Negative pH and extremely acidic mine waters from Iron Mountain, California

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 hig h as 760 g/L, have been encountered underground in the Richmond Mine at Iro n 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 g lass membrane electrodes;The calibration of pH below 0.5 with glass membran e electrodes becomes strongly nonlinear but is reproducible to a pH as low as -4. Numerous efflorescent minerals were found forming from these acid wa ters. These extreme acid waters were formed primarily by pyrite oxidation a nd concentration by evaporation with minor effects from aqueous ferrous iro n oxidation and efflorescent mineral formation.