Your water test report arrived. It is full of numbers, abbreviations, and units you may not recognize. This guide explains every line.
By the end, you will know what is safe, what needs action, and what to do next.
Quick Answer
Focus on 6 numbers first: coliform bacteriaColiform BacteriaA group of bacteria used as indicators of potential contamination. Total coliform presence suggests a pathway for pathogens to enter your well.EPA MCL: 0 (no coliform should be present)Learn more → (contamination indicator), nitratesNitratesCompounds that enter groundwater from fertilizers, septic systems, and animal waste. Especially dangerous for infants under 6 months (blue baby syndrome).EPA MCL: 10 mg/L (ppm)Learn more → (fertilizer compounds), pH, ironIronA naturally occurring mineral found in groundwater. Iron causes orange/rust staining on fixtures and laundry and gives water a metallic taste.EPA Secondary MCL: 0.3 mg/L (ppm)Learn more → (staining mineral), hardnessHardness (Calcium Carbonate)The concentration of dissolved calcium and magnesium in water. Hard water causes scale buildup in pipes, water heaters, and fixtures.No EPA standard. Soft: <1 gpg. Moderate: 1-7 gpg. Hard: 7-10.5 gpg. Very Hard: >10.5 gpg.Learn more → (dissolved calcium and magnesium), and TDSTDS (Total Dissolved Solids)The total amount of dissolved minerals, salts, and metals in water. A general indicator of water quality, though not all dissolved solids are harmful.EPA Secondary MCL: 500 mg/LLearn more → (total dissolved solids). These tell you 90% of what you need to know about your well water.
Step 1: Understand the Units on Your Report
Water test reports use specific units. Here are the ones you will see:
| Unit | What It Means | Used For |
|---|---|---|
| ppmPPM (Parts Per Million)A unit of concentration equivalent to milligrams per liter (mg/L). Used for most common contaminants like iron, manganese, and nitrates.Learn more → | Parts per million. Same as mg/L. | Iron, manganese, nitrates, fluoride, TDS |
| ppbPPB (Parts Per Billion)A unit of concentration equivalent to micrograms per liter (µg/L). Used for trace contaminants like arsenic, lead, and PFAS where even tiny amounts matter.Learn more → | Parts per billion. 1,000x smaller than ppm. | Arsenic, lead, PFAS |
| GPGGPG (Grains Per Gallon)A unit used specifically for water hardness. 1 gpg = 17.1 mg/L of calcium carbonate. Water softeners are sized based on gpg.Learn more → | Grains per gallon. 1 GPG = 17.1 mg/L. | Water hardness |
| mg/Lmg/L (Milligrams Per Liter)The standard unit for measuring contaminant concentration in water. Equivalent to parts per million (ppm). Most water test results report in mg/L.Learn more → | Milligrams per liter. Same as ppm. | Most contaminants |
| Present / Absent | Detected or not detected. | Bacteria (coliform, E. coli) |
| ND | Not Detected. Below the lab's detection limitDetection LimitThe lowest concentration a lab can reliably measure. Results below the detection limit are reported as 'ND' (not detected) or '<DL'.Learn more → (lowest measurable level). | Any contaminant |
Have your numbers? Enter them and get a plain-English diagnosis.
Analyze My Water TestStep 2: Check the 6 Most Important Numbers
1. Coliform Bacteria
What to look for: "Absent" or "Not Detected" = safe. "Present" = action needed.
Any coliformColiform BacteriaA group of bacteria used as indicators of potential contamination. Total coliform presence suggests a pathway for pathogens to enter your well.EPA MCL: 0 (no coliform should be present)Learn more → (contamination indicator bacteria) detection means a pathway exists for bacteria to enter your well. E. coliE. coliA specific type of coliform bacteria that indicates fecal contamination. Its presence in well water is a serious health concern requiring immediate action.EPA MCL: 0 (any detection requires action)Learn more → (fecal bacteria) is more serious — stop drinking the water immediately[1].
Full guide: Bacteria in Well Water
2. Nitrates
EPA limit: 10 ppmPPM (Parts Per Million)A unit of concentration equivalent to milligrams per liter (mg/L). Used for most common contaminants like iron, manganese, and nitrates.Learn more → (parts per million). Above this is unsafe, especially for infants[1].
Full guide: Nitrates in Well Water
3. pH
Safe range: 6.5–8.5. Below 6.5 means your water is acidic. It corrodes pipes and can leach leadLeadA toxic metal that typically enters water from corroded plumbing, solder, or fixtures rather than the well itself. No safe level of lead exposure.EPA Action Level: 15 ppb (0.015 mg/L)Learn more → (toxic metal) and copperCopperA metal that leaches into water from corroded copper pipes, especially in acidic (low pH) water. Causes blue-green stains on fixtures.EPA Action Level: 1.3 mg/LLearn more → (causes blue-green stains)[2].
4. Iron
EPA secondary standard: 0.3 ppm. Above this, you get orange stains and metallic taste. Not a health hazard — but it damages fixtures and clothing[2].
Full guide: Iron in Well Water
5. Hardness
No EPA limit. Measured in GPGGPG (Grains Per Gallon)A unit used specifically for water hardness. 1 gpg = 17.1 mg/L of calcium carbonate. Water softeners are sized based on gpg.Learn more → (grains per gallon). Above 7 GPG causes scale, soap scum, and appliance damage. Above 10.5 GPG is very hard[4].
Full guide: Hard Water from Wells
6. TDS (Total Dissolved Solids)
EPA secondary standard: 500 ppm. TDSTDS (Total Dissolved Solids)The total amount of dissolved minerals, salts, and metals in water. A general indicator of water quality, though not all dissolved solids are harmful.EPA Secondary MCL: 500 mg/LLearn more → (total dissolved solids) is a general water quality indicator. High TDS does not always mean unsafe — but it can affect taste[2].
Step 3: Check for Health Contaminants
If your test includes these, pay close attention. They are odorless and tasteless. Testing is the only way to find them.
| Contaminant | EPA Limit | If Above Limit |
|---|---|---|
| ArsenicArsenicA toxic element found naturally in bedrock that dissolves into groundwater. Odorless and tasteless, making testing the only way to detect it.EPA MCL: 10 ppb (0.01 mg/L)Learn more → | 10 ppb | Install RO. See guide |
| LeadLeadA toxic metal that typically enters water from corroded plumbing, solder, or fixtures rather than the well itself. No safe level of lead exposure.EPA Action Level: 15 ppb (0.015 mg/L)Learn more → | 15 ppb (action level) | Install RO. Check plumbing for lead solder. |
| NitratesNitratesCompounds that enter groundwater from fertilizers, septic systems, and animal waste. Especially dangerous for infants under 6 months (blue baby syndrome).EPA MCL: 10 mg/L (ppm)Learn more → | 10 ppm | Install RO. See guide |
| FluorideFluorideA naturally occurring mineral in groundwater. While small amounts support dental health, excessive levels can cause dental and skeletal fluorosis.EPA MCL: 4.0 mg/L. EPA Secondary MCL: 2.0 mg/LLearn more → | 4 ppm | Install RO or activated alumina. See guide |
| ManganeseManganeseA mineral that causes black or brown staining and can affect the taste of water. Often found alongside iron in well water.EPA Secondary MCL: 0.05 mg/L (ppm). EPA Health Advisory: 0.3 mg/LLearn more → | 0.3 ppm (health advisory) | Install oxidation filter. See guide |
Step 4: Understand Primary vs. Secondary Standards
Your report may reference two types of EPA standards:
- MCLMCL (Maximum Contaminant Level)The highest level of a contaminant allowed in public drinking water, set by the EPA. Private wells are not regulated by MCLs, but they serve as the best available safety benchmarks.Learn more → (Maximum Contaminant Level): Health-based limits. These are legally enforceable for public water systems. Private wells are not regulated — but use MCLs as your safety benchmark[1].
- SMCLSMCL (Secondary Maximum Contaminant Level)EPA guidelines for contaminants that affect taste, odor, or appearance but are not considered health hazards. Includes iron, manganese, and TDS.Learn more → (Secondary Maximum Contaminant Level): Aesthetic guidelines for taste, odor, and appearance. Not health-based. Iron (0.3 ppm) and TDS (500 ppm) are examples[2].
Exceeding an SMCL means annoying water, not dangerous water. Exceeding an MCL means take action.
Step 5: Decide What to Treat
Prioritize in this order:
- Health contaminants first: Bacteria, nitrates, arsenic, lead. These need immediate treatment.
- Corrosion second: Low pH corrodes pipes and leaches metals. Fix this early.
- Aesthetic issues third: Iron, hardness, sulfur, sediment. Treat for quality of life and to protect appliances.
When to Retest
- Bacteria and nitrates: At least once per year[3].
- Everything else: Every 3–5 years, or sooner if taste/smell/appearance changes.
- After flooding, well repair, or nearby construction: Test immediately.
- New baby in the home: Test for nitrates, lead, arsenic, and bacteria.
What to Do Next
Have your numbers? Enter them in our Water Test Interpreter. It translates your results into a plain-English treatment plan.
Need a test? Contact your state-certified lab or order a mail-in kit from Tap Score ($100–$200 for a full panel).
Know your problem? Jump to the specific guide: Iron | Sulfur | Hard Water | Arsenic | Bacteria | Nitrates
Frequently Asked Questions
What should I test my well water for?
At minimum: coliform bacteria, nitrates, pH, iron, hardness, and TDS. Also test for arsenic, lead, and manganese if your area has known contamination[3].
What does MCL mean on a water test?
MCLMCL (Maximum Contaminant Level)The highest level of a contaminant allowed in public drinking water, set by the EPA. Private wells are not regulated by MCLs, but they serve as the best available safety benchmarks.Learn more → (Maximum Contaminant Level) is the EPA's legal safety limit. Private wells are not regulated by MCLs, but use them as your benchmark[1].
What does ND mean on a water test?
ND means Not Detected. The lab did not find that contaminant above their detection limit. This is good news.
How often should I test my well water?
Bacteria and nitrates: at least once per year. Everything else: every 3–5 years. Also test after flooding, well repairs, or changes in taste or smell[3].
References
- U.S. EPA. "National Primary Drinking Water Regulations." 40 CFR Part 141, 2024. https://www.epa.gov/sdwa
- U.S. EPA. "National Secondary Drinking Water Regulations." 40 CFR Part 143, 2024. https://www.epa.gov/sdwa/secondary-drinking-water-standards-guidance-nuisance-chemicals
- CDC. "Private Ground Water Wells: Testing." 2024. https://www.cdc.gov/healthywater/drinking/private
- Water Quality Association. "Understanding Your Water Test Results." Consumer Guide, 2024. https://wqa.org
- Penn State Extension. "Interpreting Water Test Results." College of Agricultural Sciences, 2023.
- Virginia Cooperative Extension. "Understanding Your Water Analysis Report." 2022.
- Minnesota Department of Health. "Reading Your Well Water Test." Well Management, 2023.
- U.S. EPA. "Drinking Water Contaminant Candidate List." 2024. https://www.epa.gov/ccl