Swimming pool water that is brownish black in color may be due to:

The presence of brownish-black colored water in swimming pools is typically associated with the presence of manganese (Mn). Manganese can cause discoloration in water, leading to a brownish-black appearance. This element often enters water systems through soil and rock dissolution, especially in areas where manganese-rich minerals are present. When manganese is present in high concentrations or when the pool water chemistry is not properly balanced, it can precipitate, resulting in visible coloration.

Additionally, manganese can form compounds that are less soluble, particularly at certain pH levels or in the presence of oxidizing agents. This can cause aesthetic issues, such as staining on pool surfaces, which reinforces why it is important for pool operators to regularly monitor and manage water quality to prevent such occurrences.

In contrast, while hydrogen sulfide (H2S) may impart a color to water, it is more commonly associated with a distinct rotten egg smell and is not typically a factor in the brownish-black coloration of swimming pool water. Magnesium (Mg) and iron (Fe), although they can influence the color of water under certain conditions, are not primarily responsible for a brownish-black color in the same way manganese is. Iron, in particular, tends to create red, brown, or yellowish