Geochemical investigations of sulfide-bearing tailings at Kristineberg, northern Sweden, a few years after remediation

In the Kristineberg mining area in northern Sweden, massive, pyrite-rich Zn Cu ores are intercalated in ca. 1.9 Ga volcano-sedimentary rocks. Investigations of a tailings impoundment remediated by means of both till coverage and raising the groundwater table have been undertaken. The aim of the study was to characterise the tailings with respect to mineralogy, the chemical composition of both the tailings and the pore water, and to try to identify the significant reactions that may have occurred before and after remediation. It was found that the oxidation front had reached down to depths of between approximately 0.1 and 1.15 m before remediation. The oxidation of sulfides has produced high concentrations of some metals in the pore water; up to 26, 16, 4.1, 2.7 and 82 mg l have been measured for Al, Mn, Fe and Zn, respectively. Concentrations of metals such as Cd, Co, Cu, Ni and Pb are lower, with average concentrations of 18.4, 83.8, 45, 79.6 and 451 g l, respectively. Higher concentrations of major elements such as Ca, Fe, Mn, Mg and S have been measured at depth in pore water than at shallower levels. This is probably caused by flush out of elements after remediation and vertical transport from the upper parts before remediation. The pH is relatively high, approximately 5.5 at most depths in the tailings, except in and around the former oxidation zone where it is lower, and where the highest dissolved concentrations of elements such as As, Cd, Co, Cu, Pb and Zn occur. This is probably due to the release of metals secondarily retained below the oxidation front prior to the remediation. Since the groundwater table is raised, the groundwater reaches the retained metals, which leads to desorption of metals and dissolution of secondary minerals.