Which substance has a secondary maximum contaminant level (MCL)?

Manganese has a secondary maximum contaminant level (MCL) because it is primarily considered a nuisance contaminant that can affect the aesthetic qualities of drinking water, such as color, taste, and odor, rather than posing a direct health risk at lower levels. The secondary MCL for manganese is set to protect consumers from these aesthetic issues, as it can cause staining of laundry and fixtures, and its presence in water can be a sign of other underlying issues in the water supply.

In contrast, lead and nitrates fall under primary MCLs due to their direct health risks, especially in vulnerable populations. Lead can lead to significant health problems, particularly in children, while elevated nitrate levels can be dangerous, particularly for infants, as they can cause methemoglobinemia or "blue baby syndrome." Fluoride, while it has a beneficial effect in preventing tooth decay at optimal levels, is also regulated under a primary MCL to prevent dental fluorosis when levels are too high, which can cause cosmetic harm to teeth.

Thus, the secondary maximum contaminant level applies to substances like manganese that may not necessarily pose a direct health threat but have the potential to affect the quality of water for consumers.