Water Pollution
Water pollution is the contamination of water bodies — including rivers, lakes, groundwater, and oceans — with harmful substances such as pathogens, heavy metals, nutrients (causing eutrophication), synthetic chemicals, microplastics, and thermal effluents, rendering water unsafe for human use, aquatic life, and ecosystems. The UN estimates that over 2 billion people lack access to safe drinking water, and approximately 80% of global wastewater is discharged into waterways without adequate treatment. Water pollution is quantified using parameters such as Biochemical Oxygen Demand (BOD), dissolved oxygen (DO), and chemical oxygen demand (COD).
Key Formula
BOD5 = (Initial DO − Final DO after 5 days) / Sample dilution factor
LaTeX: BOD_5 = \frac{(DO_i - DO_f)}{S}
| Symbol | Meaning | Unit |
|---|---|---|
| BOD_5 | 5-day Biochemical Oxygen Demand | mg O₂/L |
| DO_i | Initial dissolved oxygen of diluted sample | mg/L |
| DO_f | Final dissolved oxygen after 5 days at 20°C | mg/L |
| S | Sample dilution factor (volume sample / total volume) | dimensionless |
Worked Example
Problem
A water sample is diluted 1:10 (S = 0.1) before incubation. Initial DO = 8.2 mg/L, final DO after 5 days at 20°C = 3.4 mg/L. Calculate the BOD₅ of the original sample.
Solution
Step 1: Calculate the DO change in diluted sample. ΔDO = DO_i − DO_f = 8.2 − 3.4 = 4.8 mg/L Step 2: Apply dilution factor to get original sample BOD. BOD₅ = ΔDO / S = 4.8 / 0.1 = 48 mg O₂/L
Answer
BOD₅ = 48 mg O₂/L, indicating significant organic pollution (clean water typically has BOD < 2 mg/L).
Types of Water Pollution and Their Sources, Effects, and Indicators
| Pollution Type | Common Source | Key Indicator | Major Effect |
|---|---|---|---|
| Nutrient (Eutrophication) | Fertiliser runoff, sewage | Nitrate, phosphate levels | Algal blooms, dead zones |
| Pathogenic | Untreated sewage, animal waste | E. coli count, BOD | Waterborne disease outbreaks |
| Heavy metals | Mining, industry, e-waste | Lead, mercury, arsenic (ppb) | Neurological damage, bioaccumulation |
| Microplastics | Plastic waste, synthetic textiles | Particle count/L | Marine organism ingestion, food chain |
| Thermal | Power plant cooling water | Temperature anomaly (°C) | Reduced DO, fish kills |
| Oil/Petroleum | Spills, runoff | Total petroleum hydrocarbons | Smothering wildlife, toxic effects |
Interactive Tools
Khan Academy – Environmental Science
Lessons covering water quality, pollution types, and ecosystem impacts
Open ToolNIST – Water Quality Standards
Measurement standards and reference materials for water contaminant analysis
Open ToolWolfram Alpha – BOD and Water Chemistry
Calculate BOD, dissolved oxygen saturation, and pollutant concentrations
Open Tool
Wikimedia Commons, CC BY-SA
Related Terms
Air Pollution
Air pollution is the presence of harmful substances in the atmosphere — including particulate matter (PM2.5, PM10), nitrogen oxides (NOₓ), sulfur dioxide (SO₂), carbon monoxide (CO), ozone (O₃), and volatile organic compounds (VOCs) — at concentrations that pose risks to human health, ecosystems, and climate. The World Health Organization (WHO) estimates that air pollution causes approximately 7 million premature deaths annually, making it the world's largest environmental health risk. Sources range from vehicle emissions, industrial combustion, and agricultural burning to natural events such as wildfires and volcanic eruptions.
Ecosystem Services
Ecosystem services are the direct and indirect benefits that humans derive from functioning natural ecosystems, encompassing provisioning services (food, fresh water, timber), regulating services (climate regulation, flood control, disease regulation), cultural services (recreation, spiritual value), and supporting services (nutrient cycling, soil formation). The Millennium Ecosystem Assessment (2005) estimated the total annual value of ecosystem services globally at over $33 trillion, exceeding global GDP at the time. Understanding and valuing ecosystem services is fundamental to environmental policy, conservation economics, and sustainable development planning.
Biodiversity Loss
Biodiversity loss refers to the reduction in the variety of life on Earth at genetic, species, and ecosystem levels, currently occurring at rates estimated to be 100–1,000 times higher than natural background extinction rates due to human activities. The primary drivers include habitat destruction, overexploitation, invasive species, pollution, and climate change, collectively recognised as the "five drivers of biodiversity loss" by the IPBES (Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services). The current mass extinction event, sometimes called the Sixth Mass Extinction, threatens the ecological stability and services upon which all human civilisation depends.
From Old English "wæter" (water) and Latin "pollutio" (contamination, defilement, from "polluere", to soil). Water quality and pollution as scientific disciplines developed in the 19th century, accelerated by John Snow's 1854 identification of contaminated water as the source of London's cholera outbreak, leading to modern sanitary engineering and water treatment standards.