Dispersion of metals in San Giorgio river (Sardinia SW): interaction between minerals and biosphere

Trace metals, particularly Zn, are nutrients for living organisms, however, when present in the environment in high concentrations, trace metals can pose a serious threat to ecosystem and human health. Geochemical behaviour of Zn is characterized by high mobility and bioavailability, and although Zn is an essential trace element in plant metabolism, its excess can have toxic effects in plants, limiting their growth and/or diffusion. In mine environments, dispersion and attenuation of trace metals can be mediated by processes occurring at the critical zone where water, sediments and biosphere meet. As the interface between surface and groundwater, the hyporheic zone of riverbed sediments is recognized as a critically important ecotone where biogeochemical reaction takes place. Knowledge of the hyporheic zone is of interest to develop sustainable techniques such as phytoremediation. These techniques can be targeted to the stabilization and/or to the extraction of metals, their success depends upon the type of plant used and the specific geological conditions. The use of autochthonous plants for stabilization and pollution control is probably the most realistic and effective method to rehabilitate land affected by these threats. The purpose of the present study is to investigate how the interaction between mineral and biosphere can mitigate the dispersion of trace metals in mine environments. The focus of the study is on i) understanding the overall process ruling attenuation of trace metals, and ii) understanding the specific microscopic processes. Different scales of study and different investigation methods were used. Catchment scale investigation was carried out by applying hydrological tracer to the basin of Rio San Giorgio (SW, Sardinia). The riverbed of Rio San Giorgio is characterized by the presence of a natural dense vegetation comprising mainly Juncus acutus and Phragmites australis, that rules the streambed morphology, erosional processes and water flow velocity. The riverbed sediments are mine wastes with up to 1-2 % in Zn and Pb. Biogeochemical barriers were recognized to be active in the hyporheic zone of Rio San Giorgio, presence of framboidal pyrite in sediments of Rio San Giorgio and concentration of elements at the surfaces of roots is a first mechanism limiting trace metal mobility. Plants build up a second biogeochemical barrier. In fact they i) uptake significant amounts of Fe and Zn; ii) drop the excess of Fe and Zn inside the plants is by biomineralization processes leading to the formation of Zn hydroxyapatite-like or similar phases occurring in the roots; iii) translocate as Zn-organic-complexes (amino acids as Zn cysteine and Zn acetate hydrate) into stems and leaves. Rhizobox experiments and isotopic analyses were conceived to make a step forward in understanding the mechanisms that rule the interaction between mineral, water and plants under controlled conditions. A different response of the plants to Zn supply was found and this depended on mineralogy, Zn concentration in waters, and plant species. Results from rhizobox experiments indicate that the response of the plants to Zn supply can slightly change depending of the amount and the way Zn is supplied, if it is present in highest concentration in water or substrate. Overall, Phragmites a. and Juncus a. are metal tolerant species that are suitable for phytoremediation. Particularly, they are excellent candidates for phytostabilization of metals. The results obtained in this work have clarified natural attenuation processes that occur in the hyporheic zone of rio San Giorgio. The resulting information provides useful knowledge to design phytoremediation techniques in areas affected by mine pollution.