A water treatment plant, potentially operating forever, could be expensive. Yet feeding heavy metals from the pit pond directly into creeks would have negative impacts on downstream fisheries. Waste rock, or overburden, is material removed from the pit and dumped aside. Because it has been blasted into smaller pieces than native rock, it is more chemically reactive and oxygen-rich. 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.