Evolution of the groundwater geochemistry in sulphide bearing tailings remediated by applying till cover, Kristineberg, Northern Sweden

Sammanfattning: At the Kristineberg mine in northern Sweden, sulphide-rich tailings were remediated in 1996 by applying till covers. The sulphides were isolated from the atmosphere to prevent oxidation. This was realised either by covering with unspecified till in combination with a raised groundwater level making the tailings water saturated, or by covering with compacted clayey till overlain by unspecified till (dry cover). During and after 1998, 14 groundwater pipes were installed in the tailings impoundment to study the effects of the remediation. Sampling has been performed on more than 32 occasions during more than three years, and the samples were analysed for major elements and metals, anions, pH, redox and conductivity. The results show that the groundwater characteristics vary considerably in the impoundment, even under the same type of cover. In the tailings with simple till cover and raised groundwater, in year 2001, pH ranges between 3.9 and 7, Fe-concentration between 30 and 19,000 mg/L, and Zn-concentration between 16 and 2,500,000 µg/L. The pH is generally increasing and redox is generally decreasing across the studied impoundment, where some areas showed quicker changes than others. The source reaction of the pollution seems to have stopped or slowed down. Natural buffering by chlorite dissolution occurred in the tailings. Metals such as Pb, Cu, and Cd decreased drastically after remediation, probably principally by adsorptions in connection with the raised pH, cutting the average concentrations across Impoundment 1 by up to 270 times for Cu. Before the remediation, metals released by sulphide oxidation were partly secondarily retained in the tailings below the oxidation front. Among them, Fe, S, Mg and Zn, have been remobilised by a first washout as a result of the raised groundwater level. A second washout by the clean groundwater from the western till slope, combined with the slowing down of the acid-producing reactions should lead to significant and general decreases in the observed concentrations in 2004-2005. However, iron precipitations occur in some places, creating a secondary acidification, and resulting in a secondary buffering by micas and release of adsorbed metals like Zn. The pathways and mixings of the groundwater in the impoundment which were first outlined by an hydrological study could be confirmed by the study of K, Ba and Sr. One of the main drawbacks of the remediation is an increased release of As, possibly in response to an increased pH, reaching up to 4350 mg/L As, as an average for year 2001 in one of the groundwater pipes.