How Drinking Water Is Tested

When you turn on a tap, you’re drinking the end product of a huge analytical chemistry operation. Water suppliers test samples from rivers, reservoirs and boreholes, from every stage of treatment, and from taps across their network, sometimes thousands of times a day. This article explains what they’re looking for, the chemistry behind the tests, and how you can interpret a water quality report.

Why so much testing?

Water is an excellent solvent. As it flows over rocks and soil and through pipes, it dissolves minerals, gases and sometimes pollutants. It can also carry microorganisms. Some dissolved substances are harmless or even beneficial (calcium, magnesium), some affect taste and appearance (iron, manganese), and some are toxic at low concentrations (lead, arsenic, nitrate in large amounts).

National regulations and World Health Organization guidelines set maximum levels for dozens of substances. Testing checks that each one stays within its limit.

Units: how small is small?

Water results are often given in:

• mg/L (milligrams per litre), which for dilute water is the same as parts per million (ppm)
• μg/L (micrograms per litre), the same as parts per billion (ppb)

One part per billion is like one second in about 32 years. Modern instruments routinely measure at this level, and some go a thousand times lower.

1. Microbiological tests

The most urgent safety question is whether water is contaminated by sewage. Testing for every possible pathogen would be impractical, so labs test for indicator organisms, especially E. coli and coliform bacteria, which live in the intestines. If they’re present, sewage contamination is likely.

A measured volume of water is passed through a membrane filter, which is placed on a growth medium and incubated. Colonies are counted. Other methods use a medium that changes colour or fluoresces when specific bacterial enzymes are present. For drinking water, the target is zero E. coli in a 100 mL sample.

2. Disinfectant residual (chlorine)

Most supplies are disinfected with chlorine, which forms hypochlorous acid (HOCl) in water. A small residual is kept in the pipes to stop regrowth of microbes.

Free chlorine is commonly measured with the DPD method: the reagent DPD (N,N-diethyl-p-phenylenediamine) is oxidised by chlorine to a pink product. The intensity of the pink, measured with a colorimeter, is proportional to the chlorine concentration. See colorimetry and the Beer–Lambert law.

3. pH

Water is usually kept between about pH 6.5 and 9.5. Low pH makes water corrosive: it dissolves metals such as copper and lead from pipes. High pH can affect taste and reduce the effectiveness of chlorine. pH is measured with a glass electrode meter. See how a pH meter works and the pH scale.

4. Hardness

Hardness (calcium and magnesium) isn’t a health concern, but it affects scaling and soap use. It’s measured by EDTA titration or by atomic spectroscopy, and reported as mg/L of CaCO₃. See EDTA titrations and water hardness.

5. Nitrate and nitrite

Nitrate enters water from fertilisers, manure and sewage. High levels are a particular risk for bottle-fed infants, because nitrite (formed from nitrate in the body) interferes with the blood’s ability to carry oxygen. The WHO guideline for nitrate is 50 mg/L (as NO₃⁻).

Labs measure nitrate by ion chromatography, which separates anions (chloride, nitrate, sulfate, fluoride) on an ion-exchange column and detects each by conductivity. Nitrite can be measured colorimetrically: it reacts with reagents to form an intensely coloured pink azo dye.

6. Metals: lead, arsenic, copper and more

Metals are measured by ICP-MS or ICP-OES, which can detect dozens of elements in one run at μg/L levels. See ICP-MS and atomic absorption spectroscopy.

• Lead usually comes from old lead pipes, solder or fittings, not from the source water. That’s why lead samples are often taken from the consumer’s tap, sometimes after water has stood in the pipes overnight. The WHO guideline value is 10 μg/L, and many countries aim lower, since no level of lead is considered completely safe.
• Arsenic occurs naturally in some groundwaters, notably in parts of South and Southeast Asia and South America. Guideline: 10 μg/L.
• Copper from copper pipes can cause blue-green staining and a metallic taste.
• Iron and manganese cause brown or black staining and taste problems.

7. Fluoride

Fluoride occurs naturally and is added in some areas to reduce tooth decay. Too much causes dental and skeletal problems. The WHO guideline is 1.5 mg/L. It’s measured with a fluoride ion-selective electrode, whose potential depends on fluoride concentration (see potentiometric titrations for the electrode principle), or by ion chromatography.

8. Organic compounds

Labs test for pesticides, industrial solvents and disinfection by-products such as trihalomethanes, which can form when chlorine reacts with natural organic matter. These are measured by gas chromatography–mass spectrometry or liquid chromatography–mass spectrometry, often after concentrating the sample.

Increasing attention is being paid to PFAS (“forever chemicals”), very stable fluorinated compounds that are measured at nanogram-per-litre levels by LC-MS/MS.

9. Physical tests

• Turbidity (cloudiness) is measured by shining light through the water and detecting light scattered by particles. High turbidity can shield microbes from disinfection.
• Conductivity gives a quick estimate of total dissolved ions.
• Colour, taste and odour are assessed too, because people judge water with their senses.

Home test kits: what they can and can’t do

Home kits typically use test strips or colour-comparison tablets for pH, hardness, chlorine, nitrate and sometimes lead. They’re useful for a quick check, but:

• colour matching is subjective and less precise than a lab instrument
• many kits can’t reach the low detection limits needed for lead or arsenic
• bacterial tests from kits are less reliable than accredited lab tests

If you’re concerned, especially about lead or a private well, use a certified laboratory.

How to read a water quality report

Most suppliers publish reports listing each parameter, the regulatory limit, and the measured average and range. Look for:

• whether any result exceeded its limit
• the units (mg/L vs μg/L)
• hardness, which explains limescale
• lead, which depends partly on your own home’s plumbing

Where and when samples are taken

A result is only as good as the sample. Water companies sample at the source, after each treatment stage, at service reservoirs and at randomly chosen consumer taps, following a schedule set by regulations. Samples for bacteria go into sterile bottles containing a chemical that neutralises chlorine, so disinfection doesn’t continue in the bottle. Samples for metals are acidified to keep the metals in solution, and samples for organic compounds go into glass, because plastics can absorb or release organic molecules. Most samples must be kept cool and analysed within a set time.

Key takeaways

• Drinking water is tested for microbes, disinfectant, pH, minerals, nitrate, metals, fluoride and organic contaminants.
• Indicator bacteria such as E. coli show possible sewage contamination.
• Colorimetry, titration, ion chromatography, ICP-MS and GC-MS each suit different contaminants.
• Lead usually comes from pipes, so it’s tested at the tap.
• Home kits give quick indications; accredited labs give reliable, low-level results.