Some of the waste rock contains sulfides. Rainwater seeps into waste rock piles and combines with oxygen and sulfides to form sulfuric acid. Sulfuric acid is strong enough to dissolve metals in the waste rock. Plumes of acid mine drainage with heavy metals are known to snake their way through alkaline limestone and move into ground and surface water. Tailings are spent ore mixed with cyanide and water.
Tailings contain heavy metals attached to sediments at the bottom of the tailings impoundment.
BMG's tailings would be separated from Marias Creek by a triple liner system, yet all liners are known to leak. Metals-rich sediments could escape from the tailings impoundment and move down Marias Creek. It is biologically dead. Sediments, even without metals, can be deadly to fish because they smother the gravel on which eggs hatch and kill the water bugs on which they feed. Extremely high sediment levels can actually smother fish.