You just opened the PDF from the lab, and it’s a wall of abbreviations, decimal points, and columns with headers like “MCL” and “CFU/100mL” that mean nothing to you. Somewhere in there is the answer to the only question you actually care about: is this water safe to drink. If your well water test results have you more confused than reassured, you’re not alone, and this guide walks through exactly what every line means.
We’ll cover the units labs use, the difference between a legal limit and a nuisance guideline, and what the most common results, bacteria, nitrates, arsenic, iron, and the rest, actually tell you about your water. If a number on your report doesn’t add up or you just want someone to talk you through it, reach out for a free estimate and we’ll go through the report with you, no charge for the conversation.
The Units on Your Well Water Test Results: mg/L, ppm, and CFU/100mL
Labs don’t use one universal unit, and that trips people up before they even get to the actual numbers. Here’s what you’re looking at.
mg/L (milligrams per liter) is the standard unit for most chemical results: nitrate, arsenic, lead, iron, manganese, and total dissolved solids. It’s a measure of mass per volume of water.
ppm (parts per million) shows up on some reports, especially for hardness or general mineral content. For water, mg/L and ppm are essentially the same number, because a liter of water weighs almost exactly one kilogram. A result of 8 mg/L nitrate and 8 ppm nitrate are the same result. Don’t let the different label make you think you’re looking at two different problems.
ppb (parts per billion), sometimes written as µg/L, shows up for contaminants that matter at much smaller concentrations, most often arsenic and lead. One ppb is a thousand times smaller than one ppm, so 10 ppb arsenic is a tiny fraction of 10 ppm. Mixing these two up is the single most common misread we see on a lab report.
CFU/100mL (colony-forming units per 100 milliliters) is the unit for bacteria results. Instead of a mass, it’s a count of live bacteria colonies that grew from your sample. Many Washington labs simplify total coliform to a plain “Present” or “Absent” instead of a number, since for drinking water, any detection at all is the meaningful result, not the exact count.
One more thing worth knowing: nitrate sometimes gets reported two ways, as “nitrate” (NO3) or as “nitrate-nitrogen” (NO3-N). The nitrate-N number is about 4.4 times smaller than the plain nitrate number for the same water, because it’s only counting the nitrogen atom’s weight. Check which one your lab used before you compare your result to the 10 mg/L limit, since that limit is written as nitrate-N.
MCL vs. Secondary Standards: Not All Limits Mean the Same Thing
This is the distinction that confuses more well owners than anything else on the report, and it changes how worried you should actually be.
MCL stands for Maximum Contaminant Level. These are legally enforceable, health-based limits the EPA sets for public water systems. Your private well isn’t legally required to meet them, nobody’s checking, but they’re still the correct yardstick to judge your own health risk against. Bacteria, nitrate, arsenic, and lead all have MCLs, and going over one is a real health concern, not a cosmetic one.
Secondary standards, sometimes labeled SMCL, are non-enforceable guidelines for things that affect taste, smell, color, or your plumbing, not your health. Iron, manganese, hardness, pH, and total dissolved solids fall here. A high result on one of these means stained sinks, chalky buildup, or water that tastes a little off. It does not mean you’re in danger.
That distinction is exactly why two people can look at a lab report with several “high” flags and need completely different responses. Someone with high iron and hardness needs a filtration conversation. Someone with elevated nitrate or arsenic needs to stop drinking the water until it’s addressed. The EPA keeps the full official list of primary and secondary standards at epa.gov, and it’s worth five minutes to see exactly where your results land.
Anne in Chelan found this out during a home sale last spring. Her lender-required panel came back with four flags: hardness, iron, pH, and arsenic. She assumed all four meant the same level of trouble and nearly walked away from the purchase. A quick call sorted it out: the hardness, iron, and pH were secondary, cosmetic issues any filtration company could fix. The arsenic result, 14 ppb against a 10 ppb MCL, was the only one that actually mattered for her health. She closed on the house and had a whole-house arsenic adsorption system installed for $2,400, which brought her water to under 3 ppb on the retest.
Reading Your Bacteria Results: Coliform and E. coli
Bacteria is almost always the first line on the report, and it’s the simplest to read correctly, though it’s also the one people panic over unnecessarily.
Total coliform bacteria is an indicator organism. On its own, it’s not usually dangerous, but its presence means something, surface water, soil, or an insect, is finding a way into your well that shouldn’t be there. The MCL is zero. “Present” is a fail, full stop, even if the number attached to it is low.
E. coli is a specific type of coliform that comes from animal or human waste. Any detection of E. coli, even a single colony, is a stop-drinking-the-water-now result until it’s resolved. This is different from a nuisance flag; it’s a real, immediate health signal.
A positive bacteria result almost always traces back to a physical entry point: a cracked or unsealed well cap, a corroded casing seam below grade, or spring runoff pooling around a low wellhead. Fixing the entry point and shock chlorinating the well resolves the vast majority of cases within a week or two.
Roy in Moses Lake had six years of clean tests before a wet spring changed that. His April sample came back “Present” for total coliform, no E. coli, after snowmelt pooled around a wellhead sitting a few inches below grade. A well cap reseal and shock chlorination ran him $265, and his retest ten days later came back clean. If your bacteria result comes back positive, our guide to shock chlorinating a well walks through what that process actually involves, and our full well water testing article covers how often to retest going forward.
Reading Your Chemistry Results: Nitrates, Arsenic, Lead, and the Nuisance Minerals
Once you’re past bacteria, the rest of the report is a list of chemical concentrations, each measured against either an MCL or a secondary standard.
Nitrate (as N): MCL is 10 mg/L. This is especially serious for infants under six months and pregnant women, since high nitrate can interfere with how a baby’s blood carries oxygen. There’s no taste or smell warning, so the number on the page is the only signal you get. Our nitrates in well water article covers where nitrate comes from locally and how treatment works.
Arsenic: MCL is 10 ppb (0.010 mg/L). Chelan and Douglas counties have both a legacy source, decades of lead arsenate pesticide use on old orchard ground, and a natural one, arsenic occurring in local bedrock. Neither taste nor color gives it away, which is why periodic testing matters even on wells that have always looked and tasted fine.
Lead: Regulated through an action level of 15 ppb rather than a traditional MCL, but the practical read is the same, you want to be under it. Lead in a well sample usually comes from old plumbing, solder, or fixtures rather than the aquifer itself, especially in acidic water that corrodes metal over time.
Iron and manganese: Secondary standards of 0.3 mg/L and 0.05 mg/L. Both are extremely common in valley wells and cause reddish-brown or black staining on fixtures, laundry, and tile. Manganese also has a separate, higher health advisory level for infant formula mixing, so a high result is worth a second look if there’s a baby in the house, even though it’s not a primary MCL violation.
Hardness, pH, and TDS: None of these have a formal health-based limit. Hardness above roughly 180 mg/L (about 10.5 grains per gallon) is considered very hard and accelerates scale buildup in pipes, water heaters, and pressure tanks. A pH under 6.5 is acidic and can slowly corrode copper plumbing and pull lead from old solder. TDS above 500 mg/L often shows up as a flat or mineral taste.
Teresa in Leavenworth ran her first-ever full panel after nine years on the same well and got back a nitrate-N result of 8.5 mg/L, close enough to the 10 mg/L limit that she wanted it handled immediately. An under-sink reverse osmosis system for the kitchen tap solved the drinking water problem for about $850 installed, and she now retests annually every spring instead of waiting another decade.
Quick Reference: What Common Well Water Test Results Mean
| Result | Reported As | Standard Type | Limit | What It Means |
|---|---|---|---|---|
| Total coliform | Present/Absent | Primary (MCL) | 0 (absent) | Any detection means retest and find the entry point |
| E. coli | Present/Absent | Primary (MCL) | 0 (absent) | Any detection means stop drinking until resolved |
| Nitrate (as N) | mg/L | Primary (MCL) | 10 mg/L | Above this is a real health risk, especially for infants |
| Arsenic | ppb (µg/L) | Primary (MCL) | 10 ppb | Long-term exposure concern above this level |
| Lead | ppb (µg/L) | Action level | 15 ppb | Corrosion issue, usually from plumbing, not the aquifer |
| Iron | mg/L | Secondary (SMCL) | 0.3 mg/L | Staining and taste, not a health limit |
| Manganese | mg/L | Secondary (SMCL) | 0.05 mg/L | Staining; separate infant health advisory at higher levels |
| Hardness | mg/L or GPG | No formal standard | 180+ mg/L is hard | Scale buildup, not a health concern |
| pH | Standard units | Secondary (SMCL) | 6.5-8.5 | Outside range can corrode pipes or taste off |
| TDS | mg/L | Secondary (SMCL) | 500 mg/L | Taste and clarity threshold, not health-based |
Your Well Water Test Results Came Back High. Now What?
A flagged result feels urgent, but the right next step depends entirely on which line is flagged.
If it’s bacteria, retest first. A single dirty hose bib or a touched bottle cap causes false positives more often than people expect. If the retest confirms it, look for the physical entry point, fix it, then shock chlorinate.
If it’s nitrate or arsenic, don’t boil the water. Boiling concentrates both instead of removing them. Reverse osmosis handles nitrate at the tap, and arsenic typically needs RO or an adsorption media system, whole-house or point-of-use depending on the level and how many taps you want protected.
If it’s iron, manganese, hardness, or pH, there’s no health emergency, but there is an equipment cost hiding downstream. Iron-heavy, acidic, or very hard water eats through pressure switches, check valves, and tank fittings years before they should fail. If your pressure tank is already short cycling or waterlogged on top of a water chemistry issue, that’s worth addressing together rather than one at a time.
Sediment or grit showing up in a sample is its own category. It’s not usually a chemistry problem at all, more often a sign of a worn pump screen or a failing pump pulling material up from the bottom of the well. That grit then damages the pump itself, so if your water turned gritty out of nowhere, get the well pump inspected before the abrasion shortens its life further.
Testing costs stay modest whichever direction you go. A basic bacteria and nitrate combo typically runs $50 to $90, and a fuller panel covering arsenic, lead, iron, hardness, and pH lands in the $150 to $400 range depending on the lab. Compare that to what a single missed contamination issue or a prematurely failed pressure tank costs, and retesting on a schedule is the cheap half of well ownership.
None of this is something you need to sort out alone. If your report has a result you can’t make sense of, call us at (509) 300-5151 and we’ll go through it with you and tell you plainly whether it’s a filtration conversation or a call-your-doctor conversation. Anything involving wellhead electrical work or down-hole repairs is licensed-professional territory, always, no exceptions.
Frequently Asked Questions
Is a well water test result in mg/L the same as ppm?
For water, yes, they’re functionally identical, since a liter of water weighs almost exactly one kilogram. A nitrate result of 8 mg/L and 8 ppm describe the same concentration. The units that do matter to double-check are ppb versus ppm, since ppb is a thousand times smaller and shows up on arsenic and lead results.
What’s the difference between an MCL and a secondary standard on my report?
An MCL, Maximum Contaminant Level, is a legally enforceable, health-based limit for things like bacteria, nitrate, arsenic, and lead. A secondary standard is a non-enforceable guideline for aesthetic issues like iron, hardness, and pH that affect taste or plumbing but not health. Knowing which type a flagged result is tells you whether it’s a health issue or a nuisance issue.
My total coliform result says “Present.” Does that mean my water made someone sick?
Not necessarily, but it does mean you should stop drinking the water and act on it right away. “Present” means bacteria that shouldn’t be in a sealed well got in somehow, usually through a cracked cap or a casing issue, and it needs a retest plus a fix at the entry point. It’s different from an E. coli detection, which is a more direct indicator of contamination.
Do I need a professional to interpret my well water test results, or can I do it myself?
Simple flags like hardness or iron are easy to read on your own once you know they’re secondary standards, not health limits. For anything near or over an MCL, bacteria, nitrate, arsenic, or lead, it’s worth a second opinion, either from the lab, the Washington Department of Health, or a local well service that sees these reports regularly. Call (509) 300-5151 any time and we’ll walk through a report with you at no charge.