Pit lakes from sulphide ore mining, geochemical and limnological characterization before treatment, after liming and sewage sludge treatments : cases studies at Rävlidmyran and Udden, Sweden

Abstract: Due to the increasing number of acidic mining-pit lakes and a growing awareness of the environmental risks associated with them, pit lakes have attracted more and more attention. This study started with a full-year- round investigation of two abandoned sulphide mine pit lakes, the Rävlidmyran and Udden pit lakes in northern Sweden, followed by studies of two full-scale remediation treatments, liming and sewage sludge treatment, respectively, in the Rävlidmyran pit lake. The aim of this study is to understand the geochemistry of pit lakes and the factors influencing pit lake water quality. Based on this knowledge, effects of different remediation methods are investigated and possible improvement of these methods is explored. Furthermore, the possibility of manipulating the pit lakes to obtain efficient metal attenuation is also explored in this study. The results showed that these two pit lakes have some common characteristics; they have low pH, they are oligotrophic and have high metal and sulphate contents. However, they have developed distinctly different limnology and geochemistry. Rävlidmyran is a meromictic lake, which has a rather stable stratification due to more saline groundwater inflow at depth. Three layers have been developed: the mixolimnion at the top, in which the water is well mixed and saturated with oxygen; the monimolimnion in the deeper lake, which is poor in oxygen and which does not mix with upper water; and the chemocline, which separates the upper and deeper water. The distribution of elements such as Ca, Mg, Na, K, S, Mn, Fe and Zn is seen to be strongly controlled by the stratification with lower concentrations in the upper layer and higher concentrations in the deeper water. The Udden pit lake is a dimictic lake without permanent stratification; only seasonal thermal strata develop. The water is well mixed, which results in rather uniform distribution of parameters such as pH, dissolved oxygen, conductivity and elemental concentrations. It is concluded that a careful characterization of the limnological situation of a pit lake is very important, both for designing reasonable remediation treatment methods or being able to use pit lakes for disposal of contaminants such as mine waste. Rävlidmyran was chosen for full-scale studies of remediation. Liming treatment proved to be an effective method with respect to increased pH. The effects of the liming on the dissolved elemental concentrations in the lake water were distinct, especially in the upper oxic water layer. Elements such as Fe, Mn, Zn and Mg were largely removed in the upper oxic water, probably due to formation of oxyhydroxides. Some elements such as Na, K and S were barely influenced. Al and Cu were almost completely removed from the water throughout the profiles, due to pH increase and precipitation of minerals such as gibbsite (Al(OH)3). Sorption of Cu was probably removed from the water by to gibbsite and Fe-oxyhydroxides, since pH was rather high. However, liming is not a long-term solution as long as the acidic sources flow continuously into the lake. Therefore, the liming effect will not last long and it will be rather costly to continue liming. The sewage sludge treatment did not show distinct effects on the water quality during the monitoring period. However, particulate elemental concentrations were influenced, especially during the early stages of the treatment. Concentrations of total organic carbon increased in the water column after sewage sludge treatment. However, most of the organics and nutrients settled rapidly and accumulated at the lake bottom.