The long-term interaction of mine tailings with soils and the wider environment: Examples from Mont Chemin, Switzerland

; ; ; ; ; ; (). The long-term interaction of mine tailings with soils and the wider environment: Examples from Mont Chemin, Switzerland. Journal of Geochemical Exploration, 182. 53-69. Peer reviewed.

There is no doubt that mine tailings and waste are an environmental problem and can be a risk to human health and other organisms. Although many studies about acid rock drainage have been performed, the dispersal of trace elements and their impact on the environment (groundwater, soils and agricultural use) and the long-term (i.e., decades to centuries) behaviour of mine tailings on silicate parent rock is still a matter of debate. Our main scope was, therefore, to develop an approach that enables a relatively fast and reliable prediction of the leaching
of potentially toxic elements and, thus, to determine the reactivity of former mining sites. To do so, we traced back the leaching behaviour of metals such as copper (Cu), chromium (Cr), cadmium (Cd) and lead (Pb) in soils formed on tailings from small mines over a relatively long time period. Using a chronosequence approach based on historical records of mining activities and C-14 dating, element leaching could be traced over a time period of approximately 1000 years for the historic mining sites at Mont Chemin (Switzerland). The weathering index B, that expresses chemical leaching of base cations and weathering of silicate minerals, and trace element ratios to immobile compounds (such as hafnium (Hf)) indicated that element leaching, and thus reactivity of the material, was very intense during the first 200 years after abandonment of the site. Between approximately 200–400 years, the rate of elemental change in the soils was strongly reduced tending (moderate reactivity) to very small values, and consequently low reactivity, for exposure ages of> 400 years. Thus, the chronosequence approach enabled to determine the reactivity of the sites. Soil organic carbon and oxyhydroxides were not effective enough to completely retain trace metals. Many former small mining sites exist in the Swiss Alps. Using
a database that contains information about the period of activity, date of abandonment, elements used and geology, we extrapolated our findings to sites in Switzerland with moderately comparable conditions to Mont Chemin. This relatively simple empirical approach enabled a spatialisation and rough estimation of the sites regarding their reactivity. A large number of small mining sites in Switzerland were characterised as still highly reactive and only a few were considered to be weakly reactive. With the help of the chronosequence approach
and extrapolation of the small former mining sites, a relatively fast overview of the reactivity of sites could be achieved. This approach, however, cannot substitute for a detailed risk analysis of such sites.