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Leaching of heavy metals by ground water passing through mine tailings is caused by the oxidation of sulfides through the action of microorganisms. The pH of the ground water entering the tailings system is reduced by mechanisms within the tailings system itself. Action of sulfide‐oxidizing and sulfur‐oxidizing bacteria causes the formation of sulfuric acid, thereby increasing the H+ and SO42− concentration within the system. Metal ions go into solution as metal sulfates, and the concentration of H2S increases by the interaction of H+ and S2− Increase in H2S creates an environment suitable for the growth of sulfate‐reducing bacteria which converts SO42− to S2− and simultaneously precipitates metal sulfides at a pH near 6.6. Decrease in pH with depth destroys sulfate‐reducing bacteria, and dissolution and leaching of the tailings increase. Four lines of evidence demonstrate the presence of microorganisms in the tailings system: (1) Fe‐Mn concretions below the water table at Cataldo Mission Flats, (2) fixation of Na and K below the water table, (3) precipitation of sulfate salts on the surface of Cataldo Mission Flats, and (4) experiments on two sets of tailings samples, one from the surface which showed predominant aerobic activity and one from below the water table which showed predominant anaerobic activity. Where oxygen is sufficient, iron oxidizing bacteria oxidize Fe2+ to Fe3+ which forms an insoluble hydroxide. Consequently, very little iron is found in the ground water coming from the system. The combination of these mechanisms produces poor‐quality ground water with respect to metals.

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