Iron in Drinking Water: The 0.3 mg/L Secondary Standard and How to Remove It

Iron is the contaminant most people recognize the moment they see it, because it leaves a mark: reddish brown stains in sinks, toilets, and tubs, laundry that comes out of the wash tinted orange or yellow, and a metallic taste in the water. It is common in groundwater, especially where wells draw from rock and soil that naturally release it, and it is the reason a customer calls to ask why the water is "rusty." One number governs it as a secondary concern — the 0.3 mg/L secondary maximum contaminant level in 40 CFR § 143.3 — and that number is about appearance, not health. If you are not sure whether your system has an iron issue on file, check your water system's compliance status free at orevant.com before you read on.

The 0.3 mg/L secondary standard is about appearance, not health

The secondary maximum contaminant level (SMCL) for iron is 0.3 milligrams per liter, set in 40 CFR § 143.3 under the National Secondary Drinking Water Regulations. A secondary standard is not federally enforceable the way an MCL is. It is the level above which water starts to look, taste, or stain — reddish brown deposits, a metallic taste, and orange-tinted laundry all show up around or above 0.3 mg/L. Iron has no primary MCL, because it is not a health hazard at the levels found in drinking water; the standard exists to protect appearance and plumbing. See the difference between an MCL and an action level for how the federal rulebook sorts enforceable limits from advisory numbers.

Why iron shows up in groundwater systems

Iron dissolves into water under low-oxygen (reducing) conditions, the same conditions that release manganese and, in some aquifers, arsenic. A well that pulls from a reducing aquifer can carry iron and manganese together, which is why the two are treated side by side. When that water hits the air — at the tap, in a toilet tank, in a washing machine — the iron oxidizes and drops out of solution as the reddish brown rust you see. See how manganese behaves under the same conditions, and how naturally occurring metals enter groundwater for the geology.

Iron bacteria are a separate, nastier problem

Iron can also show up through biology. Iron bacteria feed on dissolved iron and leave a reddish brown or rust-colored slime in toilet tanks, pipes, and screens. They are not a health hazard, but they clog wells, pumps, and distribution lines, and their slime can hide other contamination. A sudden appearance of slime plus a rotten-egg or swampy odor points to iron bacteria rather than plain dissolved iron, and the treatment is different. See the IOC list for how iron's regulated neighbors are monitored.

What a small system actually does about it

Treatment depends on whether you are removing dissolved iron, oxidized iron, or iron bacteria. For dissolved iron the standard approach is oxidation followed by filtration: aerate or oxidize with chlorine or potassium permanganate, then filter the precipitated iron through a greensand, manganese-dioxide, or multi-media filter. Ion-exchange water softening removes low levels of dissolved iron along with hardness. Polyphosphate sequestering keeps iron in solution so it cannot stain, but it is a cosmetic fix, not removal. The first step is knowing your raw-water iron, whether it is climbing, and whether bacteria are involved, which a monitoring calendar tracks. See how to know if your water system is compliant.

Know the number before a customer calls about stains

Iron rarely triggers an enforceable violation, which is exactly why it gets ignored until a customer calls about a rust ring in the toilet or a load of orange laundry. But a system that knows its iron number can head off the complaint, decide whether it crosses the 0.3 mg/L secondary standard, and pick the right treatment before the water becomes a visible problem. The $199 compliance scan maps every monitored contaminant on your system, including iron and its secondary-standard neighbors, against the numbers you are actually up against, so you know what to fix first.

Run the $199 Compliance Scan — see your iron level and the exact step to fix it.

Prefer to see the public record first? Check your water system free at orevant.com.

FAQ

Is iron in drinking water regulated?

Iron has a secondary standard of 0.3 mg/L (40 CFR § 143.3), which is not federally enforceable. It is an aesthetic standard for staining and taste. States may set their own limits, so check your state's drinking water rules.

Does iron in my water hurt my health?

At the levels found in drinking water, iron is not a health hazard, which is why it carries no primary MCL. It is a taste, staining, and plumbing concern. Excess iron in a person's body is a medical matter, not a drinking-water matter.

How do I remove iron from my water?

Oxidation followed by filtration is the standard method: aerate or dose an oxidant, then filter the precipitate through greensand or manganese-dioxide media. Ion-exchange softening handles low dissolved iron, and polyphosphate only hides the staining. Iron bacteria are treated by shock chlorination and cleaning.

Is iron linked to manganese?

Yes. Both dissolve under low-oxygen groundwater conditions and frequently appear together, which is why they are usually treated with the same oxidation-and-filtration train. See the manganese explainer.