What Is a Secondary Drinking Water Standard (SMCL)?

Some things in water are a nuisance rather than a danger: they stain sinks, leave a metallic taste or make the water smell. For those, the EPA sets secondary standards instead of legal limits.

What is a secondary standard?

According to the EPA, secondary maximum contaminant levels (SMCLs) are non-mandatory standards for 15 contaminants, set “as guidelines to assist public water systems in managing their drinking water for aesthetic considerations, such as taste, color, and odor.” The EPA doesn’t enforce them, and says these contaminants “are not considered to present a risk to human health at the SMCL.”

The EPA groups the problems they prevent into three kinds: aesthetic (taste, odor, color), cosmetic (such as tooth discoloration from fluoride or skin discoloration from silver) and technical (corrosion, staining, scale and sediment that damage pipes and fixtures).

How is it different from an MCL?

An MCL is a legal limit set to protect health, and public water systems must meet it. A secondary standard is advice about how water looks, tastes and smells. One contaminant can have both: fluoride’s MCL is 4.0 mg/L and its secondary standard is 2.0 mg/L, and the EPA requires community water systems above 2.0 mg/L to notify the people they serve.

A secondary standard also isn’t proof that higher levels are safe. Manganese is the clearest case: its secondary standard is 0.05 mg/L for taste and staining, but the EPA’s separate health advisory sets a lifetime level of 0.3 mg/L to protect against possible neurological effects.

Do secondary standards apply to private wells?

No. The EPA says private wells aren’t regulated by the federal government, so no federal standard applies to them, and most states don’t regulate them either. Well owners can still use the secondary standards as a yardstick: iron above 0.3 mg/L, for example, is the level the EPA links to rusty color, sediment, a metallic taste and orange stains. See iron and sulfur in well water.