Impoundment 1 at the Kristineberg mine, northern Sweden, was remediated in 1996 with the installation of a soil cover. Since the installation of the soil cover, oxygen diffusion and water infiltration into the tailings has been greatly reduced, with a resulting decrease in sulfide oxidation in the impoundment. This study investigates the pore water chemistry below the soil cover in order to determine if the pore water composition has been affected by the cover application. In addition, this study assesses vertical variations in tailings composition, in terms of magnetic susceptibility and density, and assesses the impact of these variations on pore water composition. These parameters are studied in tailings cores collected in October 2001.

Dissolved oxygen was analyzed in cores from an area of the Impoundment where only a single layer cover exists. Microelectrode analyses in the field showed relatively high dissolved oxygen concentrations (20 – 110 µM) below the cover in cores collected from unsaturated tailings. In saturated tailings, the dissolved oxygen concentrations were generally <2 µM.

The results from a tailings core collected in the vicinity of the former dam wall indicate that pore water pH increases from ca. 4.2 to 6.1 over an interval of about 200 cm below the oxidation front, with the lowest pH levels closest to the oxidation front (265 cm depth). The greatest metal and sulfate concentrations are measured in the unoxidized tailings at ca. 300 cm depth, with lower concentrations at the oxidation front and at deeper intervals. Tailings density appears to increase slightly with depth, and magnetic susceptibility greatly varies, reflecting the presence of ferromagnetic mineral phases. Iron redox chemistry in the saturated tailings was investigated with the determination of Fe(II) and Fe(III) in pore water. The measured Fe(III) concentrations are high and exceed ferrihydrite solubility. Ferric iron complexation is discussed as a potential process for maintaining such high Fe(III) levels in solution.