Bacterial enumeration and geochemical characterization were undertaken at three sites an the sulfide-rich railings impoundment at the Kidd Creek metallurgical site, Timmins, Ontario, Canada. The three sites were selected to represent varying degrees of sulfide oxidation to assess the changes in water chemistry, in the mineralogical composition of the railings, and in bacterial populations as the sulfide oxidation process proceeds under natural field conditions. The first site was characterized as having negligible oxidation-derived alteration, the pH of the porewater varied from 6.5 to 7.5, and the concentrations of dissolved constituents were similar to those observed in the deeper, unaltered tailings. Mineralogical examination of the tailings grains indicated that the sulfide surfaces were sharp and unreplaced. At this site, the predominant sulfur-oxidizing bacteria were Thiobacillus thioparus and related species. The second site showed evidence of the onset of acidification, the pH of the near-surface porewater had decreased to 5.5, and the concentrations of some sulfide oxidation products, principally SO4 and Zn, had increased beyond those observed in the unaltered railings. At the second site, T. thioparus and related species predominated. Al the third site the acidification process was well established. The pH of the shallow porewater had decreased to <4.0, and high concentrations of several sulfide oxidation products were observed. Mineralogical examination of the tailings indicated the presence of alteration rims of ferric oxyhydroxide minerals that surrounded the oxidizing sulfide grains. At this site the predominant bacterial groups,were T. ferrooxidans and T. thiooxidans types. At the third site the bacterial abundance reached a maximum at 25-45 cm below the tailings surface. The occurrence of the peak abundances coincided with the interface between the zones of the unaltered tailings and the overlaying altered sulfides. This coincidence suggests that the iron oxyhydroxide coatings, which surround the partly oxidized sulfide grains, may shield the partly altered grains, thereby inhibiting bacterial growth. The data are consistent with an initial chemical oxidation of the metal sulfides to produce elemental sulfur and polysulfides, which are subsequently oxidized by neutrophilic thiobacilli to produce sulfuric acid. After the pH has decreased to the 3.5-4.0 range, the acidophilic thiobacilli, such as T. ferrooxidans, can directly oxidize the metal sulfides.
