Reading a lab report: action levels in plain English

Last updated: 2026-09-21 · 9 min read · How-to

Your PDF is full of acronyms (MCL, SMCL, ND, µg/L). This guide translates the common lines so you can decide whether to retest, boil, call the health department, or shop treatment — without buying gear from a verbal “your water is bad.” Pair it with which panels to order and clean sampling.

What you’ll learn

Primary (health) vs secondary (taste/staining) benchmarks
What MCL, action level, and “non-detect” mean on a sheet
How to read coliform / E. coli and nitrate lines
A simple next-step order when something is elevated

Private wells are not city water — but the numbers still help

Public systems must meet EPA National Primary Drinking Water Regulations. Private wells usually do not. Still, most homeowners and many counties use those same numbers as a health reference. Your state or county may publish private-well guidance that is stricter or more local — when it does, follow that first.

Rule of thumb Compare every result to (1) the lab’s listed limit of detection, (2) EPA primary/secondary tables or your state’s private-well sheet, and (3) your prior year’s report. A jump matters even when both years sit under a limit.

The vocabulary on the page

Label	Plain English
MCL (Maximum Contaminant Level)	Health-based limit for public water systems. Use as a private-well benchmark unless your state says otherwise.
SMCL / secondary MCL	Taste, odor, staining, or corrosion guidance — not primarily a toxicity line. Iron and manganese live here.
Action level (e.g. lead)	A trigger for public systems to take steps (often corrosion control), not a true “safe/unsafe” cliff for every home.
ND / < MDL / non-detect	Not found above the lab’s method detection limit. It is not “zero forever” — only “below what this method can see.”
mg/L vs µg/L	mg/L ≈ parts per million. µg/L ≈ parts per billion (1 mg/L = 1,000 µg/L). Match units before you compare.
Present / Absent	Common for total coliform and E. coli. Presence is the finding — there is often no “ppm” number.

Primary vs secondary — which panic button?

Primary (health): bacteria indicators, nitrate/nitrite, arsenic, lead, some VOCs and pesticides. Treat elevated results as priority: retest or mitigate, don’t softener-shop first.
Secondary (aesthetic / equipment): iron, manganese, hardness, sulfate, chloride, TDS, low/high pH. Annoying and equipment-wrecking, but usually not the same urgency as E. coli or high nitrate.

EPA publishes both tables; start from primary regulations and the secondary nuisance guidance. Private-well overview: Protect Your Home’s Water.

Common lines and reference numbers (estimates)

These are EPA public-system reference values used as homeowner benchmarks. Confirm the exact row on your report and any state private-well sheet.

Analyte	Typical reference	What it usually means for you
Total coliform	Absent preferred	Indicator of well/plumbing integrity. Presence → retest carefully; check sanitary seal, well cap, and sample technique.
E. coli	Absent	Fecal contamination signal. Do not drink untreated. Contact local health; boil or use bottled until cleared.
Nitrate (as N)	MCL 10 mg/L	High risk for infants (blue-baby syndrome context). Needs a treatment class that removes nitrate — not a softener alone.
Nitrite (as N)	MCL 1 mg/L	Same family as nitrate; treat elevated results as priority with health dept guidance.
Arsenic	MCL 0.010 mg/L (10 µg/L)	Geology-driven in many regions. Confirm units; ask about treatment that is certified for arsenic reduction.
Lead	Action level 0.015 mg/L (public systems)	Often plumbing. Sample first-draw vs flushed if the lab allows; fix corrosion/sources before “filter shopping” blindly.
Iron	SMCL 0.3 mg/L	Staining, taste, media fouling. Secondary — still important for softener/filter design.
Manganese	SMCL 0.05 mg/L	Black stains, taste; some states also discuss health advisories at higher levels — check local guidance.
pH	SMCL roughly 6.5–8.5	Corrosion and treatment chemistry. Acidic water can pull metals from pipes.
TDS	SMCL 500 mg/L	Broad “dissolved stuff” indicator. High TDS alone does not tell you which treatment you need.

How to walk a report in five minutes

Confirm sample ID and date. Match the bottle label. Wrong address or a Friday bacteria hold-time miss can void the sheet.
Find bacteria first. Any E. coli “present” stops the rest of the shopping trip — health response first.
Find nitrate/nitrite. Compare to 10 mg/L and 1 mg/L (as N). Note whether the lab reported “as nitrogen” or “as nitrate” — the numbers are not interchangeable.
Scan primary metals (arsenic, lead, etc.) with units converted so µg/L and mg/L match the MCL row.
Then read secondary rows (iron, manganese, hardness, sulfate) for equipment and staining decisions.
Circle every ND and note the detection limit. An ND with a high detection limit is weaker reassurance than an ND with a low one.

When something is elevated — action order

Rule out bad sampling for bacteria: aerator left on, rinsed sterile bottle, warm hold time. Retest with the lab’s sheet before you chlorinate the whole system on a hunch. See collection steps.
Call local environmental health for E. coli, high nitrate with infants in the home, or anything you don’t understand. They know county geology and outbreak context.
Retest to confirm before expensive treatment — except when E. coli or a clear acute risk says don’t wait to protect drinking water.
Match treatment to the analyte. Softeners do not fix bacteria or nitrate. Bring the PDF to a WQA-listed dealer and ask how each number changes the proposal — or compare paths in WQA dealers vs big-box softeners.
Retest after install on the problem you paid to fix, and keep a raw-well sample in the file so you can tell aquifer change from filter failure.

Walk away if A salesperson won’t look at your certified report; pushes one product for every row; or quotes only after an in-home “free test” with no written analytes. Numbers first, gear second.

FAQ

Is “non-detect” the same as safe forever?

No. It means below that method’s detection limit on that day. Keep the annual baseline. Retest when land use, floods, plumbing, or taste/staining change.

Why does my report show different units than EPA’s table?

Labs pick mg/L or µg/L by method. Convert before comparing: multiply mg/L by 1,000 to get µg/L. If nitrate is reported “as NO₃” instead of “as N,” ask the lab for the as-N equivalent or use their conversion note.

Total coliform present but E. coli absent — now what?

Often a hygiene or well-integrity issue rather than confirmed fecal contamination. Retest carefully, inspect the wellhead, and follow your health department’s disinfection guidance. Don’t ignore repeat positives.

Can I use a softener brochure’s “ideal ranges” instead of EPA?

Prefer EPA/state health numbers for health decisions. Dealer “ideal” charts are sales tools; they can still help with hardness and iron for equipment sizing once health risks are handled.

Are WellPath listings paid?

Featured or referral placements are labelled when used. Listings are informational, not endorsements. Verify lab certification and dealer credentials yourself.

Next steps

Save this year’s PDF beside last year’s. If bacteria or nitrate need a clean retest, follow sampling without contaminating the bottle. If you’re still choosing analytes, see test panels: bacteria, nitrate, metals. Shopping treatment for hardness? See WQA dealers vs big-box softeners. Browse WellPath listings for labs and treatment pros, or see the guides index.

Disclosure: WellPath may earn lab-kit affiliate commissions or dealer lead fees. This page is general guidance, not medical or legal advice. Confirm analytes, units, and action levels with a state-certified lab and your local health department. EPA MCL/SMCL figures are public-system references used here as homeowner benchmarks.