Which of the following is true regarding manganese in drinking water?

Manganese in drinking water is regulated under the U.S. Environmental Protection Agency's guidelines due to its potential impact on water quality and health. The correct answer indicates that it has a secondary Maximum Contaminant Level (MCL). Secondary MCLs are non-enforceable guidelines intended to protect the aesthetic qualities of drinking water, including taste, odor, and appearance. Manganese can affect the water's taste and may stain plumbing fixtures or laundry, leading to undesirable aesthetic issues.

While manganese is important for human health in trace amounts—serving roles in enzyme systems and metabolism—high concentrations can lead to health risks, particularly neurological issues in children. The presence of manganese in higher levels than the secondary MCL can necessitate treatment and management strategies in municipal water supply systems.

The other options do not accurately reflect the regulations and issues concerning manganese in drinking water. It is not correct to state that manganese has a primary MCL because it is only regulated under secondary standards. Furthermore, claiming that manganese has no health effects overlooks the potential risks associated with elevated levels of exposure. Finally, while manganese is not highly toxic at low concentrations, it certainly can be harmful at excessive levels, thus deeming it non-toxic is misleading.