Samantha Ying, an assistant professor of environmental
While groundwater can be contaminated with a number of heavy metals, more emphasis has been placed on assessing the levels of arsenic than manganese, although the latter also poses a threat to human health. Levels of arsenic, a known carcinogen above the WHO's guideline of 10 parts per billion (ppb), are enforced by the Environmental Protection Agency (EPA) in the U.S. and similar agencies in other countries. Although the WHO suggests a health-based limit of 400 ppb, manganese is not listed as a contaminant on the EPA's National Primary Drinking Water Regulations, and therefore the levels are not monitored or enforced.
Read more at: https://phys.org/news/2017-08-manganese-underground.html#jCp