When certain metal sulfides are exposed to air and water, they react to form a sulfuric acid solution that is capable of dissolving harmful minerals from rock in a process known as acid mine drainage (AMD). Many of the chemical constituents contained in these acidic effluents are highly toxic to aquatic life, even in very low concentrations.
The type of ore present at Pebble contains sulfide-sulfur concentrations ranging from 1-5%, with a maximum near 9%. As little as 0.2% sulfide can generate acidic water. This indicates a high likelihood of acid mine drainage at the Pebble site, which if not controlled, would have a severe detrimental impact on aquatic life in affected watersheds.
Taken from "Bristol Bay's Wild Salmon Ecosystems and the Pebble Mine". For a more detailed explanation of this graphic, please see the full report available online. Text and images copyright © 2012 Wild Salmon Center.