Potential biogeochemical and ecological development of a flooded tailings impoundment at the Kristineberg Zn–Cu mine, northern Sweden

The potential short-term ( V 102 years) and long-term (>102 years) biogeochemical and ecological effects of diverting stream water (pH 4.9–6.7) into a limed, flooded tailings impoundment (pH 8–12) were studied by combining geochemical and biological data. In the long-term perspective, the successional development of lakes was used as a natural analogue. Based on the vertical distribution of temperature and total dissolved solids (TDS < 0.22 Am), the impoundment can be characterised as a continuous/discontinuous cold polymictic lake, with holomictic summer circulation. A re-inoculation study indicated that the growth of autotrophic, aerobic bacteria (presumably Acidithiobacillus ferrooxidans) presently is inhibited by the high pH in the impoundment. In a short-term perspective, termination of liming and diversion of stream water into the impoundment will result in a complex interplay between physical, biogeochemical and ecological effects. A reduced vertical mixing of the f2-m-deep water column, dissolution of calcite and gypsum (compounds of a sludge formed in the impoundment) and an enhanced microbiological activity are major expected effects. The dissolution of calcite may act as a pH buffer and result in metal remobilisation from the sludge. Excluding autochthonous organic matter produced in the impoundment, streamwater input of suspended matter and formation of settling flocculants are expected to result in a sediment accumulation rate of f1.5 mg cm 2 year 1 (1.6–3.3 cm/102 years). Settling allochthonous organic C (0.15–0.30 mg C cm 2 year 1) may serve as an oxygen barrier and as a reservoir of organic compounds capable of driving redox reactions. In a long-term perspective, a hydroseral development into a wetland/peatland can be expected, with a bog lake, poor fen or flat bog as final stage. This development presupposes a decreasing pH when liming is terminated and stream water is diverted into the impoundment. It also assumes that the impoundment will be similar to an acidified lake, and that the succession is driven by Sphagnum colonizing the impoundment. If the hydrological conditions/water level is affected (e.g., by climatic changes or a dam failure), a terrestrialization culminating in coniferous forest on peat soil may occur.