How to read a water quality report
Water quality reports are written for regulators, not readers. Here is what each part actually means, and the four things people most often misread.
Whether it comes from your supplier or a laboratory, a water quality report is a table of numbers against limits. It is comprehensible, but there are four things about it that mislead people almost universally.
1. A zone result is not your tap
Supplier reports are published per water supply zone, an area that can contain up to 100,000 people. The figures are a summary of samples taken across that zone.
That tells you what the supplier delivered to the area. It cannot tell you what comes out of your tap, because the last part of the journey — the service pipe and your building’s plumbing — is not represented. This matters most for lead, which almost always originates in that final stretch, and for anything affected by stagnation in a building.
2. Units are not interchangeable
| Parameter | The trap | Conversion |
|---|---|---|
| Nitrate | Reported as nitrate ion (NO₃) or as nitrate-nitrogen (NO₃-N) | Divide by 4.43 to convert NO₃ to N |
| Hardness | At least five units in common use worldwide | 1 °dH = 17.85 mg/l CaCO₃; 1 °fH = 10; 1 gpg = 17.12 |
| Concentration | mg/l, µg/l and ng/l differ by factors of a thousand | 1 mg/l = 1,000 µg/l = 1,000,000 ng/l |
| ppm and ppb | Used interchangeably with mg/l and µg/l in water, but not in other media | In water, 1 ppm ≈ 1 mg/l |
A US nitrate limit of 10 mg/l as N and an EU limit of 50 mg/l as NO₃ look very different and are in fact nearly the same — the US limit is slightly stricter.
3. Summary statistics matter enormously
A report giving "mean nitrate: 32 mg/l" and one giving "maximum nitrate: 32 mg/l" describe very different situations. Look for which statistic is quoted: mean, median, maximum, or 95th percentile. Regulatory compliance is usually assessed against maximum or percentile values, while summary tables often present means.
4. A limit is not a safety cliff
What the common parameters mean
- E. coli and coliforms
- Indicators of faecal contamination. In treated drinking water the standard is absence, so any detection is a barrier failure, not a matter of degree.
- Turbidity
- Cloudiness. Operationally the single most important number, because particles shield micro-organisms from disinfection.
- pH
- Acidity. Managed for corrosion control and disinfection efficiency rather than for direct health effect.
- Hardness
- Calcium and magnesium. No health significance; determines scaling and soap use.
- Nitrate
- Chiefly agricultural. The limit protects bottle-fed infants and is therefore comfortably protective for adults.
- Lead
- From plumbing, not from source water. Sampling method dominates the result.
- Trihalomethanes
- Disinfection by-products. Their presence means the water was disinfected, which is overwhelmingly a good thing.
- Chlorine residual
- Deliberate. It protects the water between the works and your tap.
Sources
- Drinking Water Inspectorate — Drinking water quality in England and Wales. Open — attribution required · Open Government Licence v3.0
- World Health Organization — Guidelines for Drinking-water Quality. Public but restricted · CC BY-NC-SA 3.0 IGO
- United States Environmental Protection Agency — Safe Drinking Water Information System. Open · US Government work — public domain