What makes water safe to drink?
Water that is safe to drink is called potable. It is not "pure": it always has a few dissolved salts. What matters is that harmful things stay below safe limits. Drinking-water standards (such as the WHO guidelines and national rules) check these groups:
- Germs (microbes): no disease germs. A test looks for E. coli, which comes from faeces; the safe value is 0 in a 100 mL sample.
- Looks, taste and smell: clear (low turbidity, which means cloudiness from mud), no colour, no bad smell.
- pH: about 6.5 to 8.5. Very acid water eats pipes and dissolves metals; very basic water tastes bad.
- Dissolved chemicals: nitrate (about 50 mg/L at most), arsenic and lead (about 0.01 mg/L at most), fluoride, and total dissolved salts (TDS, about 500 mg/L or less for good taste).
- Disinfectant left over: a little chlorine is kept in tap water to stop germs growing in the pipes.
Limits are set from health studies: a safe amount for a person who drinks about 2 litres a day for a whole life. Exact numbers differ a little between countries.
Where does water pollution come from?
Water pollution is anything added to water that makes it unsafe or harms living things. The main sources are:
- Farming: extra fertiliser (nitrate, phosphate) washed in by rain, and pesticides. Too many nutrients cause eutrophication: algae grow wildly, die, rot and use up the oxygen, so fish die.
- Industry: heavy metals (lead, mercury, cadmium), solvents, acids, oils and hot water.
- Homes and towns: sewage (germs and rotting matter), detergents, plastics and rubbish.
- Other: oil spills, mining waste, acid rain, leaking fuel tanks and landfills.
A point source is one pipe or drain (a factory outlet). A diffuse source is spread over a wide area (a whole farmland). Diffuse pollution is harder to control. Groundwater is polluted too; it cleans itself very slowly.
Soils as exchange and filter media
Soil is a mix of sand, silt, clay, humus (rotted plant matter), water and air. Clay and humus have a negative charge on their surface. They hold positive ions (cations) such as calcium (Ca²⁺), potassium (K⁺), magnesium (Mg²⁺), ammonium (NH₄⁺) and heavy metals.
These ions are not stuck for ever. A plant root gives out H⁺ and takes a nutrient ion in exchange. This is cation exchange. The total number of positive charges soil can hold is its cation exchange capacity (CEC); clay and humus soils have a high CEC, sandy soil a low one.
What this does for water: soil holds nutrients for plants, holds back many metals and ammonium, and its tiny gaps filter out mud and many germs. Limits: nitrate is negative, so it is not held and washes down to groundwater. A soil that is full of pollutant cannot hold more, and very acid soil can release metals back into water.
Try it
In the 3D: on the free-play step, switch on only Farm and read which check fails. Then add Soil. Now switch on Town too. Which source is hardest for the soil to fix?
At home: fill two clear bottles, one with tap water and one with muddy water. Pour the muddy water through a cloth, then through sand in a cut bottle. Compare cloudiness. (Never drink it: the water is still not safe.)
Key formulas and definitions
- Concentration (mg/L) = mass of substance (mg) ÷ volume of water (L)
- 1 mg/L is the same as 1 ppm in water
- Dilution: C₁V₁ = C₂V₂ (the amount of substance does not change)
- Percent removed = (before − after) ÷ before × 100
- Safe pH for drinking: about 6.5 to 8.5
Worked examples
1. A 2 L sample holds 150 mg of nitrate. Find the concentration and compare with the 50 mg/L limit.
c = 150 / 2 = 75 mg/L. It is more than 50 mg/L, so the water is NOT safe for nitrate.
2. 10 L of river water has 100 mg/L of a pollutant. It is mixed with 40 L of clean water. What is the new concentration?
Amount = 10 × 100 = 1000 mg. New volume = 50 L. c = 1000 / 50 = 20 mg/L.
3. Soil removes 70% of 80 mg/L ammonium from water passing through. How much is left?
Removed = 0.70 × 80 = 56 mg/L. Left = 80 − 56 = 24 mg/L.
4. A person drinks 2 L of water a day containing arsenic at the limit of 0.01 mg/L. How much arsenic do they take in per day?
Intake = 0.01 mg/L × 2 L = 0.02 mg per day.
5. A water sample has pH 5.9, 0 E. coli per 100 mL and clear water. Is it safe?
Germs and clarity pass, but pH 5.9 is below 6.5, so it is not within the usual limit. It is acidic and needs treatment before use.
Common mistakes
- Thinking clear water is safe water. Germs, nitrate, arsenic and fluoride cannot be seen.
- Calling potable water "pure water". Drinking water has dissolved salts; it only has to be below safe limits.
- Saying soil holds all pollutants. Soil holds positive ions well, but negative nitrate washes through to groundwater.
- Mixing up point and diffuse sources. A factory pipe is a point source; fertiliser on a whole field is diffuse.