How to Read a Water Test Report Without a Chemistry Degree

A water test report is a list of substances and numbers, usually with a limit beside each. It is not difficult to read once you know three things: what the units mean, the difference between the two kinds of standard, and which numbers actually change a decision.

The units

mg/L is milligrams per litre, and it is the same as ppm, parts per million. Most results are reported this way.

µg/L is micrograms per litre, the same as ppb, parts per billion. Used for substances that matter at very low concentrations — lead, arsenic, PFAS. One thousand ppb equals one ppm, and mixing these up is the most common misreading there is.

gpg is grains per gallon, used almost exclusively for hardness. One gpg is about 17.1 mg/L.

pH is a scale from 0 to 14. Seven is neutral, below is acidic, above is alkaline. It is logarithmic, so a pH of 5 is ten times more acidic than a pH of 6.

Nitrate deserves a warning. It can be reported as nitrate-nitrogen (NO3-N) or as nitrate (NO3). The federal standard of 10 mg/L is as nitrogen. The same water reported as nitrate is roughly 4.4 times higher. Check which your report uses.

Two kinds of standard

Primary standards are enforceable and health-based. Lead, arsenic, nitrate, bacteria and a long list of others. Exceeding one of these is a genuine finding that warrants action.

Secondary standards are non-enforceable guidelines about aesthetics — taste, colour, odour and staining. Iron, manganese, TDS, pH, chloride, sulfate and zinc live here. Exceeding a secondary standard means your water may be unpleasant or may stain things. It does not mean it is unsafe.

This distinction is routinely blurred in sales conversations. A report showing iron at 0.8 mg/L is above the secondary guideline of 0.3, and that genuinely explains orange staining. It is not a health finding, and presenting it as one is misleading.

The numbers that change decisions

Hardness

There is no health standard. Roughly: under 3 gpg is soft, 3–7 moderate, 7–10 hard, above 10 very hard. This number sizes a softener, and it is the one to insist on before accepting any softener quote.

Iron and manganese

Secondary guidelines are 0.3 mg/L for iron and 0.05 mg/L for manganese — both roughly where staining begins. Manganese has a much lower threshold than people expect, and it stains black rather than orange.

pH

The recommended range is 6.5 to 8.5. Below 6.5 the concern is corrosion: acidic water dissolves copper, and in older homes, lead. If your pH is low, look for blue-green staining and consider testing first-draw copper and lead.

Nitrate

10 mg/L as nitrogen. A primary, health-based standard set to protect infants. A result approaching it on a well used for formula preparation is a reason to act now rather than at your convenience.

Bacteria

Reported as present or absent rather than as a concentration. Any presence of total coliform means a contamination pathway exists. Presence of E. coli is more serious.

Lead

The federal action level is 15 ppb, but note what that is: a regulatory trigger for utility action, not a health-based safe threshold. The health goal is zero, and the CDC states there is no known safe blood lead level in children.

TDS

Secondary guideline of 500 mg/L, about taste rather than safety. As covered elsewhere on this site, TDS on its own tells you very little.

Three questions worth asking about any report

Where was the sample taken? A sample from an outside tap tells you about the supply. A first-draw sample from the kitchen tells you what you are actually drinking, including anything your own plumbing contributes.

Was it first-draw or flushed? For lead and copper this changes the result dramatically, and it is the difference between a useful test and a meaningless one.

What was not tested? A clean report only covers what was analysed. A basic panel that came back perfect says nothing about PFAS, pesticides or anything else that was not on the list.

The most important line on any report

The results that are unremarkable. A good report has plenty of them, and a good explanation says so. If someone walks you through your results and every single line is a problem requiring equipment, you are being sold to rather than informed.