The effect of bioremediation methods involving different degrees of soil disturbance on the export of metals by leaching and by plant uptake

Lysimeter experiments were performed in order to investigate the effects of two bioremediation methods on the export of metals from a former uranium mine site located near Ronneburg, Thuringia, Germany. The first method consisted of the application of topsoil and compost to contaminated soil. Soil was sampled from two amended plots and from an unamended control, all located on a former uranium-leaching heap. The second method involved the mixing of the soil from the control plot with expanded clay, mycorrhizal fungi and Streptomyces sp. The lysimeters were sown with a mixture of two plant species. The export of metals by leaching and by plant uptake from the amended plots was higher than in the control, mainly because of the higher hydraulic conductivity and the higher plant biomass. The inoculation of control soil with mycorrhizal fungi decreased the leaching of metals, but the effect on metal uptake by plants was metal and species dependent. The extra inoculation with Streptomyces sp. increased the leaching of many metals, and also in many cases the metal uptake by plants. The cumulative export by plants and leaching was higher in the treatments with expanded clay in the case of Mg, Mn, Ca, Ni and Pb, smaller in the case of U and Cu, and did not differ in the case of Cr. Copper, Cr, Pb and, to some extent, U tended to accumulate in the top horizon. For these metals, plants played a more important role than the leaching in the total export. For the other metals, a significant decrease in concentrations in the top horizon was associated with a moderate to high export dominated by leaching.