Negative pH and Extremely Acidic Mine Waters from Iron Mountain, California

Darrell Kirk Nordstrom,* Charles N. Alpers, Carol J. Ptacek, and David W. Blowes

U.S. Geological Survey, Water Resources Division, 3215 Marine Street, Boulder, Colorado 80303, U.S. Geological Survey, Water Resources Division, 6000 J Street, Placer Hall, Sacramento, California 95819-6129, National Water Research Institute, 867 Lakeshore Road, Burlington, Ontario, Canada L7R 4A6, and Department of Earth Sciences, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1

Extremely acidic mine waters with pH values as low as -3.6, total dissolved metal concentrations as high as 200 g/L, and sulfate concentrations as high as 760 g/L, have been encountered underground in the Richmond Mine at Iron Mountain, CA. These are the most acidic waters known. The pH measurements were obtained by using the Pitzer method to define pH for calibration of glass membrane electrodes. The calibration of pH below 0.5 with glass membrane electrodes becomes strongly nonlinear but is reproducible to a pH as low as -4. Numerous efflorescent minerals were found forming from these acid waters. These extreme acid waters were formed primarily by pyrite oxidation and concentration by evaporation with minor effects from aqueous ferrous iron oxidation and efflorescent mineral formation. In this paper, it is demonstrated that pH can be defined and measured below 0.0 and that waters of such low pH exist in nature.

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