A water test report is written for regulators, not for the person who owns the well. It arrives as a grid of abbreviations, and the most common outcome is that the owner reads the one line they recognise, files the rest, and never looks at it again.
Almost all of the confusion comes down to four things: units, bases, limits, and what "not detected" really means.
Units: Fewer Than It Looks Like
Reports switch between units freely, which makes results look wildly different when they are not.
| Unit | Equals | Used for |
|---|---|---|
| mg/L | ppm (parts per million) | Most contaminants: nitrate, iron, hardness, TDS |
| µg/L | ppb (parts per billion) | Trace metals: arsenic, lead, uranium |
| 1 mg/L | 1,000 µg/L | The conversion that catches people |
| ppt | parts per trillion | PFAS |
| NTU | turbidity units | Cloudiness |
| pH | 0–14 scale | Acidity, no units |
mg/L and ppm are the same number. So are µg/L and ppb. A litre of water weighs roughly a million milligrams, which is where the equivalence comes from.
The trap is the factor of a thousand between mg/L and µg/L. Arsenic reported as 0.012 mg/L and arsenic reported as 12 µg/L are the identical result, and both exceed the 10 µg/L limit. One of them looks alarmingly small.
Before reacting to any number, find the unit column.
The Nitrate Basis Trap
This one is worth its own section, because it is the single most misread line on well reports.
Nitrate is reported two ways:
- As nitrate-nitrogen (NO₃-N) — counts only the nitrogen atom. Limit: 10 mg/L.
- As nitrate (NO₃) — counts the whole molecule. Equivalent limit: about 45 mg/L.
The conversion factor is roughly 4.4.
So a result of 20 mg/L as NO₃ is about 4.5 mg/L as nitrogen — comfortably under the limit. A result of 20 mg/L as NO₃-N is double the limit and unsafe for an infant.
Same digits. Opposite conclusions. Look for "as N" or "NO₃-N" on the line, and if the report does not say, phone the lab and ask rather than guessing.
Primary Versus Secondary Standards
Not every exceedance means the same thing.
Primary standards (maximum contaminant levels) are health-based and legally enforceable for public water systems. Over the limit means a health risk: bacteria, nitrate, arsenic, lead, uranium, fluoride at 4.0 mg/L.
Secondary standards cover aesthetics — taste, odour, colour, staining. Not enforceable, not health-based. Iron at 0.3 mg/L, manganese at 0.05 mg/L, TDS at 500 mg/L, sulfate, chloride, pH's recommended 6.5 to 8.5 range.
Exceeding a secondary standard means your laundry suffers, not your health. With one caveat worth remembering: low pH is a secondary standard that causes a primary problem, because acidic water dissolves lead and copper out of your own plumbing.
Private wells are not covered by either set of standards — nobody enforces anything on your well. They remain the right yardstick because they are the numbers everyone else's water is held to.
What "Non-Detect" Actually Means
"ND", "<0.005", or "not detected" does not mean zero. It means the contaminant was not found above the laboratory's reporting limit, which is printed alongside.
That distinction matters when the reporting limit is high relative to the health standard. A lead result of "<0.020 mg/L" is technically a non-detect, but the action level is 0.015 mg/L — so the water could be over the limit and still report as not detected. That is not a clean result. It is an unusable one, and it needs re-running by a lab with better sensitivity.
Check the reporting limit against the standard before you relax.
Reading Your Own Report, in Order
- Bacteria first. Total coliform and E. coli are presence/absence, and any detection is a fail. There is no "slightly positive."
- Then the health contaminants — nitrate (check the basis), arsenic, lead, uranium, fluoride — against their primary limits.
- Then pH. Below 6.5 is corrosive and drives lead and copper into your water. It explains other results.
- Then the aesthetics — iron, manganese, hardness, TDS, sulfate — which tell you what treatment you need and how to size it.
- Then check what is missing. A panel that did not include arsenic tells you nothing about arsenic.
The Habit That Makes Reports Worth Something
One report is a snapshot: what was in that sample, from that tap, on that day, for the parameters someone chose to run.
Two reports are a direction. Five are a record — the thing that tells you whether nitrate is creeping up as the farm upstream expands, whether your new filter is actually working, and whether the water was always like this or changed after the drought.
Keep every report, log every value with its date and its units, and note anything unusual about the sampling. That archive is worth more than any single number in it — and it is the document a buyer, a lender, or a health department will ask you for.
Frequently Asked Questions
Is mg/L the same as ppm? Yes, for drinking water. One milligram per litre is one part per million, because a litre of water weighs about a million milligrams. Similarly, micrograms per litre (µg/L) equals parts per billion (ppb), and 1 mg/L equals 1,000 µg/L.
What is the difference between nitrate as N and nitrate as NO3? They measure the same contaminant on different bases. Nitrate-nitrogen (NO₃-N) counts only the nitrogen atom; nitrate as NO₃ counts the whole molecule and is about 4.4 times larger. The EPA limit is 10 mg/L as NO₃-N, equivalent to roughly 45 mg/L as NO₃. Reading the wrong basis is the most common misinterpretation on a well report.
What does non-detect mean on a water test? It means the contaminant was not found above the laboratory reporting limit — not that it is absolutely absent. The reporting limit is usually printed alongside. A non-detect at a reporting limit above the health standard is not a clean result and should be re-run by a lab with better sensitivity.
What is the difference between a primary and secondary drinking water standard? Primary standards, called maximum contaminant levels, are health-based and enforceable for public systems. Secondary standards cover taste, odour, colour and staining, are not enforceable, and exceeding one is unpleasant rather than unsafe.
Does a certified lab report guarantee my water is safe? It confirms what was measured in that sample on that day. It says nothing about contaminants that were not on the panel, or about water quality after heavy rain, well work, or a season change. Testing is a snapshot, which is why the schedule matters as much as any single result.