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This paper outlines general trends in the geochemistry of the more than 10,000 km of flooded underground mine workings in the Butte mining district. The waters in question range in pH from 4 to 8, are all moderately to strongly reducing, and show a huge range in concentration of dissolved metals such as Al, As, Fe, Mn, and Zn. Metal concentrations and total acidity are highest in the Kelley mine shaft, which was the main dewatering station used to pump ground water from the underground mine complex during active mining operations. In contrast, metal concentrations are much lower in the outer portions of the district where many of the mines contain hydrogen sulfide formed by sulfate-reducing bacteria. In comparison to the other heavy metals, concentrations of Pb and Cu are quite low in the flooded mine shafts. An interesting inverse correlation between pH and water temperature is noted, which may be partly caused by exothermic pyrite oxidation reactions in the central portion of the district.

Over a century of mining and smelting of the world-class porphyry lode ore deposit at Butte, Montana, has resulted in extensive environmental damage. In addition to its being the location of one of the world's largest and most acidic mining pit lakes, Butte is host to over 16,000 km of flooded underground mine workings. Of the more than 60 mine shafts that have historically operated in Butte, approximately one dozen are presently accessible for groundwater sampling. The geochemistry of the mine shaft waters is zoned and roughly coincides with a district-wide zonation in hydrothermal alteration and mineralization. Mine waters in the so-called "Central zone" of intense phyllic and advanced argillic alteration have lower pH and very high concentrations of As, Fe, Mn, and Zn, but very low concentrations of dissolved Cu. The scarcity of Cu is attributed to cementation onto scrap iron left in the mines, and/or to replacement of preexisting sulfide minerals below the water table in a manner analogous to supergene enrichment processes. At the other extreme, mine waters in the "Peripheral zone" of weakest alteration have near-neutral pH, low metal concentrations, and contain dissolved sulfide (H2S, HS -). These waters are close to equilibrium with calcite, siderite, crystalline or amorphous MnCO3, and mackinawite (poorly crystalline FeS). A suite of deep groundwater monitoring wells completed †