The paperwork
How to read your lab report
A lab report is a stack of rows, and every row says the same five things: what was measured, what your water came back at, in what unit, what the limit is, and whether you are over it. Once you can read one row you can read all of them.
SimpleLab’s report compares each result with four reference points: the EPA Maximum Contaminant Level, the EPA secondary (aesthetic) standard, the World Health Organization guideline and the Health Canada guideline. Anything above an MCL is flagged red.
Take a real one. Arsenic, result 18 µg/L, MCL 0.010 mg/L, flagged.
What MCL means
MCL is the Maximum Contaminant Level. EPA defines it as “the highest level of a contaminant that is allowed in drinking water,” set “as close to MCLGs as feasible using the best available treatment technology and taking cost into consideration.” MCLs are enforceable standards — on a public water system. On a private well nobody enforces anything, so the MCL is a reference line you decide whether to respect.
Sitting behind it is the MCLG, the Maximum Contaminant Level Goal: “the level of a contaminant in drinking water below which there is no known or expected risk to health.” The gap between an MCLG and an MCL is the gap between what is safe and what is achievable at a cost. For arsenic that gap is real. For lead the MCLG is zero.
The units, and why there are three of them
EPA's table states it in one line: “Units are in milligrams per liter (mg/L) unless otherwise noted. Milligrams per liter are equivalent to parts per million (PPM).” AZ1486f gives the other half: “Part per billion (ppb) = microgram per liter of water = µg/L.”
So there are two units wearing four names:
- 1 mg/L = 1 ppm
- 1 µg/L = 1 ppb
- 1 mg/L = 1,000 µg/L
Which makes the arsenic MCL of 0.010 mg/L the same number as 10 µg/L and the same number as 10 ppb. A result of 18 µg/L is 0.018 mg/L, which is 1.8 times the limit. Most of the confusion in a lab report is a decimal point, not a chemical.
What a non-detect means
ND, or <0.5, or “non-detect,” does not mean zero. It means the concentration was below the lowest level that method could reliably measure. AZ1486f has you compare three numbers to read a row: the laboratory's minimum detection limit, EPA's MCL, and your own result. If the detection limit is higher than the limit you care about, a non-detect tells you very little — which is why the method matters as much as the answer.
Action level is not an MCL
Lead and copper have no MCL. EPA regulates both with a Treatment Technique instead — “a required process intended to reduce the level of a contaminant” — with an Action Level of 0.015 mg/L for lead and 1.3 mg/L for copper. EPA describes it as “the concentration of a contaminant which, if exceeded, triggers treatment actions or other requirements.”
Two things follow, and both matter when you are holding your own report.
First, the action level is a system-wide statistic. It applies to the 90th percentile of tap samples across a whole water system — if more than ten percent of samples exceed it, the utility has to act. Your house is one sample. There is no 90th percentile of one.
Second, it was never a safety line. EPA has “set the maximum contaminant level goal for lead in drinking water at zero because lead is a toxic metal that can be harmful to human health even at low exposure levels.” The action level is where a utility's obligations begin, not where harm does. EPA has already scheduled it downward: “Beginning on the LCRI compliance date of November 1, 2027, systems must comply with the revised lead action level of 0.010 mg/L.”
Lead also does not arrive from the aquifer. It “can enter drinking water when plumbing materials that contain lead corrode,” which is EPA's way of saying a lead result is a fact about your house, not about your source. And “since you cannot see, taste, or smell lead dissolved in water, testing is the only sure way of telling whether there are harmful quantities of lead in your drinking water.”
The rows with no health limit at all
Hardness, iron, manganese, total dissolved solids, sulfate, chloride and pH are compared to Secondary Maximum Contaminant Levels — iron 0.3 mg/L, manganese 0.05 mg/L, TDS 500 mg/L, pH 6.5 to 8.5. These are “non-mandatory water quality standards,” EPA does not enforce them, and “these contaminants are not considered to present a risk to human health at the SMCL.” They are the taste, staining and scale rows. Being over one of them is an appliance problem. It is not a health finding, and anybody who presents it as one is selling.
We read your report with you, free, whether or not you ever buy anything from us. We turn “arsenic 18 µg/L” into “1.8 times the EPA limit — here is what it means, and here is the treatment” in plain English. If the test comes back clean, we tell you that, and you don’t buy a system you don’t need. Trade, not sales.
