Definition

Water pollution is the addition/presence of undesirable substances (organic, inorganic, biological, radiological, or heat) in water, degrading its quality and making it unfit for use.

Sources of Water Pollution

1. Natural Sources

• Soil erosion
• Leaching of minerals from rocks
• Decay of organic matter
• Acid rain increasing solubility of toxic elements

2. Point and Non-Point Sources

• Point Source: Pollutants discharged from a specific location (e.g., drain pipes carrying industrial effluents).
• Non-Point Source: Diffuse sources like agricultural runoff, grazing lands, construction sites, abandoned mines.

Major Causes of Water Pollution

Sewage Water

• Domestic & hospital waste contain pathogens.
• Contains excreta, detergents, food waste, and cleaning agents.
• Causes diseases: Cholera, Typhoid, Dysentery.

Dissolved Oxygen (DO)

• Essential for aquatic life.
• DO < 8 mg/L → contaminated
• DO < 4 mg/L → highly polluted
• Decreases due to decomposition of organic waste.

Factors affecting DO:

• Surface turbulence
• Photosynthesis
• Decomposition
• Respiration by organisms

Biological Oxygen Demand (BOD)

• Measures the oxygen required by bacteria to decompose organic waste.
• Expressed in mg/L.
• High BOD → Low DO → High pollution.
• Limited to biodegradable matter only.

Chemical Oxygen Demand (COD)

• Measures the oxygen required to oxidise both biodegradable and non-biodegradable substances.
• Better indicator than BOD for total pollution load.

Industrial Wastes

• Petroleum, paper, chemical, and metal industries discharge heavy metals.
• Heavy metals: Mercury, Cadmium, Lead, Arsenic, Copper (>5 g/cm³ density).
• Toxic and carcinogenic.

Agricultural Pollution

• Fertilizers (N, P, K) → Eutrophication
• Pesticides (DDT, Endosulfan, Organophosphates)
• Runoff from poultry farms and slaughterhouses
• Many pesticides are non-biodegradable → Biomagnification

Thermal Pollution

• Discharge of hot water from power plants.
• Raises the temperature by 10–15°C.
• Reduces DO levels.
• Sudden temperature rise kills aquatic organisms.

Radiation Pollution

Example: Fukushima Daiichi nuclear disaster

• Causes a DNA mutation.
• Radioactive iodine → Thyroid cancer.

Marine Pollution

Oceans act as ultimate sinks for pollutants.

Causes

• Oil spills
• Sewage dumping
• Offshore drilling
• Navigational discharge

Oil Spills

Effects

• Oil forms a thin layer blocking oxygen exchange.
• Fish & plankton die due to suffocation.
• Birds & mammals die due to poisoning.

Invasive Species

Water Hyacinth – “Terror of Bengal”

• Grows in eutrophic waters.
• Causes stagnation & ecological imbalance.

Groundwater Pollution

Common Pollutants

• Nitrates
• Fluoride
• Arsenic
• Uranium
• Heavy metals

Nitrates

• Cause Methemoglobinemia (Blue Baby Syndrome).
• React with haemoglobin forming non-functional methaemoglobin.
• Accelerates eutrophication.

Arsenic

• Major issue in the Ganga Delta (India & Bangladesh).
• Causes Black Foot Disease.
• Leads to lung & skin cancer.

Fluoride

• Causes Fluorosis.
• Skeletal deformities, knock-knee syndrome.

Effects of Water Pollution

On Human Health

• Mercury → Minamata disease
• Lead → Anaemia, neurological damage
• Cadmium → Itai-itai disease
• Arsenic → Skin cancer

On Environment

• Oxygen depletion kills fish.
• Biomagnification (DDT, Mercury).
• Eggshell thinning in birds due to DDT.

Eutrophication

Types

• Natural eutrophication
• Cultural eutrophication (human-induced)

Lake Classification

• Oligotrophic – Low nutrients
• Mesotrophic – Moderate
• Eutrophic – High nutrients

Mechanism of Algal Bloom

• Excess nitrates & phosphates → Rapid algal growth.
• Night respiration depletes oxygen.
• Fish die due to hypoxia.
• Anaerobic bacteria (e.g., Clostridium) release toxins.

Dead Zones

• Hypoxic marine regions.
• Caused by nutrient runoff.
• Example: Gulf of Mexico.

Mitigation Measures

• Wastewater treatment plants
• Riparian buffers (Wetlands, estuaries)
• Organic farming
• Nitrogen testing
• Reduce vehicular nitrogen emissions
• Cooling ponds for thermal discharge

International Conventions

London Convention (1972)

• Control the dumping of wastes at sea.
• The 1993 amendment banned radioactive waste dumping.

1996 Protocol

• Replaced the 1972 Convention.
• “Polluter Pays Principle”
• Prohibits incineration at sea.

United Nations Convention on the Law of the Sea (UNCLOS)

• Protects the marine environment.
• Establishes the International Seabed Authority.
• Holds states liable for marine pollution.