A water report is mostly numbers next to abbreviations, and the abbreviation decides what the number means. Five of them carry almost all the weight.
MCL — Maximum Contaminant Level
The enforceable legal limit. Exceed it and the system is in violation, with obligations that follow: notify customers, fix it, report it.
This is the number to check a detection against if the question is "is this legal."
MCLG — Maximum Contaminant Level Goal
The health target, and not enforceable. It is the level at which no known or anticipated health effect occurs, with a margin of safety, set without regard to cost or technical feasibility.
That last clause is what separates the two. The MCL is what is achievable; the MCLG is what would be ideal. For several carcinogens the MCLG is zero, because no threshold below which risk disappears has been identified — so any detection is above the goal while being comfortably legal.
Both numbers are honest. They answer different questions.
SMCL — Secondary Maximum Contaminant Level
Not enforceable, and not about health. Secondary standards cover the properties that make water unpleasant rather than unsafe: iron, manganese, total dissolved solids, chloride, colour, odour, pH.
Iron's secondary standard is 0.3 mg/L. Exceed it and you get orange staining and a metallic taste, not a violation. States may adopt SMCLs as enforceable if they choose; EPA sets them as guidance.
This category matters locally, because most of what bothers people about well water here is secondary rather than primary.
MRDL — Maximum Residual Disinfectant Level
The same idea as an MCL, applied to the disinfectant itself rather than a contaminant. Chlorine has to be present in the distribution system to keep water safe on the journey; the MRDL caps how much. There is an MRDLG alongside it, on the same enforceable-versus-goal split.
Action level — lead and copper
Not an MCL at all, and the difference is genuinely important.
Lead and copper are regulated by treatment technique rather than by a contaminant limit, because they mostly enter water from household plumbing after the utility's responsibility ends. The action level — 15 ppb for lead, 1.3 mg/L for copper — is the threshold at which a system must take specified steps: corrosion control, public education, sometimes service-line replacement.
Exceeding it triggers obligations. It is not automatically a violation, and it is not a line between safe and unsafe.
Reading a number properly
Put together, three questions settle almost any detection:
- Which limit is it being compared to — enforceable or goal, primary or secondary?
- Is it an average or a single result? Several standards are annual running averages, which can hide a bad quarter.
- Where was the sample taken — at the plant, in the distribution system, or at a tap? For lead and copper this changes the answer completely.
We list detections against both the legal limit and the stricter health guideline for every system in Mobile and Baldwin County, because showing one without the other is how a number gets misread. For what is happening at your own tap, the free test is the direct answer.