Sulphide-rich tailings remediated by soil cover evaluation of cover efficiency and tailings geochemistry, Kristineberg, northern Sweden

Sammanfattning: The efficiency of soil cover as a method of remediation of sulphide-rich tailings has been studied at an impoundment at the Kristineberg mine, Northern Sweden. Two variations of soil cover were used in the remediation. The major part of the impoundment was covered with a 1.0 m layer of till where the groundwater table was shallow. In combination with the removal of the water dividing ditches surrounding the impoundment, saturation of the tailings as well as the till cover was achieved. In areas with a deeper groundwater it was not possible to saturate the tailings by means of this method. Instead, a sealing layer consisting of a 0.3 m compacted clayey till, acting as an oxygen diffusion barrier, situated underneath a 1.5 m protective cover was used. Field studies at the impoundment cover pore-water extraction and solid-sample collection at five locations. Solid tailings were subject to sequential extractions in the laboratory. Open groundwater pipes for measuring groundwater levels as well as BAT® groundwaterpipes for geochemical sampling of the groundwater were installed over the entire impoundment. At a location in the area with the sealing layer, tension lysimeters were installed in a profile in the vadose zone down to the unoxidised tailings. Nearby, one oxygen diffusion lysimeter and one water infiltration lysimeter were installed below 1.5, 1.0, and 0.3 m of protective till cover, respectively. The sealing layer has been investigated in the laboratory with respect to its susceptibility to the effects of freezing and thawing. The solid samples from the tailings revealed that in some areas, the sulphide oxidation prior to the remediation had been intense. In other areas, with a shallower pre-remediation groundwater table, the oxidation seemed to have ceased upon reaching it. In the area with water- saturated tailings increased pore water concentrations around and below the oxidation zone were visible, due to release of secondarily retained elements. Elements with peaks at this level were As, Cd, Co, Cr, Cu, Mo, Ni, Pb, and Zn. However, compared with pre-remediation data the concentrations are generally lowered, indicating that sulphide oxidation has slowed down. Sequential extraction of the solid tailings samples showed that a large part of the elements below the oxidation front, in the secondary enrichment layer, are relatively mobile and are released within the adsorbed, or the amorphous iron (oxy)hydroxide fractions. This was the case for elements such as Fe, As, Ba, Cd, Co, Cr, Cu, Ni, and Zn. The continuous measurements performed in the groundwater pipes show that elements released by raising the groundwater table are transported out of the impoundment, and that the overall water quality is constantly improving due to the inflow of uncontaminated groundwater from the adjacent hill slope. A model for the water transport has been developed and prediction of the future behaviour of the impoundment is proposed. The tension lysimeter measurements show that infiltrating water and diffusing oxygen cause remobilisation of metals around the oxidation front. However, most of these metals are retained again prior to reaching the groundwater table. The mass flow caused by this mobilisation is very small compared to that of the laterally flowing groundwater. Mobilised elements are Fe, S, Si, Al, Cd, Co, Hg, Mg, Mn, Mo, Ni, Pb, and Zn. The freeze/thaw laboratory experiments stress that the compaction degree is very important for achieving a hydraulic conductivity low enough for the requirements of a sealing layer. If a high enough compaction degree is obtained, the corresponding hydraulic conductivity is very low, approximately 5x10-10 m/s with the clayey till used at the study site. The freeze/thaw experiments also revealed that when properly compacted the clayey till is sensitive to frost penetration, leading to an increase of hydraulic conductivity, up to ~10-8 m/s. Oxygen diffusion measurements indicate that the effective diffusion through the sealing layer is low for all three different protective till-cover thicknesses, and so is the water infiltration. However, during the field measurements, no frost penetration into the sealing layer was monitored.