Water pollution: sources, measurement and health

Detail

1. What water pollution means

• Surface water is never pure. River, lake and pond water always carries some soil, plant matter and dissolved salts.
• Self-purifying capacity: water's natural ability to clean itself. Flowing water mixes in oxygen from the air. Microbes use this oxygen to break down dirt. Sunlight and dilution also help.
• Water pollution: water becomes polluted when suspended (floating), organic (from living things, such as sewage) and inorganic (minerals, metals, chemicals) substances go beyond its self-purifying capacity.
• Too much waste enters → microbes use up the dissolved oxygen → fish die and the water smells → the water can no longer clean itself.

2. Sources of water pollution

Source type	Examples	Main pollutants	Importance
Natural	Erosion, landslides, decay of plants and animals	Silt, organic matter	Minor
Industrial	Leather (tanneries), pulp and paper, textiles, chemicals	Effluents, heavy metals, poisonous gases	Biggest human source
Agricultural	Chemical fertilisers, pesticides, herbicides	Nitrates, toxic chemicals	Large and spread out (no single outlet)
Cultural	Pilgrimages, religious fairs, tourism	Organic waste, ashes, flowers, plastics, bathing load	Seasonal, but heavy at holy sites

• Industry:
• Effluent: liquid waste that a factory releases.
• Heavy metals: metals such as lead, mercury, cadmium and chromium. They do not break down. They build up in fish and then in humans.

• Factories dump effluents, heavy metals and poisonous gases straight into rivers and lakes.

• Agriculture:

• Farmers use too much fertiliser → crops cannot take it all up → rain and irrigation water wash it into streams, and it seeps into groundwater (water stored under the ground in rocks and soil) → nitrate levels rise.

• Pesticides and herbicides also reach rivers and groundwater.

• Cultural activities: large crowds at pilgrimages and fairs bathe in rivers and throw offerings into them. This adds organic load to stretches that are already stressed.

• Result: almost all surface water sources in India are contaminated and unfit to drink without treatment. NCERT shows this with toxic white foam floating on the Yamuna (Fig. 9.1).

3. Measuring water pollution — BOD and quality criteria

• Biochemical oxygen demand (BOD): the amount of dissolved oxygen that microbes use to break down the organic matter in water.
• More sewage → microbes need more oxygen → higher BOD → dirtier water.

• Low BOD = clean water. High BOD = organic pollution.

• CPCB primary water quality criteria for outdoor bathing (CPCB: Central Pollution Control Board, India's national pollution watchdog) [5]:

Parameter	Limit for bathing water
pH (how acidic or basic water is)	6.5–8.5 [5]
Dissolved oxygen (DO) (oxygen dissolved in water, needed by fish)	5 mg/l or more [5]
BOD	3 mg/l or less [5]
Faecal coliform (bacteria from human and animal faeces; a sign of sewage)	500 MPN/100 ml desirable; 2,500 MPN/100 ml maximum [5]

• Why 3 mg/l? BOD of 3 mg/l or less means the water is fairly free of pollutants that eat up oxygen, and it stops foul-smelling gases from forming [5].
• Why 5 mg/l DO? It stops sediment from rotting without oxygen and giving off bad-smelling gases (anaerobic gases) [5].

4. Polluted river stretches (PRS)

• Polluted river stretch: a part of a river where BOD is above 3 mg/l, so the water fails the bathing standard.
• CPCB assessment (2022): 311 polluted stretches on 279 rivers [2].

• Down from 351 stretches in 2018 [2].

• Priority classes: stretches are put into five classes (I to V) by the highest BOD recorded there [2].

Priority class	BOD (mg/l)	Number of stretches (2022)
I (worst)	More than 30	46 [2]
II	20–30	16 [2]
III	10–20	39 [2]
IV	6–10	65 [2]
V (least bad)	3–6	145 [2]

• States with the most polluted stretches (2022): Maharashtra 55 → Madhya Pradesh 19 → Bihar 18 = Kerala 18 → Karnataka 17 [2].

• Maharashtra has 4 Priority I, 5 Priority II, 18 Priority III and 17 Priority IV stretches [2].

• Some of the worst stretches:

• Yamuna below Delhi. The Yamuna rises at the Yamunotri glacier in Uttarakhand and joins the Ganga at Prayagraj (Uttar Pradesh).
• Sabarmati at Ahmedabad, Gujarat.
• Musi at Hyderabad, Telangana. It is a tributary of the Krishna.
• Gomti at Lucknow, Uttar Pradesh. It is a tributary of the Ganga.
• Cooum at Chennai, Tamil Nadu.

• Ganga at Kanpur (tanneries) and Varanasi.

• Pattern: the worst stretches lie downstream of big cities and industrial clusters. The main cause is untreated city sewage, not natural sources.

5. The sewage treatment gap — the core cause

• Sewage treatment gap: the difference between the sewage a city produces and the sewage it actually treats.
• CPCB National Inventory of STPs (March 2021) (STP: sewage treatment plant):
• Cities produce 72,368 MLD of sewage (MLD: million litres a day) [3].
• Installed treatment capacity is 36,668 MLD, about half of what is needed [3].
• Capacity actually used is only 20,235 MLD, about 28% of the sewage produced [3].

• 52,133 MLD goes into rivers, lakes and the ground untreated [3].

• STP status (2021): 1,631 STPs in 35 states and UTs [3].

• 1,093 working, 102 not working, 274 under construction, 162 proposed [3].

• Of the 900 working STPs with known status, only 578 STPs (12,200 MLD) met the norms set by the state boards [3].

• Chain of the problem:

• Fast urban growth → sewers and STPs fall behind → raw sewage flows into drains and rivers.
• Organic load rises → BOD rises and dissolved oxygen falls → the river can no longer clean itself.
• Faecal coliform rises → water-borne disease spreads downstream.

6. Health effects — water-borne diseases

• Water-borne diseases: illnesses caused by germs (bacteria, viruses, worms) that people take in through dirty water.

• Diarrhoea, intestinal worms, hepatitis, typhoid and cholera.

• WHO estimates that about one-fourth of the communicable diseases in India are water-borne. (A communicable disease is one that spreads from person to person.)

• Heavy metals in fish and crops: these cause long-term poisoning of the nervous system, kidneys and bones.

7. Groundwater contamination (link to water resources of India)

• Arsenic:
• Found mainly in the Ganga-Brahmaputra delta and plains: West Bengal, Bihar and Assam.
• Arsenic has been found at certain points in 221 districts of 25 states/UTs [6]. (Earlier figure: 152 districts in 21 states [7].)
• Worst districts in West Bengal are Nadia (86.4% of samples), Murshidabad (76.7%) and North 24 Parganas (52.2%) [8].

• Long exposure causes skin lesions and cancer.

• Fluoride:

• Found mainly in Rajasthan, Telangana and Gujarat.
• Fluoride above the permissible limit is found in pockets of 370 districts in 23 states [7].

• Too much fluoride causes fluorosis: mottled teeth and bent, stiff bones.

• Nitrate:

• Comes from fertilisers and sewage.
• The BIS permissible limit is 45 mg/l [8]. (BIS: Bureau of Indian Standards.)
• In infants it causes "blue baby syndrome".

8. Law — Water (Prevention and Control of Pollution) Act 1974

• India's first major pollution law.
• Consent rule: an industry needs consent (permission) from the pollution control board before it sets up a unit or discharges effluent into a river, a sewer or land.
• Weak implementation: NCERT (1997) noted 251 polluting industries along rivers and lakes. Enforcement was poor, so effluents kept flowing in.
• Follow-up programmes: the National River Conservation efforts and Namami Gange (2014), which mainly builds STPs and checks industrial outflow in the Ganga basin.

Prelims Hooks

• BOD measures organic pollution. High BOD = dirtier water. The bathing limit is ≤3 mg/l, and DO must be ≥5 mg/l [5].
• Faecal coliform limit for bathing: 500 desirable, 2,500 MPN/100 ml maximum [5]. Bathing pH: 6.5–8.5 [5].
• CPCB 2022: 311 polluted stretches on 279 rivers, down from 351 (2018) [2].
• A stretch is Priority I when BOD is above 30 mg/l. A stretch is Priority V when BOD is 3–6 mg/l. Priority V has the most stretches (145) [2].
• Most polluted stretches: Maharashtra (55) > Madhya Pradesh (19) > Bihar = Kerala (18) > Karnataka (17) [2].
• Urban sewage (2021): 72,368 MLD produced. Treatment capacity is 36,668 MLD. Only about 28% (20,235 MLD) is actually treated [3].
• River–city matches: Musi–Hyderabad, Cooum–Chennai, Gomti–Lucknow, Sabarmati–Ahmedabad.
• Contaminant–region matches: arsenic → West Bengal, Bihar, Assam (Ganga-Brahmaputra delta); fluoride → Rajasthan, Telangana, Gujarat.
• Trap: NCERT names industry as the biggest human source of water pollution. For river stretches today, CPCB data show untreated urban sewage as the main cause.
• Water Act 1974 came before the Air Act (1981) and the Environment (Protection) Act (1986). Its key tool is consent to discharge.

Mains Points

• Cause–distribution pattern: polluted stretches cluster below big cities and industrial belts (Yamuna–Delhi, Ganga–Kanpur, Musi–Hyderabad). Urban growth has run ahead of sewers and STPs. Only about 28% of 72,368 MLD of sewage is treated [3], and about half of working STPs that were checked failed to meet norms [3].
• Health and economic cost: about one-fourth of India's communicable diseases are water-borne (WHO). Geogenic and farm-linked groundwater contamination (arsenic in 221 districts [6], fluoride in 370 districts [7]) adds long-term disease. This hits the rural poor in the Ganga plains and dry western India hardest.
• Policy response and gaps: the Water Act 1974 consent system was weakened by poor enforcement (NCERT: 251 polluting industries, 1997). Namami Gange (2014) moved the focus to building STPs. Polluted stretches fell from 351 (2018) to 311 (2022) [2]. Remaining needs: run existing STPs fully, reuse treated sewage, apply real-time effluent monitoring, and control farm runoff and crowd pollution at religious fairs.