On this page
What’s on a typical report
- Analyte: what was measured (e.g., nitrate, arsenic).
- Result: the amount found, often with a qualifier.
- Units: mg/L, µg/L, ng/L, pCi/L, or “present/absent” for bacteria.
- Reporting limit: the lowest amount the lab can reliably report. “ND” (not detected) means below that limit. It doesn’t necessarily mean zero.
- Reference level: many labs print the EPA standard next to each result. If yours doesn’t, use the table below.
Bacteria results: usually “present/absent,” or a count per 100 mL. Penn State Extension explains that “TNTC” (too numerous to count) generally means more than 200 colonies per 100 mL. “Confluent” means other bacteria overgrew the plate. Ask the lab whether you need to resample.
Units decoded
| Unit | Roughly equal to | Typically used for |
|---|---|---|
| mg/L (milligrams per liter) | ppm (parts per million) | Nitrate, iron, TDS, hardness |
| µg/L (micrograms per liter) | ppb (parts per billion) | Arsenic, lead, uranium |
| ng/L (nanograms per liter) | ppt (parts per trillion) | PFAS |
| pCi/L (picocuries per liter) | n/a (a measure of radioactivity) | Radon, radium, gross alpha |
1 mg/L = 1,000 µg/L. So EPA’s arsenic limit of 0.010 mg/L is the same as 10 µg/L, or 10 ppb.
Nitrate check: 10 mg/L of nitrate-nitrogen equals 45 mg/L of nitrate (Penn State Extension). Make sure you know which one your lab reported.
Primary vs secondary standards
- Primary standards (MCLs) are health-based limits that public water systems must meet (EPA). The MCLG is the health goal; for many contaminants it’s zero.
- Secondary standards (SMCLs) cover taste, odor, color and staining, and aren’t enforceable (EPA).
- Action levels (lead, copper) trigger corrective steps at public water systems.
- Health advisories are non-enforceable guidance levels (e.g., manganese).
None of these legally apply to private wells, but EPA points well owners to them as the reference for test results.
Reference table
Values from EPA primary and secondary standards, the EPA manganese health advisory, and EPA’s PFAS rule page. Checked October 11, 2026.
| Contaminant | EPA level | Type | More |
|---|---|---|---|
| Total coliform | Goal: zero | Primary | Coliform and E. coli in Well Water: What a Positive Test Means |
| Nitrate (as N) | 10 mg/L | Primary | Nitrates in Well Water: Health Risks, Limits and Fixes |
| Nitrite (as N) | 1 mg/L | Primary | |
| Arsenic | 0.010 mg/L | Primary | Arsenic in Well Water: Where It Comes From and How to Remove It |
| Fluoride | 4.0 mg/L | Primary | |
| Uranium | 30 µg/L | Primary | |
| Gross alpha | 15 pCi/L | Primary | |
| Radium 226 + 228 | 5 pCi/L | Primary | |
| Lead | Goal: zero; action level 0.015 mg/L (0.010 mg/L for public systems from Nov 1, 2027) | Action level | Lead in Well Water: Testing Your Pipes, Pump and Fixtures |
| Copper | 1.3 mg/L | Action level | |
| PFOA / PFOS | 4.0 ppt each (rule changes proposed May 2026) | Primary | PFAS in Private Wells: Testing and Treatment |
| Iron | 0.3 mg/L | Secondary | Hard Water, Iron and Manganese in Wells: Stains, Smells and Fixes |
| Manganese | 0.05 mg/L (secondary); 0.3 mg/L (health advisory) | Secondary / HA | |
| pH | 6.5–8.5 | Secondary | |
| Total dissolved solids | 500 mg/L | Secondary | |
| Chloride | 250 mg/L | Secondary | |
| Sulfate | 250 mg/L | Secondary | |
| Hardness | No EPA standard. USGS: 0–60 soft, 61–120 moderately hard, 121–180 hard, >180 very hard (mg/L as CaCO₃) | n/a | USGS |
| Radon (water) | No federal standard | n/a | Radon in Well Water: Should You Test? |
The 0.010 mg/L lead figure is from EPA’s 2024 Lead and Copper Rule Improvements.
A result is over the line: what now?
- Don’t buy equipment yet.
- For a health-based contaminant, CDC suggests considering bottled water or another safe source and contacting your local health or environmental department.
- Retest to confirm the result and concentration (EPA).
- Match treatment to the contaminant, and buy equipment certified for that specific contaminant. Well Water Treatment Options: UV vs RO vs Softener vs Whole-House Filter
- After installing treatment, test the treated water to confirm it works.
Need a confirmation test? Use a state-certified lab (often run by your county or state health department), or a mail-in panel from a reputable lab such as Tap Score: Essential well water test →.
Sources
- EPA, National Primary Drinking Water Regulations: https://www.epa.gov/ground-water-and-drinking-water/national-primary-drinking-water-regulations
- EPA, Secondary Drinking Water Standards: https://www.epa.gov/sdwa/secondary-drinking-water-standards-guidance-nuisance-chemicals
- EPA, Protect Your Home’s Water: https://www.epa.gov/privatewells/protect-your-homes-water
- EPA, PFAS NPDWR page (updated May 18, 2026): https://www.epa.gov/sdwa/and-polyfluoroalkyl-substances-pfas
- EPA, Manganese Health Advisory: https://www.epa.gov/sites/default/files/2014-09/documents/support_cc1_magnese_dwreport_0.pdf
- Federal Register, LCRI: https://www.federalregister.gov/documents/2024/10/30/2024-23549/national-primary-drinking-water-regulations-for-lead-and-copper-improvements-lcri
- CDC, Guidelines for Testing Well Water: https://www.cdc.gov/drinking-water/safety/guidelines-for-testing-well-water.html
- USGS, Hardness of Water: https://www.usgs.gov/water-science-school/science/hardness-water
- Penn State Extension, Coliform Bacteria: https://extension.psu.edu/coliform-bacteria
- Penn State Extension, Nitrates in Drinking Water: https://extension.psu.edu/nitrates-in-drinking-water
Private Well Water Testing: The Complete Guide for Rural Homeowners (2026)