Rio San Giorgio (Iglesiente, Sardinia, Italy), a stream affected by abandoned mine wastes, is characterized by dense
vegetation in the streambed, mainly comprised of Phragmites australis and Juncus acutus. This vegetation creates
natural biogeochemical barriers that drive mineralization processes and attenuate metals load in the stream. Several
techniques, covering scales from micrometres to kilometres, were applied to investigate the biogeochemical processes:
water chemistry, injected hydrologic tracer, mineralogy, microscopic investigation and X-ray spectroscopy. From this
multiscale and multimethod approach, we recognized two predominant sets of biogeochemical processes: microbially
driven metal sulphide precipitation, mainly resulting in pyrite formation; and plant uptake of metals that leads to
formation of iron oxide-hydroxide and incorporation of Zn within the roots and aerial part (stem and leaves). The dense
vegetation in the Rio San Giorgio streambed controls its morphology, velocity of streamflow, and, as reflected by
observed bromide-tracer loss, enhanced water exchange between the streambed and the hyporheic zone. The combined
effect of these vegetative controls is to establish biogeochemical barriers that greatly retard trace-metal mobility in the
hyporheic zone. We estimated this effect can led to an apparent decrease in Zn load up to 60%