Which of the following can cause a color change in water often due to the decomposition of organic materials?

The presence of iron in water can lead to a color change, often resulting in a reddish or brownish tint. This color change typically occurs due to the oxidation and subsequent precipitation of ferrous iron (Fe2+) as it transforms to ferric iron (Fe3+) when exposed to oxygen. This process can be exacerbated by the decomposition of organic materials, which provides nutrients that promote microbial activity. When organic materials decompose, they can release iron into the water, leading to an increased concentration that can affect color.

In contrast, magnesium, calcium, and lead do not commonly cause similar color changes associated with organic decomposition. While magnesium and calcium are important minerals found in water, they do not typically lead to noticeable color alterations. Lead, although a heavy metal of concern for health reasons, does not cause discoloration in the same manner and is typically present at very low concentrations, making color changes less likely in most water sources.