How to Read a Water Test Report

Why this matters

Most of the bad calls made off a water report are not chemistry errors. They are unit errors, basis errors, and reading a number that the sample was never capable of producing. A hardness of 13 and a hardness of 222 can be the same water reported two ways. A "not detected" can mean the lab looked and found nothing, or that the lab's method could not have seen the problem even if it were there. A number from the wrong tap on the wrong day is a fact about some other water.

So read the report the way you would read a nameplate: top to bottom, header first, and confirm what the number is before you decide what it means. The chemistry is the easy part. A sibling article covers what each parameter does to which metal; this one is about whether you are allowed to use the number at all.

Before you act on what it says

A report is paper, and reading it is not hazardous. Acting on it is, and two results change what you are doing rather than how you are doing it.

A microbiological result, including a coliform, Legionella or heterotrophic plate count exceedance, is not a maintenance item. It goes to the building's water management program and to whoever holds the notification duty for that system (ASHRAE Standard 188 for building water systems), and the response may include restricting use of the fixture before anyone works on it. Do not flush, dose or open a system on a positive microbiological result without that decision being made first, because flushing or opening it generates the aerosol that is the exposure route.

A lead result at or above the tap action level triggers a notification path, not a fitting swap. The tap action level for lead is 0.015 mg/L and for copper is 1.3 mg/L (40 CFR Part 141, Subpart I), and where the sample was a compliance sample the reporting obligations sit with the system, not with the shop that pulled it.

Any correction the report leads you to dose carries the chemical hazards of that product: get the glove class and respiratory control from Section 8 of its safety data sheet (29 CFR 1910.1200), add acid to water and never the reverse, and never combine a hypochlorite product with an acid or an ammonia-bearing product, since both combinations release gas into the room you are standing in.

Step 1: read the header before a single value

If any of these is missing, get it before you read further, because each one can invalidate everything below it.

• Sample point. "Kitchen tap" and "boiler makeup" are different waters in the same building.
• Date and time drawn, and how long before the lab received it. Some parameters expire.
• Sample type. First-draw or flushed, running or standing, hot side or cold side.
• Preservation. Whether the container was acidified or chilled, which determines whether the metals or the alkalinity on the page are believable.
• Who drew it. A customer-drawn sample into a rinsed drink bottle is a screening result, not evidence.

Step 2: fix the units and the basis

This is where the reversible errors live.

• mg/L and ppm are interchangeable at these dilutions. Treat them as the same number.
• Grains per gallon is the plumbing trade's hardness unit and the lab's is mg/L. One grain per gallon is 17.1 mg/L as CaCO3. A softener sized off 13 when the report said 222 is sized for about a seventeenth of the load.
• "As CaCO3" versus "as the ion." Hardness and alkalinity are both conventionally reported as calcium carbonate equivalents. Calcium reported as the ion converts to hardness-as-CaCO3 by multiplying by about 2.5; magnesium by about 4.1. A report listing calcium at 60 and hardness at 224 is not contradicting itself.
• uS/cm versus mS/cm. A factor of 1,000 sits between them and both appear on handhelds.
• Nitrate as N versus nitrate as NO3. The same water reads about 4.4 times higher on the second basis.

Step 3: find the reporting limit, and read "ND" correctly

Not detected means below the lab's reporting limit for that method. It does not mean zero. The number that matters is the limit itself, printed in its own column and skipped by almost everybody.

The test that makes this concrete: if a method's reporting limit for lead is above 0.015 mg/L, an ND on that report cannot tell you whether the water is above or below the action level. The test was incapable of answering the question that was asked. That is not a clean result, it is no result, and the fix is to order the analysis at a lower reporting limit rather than to relax.

Same logic in the other direction on a trace parameter. An iron result of 0.02 mg/L against a reporting limit of 0.02 is a number sitting on the floor of what the method can see, and it should be treated as "at or below," not as a trend point.

Step 4: check the report against itself

Three arithmetic checks, none of which needs the lab's cooperation. A report that fails them has a transcription or analysis problem and no value on it should be acted on.

• Ion balance. Convert every cation and every anion to milliequivalents per liter and total each side. They should agree closely, because water is electrically neutral. Divide each mg/L figure by its equivalent weight: calcium 20.04, magnesium 12.15, sodium 22.99, potassium 39.10, chloride 35.45, sulfate 48.03, and alkalinity as CaCO3 by 50.
• Hardness against its own cations. 2.5 times calcium plus 4.1 times magnesium should land on the reported hardness.
• TDS against conductivity. Divide reported TDS in mg/L by conductivity in uS/cm. Ordinary natural waters land around 0.55 to 0.7. A ratio well outside that band means one of the two is wrong, or the water carries something unusual, and either way it needs an explanation before use.

Step 5: name what is missing

A report answers what was ordered. The gaps are yours to notice, and the four most commonly absent from a general potability panel are the four that decide equipment questions: dissolved oxygen (the driver of general steel corrosion, and near zero in a healthy closed loop), chloride reported separately rather than buried in TDS, silica, and the temperature at the moment of sampling. Add iron and manganese on any well supply.

Worked example: one report, walked

A small commercial building, cold supply, sample drawn at the mechanical room makeup connection after a two-minute flush. Illustrative values, all mg/L except pH and conductivity:

Header check. Sample point named, cold side, flushed. So this is what the main delivers, not what the building's piping gives after standing, and it cannot be used to answer a lead or copper question at a fixture - that needs a first-draw sample after the minimum six-hour stagnation the tap protocol specifies (40 CFR Part 141, Subpart I).

Ion balance. Cations: 60 / 20.04 = 2.99, plus 18 / 12.15 = 1.48, plus 25 / 22.99 = 1.09, plus 3 / 39.10 = 0.08. Total 5.64 milliequivalents per liter. Anions: 180 / 50 = 3.60, plus 45 / 35.45 = 1.27, plus 38 / 48.03 = 0.79. Total 5.66. The two sides differ by 0.02 on about 5.65, under half a percent. The report is internally consistent.

Hardness check. 2.5 x 60 = 150, plus 4.1 x 18 = 73.8, giving 223.8, against a reported 224. Consistent.

TDS check. 400 / 620 = 0.645, inside the 0.55 to 0.7 band. Consistent.

Now read it. Hardness 224 is in the hard band and this water will deposit on hot surfaces. Alkalinity 180 is comfortably buffered, so the pH will hold still against ordinary disturbances. Since hardness exceeds alkalinity, about 44 mg/L as CaCO3 is non-carbonate hardness (224 minus 180), which will not precipitate as carbonate on heating the way the rest will. Chloride at 45 is unremarkable and well under the 250 mg/L secondary standard (40 CFR Part 143).

What is missing, and it is the actionable part. No dissolved oxygen, so this report cannot settle whether the closed loop it feeds is taking air. No silica, so a concentration limit for any evaporative equipment on this supply cannot be set. No sampling temperature, which the scaling calculation needs. And it is a cold flushed sample, so it says nothing about the hot side, where the actual deposit complaint lives.

What the answer would have been on a bad basis. Had the hardness arrived as 13 grains per gallon and been typed into a softener sizing sheet as 13 mg/L, the unit would have been sized for roughly a seventeenth of the actual load and would exhaust between regenerations from the first week, presenting as a failed softener.

The failure mode: the report gets filed as "water is fine," because nothing on it is out of range for drinking. Every value on the page is a drinking-water value, and none of the four parameters that would have answered the equipment question was ordered.

How to verify you read it right

Write your conclusion as one sentence naming the sample point and the date it came from - "the cold makeup at the mechanical room, on this date, is hard and well buffered" - and see whether the sentence still supports the decision you were about to make. If the decision is about the hot side, a recirculating loop, or a fixture after standing, the sentence will not reach it, and you need another sample rather than a bolder reading of this one.

Then run the three internal checks above before anything gets ordered or dosed, and keep the report with the sample point in the equipment file. A single report is a snapshot; two reports from the same point a year apart are the first real information you will have about whether anything is changing.

References

• 40 CFR Part 141, Subpart I, for the lead and copper tap action levels of 0.015 mg/L and 1.3 mg/L and the first-draw sampling protocol
• 40 CFR Part 143 (National Secondary Drinking Water Regulations) for the secondary standards, which are aesthetic values rather than equipment limits
• ASHRAE Standard 188, Legionellosis: Risk Management for Building Water Systems, for the response path on a microbiological result
• 29 CFR 1910.1200 (Hazard Communication) and Section 8 of the product safety data sheet for any chemical correction the report leads to
• Standard Methods for the Examination of Water and Wastewater (APHA, AWWA, WEF) for reporting limits, the hardness calculation and the gravimetric TDS determination
• See related: What Conductivity and Total Dissolved Solids Actually Measure; Hardness, Alkalinity and pH, and What Each One Does; The Water Chemistry That Attacks a System