine plants.
- Birds and mammals become coated with oil, losing insulation and buoyancy.
Deepwater Horizon (2010, Gulf of Mexico):
- Over 780,000 m³ of oil spilled into the sea, devastating ecosystems and coastal economies.
- Cleanup included skimming, chemical dispersants, and bioremediation, using oil-degrading bacteria to break down hydrocarbons.
The Flint Water Crisis, USA
- Location: Flint, Michigan
- Source: Corrosive water from the Flint River leaching lead from old pipes.
- Pollutant: Lead and other heavy metals.
- Impacts:
- Severe lead poisoning in children causing neurological and developmental issues.
- Public distrust and long-term infrastructure costs.
- Highlighted the consequences of poor water governance.
- Management Strategies:
- Replacement of all lead pipes.
- Distribution of bottled water and filters.
- Federal and state-level interventions under the Safe Drinking Water Act.
