Biogeochemical evaluation of soil covers for base metal tailings, Ag-Pb-Zn Cannington Mine, Australia

This study reports on the transfer of metals from soil covered tailings into native plants at the Cannington Ag-Pb-Zn mine in semi-arid northwest Queensland, Australia. A number of field trial plots were established over sulfidic metal-rich tailings in 2001. The plots differed in either soil depth (200, 500, 800 mm) or the combination of local soil and waste rock (1600 mm) used for the construction of the trialed capping strategy. In 2004, the plots were sampled for their cover materials, cover plants and tailings to evaluate the performance of the different cover designs. In all field trial plots, the roots and to a lesser degree the foliage of native plant species (Triodia longiceps, Astrebla lappacea, Astrebla squarrosa, Iseilema membranaceum, Rhynchosia minima, Sclerolaena muricata), growing on the soil covered tailings, display evidence of biological uptake of Ag, As, Cd, Pb, Sb and Zn, with values being up to one order of magnitude above background samples for the same species. The plants acquired their detected metal distributions from the tailings and mineralized waste rocks as evidenced by the penetration of plant roots through the entire soil cover to the top of the tailings or the mineralized waste rock layer. In general, the plant species growing on the soil covered tailings have bioconcentration factors (BCF, metal concentration ratio in plant roots to DTPA-extractable soil) and translocation factors (TF, metal concentration ratio of plant foliage to roots) for As, Cd, Cu and Zn and for Ag and Pb greater than one, respectively.\ud \ud The trialled covers allow the translocation and accumulation of trace metals into the above-ground biomass of metal-tolerant native plants. Such processes may introduce metals and metalloids into surrounding ecosystems despite the waste remaining physically isolated. Hence, engineered dry covers of mine waste repositories need to consider the root penetration depth of native plants as well as the bioavailability of metals and their possible translocation and accumulation into the above-ground tissue of cover plants