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

Manganese is the substance associated with a secondary maximum contaminant level (MCL). Secondary MCLs are set for contaminants that may affect the aesthetic quality of drinking water, such as taste, color, and odor, rather than directly impacting health. Manganese can contribute to issues related to staining, taste, and turbidity in water, which is why it has a secondary standard.

In contrast, Lead, Arsenic, and Nitrate are regulated primarily due to their potential health effects. Lead and Arsenic have primary MCLs due to their toxicity and risk of causing serious health problems, while Nitrate is also regulated primarily for health reasons, particularly concerning its impact on infants and potential methemoglobinemia, commonly known as "blue baby syndrome." Thus, the regulatory focus for manganese centers more on water quality and user experience rather than direct health hazards, distinguishing it as the correct answer regarding secondary MCLs.