 Research geologist Andrew Manning with the U.S. Geological Survey co-authored the study. He says one explanation is disappearing permafrost. When you warm the ground and you melt the permafrost, groundwater and oxygen seep down deeper into fresh, unweathered parts of the subsurface, pick up more metals and carry those to the stream. Manning says similar effects can be found in other parts of the world too, notably the so-called rusting rivers of the Arctic. For NPR News, I'm Rae Solomon.