Causes of Water Pollution

(i) Pathogens

The most serious water pollutants are disease-causing agents called pathogens — bacteria and other organisms entering water through domestic sewage and animal excreta. Human excreta, for example, carries bacteria such as Escherichia coli and Streptococcus faecalis, which cause gastrointestinal diseases.

(ii) Organic Wastes and Dissolved Oxygen

Organic matter such as leaves, grass and trash reaches water bodies through run-off, and excessive phytoplankton growth adds further biodegradable material. Because these wastes are biodegradable, bacteria decompose them — but in doing so, the bacteria consume the dissolved oxygen (DO) in the water.

• Cold water can hold up to about 10 ppm of dissolved oxygen, compared with roughly 200,000 ppm of oxygen in air — water simply cannot store much oxygen.
• If DO falls below 6 ppm, fish growth is inhibited.
• Oxygen enters water from the atmosphere and from photosynthesis by aquatic plants during daylight; at night, photosynthesis stops but respiration continues, further depleting DO.

When too much organic matter is added, all available oxygen gets used up. Oxygen-dependent aquatic life then dies, and anaerobic bacteria (which do not need oxygen) take over decomposition, producing foul-smelling, health-harming chemicals. In healthier conditions, aerobic bacteria degrade organic waste while still depleting some dissolved oxygen.

This leads to the concept of Biochemical Oxygen Demand (BOD) — the amount of oxygen required by bacteria to break down the organic matter in a given volume of water. BOD is therefore a direct measure of organic pollution:

• Clean water: BOD < 5 ppm.
• Highly polluted water: BOD ≥ 17 ppm or more.

(iii) Chemical Pollutants

Because water is such an effective solvent, many water-soluble inorganic and organic chemicals become pollutants.

• Heavy metals — cadmium, mercury, nickel and others are dangerous because the human body cannot excrete them; over time they accumulate past the body's tolerance limit and damage the kidneys, central nervous system and liver.
• Acids and salts — sulphuric acid from mine drainage, and de-icing salts (sodium and calcium chloride) used in colder climates, are common water-soluble pollutants.
• Organic chemicals — petroleum products (e.g., major oil spills), pesticide residues that drift or run off from farmland, and industrial chemicals such as polychlorinated biphenyls (PCBs, used as cleansing solvents), suspected to be carcinogenic. …