The Paint Pots in Kootenay National Park (British Columbia) appear to derive the Fe, Zn, Pb, and contents of their water from sulfide mineralization in Lower and Middle Cambrian carbonates. The Fe, Zn, Ni, and contents of groundwater discharging into a tributary of Engineer Creek (Yukon) are likely derived from sulfide mineralization in Devonian or Ordovician black shales exposed in the area. The high Fe and contents of a natrojarosite deposit northeast of Fort Norman (Northwest Territories) are probably derived from pyritiferous Cretaceous shales in that area. Isotope analyses of water and of dissolved and precipitated sulfur species from these three sites where acidic, heavy-metal-bearing groundwater is being discharged revealed that between 38 and 74% of the oxygen used in the subsurface oxidation of metal sulfides is supplied by H2O molecules rather than by molecular (dissolved) oxygen. The available data also suggest that lower percentages of water oxygen in the secondary sulfates reflect increasing activity of Thiobacillus ferrooxidans or similar bacteria in the oxidation process.