Manganese is an essential trace element. However, in excessive concentrations, the metal can cause health problems. Two Eawag researchers have now produced a global risk map for manganese in groundwater. Half of the world’s population uses groundwater as drinking water. According to the study, between 180 and 220 million people could be using water with elevated manganese concentrations as drinking water – significantly more than previously thought. Densely populated regions of Asia are particularly affected.

Manganese is one of the most common metals in the Earth’s crust and is essential for the human body. Normally, people get it mainly from food. In certain regions, however, even drinking water may contain considerable amounts. For a long time, this was regarded primarily as a problem because manganese affects the taste of the water and causes dark discolouration around the well outlet.
In recent years, however, there has been a growing body of evidence pointing to the health consequences of excessive manganese intake. Studies show that manganese can damage the nervous system, particularly in infants and children. The World Health Organisation (WHO) therefore lowered its guideline value for manganese in drinking water from 400 to 80 micrograms per litre in 2021. The limit values in Switzerland and the EU are even lower, at 50 micrograms per litre.
However, it has not yet been clear how many people drink water that exceeds these levels. A new study by two researchers from Eawag’s Department of Water Resources and Drinking Water – Joel Podgorski, an environmental scientist and lead author, and geochemist Michael Berg – now provides an important basis for changing this. “The main focus so far has been on risks to drinking water such as arsenic, fluoride and nitrate,” says Michael Berg. "In contrast, manganese is still largely overlooked – and monitored only sporadically."

300,000 measurements compiled
The two researchers produced a global forecast map of manganese contamination in groundwater. For the study, which has just been published in the journal Nature Water, they compiled just under 300,000 groundwater measurements from around the world, drawn from scientific publications, government archives and public databases. They then linked this data to more than 50 environmental parameters, such as climate, geology, topography and soil properties.
The data was fed into a machine learning model developed in-house. The model identifies patterns associated with high or low concentrations of manganese in groundwater. This makes it possible to make forecasts for areas where no measurement data is available. The researchers thus created a world map showing where the guideline value for manganese in groundwater is likely to be exceeded.
The models highlight high-risk areas on every continent. Large river deltas and young sedimentary landscapes in South and South-East Asia are particularly affected. Hotspots can be found, for example, in the Ganges-Brahmaputra Delta in Bangladesh and India, in the Mekong Delta, in the Red River Delta near Hanoi, in parts of Pakistan, and also along the Mississippi in the USA. The reason for this distribution lies in geochemical processes taking place underground: Manganese is often found bound as manganese oxide in sediments. This is reduced by microorganisms under low-oxygen conditions and dissolves in the groundwater.

Nine out of ten people affected live in Asia
In a second step, the researchers combined the pollution map with population data and information on the drinking water supply. This meant that between 180 and 220 million people worldwide could be consuming water with manganese concentrations above the WHO guideline value. More than 90 per cent of these people live in Asia. “This is not only due to the young river sediments, which promote the accumulation of manganese in the groundwater there,” says Joel Podgorski. "These regions are also densely populated – and a large proportion of the population relies on untreated groundwater."
According to the researchers, manganese levels across Switzerland are safe. And the risk is also low in the rest of Europe. Admittedly, the study identifies areas with high levels of pollution, including the Po Delta in northern Italy and a belt stretching from Poland to eastern Germany. “But here, the vast majority of people have drinking water from the tap, which is treated using modern technology,” says Michael Berg. "That is why the maps showing geological risk and those showing the affected population differ significantly."
According to him, a simple treatment plant is sufficient to remove manganese from drinking water. This is because as soon as water containing manganese comes into contact with oxygen, it precipitates as a metal oxide. It turns back into a solid, insoluble form – and can be filtered out of the water. “That can be achieved simply with a water aeration system and a sand filter,” says Michael Berg.

Manganese and arsenic hardly overlap at all
Finally, the researchers compared the newly created manganese map with the arsenic risk maps they had published in 2020. Although arsenic and manganese often arise from similar geochemical processes, there is little overlap between them at the local level: According to the calculations, both substances are found simultaneously in only around four per cent of high-risk areas. This has implications for water monitoring: “Anyone who tests a well for arsenic and finds no contamination cannot automatically declare it safe – you also have to test for manganese,” says Joel Podgorski.
In any case, the study is no substitute for on-site measurements. “Our maps show probabilities, not certainties,” says Berg. However, the new data provide an important basis for authorities, policy-makers and international organisations. On this basis, priorities can be set in future for the construction of drinking water treatment plants or for additional monitoring programmes.

All data on a single platform Over the past few years, the team led by Michael Berg and Joel Podgorski has compiled a whole series of global and regional hazard maps of groundwater contamination. As well as manganese, these also include pollutants such as arsenic, fluoride, salinity and nitrate. All maps are freely available on the website www.gapmaps.org . The platform is aimed at experts and authorities from around the world who are looking for groundwater data – or who wish to create their own risk maps using the Eawag model.