Why are higher concentrations of iron and manganese found in lower portions of raw water reservoirs?

Higher concentrations of iron and manganese in the lower portions of raw water reservoirs are primarily attributable to anaerobic conditions. In these areas, the lack of oxygen creates a favorable environment for the dissolution of these metals from sediments into the water. Under anaerobic conditions, iron and manganese can remain soluble and accumulate, leading to increased concentrations compared to surface water layers where oxygen is present and can promote the oxidation and precipitation of these metals.

In contrast, while increased pH levels and rapid water flow can influence the chemistry of water, they do not specifically account for the accumulation of iron and manganese in deeper layers of a reservoir. Chemical treatments are typically applied to water to remove or manage concentrations of these metals, rather than contributing to their accumulation in specific areas of the water column. Thus, the anaerobic environment at lower depths is the key factor in the higher concentrations observed.