THE ROLE OF FERRIC IRON IN SUBAQUEOUS OXIDATION OF PYRITE IN THE BERKELEY PIT-LAKE, BUTTE, MONTANA

The Berkeley pit-lake contains over 100 billion L of acidic, metal-rich water (pH ~ 2.5, Fe > 800 mg/L, Zn > 600 mg/L, Cu > 150 mg/L). Water quality in the pit-lake is much worse than all influent ground and surface waters, and has shown no improvement over 20 years of mine flooding. To explain these observations, we propose that much of the acid in the pit-lake is being generated in-situ, via anaerobic oxidation of pyrite by dissolved Fe(III). Vertical profiles in the Berkeley pit-lake from 1997 to 2003 show a distinct “ferrocline” separating near-surface water with Fe(III) >> Fe(II) from deeper water with Fe(II) > Fe(III). However, even below the ferrocline, dissolved ferric iron concentrations are still appreciable (~ 5 mmol). Using published rate data for oxidation of pyrite by ferric iron, an estimated 6 x 10⁶ to 600 x 10⁶ moles of acid may be generated via subaqueous pyrite oxidation in the Berkeley pit-lake each year. Seasonal lake overturn may play a key role in redistributing solutes in the pit-lake, allowing pyrite oxidation to continue. Water-d¹⁸O, sulfate-d¹⁸O and sulfate-d³⁴S results are consistent with this hypothesis. Sulfate in the pit-lake has d³⁴S similar to pyrite in the adjacent ore body, and d¹⁸O similar to pit-lake water, indicating local derivation of S, and minimal incorporation of atmospheric O₂ during pyrite oxidation. The possibility of low temperature isotopic exchange between sulfate and water is also being evaluated. In contrast, sulfate in groundwater surrounding the Berkeley pit-lake contains isotopically heavier oxygen and a wide range in d³⁴S, consistent with a conventional, aerobic pyrite oxidation mechanism and derivation of sulfate from multiple sources.

The anaerobic pyrite oxidation model advocated here for the Berkeley Pit has major implications to other acidic pit lakes that contain high concentrations of ferric iron. In such cases, pit flooding may be ineffective in curbing pyrite oxidation.

The information in this document has been funded in part by the U.S. Environmental Protection Agency (EPA), Contract #DW89938870-01-1, under an Interagency Agreement (IAG) between EPA and the U.S. Department of Energy, IAG No. DE-AC22-96EW96405.