Water Chemistry: What's Actually in Your Tap Water in India, the US, and the UK

Water isn't just H2O. Every glass of tap water contains dissolved minerals, treatment chemicals, and trace elements that vary dramatically depending on where you live. Let's look at the chemistry of what you're actually drinking.

Pure Water Doesn't Exist in Nature

Chemically pure water (H2O and nothing else) is almost never found in nature. Water is an excellent solvent — often called the "universal solvent" — because of its polar molecular structure.

Why water dissolves so many things:

• The oxygen atom is highly electronegative, creating a partial negative charge
• The hydrogen atoms carry partial positive charges
• This polarity allows water to surround and separate ions from ionic compounds
• Water also forms hydrogen bonds with many organic molecules

As water flows through rocks, soil, and pipes, it picks up minerals, metals, and other dissolved substances.

Common Substances in Tap Water

Minerals (The Good Stuff)

Calcium (Ca2+) and Magnesium (Mg2+):

• Primary contributors to water hardness
• Come from dissolving limestone (CaCO3) and dolomite (CaMg(CO3)2)
• Actually beneficial for health — contribute to daily mineral intake
• Cause limescale in kettles and boilers

Sodium (Na+):

• Naturally present in most water sources
• Higher in areas with salt deposits
• Water softeners exchange calcium/magnesium for sodium

Potassium (K+):

• Present in small amounts
• Comes from mineral dissolution
• Essential nutrient

Bicarbonate (HCO3-):

• Acts as a natural pH buffer
• Keeps water from being too acidic or too basic
• Produced when CO2 dissolves in water and reacts with minerals

Treatment Chemicals

Chlorine (Cl2) or Chloramine (NH2Cl):

• Added to kill bacteria and viruses
• Chlorine dissipates relatively quickly (you can smell it in freshly filled glasses)
• Chloramine (chlorine + ammonia) lasts longer in the distribution system
• WHO guideline: up to 5 mg/L is safe for drinking

Fluoride (F-):

• Added to many water supplies to prevent tooth decay
• Controversial in some regions
• Typical concentration: 0.5 - 1.5 mg/L where added
• Natural fluoride levels vary widely

Trace Elements and Contaminants

Iron (Fe2+/Fe3+):

• Causes reddish-brown staining
• Comes from pipes or natural groundwater
• Generally not harmful at levels found in tap water

Lead (Pb2+):

• Leaches from old lead pipes and solder
• No safe level of lead exposure (WHO)
• A major concern in older buildings and infrastructure

Arsenic (As):

• Naturally occurring in groundwater in some regions
• A significant problem in parts of India and Bangladesh
• WHO guideline: maximum 10 ppb (parts per billion)

Water Quality: A Country-by-Country Look

India

The challenge: India's water quality varies enormously between cities, states, and urban vs rural areas.

Key facts:

• BIS (Bureau of Indian Standards) IS 10500 sets drinking water standards
• Acceptable TDS (Total Dissolved Solids): up to 500 mg/L (permissible limit: 2000 mg/L)
• Fluoride is a major issue in Rajasthan, Gujarat, Andhra Pradesh, and Telangana — natural levels can exceed 5 mg/L, causing fluorosis
• Arsenic contamination affects West Bengal, Bihar, Jharkhand, and parts of UP — the Indo-Gangetic plain has naturally high arsenic levels
• Nitrate contamination from agricultural runoff is increasing in Punjab and Haryana

Common water treatment in Indian homes:

• RO (Reverse Osmosis) purifiers: Remove up to 95% of dissolved solids. Very popular but also remove beneficial minerals
• UV purifiers: Kill bacteria and viruses without removing minerals
• Activated carbon filters: Remove chlorine taste and some organic contaminants
• Boiling: Still the most common method in rural areas — kills pathogens but doesn't remove chemical contaminants

TDS levels in major Indian cities:

• Delhi: 300-700 mg/L (varies by area, some areas exceed 1000)
• Mumbai: 50-150 mg/L (relatively soft water from lake sources)
• Chennai: 400-1200 mg/L (groundwater areas have high TDS)
• Bangalore: 200-600 mg/L
• Kolkata: 150-400 mg/L

United States

Regulation: The EPA (Environmental Protection Agency) sets legally enforceable standards under the Safe Drinking Water Act.

Key standards:

• Lead: Action level of 15 ppb at the tap
• Arsenic: Maximum 10 ppb
• Fluoride: Maximum 4 mg/L (recommended: 0.7 mg/L for dental health)
• Chlorine: Maximum residual disinfectant level of 4 mg/L
• PFAS (forever chemicals): New standards set in 2024 for several PFAS compounds at 4-10 ppt (parts per trillion)

Regional variations:

• Flint, Michigan became a global symbol of water infrastructure failure when lead contamination was discovered in 2014
• Southwest US water often has high mineral content due to desert geology
• Northeast US generally has softer water from surface reservoirs
• Agricultural states face nitrate contamination from fertilizer runoff

Water hardness across the US:

• Very soft: Pacific Northwest (Seattle: ~30 mg/L CaCO3)
• Moderate: Northeast (New York: ~50-100 mg/L)
• Hard: Midwest (Chicago: ~150 mg/L)
• Very hard: Southwest (Phoenix: ~300-500 mg/L)

United Kingdom

Regulation: The Drinking Water Inspectorate (DWI) enforces standards based on UK and WHO guidelines.

Key facts:

• UK tap water is among the safest in the world
• Fluoridation is less common than in the US — only about 10% of the UK population receives fluoridated water (parts of the West Midlands, Northeast England)
• Water hardness varies dramatically: very hard in London and Southeast England (chalk and limestone geology), very soft in Scotland and Wales (granite geology)

London water:

• Very hard: 200-400 mg/L CaCO3
• High calcium and magnesium content
• Causes significant limescale buildup
• Tastes distinctly different from Scottish water

Scottish water:

• Very soft: 20-80 mg/L CaCO3
• Lower mineral content
• Less limescale
• Different taste profile

Thames Water facts:

• Supplies 9 million people in London
• Water is sourced from the Thames and underground aquifers
• Treated with chlorine (not chloramine)
• Contains naturally occurring fluoride at low levels (~0.1-0.3 mg/L)

The Chemistry of Water Hardness

Water hardness is caused by dissolved calcium and magnesium ions:

Temporary hardness:

• Caused by calcium bicarbonate Ca(HCO3)2
• Removed by boiling (bicarbonate decomposes, CaCO3 precipitates as limescale)
• Ca(HCO3)2 → CaCO3 + H2O + CO2

Permanent hardness:

• Caused by calcium sulfate (CaSO4) and similar compounds
• NOT removed by boiling
• Removed by water softeners (ion exchange) or RO filtration

Measuring hardness:

• Measured in mg/L of CaCO3 equivalent
• Soft: 0-60 mg/L
• Moderately hard: 61-120 mg/L
• Hard: 121-180 mg/L
• Very hard: >180 mg/L

pH of Tap Water

Safe range: 6.5 - 8.5 (WHO guideline)

Water pH is important because:

• Too acidic (< 6.5): Corrodes metal pipes, leaching lead and copper
• Too basic (> 8.5): Can cause scale buildup and taste issues
• Most tap water falls between 7.0 and 8.0

Buffer system: The bicarbonate-carbonate system naturally buffers water pH:

CO2 + H2O ⇌ H2CO3 ⇌ H+ + HCO3-

This is why adding acid or base to water doesn't change its pH dramatically — the buffer absorbs the change.

How to Test Your Water

Home testing options:

• TDS meter: Measures total dissolved solids (a rough indicator of mineral content). Costs around 200-500 rupees / $10-20 / 8-15 pounds
• pH strips: Quick and cheap, but imprecise
• Water testing kits: Test for specific contaminants (lead, bacteria, chlorine)
• Lab analysis: Most comprehensive — send a sample to a certified lab

What the numbers mean:

• TDS 50-150 mg/L: Excellent drinking water
• TDS 150-300 mg/L: Good, acceptable mineral content
• TDS 300-500 mg/L: Fair, consider filtration
• TDS > 500 mg/L: Poor, filtration recommended
• TDS > 1200 mg/L: Unacceptable without treatment

Conclusion

The chemistry of your tap water tells a story about your local geology, infrastructure, and water treatment practices. Whether you're in Delhi, Detroit, or Devon, understanding what's in your water helps you make informed decisions about filtration and consumption.

Use our interactive periodic table to explore the elements found in your water — click on calcium, magnesium, chlorine, fluorine, and other elements to learn about their properties and why they end up in your glass.