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

Black stains on plumbing fixtures are primarily associated with manganese. Manganese is a naturally occurring mineral found in water and can precipitate out when certain conditions are met, such as low pH levels or high water temperatures. These precipitated manganese particles can leave annoying black stains on fixtures, leading to the appearance of unsightliness in bathrooms and kitchens.

While other substances like iron and calcium may cause discoloration—iron typically leads to reddish-brown stains and calcium contributes to hard water buildup—manganese specifically results in the distinct black appearance that can be easily mistaken for other contaminants. Therefore, understanding the specific characteristics of manganese is essential for diagnosing and addressing water quality issues in plumbing systems.