Henning Holmström

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 microg/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.

Sequential extraction of sulfide-rich tailings remediated by the application of till cover, Kristineberg mine, northern Sweden.

A sequential extraction has been carried out on sulfide-rich mine tailings. The purpose was to investigate how elements released by oxidation are secondarily retained in the tailings and the possible consequences of the remediation. After investigating the solid tailings, seven samples were chosen for sequential extractions. Two samples were oxidised, situated just above the oxidation front; two samples from just below the former oxidation front with increased concentrations of several elements; two unoxidised samples were from an intermediate depth, and the deepest sample was from the tailings-peat boundary at the bottom of the impoundment. Five phases were extracted: adsorbed/exchangeable/carbonate; labile organics; amorphous Fe-oxyhydroxides/Mn-oxides; crystalline Fe-oxides; and organics/sulfides. The addition from dried porewater to the extracted fractions has been calculated and considered as minor. In the oxidised tailings, the sulfide fraction still dominates for elements such as Fe, S, Cd, Co, Cu, Hg and Zn, although the concentrations are low compared to the unoxidised tailings. Generally, the second most important fraction is the adsorbed/exchangeable/carbonate fraction. Below the oxidation front, the sulfide content of the tailings sharply increases. In the secondary enrichment zone, the total element concentrations increase compared with the deeper unoxidised samples, mainly due to secondary retention. For some elements, secondary retention is greater than the total amount extracted for the deeper unoxidised samples. In the secondary enrichment zone the adsorbed/exchangeable/carbonate fraction represents approximately 20 wt.% or more for Cd, Co, Mn, Ni and Zn. The amorphous iron oxyhydroxide or the crystalline iron oxide fractions are less important at this level, although for As, Ba and Cu the amorphous iron oxyhydroxide fraction represents up to 20 wt.%. At the lower border of the enrichment zone, the total concentration for most metals is lower, but the importance of the adsorbed/exchangeable/carbonate fraction is further enhanced for Cd, Cu, Ni and Zn. These elements have 35-60 wt.% of the total amount from this fraction. For As, Cd, Cu, Ni and Pb, the secondary fractions extracted (extractions A-D) represent between 60 and 80 wt.% of the total content. At greater depth in the impoundment the relative importance of the adsorbed/exchangeable/carbonate fraction decreases, whilst the importance of amorphous iron oxyhydroxide and crystalline iron oxide fractions increases. The adsorbed/exchangeable/carbonate fraction is the most easily remobilised fraction. A raised groundwater table previously situated below the enrichment zone may result in the release of secondarily retained metals.

Multi-element analysis of wild berries from northern Sweden by ICP techniques

In this study, the abundances of 60 chemical elements were determined in berries of blueberry (Vaccinium myrtillus) and lingonberry (Vaccinium vitis-idaea) by a combination of inductively coupled plasma atomic emission spectrometry (ICP-AES) and inductively coupled plasma double focusing sector field mass spectrometry (ICP-SMS). Samples of both species were collected at 35 sites in northern Sweden. The sites are related to four zones representing areas affected by different types of human activity as well as an adjacent reference area. Special care was taken to keep sample contamination during sampling and sample preparation as low as possible. Different approaches such as use of an internal quality control sample, spike recovery tests and comparison between different analytical techniques, were used to ensure the quality of the results. Variations in element concentrations within individual sampling sites were estimated, based upon in-site duplicate sampling and analysis. The contribution from surface contamination to total berry concentrations was assessed by analysis of samples before and after rinsing with water. A comparison of the two species showed that, in spite of very similar concentrations for the majority of the elements, the highest Tl, Sr and Ba values were found in lingonberry while the highest Cl and Re concentrations were found in blueberry. The highest total concentrations of Ag, As, Be, Bi, Br, Cd, Hg, I, Ni, Pb, Sb and Tl were found in berries from mining areas, whereas those of Li, V, Hf, W, Ta and REE were found in the vicinity of high-traffic roads.

Layers rich in Fe- and Mn-oxyhydroxides formed at the tailings-pond water interface, a possible trap for trace metals in flooded mine tailings

Geochemical field studies of the flooded sulphide-rich mine tailings at Stekenjokk in northern Sweden have been performed. Minor diffusion of elements from the tailings to the pond water is occurri ...

Oxygen penetration and subsequent reactions in flooded sulphidic mine tailings : a study at Stekenjokk, northern Sweden

Abstract A study of O2 penetration and pore water geochemistry of the flooded tailings at Stekenjokk has been performed. The results show that there is a diffusion of elements from the tailings pore water to the overlying water. The presence of elements such as Ca, Mg, S, Si, Ba and Sr are likely the result of diffusion of older process water trapped in the tailings. Oxygen concentrations in the tailings measured with microelectrodes show that there is O2 available down to 16 to 17 mm depth in the tailings. Pore water analyses show that there are subsurface maxima for the elements Cu, Zn, Ni, Co and Cd at depths of 0.25 to 2.75 cm. The highest concentrations of almost all elements were found where previously oxidised material was deposited before the flooding. Lower pH is measured in the uppermost part of the tailings compared with the pond water and the tailings pore water at depth. Oxidation of sulphides in the uppermost part of the tailings is probably occurring. A decrease in oxidation rate can be expected in the future due to deposition of organic material at the tailings surface. Flooding seems to be an efficient remediation method at Stekenjokk.

Geochemistry of the infiltrating water in the vadose zone of a remediated tailings impoundment, Kristineberg mine, Northern Sweden

At the remediated tailings Impoundment 1 at Kristineberg, Northern Sweden, installations of tension lysimeters were performed in the protective cover (10, 50, and 100 cm), in the oxidised tailings ...

The character of the suspended and dissolved phases in the water cover of the flooded mine tailings at Stekenjokk, northern Sweden

Studies of the suspended and dissolved phases of the pond water, material collected from sediment traps, and surficial sediments/tailings from the flooded tailings pond at Stekenjokk have been performed. The aim was to characterise the material, to study the seasonal variations and to quantify possible resuspension of the tailings in the pond. The element concentrations in the pond at Stekenjokk seem to be largely controlled by processes controlling the precipitation and dissolution of Mn- and Fe-oxyhydroxides in both the water column and in the surficial tailings. Physiochemical processes such as weathering of silicates on the surrounding mountain slopes or dykes contributes both dissolved elements and detrital particles. The suspended phase consists of detrital silicate material as well as Fe- and Mn-oxyhydroxides. The average heavy metal concentrations are high, e.g. 0.42% Cu, 0.15% Pb and 3.1% Zn, which is probably due to sorption onto Fe- and Mn-oxyhydroxides. The suspended phase is richer in Fe, and particularly Mn, during the winter. The suspended phase resembles the material collected in sediment traps and the material in the surficial sediments. The pond water is well mixed during the ice-free season. The dissolved heavy metal concentrations are generally rather low with, e.g. maximum concentrations of 2.03 micrograms/l Cu, 0.23 microgram/l Pb and 268 micrograms/l Zn during the winter. Higher dissolved concentrations are found below the ice-cover above the sediment surface during the winter, caused by diffusion of elements from the sediment-water interface up into the pond water. Most of the metals occurring in the pond are dissolved and resuspension of tailings is negligible.

Desorption of metals retained secondarily after release by sulphide oxidation; the main mechanism for groundwater contamination in the tailings at the Laver mine, northern Sweden

Abstract Geochemical studies of pore water and groundwater in sulphide-bearing tailings have been performed at the Laver mine in northern Sweden. Pore water has been sampled from just above the groundwater table down to the peat and till underlying the tailings. Groundwater has been sampled weekly from April to November in pipes installed at various depths in the tailings. All samples were analysed for major and trace elements by using ICP-AES and ICP-MS. When the oxidation front in the tailings is moving downwards, metals released by weathering in this low-pH environment are to a large extent retained secondarily in the tailings below the oxidation front and do not reach the groundwater, except in areas where the oxidation front is situated close to the groundwater table. Vertical flow of precipitation water contaminated with metals released by sulphide oxidation is, thus, not the major explanation for groundwater contamination. Instead, contamination occurs when the advancing oxidation front pushes the secondary enrichments of metals ahead to meet the groundwater table and the metals are released to the groundwater. The release of metals is caused by desorption due to the low pH in this environment. Areas of the tailings deposit with shallow groundwater table are at present the main sources of metal release. There is a seasonal variation in the composition of groundwater, particularly shallow groundwater. This is caused by changing levels of the groundwater table. Rising groundwater table results in outflush of metals in areas where the groundwater reaches secondarily retained metals. A steady trend with rising groundwater table after snowmelt results in a larger proportion of contaminated water in shallow groundwater, which decreases during the autumn. Concentration peaks of Cu and other metals in shallow groundwater may be the result of small, rapid rises of the groundwater table due to strong precipitation, superimposed on the general seasonal trend.