The Indo-Gangetic Basin provides more than 750 million people in Pakistan, India, Nepal, and Bangladesh with water—but 60% of its groundwater is contaminated, The Guardian reports. In a study published in Nature Geoscience, researchers found 23% of the water too salty, and 37% with toxic concentrations of arsenic. Arsenic is naturally (and safely) present in many water sources, but it can climb to dangerous levels with the use of fertilizers and mining. High salinity can arise from inefficient farmland irrigation and poor drainage. The Indo-Gangetic Basin provides nearly a quarter of all groundwater extracted annually worldwide.