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Water, soil and noise pollution

Pollution · Part 2 / 3 · Fact-checked on 8 November 2026 · how we check

Written with AI from the sources listed at the bottom of this page, then fact-checked by a separate AI.

Point and non-point sources; dissolved oxygen, BOD and COD; eutrophication; mercury, cadmium and lead; arsenic, fluoride and nitrate in groundwater; sewage treatment, the Ganga Action Plan and Namami Gange; bioremediation and phytoremediation; soil pollution, bioaccumulation and biomagnification; the Noise Rules, 2000; thermal, radioactive and light pollution and oil spills.

The first part of this chapter dealt with air; this second part turns to water, soil and noise. Water counts as polluted once its physical, chemical or biological make-up has been altered enough to hurt the people, crops or creatures that depend on it. A point source is a single outlet that can be identified, such as a factory's effluent pipe or a town's sewage drain emptying into a river; because the outlet is known, it can be monitored, given consent conditions and penalised. A non-point source is spread out: rain washing fertiliser and pesticide off thousands of fields, or oil and litter off city streets. There is no single pipe to block, so control depends on changing what people do on the land, which makes non-point pollution the harder problem.

Three measurements tell how polluted water is. dissolved oxygen (DO) is the oxygen held in the water, which fish and other aquatic life breathe; it falls as water warms and as microbes consume it. biochemical oxygen demand (BOD) is the amount of oxygen that bacteria use up while breaking down the organic matter in a sample, usually measured over five days at 20°C. Organic waste such as sewage raises BOD and pulls DO down, so a high BOD signals heavy organic pollution. chemical oxygen demand (COD) measures the oxygen needed to oxidise the matter in the sample chemically, with a strong oxidising agent, so it also counts substances that bacteria cannot digest. For the same sample COD is normally higher than BOD. The Central Pollution Control Board's criteria for outdoor bathing water ask for DO of at least 5 mg/l and BOD of no more than 3 mg/l.

eutrophication is the over-enrichment of a lake, pond or slow-moving river with nutrients, chiefly nitrogen and phosphorus. Every lake slowly gathers nutrients and ages over centuries; human activity speeds this up enormously through fertiliser runoff, untreated sewage and phosphate detergents, and this faster version is called cultural eutrophication. Extra nutrients feed a sudden growth of algae, an algal bloom, which turns the water green and shades out the plants below. When the algae die, bacteria decompose them and use up the dissolved oxygen; BOD rises, DO crashes and fish suffocate. Some blooms of blue-green algae (cyanobacteria) also release toxins. Stretches of coastal sea left with too little oxygen for most life are called dead zones. The floating mats of water hyacinth that choke many Indian lakes thrive in the same nutrient-rich water.

Heavy metals are dangerous because they do not break down and they build up in living tissue. The best-known disaster is Minamata disease. From the 1930s a chemical factory at Minamata in Japan let mercury waste flow into the bay, where it was turned into methylmercury and concentrated in fish; people who ate the fish suffered nerve damage, numbness and birth defects, and the disease was officially identified in 1956. The Minamata Convention on Mercury, adopted in 2013 and in force from 2017, is named after the town. Itai-itai disease, also from Japan (the name means 'it hurts, it hurts'), was caused by cadmium from mining that polluted the rice fields of the Jinzu river basin; it left bones brittle and painful and damaged the kidneys. Lead harms the brain and nerves, above all in children, which is one reason leaded petrol was phased out.

Some of India's worst water pollution cannot be seen and comes from the ground itself. Arsenic occurs naturally in the young sediments of the Ganga-Brahmaputra plains, and heavy pumping of groundwater draws it into wells in states such as West Bengal, Bihar, Uttar Pradesh and Assam; years of drinking it cause skin lesions and cancers. Fluoride comes from fluoride-bearing rocks in drier regions such as Rajasthan, Gujarat and parts of Telangana and Andhra Pradesh. A little protects teeth, but too much causes fluorosis: mottled teeth first, then stiff and deformed bones. Surplus fluoride can be settled out of water by dosing it with alum and lime, a method worked out by the National Environmental Engineering Research Institute (NEERI) and known as the Nalgonda technique. Nitrate, mostly from fertilisers and sewage, seeps into groundwater and can cause 'blue baby syndrome' (methaemoglobinaemia) in infants. The Bureau of Indian Standards' drinking water standard, IS 10500:2012, sets limits of 0.01 mg/l for arsenic, 1.0 mg/l for fluoride (1.5 mg/l where no other source exists) and 45 mg/l for nitrate.

Most of the organic load in Indian rivers is untreated urban sewage, so treatment plants are the first line of defence. A sewage treatment plant works in stages. Primary treatment removes floating and settling solids by screening and sedimentation. Secondary treatment lets microbes consume the dissolved organic matter, often in aerated tanks (the activated sludge process), which cuts BOD sharply. Tertiary treatment adds further steps, such as removing nutrients and disinfecting, before the water is reused or released. River cleaning in India began with the Ganga Action Plan in 1985, which was widened in 1995 into the National River Conservation Plan for other rivers. The Ganga was declared a national river in 2008, and in 2015 the Union government approved Namami Gange, an integrated mission for the river. It is carried out by the National Mission for Clean Ganga, which works under the National Ganga Council chaired by the Prime Minister.

Living things can also be put to work cleaning up. bioremediation uses microbes, mainly bacteria and fungi, to break pollutants down into harmless substances such as carbon dioxide and water. Bacteria that digest oil, spread on spills and on oily sludge, are the usual example; Oilzapper, a microbial culture developed in India by The Energy and Resources Institute (TERI), is one such product. phytoremediation uses plants instead. Some plants draw metals or other toxins out of soil or water into their roots and leaves, and are then harvested and disposed of safely; others hold the pollutants in place or break them down. Both methods are cheap and gentle compared with digging up contaminated soil and burning it, but they are slow and work best where the contamination is moderate.

Soil is polluted when harmful substances build up faster than it can neutralise them. Farming is a major source: pesticides that linger in the ground, and fertilisers used in excess, which acidify the soil, upset its microbes and leach nitrate into groundwater. Industry and mining add heavy metals and toxic effluent, and plastic fragments persist for decades. The most harmful pesticides are persistent and dissolve in fat, as DDT does. They stay in an animal's fat instead of being excreted, which is bioaccumulation, and their concentration rises at each step up a food chain, which is biomagnification, so top predators carry the heaviest load; DDT thinned the eggshells of birds of prey. India now allows DDT only for limited public-health use against disease-carrying mosquitoes. In 2011 the Supreme Court ordered a nationwide ban on the production, use and sale of endosulfan, after illness was linked to its aerial spraying on cashew plantations in Kasaragod, Kerala.

Noise is unwanted sound, measured in decibels (dB). The scale is logarithmic: every rise of 10 dB means ten times the sound energy, so 80 dB is not slightly worse than 70 dB but ten times as intense. Long exposure to loud noise causes permanent hearing loss. A 1987 amendment brought noise within the Air Act's definition of an air pollutant, but the detailed rules are the Noise Pollution (Regulation and Control) Rules, 2000, made under the Environment (Protection) Act, 1986. They set ambient limits for four kinds of area, separately for day (6 a.m. to 10 p.m.) and night (10 p.m. to 6 a.m.). The ground within 100 metres or more of a hospital, an educational institution or a court is treated as a silence zone. Loudspeakers may not be used at night except in closed premises such as halls, though a state may allow them until midnight on up to fifteen festival days a year. In 2005 the Supreme Court upheld such limits, treating freedom from noise as part of the right to life under Article 21.

A few other kinds of pollution complete the picture. thermal pollution is the release of heated water, mostly from the cooling systems of thermal and nuclear power stations and some industries; since warm water holds less oxygen, the stretch of river or coast that receives it can lose fish and other life. Radioactive pollution comes from nuclear accidents and from careless disposal of nuclear and medical waste; radiation damages cells and genes, as the accidents at Chernobyl (1986) and Fukushima (2011) showed. Light pollution, the excess artificial light of towns and cities, hides the night sky and confuses animals that are active at night; sea-turtle hatchlings, which head for the brightest horizon, can crawl inland instead of to the sea. Oil spills coat the water, cut off oxygen and light below and mat the feathers of seabirds; they are tackled with floating booms, skimmers, chemical dispersants and bioremediation.

Two questions tie this part together. How much oxygen does a pollutant use up? Sewage and nutrients raise BOD, lower DO and, through eutrophication, can choke a lake. How long does it last? Metals, arsenic, fluoride and persistent pesticides do not break down, so they accumulate in bodies and magnify along food chains. Control follows the source: point sources through consents and treatment plants, non-point sources through changed practices on farms and in cities, and noise through zone limits and time rules. The third part turns to solid waste and the rules that decide who must handle each kind.

Exam traps

• A high BOD means more organic pollution, not cleaner water; as BOD rises, DO falls.
• For the same sample COD is normally higher than BOD, because the chemical test also counts matter that microbes cannot digest.
• In eutrophication most fish die from lack of oxygen when bacteria decompose the dead algae; the algae's growth itself is fed by nitrogen and phosphorus, not by heavy metals.
• Minamata disease is mercury; itai-itai disease is cadmium. Both cases are from Japan; do not swap the metals.
• Arsenic in the groundwater of the Ganga-Brahmaputra plains is mainly natural (geogenic), brought up by heavy pumping, not mainly industrial.
• Blue baby syndrome is caused by nitrate, not by fluoride or arsenic; fluorosis is caused by fluoride.
• The Noise Rules, 2000 were made under the Environment (Protection) Act, 1986, even though a 1987 amendment also brought noise into the Air Act's definition of an air pollutant.
• Hospitals, educational institutions and courts each carry a surrounding silence zone of no less than 100 metres, whose limits (50 dB by day, 40 dB by night) are the strictest of the four kinds of area.

Recap

• Point source: one identifiable outlet (effluent pipe, sewage drain); non-point source: diffuse runoff from farms and streets, harder to control.
• DO is oxygen in water; BOD is oxygen used by microbes (five days, 20°C); COD is oxygen for chemical oxidation and is normally above BOD.
• CPCB bathing-water criteria: DO at least 5 mg/l, BOD at most 3 mg/l.
• Eutrophication: nitrogen and phosphorus feed an algal bloom; decomposing algae use up oxygen; fish die; dead zones form.
• Minamata (mercury, identified 1956; convention 2013, in force 2017); itai-itai (cadmium); lead harms children's nerves.
• Arsenic (Ganga-Brahmaputra plains, natural), fluoride (fluorosis; Nalgonda technique), nitrate (blue baby syndrome); IS 10500:2012 limits 0.01, 1.0 and 45 mg/l.
• Sewage treatment: primary (solids), secondary (microbes, cuts BOD), tertiary (nutrients, disinfection); Ganga Action Plan 1985, NRCP 1995, Namami Gange 2015 run by NMCG.
• Bioremediation uses microbes (Oilzapper); phytoremediation uses plants; persistent chemicals bioaccumulate and biomagnify (DDT); endosulfan banned by the Supreme Court in 2011.
• Noise Rules 2000 under the EPA: four areas, day 6 a.m.-10 p.m., night 10 p.m.-6 a.m.; silence zone 100 m; no loudspeakers at night outside closed premises.
• Thermal pollution lowers oxygen; radioactive pollution damages cells and genes; light pollution misleads turtle hatchlings; oil spills are fought with booms, skimmers and microbes.

Practice questions

Sources

• Central Pollution Control Board: Primary Water Quality Criteria for designated best uses (outdoor bathing: DO 5 mg/l or more, BOD 3 mg/l or less); water quality monitoring of rivers — https://cpcb.nic.in
• The Water (Prevention and Control of Pollution) Act, 1974 (consent for discharge of effluent) — https://www.indiacode.nic.in
• United States Environmental Protection Agency: nutrient pollution and eutrophication; biochemical and chemical oxygen demand; thermal discharges; oil spill response; remediation technologies — https://www.epa.gov
• Minamata Convention on Mercury (adopted 10 October 2013, in force 16 August 2017), UNEP secretariat — https://www.mercuryconvention.org
• World Health Organization fact sheets: mercury and health; lead poisoning; arsenic; cadmium (itai-itai disease) — https://www.who.int
• Bureau of Indian Standards: IS 10500:2012, Drinking Water: Specification (arsenic 0.01 mg/l; fluoride 1.0 mg/l, 1.5 mg/l in the absence of an alternate source; nitrate 45 mg/l) — https://www.bis.gov.in
• Central Ground Water Board: groundwater quality reports (arsenic, fluoride and nitrate affected areas); NEERI's Nalgonda technique for defluoridation — https://cgwb.gov.in
• National Mission for Clean Ganga, Ministry of Jal Shakti: Namami Gange (approved May 2015); Ganga Action Plan (1985) and National River Conservation Plan; River Ganga (Rejuvenation, Protection and Management) Authorities Order, 2016 — https://nmcg.nic.in
• Press Information Bureau: bioremediation of oil spills and oily sludge with TERI's Oilzapper — https://pib.gov.in
• Supreme Court of India: Democratic Youth Federation of India v. Union of India, interim order of 13 May 2011 banning the production, use and sale of endosulfan — https://main.sci.gov.in
• The Noise Pollution (Regulation and Control) Rules, 2000, with Schedule of ambient noise standards; the Environment (Protection) Act, 1986; the Air (Prevention and Control of Pollution) Act, 1981, section 2(a) as amended in 1987 — https://www.indiacode.nic.in
• Supreme Court of India: In re Noise Pollution (Implementation of the Laws for Restricting Use of Loudspeakers and High Volume Producing Sound Systems), judgment of 18 July 2005 — https://main.sci.gov.in