Fluoride, arsenic, nitrate: what's actually in your water

Clear, odourless water is not the same as safe water. India's three most consequential water contaminants — fluoride, arsenic and nitrate — are dissolved minerals and compounds that leave no trace you can see, taste or smell, and each turns up in a different part of the country for a different reason. Which one you're actually likely to be drinking depends less on any purifier you own than on the geology under your house and where your water is sourced from.

About 85% of rural India, and a meaningful share of small-town India, draws drinking water from groundwater — borewells and hand pumps pulling straight from the aquifer rather than from treated municipal supply. Groundwater sits in contact with rock and soil for years before it reaches a tap, which is exactly how it picks up fluoride, arsenic and nitrate in the first place. The Central Ground Water Board's Annual Ground Water Quality Report 2024, based on sampling from over 15,000 monitoring locations nationwide, is the most current official picture, and it shows three distinct contamination maps, not one uniform problem.

What counts as safe

India's own standard, BIS IS 10500:2012, and WHO's guidelines are close on these three parameters but not identical:

• Fluoride — BIS acceptable limit: 1.0 mg/L; permissible up to 1.5 mg/L only if no alternative source is available. WHO's guideline value is also 1.5 mg/L.
• Arsenic — BIS acceptable limit: 0.01 mg/L (10 micrograms per litre); permissible up to 0.05 mg/L only if no alternative source is available — the same two-tier structure as fluoride. WHO sets 0.01 mg/L with no such relaxation. The acceptable limit is kept this low because even small, sustained doses carry cancer risk.
• Nitrate — BIS acceptable limit: 45 mg/L, with no relaxation permitted regardless of alternative sources — the only one of the three with zero tolerance. WHO's guideline is 50 mg/L as nitrate (NO3−). The limit exists mainly to protect infants.

Nitrate: agriculture written into the water table

Nitrate reaches groundwater almost entirely through farming — chemical fertiliser and manure that crops don't take up washes down through the soil and accumulates in the aquifer below, along with leakage from septic systems and sewage in denser areas.

Nationally, 19.8% of CGWB's 2024 groundwater samples exceeded the permissible nitrate limit — the single most common exceedance of the three contaminants, and part of why 440 districts now show excess nitrate, up from 359 in the board's 2017 assessment. That national figure understates how bad it gets in farming-heavy states: Rajasthan (49% of samples), Karnataka (48%) and Tamil Nadu (37%) all run far above it. Worst-hit pockets include Barmer and Jodhpur in Rajasthan; Wardha, Buldhana, Amravati, Nanded, Beed, Jalgaon and Yavatmal in Maharashtra; Rangareddy, Adilabad and Siddipet in Telangana; Villupuram in Tamil Nadu; Palnadu in Andhra Pradesh; and Bathinda in Punjab.

The main proven health risk is methemoglobinemia — 'blue baby syndrome' — in infants under six months, particularly those fed formula mixed with contaminated water. Gut bacteria convert nitrate to nitrite, which binds haemoglobin and blocks it from carrying oxygen; severe cases can be fatal. Adults tolerate moderate nitrate exposure without acute symptoms, though researchers are still studying possible links between high chronic exposure and thyroid function or pregnancy outcomes.

Fluoride: the western and southern belt

Fluoride isn't pollution in the usual sense — it leaches naturally out of fluoride-bearing rock such as granite and gneiss into groundwater, which is why it tracks geology rather than industry or farming.

Rajasthan is the worst affected — CGWB's 2024 survey found more than a quarter of wells in Barmer, Nagaur and Jaipur over the limit — with Gujarat, Andhra Pradesh, Haryana, Karnataka and Telangana also showing high levels, and Punjab, Madhya Pradesh and Maharashtra moderately affected. Elevated fluoride has been recorded across roughly 230 districts in 20 states over the past decade. A government estimate put 11.7 million people at direct risk as of 2014; a more widely cited independent estimate runs far higher — over 66 million people affected, including 6 million children — and the gap between the two reflects how inconsistently fluorosis has been diagnosed and counted, not a number either side has settled.

Low-grade excess causes dental fluorosis — mottled, brownish streaking on tooth enamel — which is largely cosmetic. Sustained higher exposure causes skeletal fluorosis: joint pain and stiffness that can progress to bone deformity and, in severe cases, a rigid spine. Children's developing bones are more vulnerable, and once skeletal damage sets in, it doesn't reverse.

Arsenic: the eastern river plains

Arsenic contamination follows a different geology altogether — it's concentrated in the young alluvial sediment of the Ganga and Brahmaputra flood plains, where particular aquifer conditions release naturally occurring arsenic into the groundwater.

West Bengal carries by far the largest burden, with an estimated 9.6 million people at immediate risk; some of the worst-hit blocks in Murshidabad, Malda, Nadia and North 24 Parganas have recorded arsenic levels above 300 micrograms per litre — more than 30 times the limit. Assam (about 1.6 million people at risk), Bihar (1.2 million) and Uttar Pradesh's eastern districts (roughly 500,000) round out the belt along the Ganga-Brahmaputra plains, with smaller pockets in Jharkhand and Manipur.

Arsenic's danger is that it's slow. Chronic exposure first shows up as skin lesions — keratosis (thickened, hardened patches) and dark or light pigmentation changes — typically after years of drinking contaminated water. Longer exposure raises the risk of skin, bladder, lung and kidney cancers, plus cardiovascular disease. Because symptoms can take a decade or more to surface, contamination in a given area sometimes goes unrecognised for a generation.

Why borewell water tends to be worse

None of this makes municipal supply automatically safe or borewell water automatically dangerous — it varies too much by exact location for a blanket rule. But the pattern holds directionally: treated municipal supply is usually drawn from rivers or reservoirs and processed before distribution, while borewell water is pulled straight from an aquifer that's had years or decades of contact with the surrounding rock and soil. That contact is precisely what dissolves fluoride and arsenic into the water and lets nitrate from surface runoff seep down into it — so borewell water in India typically runs higher in total dissolved solids and hardness than treated supply, and it's the source most likely to carry these three contaminants at levels that matter. If your household relies on a borewell, especially in any of the states above, a water test is the only way to know what's actually in it.