Water, Soil and Chemical Pollution in India

A real question on this chapter rarely asks "what pollutes water". It asks something narrower and easier to get wrong: which specific parameter does CPCB use to classify a river stretch fit for drinking without treatment, what number separates a Schedule I thermal power plant's sulphur dioxide limit from a newer one's, or which two elements, arsenic and fluoride, sit at opposite ends of India's groundwater contamination map for entirely different geological reasons. This note works through six clusters examiners actually draw from: water pollution and CPCB's own classification system, heavy metals and emerging contaminants in groundwater, reactive nitrogen, pollution from coal and thermal power, soil degradation and salinization, and bioremediation as a control technique, each anchored to a verified number or document rather than a general description.

Water pollution: CPCB's designated-best-use classes

India does not regulate river water quality with a single "clean or dirty" standard. The Central Pollution Control Board (CPCB) classifies water bodies into five designated-best-use classes, A to E, each with its own threshold values, and the exam trap is assuming stricter always means "more treatment" rather than "closer to the source use". Class A water (drinkable without conventional treatment, only disinfection) must carry a Biochemical Oxygen Demand (BOD) of 2 mg/l or less and dissolved oxygen of 6 mg/l or more. Class B (organised outdoor bathing) allows a BOD up to 3 mg/l with dissolved oxygen at 5 mg/l or more. Class C (drinking water after conventional treatment) also allows BOD up to 3 mg/l but with a lower dissolved-oxygen floor of 4 mg/l. Class D (propagation of wildlife and fisheries) drops the BOD criterion entirely in favour of a free-ammonia limit, and Class E (irrigation and industrial cooling) is governed by electrical conductivity and sodium absorption ratio rather than BOD or oxygen at all.

BOD, the amount of dissolved oxygen microorganisms consume while breaking down organic matter in a water sample, is the parameter to remember first: a lower BOD indicates less organic pollution, since clean water leaves more oxygen for the microbial community to use rather than needing it. This is why BOD, not a chemical toxicity measure, is CPCB's primary sorting criterion between classes A, B and C. Left unchecked, the organic and nutrient load driving BOD up also drives eutrophication: excess nitrogen and phosphorus from sewage, detergents and agricultural runoff over-fertilise a water body, triggering algal blooms; when the algae die, bacterial decomposition consumes the water's dissolved oxygen, and severe cases produce hypoxic "dead zones" where fish and other aquatic life cannot survive.

Heavy metals and emerging pollutants in groundwater

Two contaminants dominate India's groundwater pollution questions, and they are geologically unrelated even though both are often grouped under "heavy metal pollution". Arsenic contamination is concentrated in the alluvial aquifers of the Ganga-Brahmaputra floodplain, principally West Bengal, Bihar, Uttar Pradesh, Jharkhand and Assam, where arsenic occurs naturally in sediment and is mobilised into shallow groundwater under reducing conditions, not from an industrial source. Under the Bureau of Indian Standards' IS 10500:2012 drinking water specification, arsenic's acceptable limit is 0.01 mg/l, with no relaxation permitted even in the absence of an alternative source (unlike most other parameters, which carry a separate, more lenient permissible limit). Chronic exposure causes arsenicosis: skin hyperpigmentation and hyperkeratosis, and over years of exposure, elevated risk of skin, lung and bladder cancers.

Fluoride contamination, by contrast, is a geochemical problem tied to fluoride-bearing rock (granite, gneiss) rather than river-plain sediment, which is why fluorosis-endemic areas cluster differently, in parts of Rajasthan, Gujarat, Andhra Pradesh and Telangana among others. IS 10500:2012 sets fluoride's acceptable limit at 1.0 mg/l and its permissible limit (in the absence of an alternative source) at 1.5 mg/l, a genuine relaxation that arsenic does not get. Excess fluoride causes dental fluorosis (mottled, discoloured enamel) and, at higher chronic exposure, skeletal fluorosis (joint stiffness and bone deformity). The exam-relevant contrast: arsenic is a "zero tolerance" parameter under BIS rules, fluoride is not, and mixing the two limits up is a common trap.

Microplastics, plastic fragments generally under 5 mm, are the newest addition to this cluster and the one UPSC is most likely to test as an "emerging pollutant" category rather than a numeric standard. They enter water and soil through the breakdown of larger plastic waste, synthetic textile fibres, and personal care products, and have been documented in Indian rivers, coastal seafood and even air. CPCB acknowledged the presence of microplastics in water and air in 2023 and flagged the need for a standardised sampling and analysis method, a reminder that this is still a research and monitoring problem in India rather than one with a settled regulatory limit, unlike arsenic or fluoride.

Reactive nitrogen: the fertiliser-driven disruption

The nitrogen cycle question tests a specific idea: atmospheric nitrogen (78% of the air) is mostly unusable by organisms until it is converted, or "fixed", into a reactive form, and human activity, chiefly synthetic fertiliser manufacture, has multiplied that conversion far beyond what natural fixation once provided. Crops absorb only part of the nitrogen applied as fertiliser; according to UNEP, 200 million tonnes of reactive nitrogen, roughly 80% of the total applied, is lost to the environment every year, leaching into soil and water or escaping to the air, at an estimated annual cost of around US$200 billion in lost resources. This excess reactive nitrogen is the same nitrogen that drives eutrophication in water bodies described above, making nitrogen pollution and water pollution two views of the same chemistry rather than separate problems.

One specific form of this lost nitrogen matters for climate questions: nitrous oxide (N2O), released from over-fertilised soils and livestock waste, is a greenhouse gas UNEP describes as 300 times more potent than carbon dioxide, with an atmospheric lifetime of roughly 200 years, and it is also the single largest human-caused threat to the ozone layer today. India led the push for global action here: it sponsored the UN Environment Assembly resolution on Sustainable Nitrogen Management (UNEP/EA.4/L.16), adopted at UNEA-4 in March 2019, which fed into the Colombo Declaration on Sustainable Nitrogen Management, adopted in October 2019, committing signatory countries to halve nitrogen waste by 2030. A statement question naming India as the sponsor of this nitrogen-management push, rather than a passive signatory, is testing a real and specific fact.

Coal, thermal power and industrial pollution

Coal-based thermal power plants are tested through their emission standards, and the trap here is date-based: older plants are held to looser limits than newer ones, so a question quoting one number without specifying plant vintage is incomplete. Under MoEFCC Notification S.O. 3305(E) dated 7 December 2015, plants commissioned before 31 December 2003 must meet particulate matter of 100 mg/Nm3, sulphur dioxide of 600 mg/Nm3 (units below 500 MW) or 200 mg/Nm3 (500 MW and above), oxides of nitrogen at 600 mg/Nm3, and mercury at 0.03 mg/Nm3 for units of 500 MW and above. Plants commissioned between 2003 and 2016 tighten to 50 mg/Nm3 particulate matter and 300 mg/Nm3 NOx (sulphur dioxide and mercury limits stay the same as the older category). Plants commissioned from 1 January 2017 face the strictest limits: 30 mg/Nm3 particulate matter, and 100 mg/Nm3 each for sulphur dioxide and NOx. The same 2015 notification also introduced specific water consumption limits, capping new plants at 2.5 cubic metres per megawatt hour with zero wastewater discharge.

Fly ash, the fine particulate residue from burning coal, is the other half of this cluster. MoEFCC's 2009 notification (S.O. 804(E)) set a phased target of 100% fly ash utilisation for thermal power stations, climbing from 50% in the first year to 100% by the fourth or fifth year depending on when the plant was commissioned, rather than allowing indefinite ash-pond disposal. Fly ash is classified as high-volume, low-effect waste, excluded from the Hazardous Waste Rules, and is put to use in cement manufacture, fly ash bricks, road construction and mine void filling, uses that later notifications made mandatory minimum content for in nearby construction.

Soil degradation and salinization

India's own land-degradation baseline comes from the ISRO/Space Applications Centre's Desertification and Land Degradation Atlas: in 2018-19, 97.85 million hectares, 29.7% of India's total geographical area, was undergoing land degradation, up from 94.53 million hectares (28.76%) in 2003-05. Water erosion and vegetation loss are the largest contributors nationally, but salinization and waterlogging form a distinct, irrigation-driven degradation pathway that examiners test separately: sustained canal irrigation without adequate drainage raises the water table until it approaches the surface, and capillary action then draws dissolved salts upward, where they concentrate as the surface water evaporates, leaving behind land too saline for most crops.

ICAR's Central Soil Salinity Research Institute (CSSRI), headquartered in Karnal, puts the total extent of salt-affected soil in India at roughly 6.74 million hectares, split between saline soils (about 1.71 million hectares), alkali or sodic soils (about 3.79 million hectares) and coastal saline soils (about 1.25 million hectares). By state, Gujarat carries the single largest share, followed by Uttar Pradesh and Maharashtra, a useful correction against the assumption that Punjab and Haryana's canal-irrigated belt is automatically the country's worst-hit region; it is a well-documented waterlogging zone, but the national salt-affected total is led by Gujarat's coastal and arid tracts. CSSRI's subsurface drainage technology is the standard reclamation response, used to lower water tables in waterlogged saline fields and restore them to cultivation.

Bioremediation: pollution control using living systems

Bioremediation uses living organisms, bacteria, fungi, algae or plants, to break down or immobilise pollutants rather than removing and disposing of contaminated material physically. Phytoremediation, its plant-based form, relies on hyperaccumulator species that draw heavy metals out of contaminated soil through their root systems, a lower-cost alternative to excavation for sites with chromium, lead or arsenic contamination. On the marine side, bioremediation using oil-degrading microbial cultures has been field-tested on Indian oil spill sites, including monitored bioremediation of spilled sludge at the Ennore-Kamarajar Port area on the Chennai coast. CPCB has separately worked on guidelines for the scientific remediation of legacy solid waste, extending the same living-systems logic beyond water and soil pollutants to accumulated municipal waste. The exam angle here is usually conceptual: bioremediation treats pollutants in place using biological processes, distinct from physical or chemical remediation methods that remove, incinerate or chemically neutralise the contaminant.

Exam angle

This chapter rewards precision over general awareness, because most traps swap one verified number or one geographic detail for a similar-sounding wrong one. A question may cite CPCB's Class A dissolved-oxygen floor while actually describing Class C, or state arsenic's permissible limit as 1.5 mg/l (that is fluoride's number, not arsenic's, which allows no relaxation at all). Thermal power emission limits are frequently tested without specifying plant vintage, precisely because the 2015 notification's three tiers, pre-2003, 2003 to 2016, and post-2017, carry genuinely different numbers; always check which category a stated figure belongs to. Land degradation and soil salinization also get conflated: India's overall degraded-land figure (about 30% of its area) is a different statistic from its salt-affected soil figure (a much smaller 6.74 million hectares), and Gujarat, not Punjab or Haryana, leads the latter. Finally, nitrous oxide and the nitrogen cycle are tested as a climate fact wearing an agriculture disguise: it is fertiliser overuse, not direct industrial emission, that generates most of India's reactive nitrogen loss.

Quick revision points

• CPCB water classes: A (BOD under 2 mg/l, drinkable without treatment), B (bathing, BOD under 3 mg/l), C (drinkable after treatment, BOD under 3 mg/l but lower dissolved-oxygen floor), D (wildlife and fisheries, free-ammonia based), E (irrigation and cooling, conductivity based). Lower BOD means less organic pollution.
• Arsenic: BIS acceptable limit 0.01 mg/l, no relaxation; Ganga-Brahmaputra plain (West Bengal, Bihar, UP, Jharkhand, Assam). Fluoride: acceptable 1.0 mg/l, permissible 1.5 mg/l (relaxation allowed), different geology, different states.
• Reactive nitrogen: about 200 million tonnes (80% of applied nitrogen) lost to the environment yearly (UNEP); N2O is roughly 300 times more potent than CO2 as a greenhouse gas. India sponsored UNEA-4's nitrogen resolution (2019), leading to the Colombo Declaration's 2030 target to halve nitrogen waste.
• Thermal power emission norms (S.O. 3305(E), 2015): pre-2003 plants allow the loosest limits (PM 100, NOx 600 mg/Nm3); plants from 2017 onward face the tightest (PM 30, SO2 and NOx 100 mg/Nm3 each).
• Land degradation: 97.85 million hectares (29.7% of India's area) degraded in 2018-19 per the ISRO/SAC Atlas. Salt-affected soil: about 6.74 million hectares total (CSSRI), Gujarat leads, followed by Uttar Pradesh and Maharashtra.
• Bioremediation uses living organisms to treat pollutants in place; phytoremediation is its plant-based form for heavy metals.

Try a few questions now to see which specific number or state a statement is quietly testing.