Testing
EPA secondary drinking water standards, the aesthetic guidelines chart
Quick answer
Secondary standards are aesthetic guidelines, not enforceable health limits: iron at 0.3 mg/L, manganese at 0.05, TDS at 500, sulfate and chloride at 250, and pH between 6.5 and 8.5. There is no secondary standard, and no standard of any kind, for hardness.
Secondary drinking water standards exist for a completely different reason than the primary limits covered on the EPA drinking water limits chart. Primary standards are enforceable and health based. Secondary standards are aesthetic guidelines: they describe the point at which a substance starts to stain, taste, smell or otherwise become a nuisance, and utilities are not legally required to meet them the way they are required to meet a primary MCL.
That distinction gets lost constantly in marketing for treatment equipment, where a secondary guideline is frequently presented with the same weight as a health limit. Iron staining a sink is a real, annoying, expensive problem. It is not, at the concentrations that cause it, a health emergency, and treating it as one misdirects a household toward urgency it does not need while genuine health contaminants like lead, arsenic and nitrate, covered under the primary standards, get less attention than they deserve.
The single most useful fact on this page might be what is missing from it entirely: there is no EPA standard for hardness, secondary or otherwise, because hardness is not classified as a health or aesthetic contaminant at all.
The full secondary standards chart
Iron's secondary standard sits at 0.3 mg/L and manganese at 0.05 mg/L, the concentrations where staining and metallic or bitter taste typically begin.
Published figure A published standard, regulatory limit, unit conversion or manufacturer specification. It does not change because somebody disagrees with it.
| Contaminant | Limit | Unit | Aesthetic effect |
|---|---|---|---|
| Iron | 0.3 | mg/L | Rust staining, metallic taste |
| Manganese | 0.05 | mg/L | Black staining, bitter taste |
| Total dissolved solids | 500 | mg/L | Taste, scale, corrosion |
| Sulfate | 250 | mg/L | Salty taste, laxative effect |
| Chloride | 250 | mg/L | Salty taste, corrosion |
| Zinc | 5 | mg/L | Metallic taste |
| Copper | 1.0 | mg/L | Blue green staining, metallic taste |
| Fluoride | 2.0 | mg/L | Dental mottling |
| Aluminum | 0.2 | mg/L | Color |
| Silver | 0.1 | mg/L | Skin discoloration |
| pH | 6.5 to 8.5 | pH units | Corrosion below the range, scale above it |
| Color | 15 | color units | Visible tint |
| Odor | 3 | threshold odor number | Detectable smell |
These are guidelines, not enforceable health limits. A utility can exceed a secondary standard without violating the law the way exceeding a primary MCL would.
There is no standard of any kind for hardness
Hardness, the calcium and magnesium content that causes scale and reduces soap lather, is not on the secondary chart above, and it is not on the primary chart either. There is no maximum contaminant level, no secondary maximum contaminant level, and no health goal for hardness at any concentration, because it is not classified as a contaminant under EPA rules at all. A great deal of marketing for softening equipment implies otherwise, framing hardness in the same language used for a regulated contaminant, and that framing is not accurate.
That absence does not make hardness unimportant. It causes genuine, measurable, expensive problems: scale inside a water heater, reduced appliance life, and soap that will not lather properly. It simply means there is no published number to compare a test result against the way there is for iron or manganese. The relevant reference is the United States Geological Survey classification, a descriptive scale rather than a regulatory limit, covered in full on the water hardness chart.
The USGS classification is a descriptive scale, not a regulatory limit: water above 10.5 grains per gallon (180 mg/L) is classified very hard with no enforceable ceiling attached.
Published figure A published standard, regulatory limit, unit conversion or manufacturer specification. It does not change because somebody disagrees with it.
| Classification | Grains per gallon | Milligrams per liter |
|---|---|---|
| Soft | 0 to 1 | 0 to 17.1 |
| Slightly hard | 1 to 3.5 | 17.1 to 60 |
| Moderately hard | 3.5 to 7 | 60 to 120 |
| Hard | 7 to 10.5 | 120 to 180 |
| Very hard | Above 10.5 | Above 180 |
This scale describes water, it does not regulate it. There is no point on this table where a utility or a homeowner is legally required to do anything, unlike the enforceable primary standards on the EPA drinking water limits chart.
Why the aesthetic and enforceable lines matter for equipment decisions
The practical consequence of the aesthetic versus enforceable distinction is where a household should spend its testing budget first. Iron staining and TDS related scale are visible and eventually self diagnosing: a household notices orange fixtures or white crust and can measure the responsible number at home with a test kit such as the HM Digital TDS-3 Handheld TDS Meter, 0 to 9990 ppm for TDS. Contaminants on the primary chart, lead, arsenic, nitrate and bacteria among them, give no such warning. They are invisible and tasteless, and confirming any of them requires a certified laboratory test rather than a visual cue, with a positive result going to your local health department or a licensed professional rather than straight to equipment shopping.
Frequently asked questions
Is the secondary drinking water standard for iron enforceable?
No. Secondary standards, including iron at 0.3 mg/L, are aesthetic guidelines rather than enforceable health limits. A utility that exceeds a secondary standard is not violating the law the way exceeding a primary maximum contaminant level would be. Iron staining and metallic taste are real nuisances, but they are not classified as a health emergency at the concentrations that cause them.
Is there an EPA standard for water hardness?
No, none at all, primary or secondary. Hardness is not classified as a health or aesthetic contaminant under EPA rules, so there is no published limit to compare a test result against. The relevant reference instead is the USGS hardness classification, a descriptive scale rather than a regulatory limit, which runs from soft up through very hard above 10.5 grains per gallon.
What is the secondary standard for manganese?
0.05 mg/L, roughly a sixth of the iron secondary standard, reflecting how much more readily manganese causes visible black staining and a bitter taste at low concentrations. Like iron, this is an aesthetic guideline rather than an enforceable health limit, though very high manganese exposure has separate health research associated with it that a certified laboratory test and a health department can speak to directly.
What is the secondary standard for pH?
6.5 to 8.5. Below that range water is corrosive and above it water tends to scale, and both effects are aesthetic and material concerns rather than direct health limits. Low pH does carry a real health related consequence indirectly, since corrosive water mobilizes lead from plumbing, covered on the pH and corrosion chart.
Does exceeding a secondary standard mean the water is unsafe to drink?
Not by itself. Secondary standards describe aesthetic thresholds, taste, odor, staining and similar nuisances, rather than health risk. Water can exceed several secondary guidelines and still meet every enforceable primary health standard. Confirming actual safety questions, lead, arsenic, nitrate, bacteria and PFAS among them, requires checking the separate primary standards with a certified laboratory test.
What is the secondary standard for total dissolved solids?
500 mg/L. Above that level, water commonly develops a noticeable taste and contributes more readily to scale and corrosion, but TDS itself is not a single contaminant, it is a measure of everything dissolved in the water combined. A TDS meter gives a fast home reading, while a certified laboratory panel identifies what is actually contributing to that number.
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Researched, not professional advice. This page is compiled from published standards, regulatory limits, manufacturer specifications and owner-review consensus, not hands-on testing. Nothing here diagnoses a water problem or says any product makes water safe to drink. Figures described as a rule of thumb are water treatment trade convention rather than published standards, and they are labeled that way wherever they appear. Test your water before you buy equipment: on city water start with the Consumer Confidence Report your utility publishes each year, and on a private well only a certified laboratory test tells you what you have. Bacteria, nitrate, lead, arsenic and PFAS are not do it yourself topics, they are invisible and tasteless, and they need a laboratory and usually a licensed professional rather than a product. Remember that a certification covers a specific claim under a specific standard, so "NSF certified" with no number attached tells you very little.