Global assessment of manganese in groundwater

Although manganese (Mn) is an essential component of human nutrition, high levels of Mn consumption may be toxic. Despite most dietary Mn generally coming from food, Mn intake via drinking water sourced from groundwater can be substantial. With new evidence pointing to greater detrimental health effects, particularly for infants, from excess Mn in drinking water, the World Health Organization (WHO) lowered its health-based guideline for Mn in drinking water several years ago from 400 µg/L to 80 µg/L. With the new guideline value being just one-fifth of the previous one, we have employed machine learning to model Mn concentrations in groundwater globally in order to assess the implications of this change on affected regions and populations. This was done by first assembling a large dataset of groundwater Mn concentrations and relevant environmental parameters, which were then used in machine-learning modeling. Based on these results and considering national-scale rates of household use of unmanaged groundwater, we estimate that over 200 million people are at risk from consuming >80 µg/L Mn in drinking water, which happens to be about five times more than the 39 million people at risk based on the previous guideline of 400 µg/L. Although the number of people at risk due to high Mn in drinking water is comparable to that for arsenic and fluoride, Mn generally receives much less attention than do these other, also naturally occurring, groundwater contaminants. As such, the groundwater Mn hazard and risk maps produced in this study are important guides in helping identify safe groundwater sources.