Black stains on plumbing fixtures are most likely caused by what substance?

Black stains on plumbing fixtures are most commonly attributed to the presence of manganese in the water supply. Manganese can oxidize and form dark-colored staining when it accumulates in water systems, particularly when water conditions promote its precipitation.

Unlike iron, which typically causes red or brown stains, or calcium, which can lead to white, chalky deposits, manganese gives rise to black stains, making it a distinct identifier. Nickel does not typically cause staining issues in plumbing fixtures, further underscoring why manganese is the correct choice in this scenario. Understanding this key characteristic of manganese helps water operators identify water quality issues efficiently and take appropriate action to mitigate staining problems in plumbing systems.