How to Read Well Water Test Results (With Every Limit Explained)
A lab report is a list of numbers, units and abbreviations. Here's how to read it, which results are health issues and which are nuisances, and what to do about each.
Quick answer
Deal with health results first: any E. coli, nitrate over 10 mg/L, arsenic over 0.010, lead over 0.015.
Nuisance results (iron, manganese stains, hardness, pH) affect taste, staining and plumbing. Compare each
number to the limit below, check the units, and fix health issues before nuisances.
First, check the units
| Unit on report | Same as | Example |
|---|---|---|
| mg/L | ppm (parts per million) | Iron 0.8 mg/L = 0.8 ppm |
| µg/L (ug/L) | ppb (parts per billion) | Arsenic 12 µg/L = 0.012 mg/L |
| gpg | grains per gallon (hardness) | 171 mg/L ÷ 17.1 = 10 gpg |
| ND, <RL, <0.005 | Not detected / below reporting limit | Usually good news |
| P / A, Present / Absent | Bacteria result | "Present" for E. coli = act now |
The most common mistake is mixing up µg/L and mg/L. An arsenic result of "8" is fine in µg/L (0.008 mg/L) but would be terrible in mg/L. Check the unit column for every line.
Every common result, with its limit
"Health" limits are EPA maximum contaminant levels or action levels. "Nuisance" limits are EPA secondary standards for taste, color and staining.
| Result | Limit | Type | If yours is over |
|---|---|---|---|
| Total coliform bacteria | Absent Indicator organism (CDC yearly test) | Warning sign | A path exists for germs to reach your water. Not usually harmful itself, but a warning sign. Find the entry point, shock chlorinate, retest |
| E. coli | Absent Indicates fecal contamination | Health | Fecal contamination. Do not drink unless boiled; contact your health department. Stop drinking, find the source, disinfect, retest |
| Nitrate (as N) NO3-N | 10 mg/L EPA maximum contaminant level | Health | Health risk, especially for babies under 6 months. Often from fertilizer, manure or septic systems. Use other water for infants; reverse osmosis or ion exchange; find the source |
| Nitrite (as N) | 1 mg/L EPA maximum contaminant level | Health | Health risk, especially for infants. Usually points to recent contamination. Use other water; find the source; retest |
| Arsenic | 0.010 mg/L (10 µg/L) EPA maximum contaminant level | Health | Long-term exposure raises cancer and other health risks. Natural in some rock. Certified arsenic treatment (often reverse osmosis or adsorptive media); speciation test helps |
| Lead | 0.015 mg/L (15 µg/L) action level EPA action level; health goal is zero | Health | Usually from plumbing, made worse by corrosive water. No safe level. Flush taps, use certified lead filter, correct pH, replace lead parts |
| Copper | 1.3 mg/L action level EPA action level | Health | Usually from corroding copper pipes. Blue-green stains, metallic taste. Raise pH with an acid neutralizer |
| Uranium | 0.030 mg/L (30 µg/L) EPA maximum contaminant level | Health | Natural in some granite and other rock. Kidney effects with long-term exposure. Certified treatment such as reverse osmosis or anion exchange |
| Fluoride | 4.0 mg/L (secondary 2.0) EPA maximum contaminant level and secondary standard | Health | Above 2.0 can cause tooth mottling in children; above 4.0 is a health limit. Reverse osmosis or activated alumina at the kitchen tap |
| Manganese | 0.05 mg/L (health advisory 0.3) EPA secondary standard and health advisory | Nuisance | Black stains and bitter taste above 0.05. Above 0.3 it is also a health concern. Oxidizing filter, often with pH correction |
| Iron | 0.3 mg/L EPA secondary standard | Nuisance | Orange stains, metallic taste. A nuisance, not a health limit. Sediment filter, softener or oxidizing iron filter depending on level |
| pH | 6.5 – 8.5 EPA secondary standard | Nuisance | Below 6.5: corrosive, can dissolve copper and lead. Above 8.5: slippery feel, scale, bitter taste. Low pH: acid neutralizer |
| Total dissolved solids TDS | 500 mg/L EPA secondary standard | Nuisance | General mineral content. High TDS can taste salty or bitter and cause scale. Find which minerals are high; reverse osmosis for drinking water |
| Sulfate | 250 mg/L EPA secondary standard | Nuisance | Salty or bitter taste; can have a laxative effect. Feeds rotten-egg smell in water heaters. Reverse osmosis or anion exchange; water heater anode swap for odor |
| Chloride | 250 mg/L EPA secondary standard | Nuisance | Salty taste; corrosive to plumbing. Can come from road salt or saltwater intrusion. Reverse osmosis for drinking water; investigate source |
| Zinc | 5 mg/L EPA secondary standard | Nuisance | Metallic taste. Often from galvanized pipes. Check galvanized plumbing; correct pH |
| Sodium | 20 mg/L (low-sodium diets only) EPA drinking water advisory | Info | Only relevant to people on a doctor-prescribed low-sodium diet. Softeners raise sodium. Unsoftened kitchen tap or reverse osmosis for drinking |
| Hardness as CaCO₃ | Treatment usually optional below 120 mg/L (7 gpg) Extension guidance (no EPA limit) | Info | Scale and soap problems. Not a health issue. Water softener if scale bothers you |
What to do, in order
- E. coli present, or nitrate or nitrite over the limit: stop drinking it now, use bottled water (always for infant formula), and call your health department.
- Arsenic, lead, uranium or fluoride over the limit: use bottled water or a certified point-of-use filter for drinking and cooking while you plan permanent treatment.
- Total coliform present (no E. coli): find how it's getting in, shock chlorinate, and retest.
- Low pH: fix it early, because corrosive water can raise lead and copper. See corrosive water.
- Nuisances (iron, manganese, hardness, sulfur): choose treatment from the exact numbers. Start with the symptom guides for iron, manganese and hardness.
Faster way: type the numbers from your lab report into our free
well water test results interpreter. It shows what passes, what fails,
and the usual order of treatment.
Frequently asked questions
What is the difference between mg/L and ppm?
In water testing they are the same: 1 mg/L equals 1 part per million (ppm). 1 µg/L equals 1 part per billion (ppb), and 1,000 µg/L equals 1 mg/L.
My lab report says “ND” or “<0.005”. What does that mean?
ND means not detected. A “less than” value means the amount was below the lab’s reporting limit, the lowest it can reliably measure. Both are usually good news, as long as the reporting limit is below the standard.
Are EPA limits legally required for private wells?
No. EPA standards apply to public water systems. They are still the best yardstick for private well owners, and most labs compare your results to them.
What is the difference between primary and secondary standards?
Primary standards (maximum contaminant levels) protect health. Secondary standards cover nuisances like taste, color, odor and staining, and are not health limits.
Sources
- National Primary Drinking Water Regulations, U.S. EPA
- Secondary Drinking Water Standards: Guidance for Nuisance Chemicals, U.S. EPA
- Basic Information about Lead in Drinking Water, U.S. EPA
- Drinking Water Advisory: Sodium, U.S. EPA
- Guidelines for Testing Well Water, U.S. Centers for Disease Control and Prevention
- Iron and Manganese in Private Water Systems, Penn State Extension
- Water Softening (NebGuide G1274), University of Nebraska–Lincoln Extension
- Private Drinking Water Wells, U.S. EPA
Sources checked October 24, 2026. Found an error? Tell us and we will fix it.