Groundwater is routinely monitored for pesticides, heavy metals and forever chemicals — per- and polyfluoroalkyl substances — as these pollutants are acknowledged to pose major public health risks. A lesser-known threat is associated with excessive sulfate levels in groundwater, which are linked to severe gastrointestinal issues. The extent of this potable water hazard is depicted in a new high-resolution global groundwater sulfate distribution map.

Researchers from Hong Kong University of Science and Technology, Princeton University, City University of Hong Kong and Beijing Institute of Technology analyzed over 17,000 sulfate concentration measurements alongside global geospatial datasets of variables. A 1-km resolution map depicting excessive sulfate levels in groundwater, published in Environmental Science & Technology, was then prepared.

The data reveal that approximately 194 million people worldwide are exposed to water with sulfate concentrations exceeding 250 mg/L, a threshold recommended by the World Health Organization. In addition, an estimated 17 million people face substantial health risks because they reside in regions where sulfate concentrations surpass 500 mg/L, levels associated with gastrointestinal issues.

Regions like South Asia and North Africa, where billions rely on untreated groundwater for drinking, are depicted as hotspots of elevated sulfate concentrations. Natural geological conditions dominate as contributors in some areas, but anthropogenic factors such as industrial discharge and agricultural practices play a more significant role in other regions. Such variability highlights the importance of region-specific sulfate mitigation strategies.