Mobility of toxic metals from calcareous mine tailings in compost amended mesocosms.

The elevate acid neutralization capacity of dolomite and other sedimentary carbonate rocks contained in the mine tailings of the Raibl mine (Cave del Predil, Italy), a dismissed zinc mine situated in the Julian section of the Southern Limestone Alps range, has so far minimized leaching of toxic metals from abandoned mill tailing deposits. However, their reclamation by amendment with compost, as well as the natural reclamation of a boundary strip situated near a wooded zone, may increase mobilization risks. The effect of compost addition was studied by collecting leacheates from model constructed profiles (mesocosms) incubated for 100 days at 25 °C, under a simulated rainfall regime that mimicked the average precipitation patter of the summer months. A single layer profile of tailings was compared with a single profile of compost amended tailings and with a two layer profile: the 10 cm top layer made by compost amended tailings and the 10 cm bottom layer made of native tailings. Significantly more Zn and Tl leached from the composite profile while Pb and As were more mobile in the amended tailings (single profile). The lower horizon of native tailings was able to drastically reduce metals leaching from the top horizon. This different behaviour is coherent with a fundamental role of DOC in the mobilization of Pb and As and with its much lower influence on that Zn and Tl. Mobilization of Zn and Tl was governed by S oxidation and to a lesser extent by carbonates.