Updated 30 September 2026
Test every private well for four contaminants: coliform bacteria, nitrate (limit 10 mg/L as nitrogen), pH and total dissolved solids. Everything else depends on your geology, your neighbors and your plumbing. Federal numbers come in three kinds that are easy to confuse: enforceable health limits (MCLs), non-enforceable aesthetic guidelines (SMCLs) and treatment-technique action levels. The table below labels which kind applies to every contaminant, and says so explicitly where no federal number exists.
Test every private well for coliform bacteria, nitrate, pH and total dissolved solids, because the about 43 million Americans who draw their drinking water from a private well are not covered by the Safe Drinking Water Act. The limits the EPA sets apply to public utilities. For a private well they are reference points, not requirements, and nobody is checking your water against them except you.
That leaves well owners with a practical problem. A laboratory report comes back with fifteen numbers and no verdict. The Centers for Disease Control and Prevention recommends annual testing for coliform bacteria, nitrate, total dissolved solids and pH, but a result only means something once you know which number is a health limit, which is a taste-and-staining guideline, and which has no federal number at all. This page is that translation layer.
The Three Kinds of Federal Number, and Why the Difference Matters
Most confusion about well water test results comes from treating every published number as though it means the same thing. It does not.
A Maximum Contaminant Level (MCL) is an enforceable health-based standard. It is the highest concentration of a contaminant allowed in water delivered by a public system, set as close to the health goal as treatment and cost allow. Arsenic at 0.010 mg/L is an MCL. Exceeding it in a public system triggers legal consequences.
A Secondary Maximum Contaminant Level (SMCL) is not enforceable and is not about health. The EPA states plainly that it “does not enforce these secondary maximum contaminant levels” and that they exist “as guidelines to assist public water systems in managing their drinking water for aesthetic considerations.” Iron at 0.3 mg/L is an SMCL. Water above it stains laundry and tastes metallic. It does not make you ill.
An action level is different again. Lead and copper have no MCL. They are governed by a treatment technique: if more than ten percent of sampled taps exceed the action level, the utility must act. For a private well the action level is still the most useful reference point available, but it was written for system-wide sampling, not for a single kitchen tap.
An iron result of 1.2 mg/L is four times the federal guideline and is a laundry and plumbing problem. An arsenic result of 0.012 mg/L is only slightly above its federal limit and is a health problem. The size of the exceedance tells you nothing on its own. The kind of limit does.
Well Water Contaminant Reference Table
Limits below are taken from the EPA’s National Primary and Secondary Drinking Water Regulations, checked against the published tables in August 2026. Concentrations are in milligrams per liter (mg/L), which is equivalent to parts per million, unless stated otherwise.
| Contaminant | Where it comes from in a well | Federal number | Kind of limit | What an elevated result means | Common treatment |
|---|---|---|---|---|---|
| Total coliform / E. coli | Surface water entering through a damaged casing, cap or nearby septic system | 0 (MCLG) | Health, enforceable for utilities | A pathway exists between the surface and your water. E. coli specifically indicates fecal contamination | Find and fix the pathway, then shock chlorinate and retest. Continuous UV or chlorination if it recurs |
| Nitrate (as nitrogen) | Fertilizer, manure, septic effluent | 10 | Health (MCL) | Serious risk to infants under six months. Do not boil, which concentrates it | Reverse osmosis, ion exchange or distillation |
| Nitrite (as nitrogen) | Same sources as nitrate, usually alongside it | 1 | Health (MCL) | Same infant risk as nitrate, at a tenth of the concentration | Reverse osmosis or ion exchange |
| Arsenic | Natural bedrock, especially in the Northeast, upper Midwest and Southwest | 0.010 | Health (MCL) | Long-term exposure risk. Tasteless, odorless and invisible | Point-of-use reverse osmosis, activated alumina or iron-oxide adsorption |
| Lead | Almost always the plumbing, not the aquifer: solder, brass fittings, older pumps | 0.010 | Action level (treatment technique) | No safe level for children. Sample first-draw water to find plumbing sources | Replace the source fitting. Point-of-use reverse osmosis or certified lead filter |
| Copper | Corrosion of copper pipe, accelerated by low pH | 1.3 | Action level (treatment technique) | Blue-green staining and a metallic taste. Often a symptom of corrosive water | Raise pH with a neutralizing filter, which treats the cause |
| Uranium | Granite and other natural bedrock | 0.030 | Health (MCL) | Kidney risk from long-term exposure, separate from its radioactivity | Reverse osmosis or anion exchange |
| Radium 226 + 228 | Natural radioactive decay in bedrock aquifers | 5 pCi/L | Health (MCL) | Long-term cancer risk. Common in specific regional aquifers | Ion exchange, reverse osmosis or a water softener in some cases |
| Gross alpha particles | Natural radioactivity, a screening measure | 15 pCi/L | Health (MCL) | A screen rather than a diagnosis. An exceedance means test for radium and uranium specifically | Depends entirely on which isotope is responsible |
| Fluoride | Natural mineral deposits | 4.0 | Health (MCL) | Skeletal effects above the MCL. A separate 2.0 SMCL exists for tooth discoloration in children | Reverse osmosis, activated alumina or distillation |
| Barium | Natural mineral deposits, some drilling activity | 2 | Health (MCL) | Blood pressure effects at sustained high exposure | Reverse osmosis, ion exchange or lime softening |
| Cadmium | Corroded galvanized pipe, industrial and mining runoff | 0.005 | Health (MCL) | Kidney risk from long-term exposure | Reverse osmosis or ion exchange |
| Chromium (total) | Natural deposits, plating and industrial discharge | 0.1 | Health (MCL) | Total chromium only. Hexavalent chromium has no separate federal MCL | Reverse osmosis or ion exchange |
| Selenium | Natural deposits, irrigation drainage in western states | 0.05 | Health (MCL) | Circulatory and nervous system effects at high exposure | Reverse osmosis, activated alumina or distillation |
| Antimony | Solder, some electronics and ceramics manufacturing | 0.006 | Health (MCL) | Cholesterol and blood sugar effects at long-term high exposure | Reverse osmosis or distillation |
| Benzene | Leaking fuel storage tanks, industrial discharge | 0.005 | Health (MCL) | Test if a fuel tank, refuelling area or industrial site sits upgradient | Granular activated carbon |
| PFOA and PFOS | Firefighting foam, landfills, industrial sites, biosolids on farmland | 4.0 ng/L | Health (MCL) | Measured in parts per trillion, a thousand times smaller than the units above | Granular activated carbon, reverse osmosis or anion exchange |
| PFHxS, PFNA, HFPO-DA (GenX) | Same industrial sources as PFOA and PFOS | 10 ng/L | Health (MCL) | A mixture of several PFAS is also regulated by a combined hazard index | Granular activated carbon, reverse osmosis or anion exchange |
| Iron | Natural bedrock and soil, extremely common in well water | 0.3 | Aesthetic (SMCL, not enforceable) | Orange-brown staining and a metallic taste. Not a health limit | Oxidation and filtration, air injection or a softener at low concentrations |
| Manganese | Natural bedrock and soil, usually alongside iron | 0.05 | Aesthetic (SMCL, not enforceable) | Black staining. Some health agencies publish separate advisory levels for infants | Oxidising filter, often the same unit that treats iron |
| Sulfate | Natural mineral deposits | 250 | Aesthetic (SMCL, not enforceable) | Bitter taste and a laxative effect on people not used to it | Reverse osmosis, ion exchange or distillation |
| Total dissolved solids | Everything dissolved in the water combined | 500 | Aesthetic (SMCL, not enforceable) | A general indicator. A sharp change between tests matters more than the number itself | Reverse osmosis, if treatment is wanted at all |
| Chloride | Road salt, seawater intrusion, natural deposits | 250 | Aesthetic (SMCL, not enforceable) | Salty taste and accelerated corrosion of plumbing | Reverse osmosis or distillation |
| pH | Natural water chemistry | 6.5 to 8.5 | Aesthetic (SMCL, not enforceable) | Below 6.5 the water is corrosive and dissolves lead and copper out of your pipes | Neutralising filter or soda ash injection |
| Hardness | Dissolved calcium and magnesium from limestone | None | No federal limit of any kind | Scale, poor lathering and shortened appliance life. Not a health concern | Ion exchange water softener |
| Hydrogen sulfide | Sulfur bacteria or natural sulfur in the aquifer | None | No federal limit (odor SMCL is 3 TON) | Rotten egg smell. At high concentrations it is corrosive and dangerous in enclosed spaces | Aeration, oxidizing filter or chlorination with carbon filtration |
The distinction above is not our interpretation. The EPA publishes primary and secondary standards as separate regulations, and states directly that secondary standards are guidelines it does not enforce.
Source: EPA, National Primary Drinking Water Regulations and Secondary Drinking Water Standards, tables checked August 2026
What Every Private Well Should Be Tested For
Four tests apply to every private well regardless of where it sits. Coliform bacteria, because it detects a physical pathway between the surface and your water. Nitrate, because it is the one contaminant with an acute risk rather than a long-term one, and because it signals agricultural or septic influence. pH, because corrosive water manufactures its own lead and copper problem inside your plumbing. Total dissolved solids, because a large shift between annual tests is an early warning that something about your aquifer has changed.
Where a contaminant enters the system in the first place often traces back to how the well itself is built. Our how does well water work explainer covers the casing, screen, and cap that are supposed to keep contamination out.
Those four are the baseline that the CDC recommends annually. They are also cheap, and most certified laboratories sell them as a single package.
What to Add Based on Where Your Well Is
The rest of the table is conditional. Adding every test on it to every sample would cost several hundred dollars and tell most owners nothing.
- Arsenic, uranium and radionuclides. Add these if your well draws from bedrock, particularly granite. State geological surveys publish regional maps, and a single test settles the question for years because bedrock chemistry changes slowly.
- Nitrate at a higher frequency, plus pesticides. Add these if row crops, orchards or intensive livestock sit within a few hundred meters, especially upgradient of your well.
- Volatile organic compounds and fuel components. Add these if a fuel storage tank, filling station, dry cleaner or industrial site is nearby, or if the water has a chemical or solvent smell.
- PFAS. Add these if a military installation, airport, firefighting training ground, landfill or paper mill is in the area, or if biosolids have been spread on nearby farmland.
- Lead and copper. Add these if the house predates 1990, has brass fittings or copper plumbing, or if the pH result came back below 6.5. Sample the first water drawn in the morning, because that is the water that has been sitting against the metal.
- Chloride and sodium. Add these if you are near a coast, a heavily salted road, or an area with known seawater intrusion.
How Often to Retest
Annually for the bacteria and nitrate baseline. Every three to five years for the geological contaminants, which move slowly. Immediately, regardless of schedule, after any of the following: flooding of the wellhead, work on the well or pump, a change in taste, smell or color, a nearby spill, or an unexplained gastrointestinal illness in the household. Also test when buying a property, before a newborn arrives, and after any well repair that opened the system to the air.
Do not boil water to deal with a nitrate result. Boiling evaporates water and leaves the nitrate behind, so it raises the concentration rather than lowering it. Infants under six months should not drink water above 10 mg/L nitrate as nitrogen, and bottled water is the correct short-term answer while treatment is arranged.
Detection Limits Are Not Regulatory Limits
A laboratory report often shows a result as “ND” or “less than 0.002”. That is the detection limit of the method used, not a statement that the contaminant is absent. For most contaminants the distinction does not matter, because detection limits sit far below health limits. For PFAS it matters a great deal: the federal limits are 4 and 10 nanograms per liter, which is close to what many laboratories can reliably measure. If you are testing for PFAS, ask the laboratory what its reporting limit is before you pay, and make sure it is below the federal number.
What to Do After an Elevated Result
Resist buying equipment on the day the report arrives. The sequence that avoids wasted money is: confirm, identify, then treat.
Confirm by retesting, ideally with a fresh sample taken carefully and sent to the same laboratory. Sampling error is common, particularly for bacteria, where a contaminated tap or an unwashed hand produces a positive that has nothing to do with the aquifer.
A sudden change right after a storm often has a physical, not chemical, explanation. Our well water cloudy after rain guide covers what heavy rain actually disturbs underground.
Identify the source next. A coliform positive with an intact, properly sealed wellhead is a different problem from one with a cracked casing. A lead result at the kitchen tap but not at the wellhead is plumbing, and no whole-house filter will fix it as cheaply as replacing one fitting.
A confirmed coliform result calls for disinfection, not just identification. Our shock chlorination guide covers the dose and the retest schedule that closes the loop.
Only then match treatment to the confirmed contaminant, using the last column of the table as a starting point rather than a prescription. Our filtration and treatment guides work through the equipment choices in detail, and the filtration system guide covers how the units combine in sequence.
For dissolved chemical contaminants like nitrate or arsenic specifically, one treatment does most of the work. Our reverse osmosis guide covers when RO is the right call and when it is not.
Many county health departments test private wells for coliform bacteria and nitrate free or at a subsidized price, and some run seasonal clinics where you collect a bottle and drop it back the same day. Call the county before paying a commercial laboratory for the basic panel.
State Limits Can Be Stricter Than Federal Ones
Federal numbers are a floor, not a ceiling. Several states set lower limits than the EPA for specific contaminants, and several publish health advisory levels for substances with no federal limit at all, manganese being the most common example. State health departments and university extension services also publish guidance calibrated to local geology, which is more useful than a national average when deciding what to test for. Where a state number exists and is stricter, use the state number.
Frequently Asked Questions (FAQs)
Which well water test should I do first if I can only afford one?
Coliform bacteria. It is the cheapest common test, it detects a physical defect in the well rather than a slow chemical process, and a positive result carries the most immediate risk. Nitrate is the second, and most laboratories bundle the two.
Does an iron result above 0.3 mg/L mean my water is unsafe?
No. Iron sits under the secondary standards, which the EPA publishes for taste, odor and staining and does not enforce. Water at 1 or 2 mg/L of iron stains fixtures and tastes metallic, and it is worth treating for those reasons, but it is not a health limit.
Why do lead and copper have action levels instead of MCLs?
Because in almost every case the metal enters the water from the plumbing rather than the source. Regulating the water leaving a treatment plant would miss the problem entirely, so the rule regulates corrosion control and sets an action level measured at the tap.
My laboratory reported PFAS as non-detect. Am I clear?
Only if the laboratory’s reporting limit is below the federal limits of 4 nanograms per liter for PFOA and PFOS. Some methods cannot reliably measure that low, so a non-detect from a high reporting limit says less than it appears to. Ask for the reporting limit in writing.
How much of this table applies if my well is shallow?
Shallow wells are more exposed to surface activity, so bacteria, nitrate, pesticides and fuel components deserve more attention. Deep bedrock wells are better protected from the surface but more exposed to natural arsenic, uranium and radium. Depth changes which half of the table matters, not whether testing is needed.
- EPA, National Primary Drinking Water Regulations, MCL and MCLG values, table checked August 2026.
- EPA, Secondary Drinking Water Standards: Guidance for Nuisance Chemicals, SMCL values and the statement that they are non-enforceable guidelines.
- EPA, Private Drinking Water Wells, on the regulatory status of private wells.
- CDC, Drinking Water, annual private well testing recommendations.
