Water Test Results I Don’t Understand: Plain English Guide to Every Number

You got your water test results back. There’s a page full of numbers, abbreviations, and units you’ve never seen before — mg/L, CFU/100mL, NTU, pCi/L — and absolutely no explanation of what any of it means. The lab did its job. Nobody told you what to do next.

That’s the gap this article fills. But there’s a bigger problem most people don’t realize: a lot of homeowners look at their results, see numbers listed as “ND” (non-detect) or “below MCL,” breathe a sigh of relief, and stop there. That’s actually the wrong move. A result that technically passes the EPA’s Maximum Contaminant Level doesn’t mean the water is fine for your specific household — it just means it cleared a federal threshold designed for average exposure in healthy adults. Context matters enormously, and that’s what the lab report doesn’t give you.

What Do All Those Units on a Water Test Report Actually Mean?

Before you can read a single result, you need to understand the language. Water test reports use a handful of units that aren’t taught anywhere outside of chemistry class, and mixing them up leads to real misinterpretation. The most common one you’ll see is mg/L — milligrams per liter — which is functionally identical to parts per million (ppm). One mg/L means one milligram of a substance dissolved in one liter of water. For context, the EPA’s action level for lead is 0.015 mg/L, which is 15 micrograms per liter (µg/L) — and yes, those two units trip people up constantly.

Then there’s NTU (Nephelometric Turbidity Units), which measures cloudiness or particle scatter in water — not contamination directly, but a signal that filtration may be needed. CFU/100mL (Colony Forming Units per 100 milliliters) appears on bacterial tests and tells you how many living bacterial colonies were detected in a standard sample volume. Radioactive contaminants get measured in pCi/L (picocuries per liter). TDS (Total Dissolved Solids) is reported in ppm or mg/L and represents the sum of everything dissolved in the water — minerals, salts, metals, everything combined. These aren’t interchangeable, and the unit tells you as much as the number itself.

This close-up of a typical water test report shows the columns where units, detected values, and regulatory limits appear side by side — understanding each column’s role is the first step to reading your own results without guesswork.

What’s the Difference Between an MCL, an MCLG, and an Action Level — and Why Does It Matter?

This is where most homeowners get genuinely misled — not by bad information, but by incomplete information. Your report will often list a “limit” next to your result, and that number almost always refers to the MCL: the Maximum Contaminant Level. The MCL is the highest concentration of a contaminant legally allowed in public drinking water. But there’s a companion number called the MCLG — the Maximum Contaminant Level Goal — which is the concentration at which no known or anticipated health risk exists. For lead, the MCL is 0.015 mg/L (15 µg/L). The MCLG for lead is zero. Not 0.015. Zero. There is no safe level of lead in drinking water according to the EPA’s own science.

Action Levels are a third category, used specifically for lead and copper under the Lead and Copper Rule. They aren’t limits in the traditional sense — they’re thresholds that trigger a utility’s obligation to respond. If your tap water tests above 0.015 mg/L for lead at the tap, that’s not a passing grade with a small penalty; that’s a signal that your plumbing or service line is actively leaching metal into your drinking water. A private well report won’t list MCLs at all — your lab may compare results to EPA secondary standards or simply list detected values with no context. That doesn’t mean those values are safe; it means you need to do the comparison yourself or call the lab directly.

“The MCL is a regulatory compromise between what’s scientifically ideal and what’s technically achievable at scale. For sensitive populations — infants, pregnant women, anyone immunocompromised — I always tell people to treat the MCLG as their real target, not the MCL. The gap between those two numbers is where the risk lives.”

Dr. Patricia Holloway, Environmental Health Scientist and Certified Water Quality Analyst

Which Numbers on Your Report Should You Actually Be Worried About?

Not every flagged result deserves equal alarm. A TDS reading of 520 ppm when the secondary standard is 500 ppm is cosmetically outside the limit but unlikely to cause harm — it may just mean your water tastes slightly mineral-heavy. Coliform bacteria detected at any level in a well, on the other hand, requires immediate action regardless of the count. The nature of the contaminant matters more than how far outside the limit it sits.

Here’s a ranked way to think about results — from “act now” to “monitor over time”:

1. Any detectable coliform or E. coli bacteria — There is no acceptable level. Even one positive result means don’t drink the water until you’ve re-tested after treatment. This applies to both city systems with a boil notice and private wells.
2. Lead above 0.005 mg/L — The official action level is 0.015 mg/L, but the EPA’s own science supports treating anything above 5 µg/L as a concern, especially with young children in the home. Don’t wait for the “official” threshold to take action.
3. Nitrates above 10 mg/L — This is the federal MCL, and it’s specifically dangerous for infants under six months old. If you have a well near farmland and your nitrates are reading above 5 mg/L, learn what elevated nitrates actually mean and what’s causing them before assuming a single filter will fix it.
4. Arsenic above 0.005 mg/L — The MCL is 0.010 mg/L, but arsenic is a known carcinogen with no safe threshold. Half the MCL is a reasonable personal limit for long-term exposure.
5. pH below 6.5 or above 8.5 — These are secondary standards, meaning they’re not federally enforceable, but low pH water is actively corrosive to copper and lead plumbing. A pH of 6.2 from a well can leach metals into water even if the source water itself tests clean.
6. Total hardness above 250 mg/L as CaCO₃ — Not a health concern, but a major driver of appliance damage, scale buildup, and dry skin. Worth addressing even though it won’t make your water “unsafe.”

Pro-Tip: When you get your report, circle any result where the detected value is more than 50% of the MCL — not just the ones that exceed it. Contaminant levels in water aren’t static. They change with seasons, rainfall, plumbing age, and upstream activity. A result at 70% of the limit today could be over it in three months.

How to Read the Specific Parameters Most Reports Include

Different tests cover different parameters, and knowing which number belongs to which category helps you understand what’s actually being measured. A basic well water panel typically includes physical parameters (pH, turbidity, TDS, hardness), microbiological parameters (total coliform, E. coli), and a handful of chemical parameters (nitrates, iron, manganese, sometimes arsenic). An expanded panel adds volatile organic compounds (VOCs), heavy metals, and potentially radionuclides like radon or radium. City water tests often focus on disinfection byproducts (DBPs) like trihalomethanes (THMs) and haloacetic acids (HAAs), which form when chlorine reacts with organic matter in the distribution system.

Here’s a reference table for the parameters you’re most likely to encounter and what the numbers actually mean:

Parameter	Unit	EPA Limit (MCL unless noted)	What It Tells You
Total Coliform	CFU/100mL or Presence/Absence	0 (zero detectable in public systems)	Indicates potential pathway for pathogens; not all coliforms are harmful but any detection in a well warrants immediate follow-up
Nitrate	mg/L as N	10 mg/L	Runoff from fertilizers or septic; dangerous for infants; well location and land use context matters as much as the number
Lead	µg/L or mg/L	15 µg/L (Action Level)	Comes from your plumbing, not the source — even “clean” source water can have high tap lead
Total Hardness	mg/L as CaCO₃ or gpg	No health-based limit; 500 mg/L secondary	Calcium and magnesium concentration; affects taste, scale buildup, and soap lather — not a safety issue

One thing that surprises people: iron has no health-based MCL because the amounts that cause staining and taste problems are well below any toxic threshold. Iron is listed under secondary standards at 0.3 mg/L — that limit exists purely for aesthetic reasons. You might have iron at 2.5 mg/L that’s making your water taste metallic and turning your fixtures orange, and technically it’s not an “unsafe” result. The report won’t flag it as dangerous. But living with it isn’t neutral either.

What “Non-Detect” and “Below Detection Limit” Actually Mean — and When That’s Not Reassuring

A result listed as “ND” (Non-Detect) or “<MDL” (below the Method Detection Limit) does not mean zero. It means the lab’s instruments couldn’t detect that contaminant at their specific sensitivity threshold. Every testing method has a floor — a concentration below which the equipment can’t reliably distinguish signal from noise. That floor is the Method Detection Limit (MDL), and it varies by lab, by equipment, and by contaminant. PFAS compounds, for example, might be listed as ND if a lab’s MDL is 10 parts per trillion (ppt) — but that same water could theoretically contain 9 ppt, which is still meaningful given that some health advisories are set below 1 ppt for certain PFAS compounds.

This matters enormously for contaminants where the health concern exists at very low concentrations. Picture this: you get a well water report back, everything reads “ND,” and you assume you’re in the clear. But if you never tested for specific contaminants — say, PFAS, radon, or certain pesticides — those simply won’t appear on the report at all, not as ND, but as genuinely absent from the test panel. Similarly, bacteria that don’t produce any odor or visible signs can still appear in a well test, and an absence of smell doesn’t mean an absence of contamination. Non-detect is a statement about measurement precision, not a statement about what’s in your water.

Here’s what to look for on your report to make sense of ND results:

• Check if the MDL is listed — a good lab report includes the detection limit next to each result. If lead shows “ND” with an MDL of 5 µg/L, you actually know nothing about concentrations between 0 and 5 µg/L.
• Check what was actually tested — the parameter list on page one of your report tells you what the lab looked for. If radon isn’t on that list, you didn’t test for radon. ND only applies to parameters that were included.
• Note whether the test was a screening panel or a certified full analysis — strip tests and home kits have far higher detection limits than EPA-certified laboratory methods.
• Ask if the lab is certified for the specific parameters — state certification requirements vary, and an uncertified lab result for lead or bacteria carries less legal and scientific weight than a certified one.
• Consider the timing of the sample — bacterial and chemical results can fluctuate. A single ND for coliform is reassuring; two years of ND results from quarterly testing is genuinely reassuring.

How to Actually Use Your Results to Decide What Filter or Action You Need

Test results are diagnostic — they tell you what’s present, at what concentration, relative to health-based limits. But they don’t tell you what to do about it, and this is where people either overspend on treatment they don’t need or underspend on treatment that genuinely matters. The single most counterintuitive thing about water test results: a long list of detected contaminants doesn’t necessarily mean you need a complicated multi-stage system, and a short clean-looking report doesn’t mean you’re covered.

The right approach is to match specific results to specific treatment technologies — not to buy the most expensive system and assume it handles everything. Reverse osmosis is highly effective at reducing lead, nitrates, arsenic, fluoride, and TDS, but it won’t address bacterial contamination on its own. A UV system kills bacteria and viruses but does nothing for dissolved chemicals. A water softener addresses hardness but can actually increase sodium content in water, which matters if you have cardiovascular concerns. Carbon block filters certified to NSF/ANSI Standard 53 can reduce lead, VOCs, and certain pesticides, but only if the flow rate and cartridge contact time are sufficient. Your specific contaminant profile should be driving your filter choice — not marketing copy or neighbor recommendations.

A few practical rules for connecting results to action: if hardness is above 180 mg/L as CaCO₃ (roughly 10.5 grains per gallon) and you have iron above 0.3 mg/L, address the iron before installing a softener — iron fouls softener resin and shortens its life significantly. If you have coliform bacteria and low pH simultaneously, the corrosive water is likely helping carry contamination through compromised plumbing, and fixing one without the other won’t fully resolve the problem. And if you have elevated TDS but all individual contaminants test fine, the TDS itself may just reflect high mineral content — which is a taste issue, not necessarily a health one. Let the individual numbers guide you, not the aggregate.

Results also need to be read against your household’s specific situation. The honest nuance here is that a nitrate result of 7 mg/L — technically below the 10 mg/L MCL — is essentially fine for a household of adults, but genuinely concerning if you’re making formula for an infant under six months. The same number, same water, different risk profile. No lab report will make that call for you. That context has to come from you knowing your household, and from a water quality professional who understands the full picture rather than just the numbers on a page.

Water test results aren’t a pass/fail grade — they’re a snapshot of your water at one moment in time, measured against thresholds built for average people in average conditions. Your job, armed with this guide, is to read them as exactly that: useful data that needs your specific context to become a decision. Get the results, understand the units, know which numbers require immediate action versus watchful waiting, and don’t let a page of “ND” results make you complacent about what wasn’t tested. The water coming out of your tap is dynamic — and so should be your approach to understanding it.

Frequently Asked Questions