Up to 220 million people globally may be at risk of arsenic-contaminated watera6e06079e60eb1bb9615d3962014dbc5

As many as 220 million
people around the world may be at risk of drinking arsenic-contaminated groundwater, a new study finds.

Combining climate,
environmental and geologic data with machine learning, researchers made a global
map, described in the May 22 Science,
that predicts where groundwater arsenic concentrations are likeliest to exceed
10 micrograms per liter, a safe drinking water limit set by the World Health Organization.

Arsenic is present in trace
amounts in many different types of soil and rock. It becomes harmful to people when
it leaches out of these soils and into groundwater, which can occur due to a
variety of chemical processes. Long-term exposure can lead to skin lesions and
cancer.

Scientists have previously
identified many hot spots of arsenic contamination in groundwater, including
regions of Bangladesh, Argentina
and Vietnam (SN: 11/20/02; SN: 3/5/15). But data on groundwater
arsenic are lacking for many other regions.

So environmental scientist
Joel Podgorski and hydrologist Michael Berg, both of the Swiss Federal
Institute of Aquatic Science and Technology in Dübendorf, set out to create a
high-resolution global risk map based on dozens of different environmental
factors, from temperature and precipitation to soil age and pH.

“In the last 12 years,
there’s a lot more data that has become available,” Podgorski says. He and Berg
amassed data from almost 80 studies. They then used a machine learning
technique called the “random forest” method, which created predictions of
arsenic risk at a resolution of one square kilometer, based on different
subsets of the data. The researchers then averaged the results of about 10,000
different predictions together to create the final map.

The result is “really the
first truly global risk map of arsenic contamination in groundwater,” Podgorski
says. The map shows many of the expected hot spots for arsenic contamination,
particularly in Asia and South America. But it also highlights possible
elevated risk in less well-studied regions, including central Asian countries
such as Kazakhstan and Mongolia, a broad swath of countries around the Sahara
as well as in the Arctic.

Even in the United States,
more than a million rural residents may be unknowingly
exposed to arsenic contamination
in their drinking water wells. Risk maps based on statistical
models, such as the map created by Podgorski and Berg, cannot predict
individual well water arsenic concentrations, writes geochemist Yan Zheng of
the Southern University of Science and Technology in Shenzhen, China, in a commentary
in the same issue of Science. But, she notes, the map’s “greatest value
lies in identifying potential areas at risk
that have not had testing.”

Podgorski agrees. “This is a basic message of the map; it should be used as a guide to more testing.”

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