Nearly 20 million people in China are exposed to high levels of arsenic in the water they use for drinking and cooking, a new model based on geological and hydrological data and well samples shows. The model predicts high arsenic concentrations(10 micrograms per liter or greater) across more than 580,000 square kilometers, according to Chinese and Swiss researchers, who published their findings in Science. Researchers had long known that some regions had high arsenic concentrations, but it would have taken several decades to test the millions of groundwater wells in China. The new prediction combined the most recent water tests, in which 5 percent of wells were found to contain arsenic in excess of 50 micrograms per liter, with data about the underlying geology, soil characteristics, and topographic features. Regions at risk include the basins of the Tarim (Xinjiang), Ejina (Inner Mongolia) and Heihe (Gansu) rivers, as well as the North China Plain (Henan and Shandong).