Geology topics

Francesca Podda

Publications and source records attributed to Francesca Podda.

3 recordsLinked to original sources

USGS2018

Application of hydrologic-tracer techniques to the Casargiu adit and Rio Irvi (SW-Sardinia, Italy): Using enhanced natural attenuation to reduce extreme metal loads

Hydrologic tracer techniques were applied to Rio Irvi (SW Sardinia), a stream affected by mine drainage, allowing the calculation of stream discharge and metal loads and comparison to other streams. The calculated discharge showed a continuous increase from near 21.2 L/s to 29.1 L/s. Cumulative loads of mine-related constituents, including the Casargiu adit inflow, were large, with more than 9900 kg/day of SO 4 2− , 2370 kg/day of Zn, 550 kg/day of Fe and 172 kg/day of Mn. The greatest measurable inflow source of metals, other than the Casargiu adit, was an acidic tributary (L4), but most sources of instream metal load were related to dispersed groundwater inflows. Some of those groundwater inflows were related to non-flowing tributaries. Calculations of the cumulative instream metal load, excluding the Casargiu adit inflow, indicated increases of 1250 kg/day for SO 4 2 , 858 kg/day for Zn - , 137 kg/day gain for Fe and 60 kg/day for Mn. Rio Irvi Zn load was extreme for a stream of this size and discharge. A comparison with two other mine-affected rivers in Sardinia indicated the loading in Rio Irvi was two to three orders of magnitude greater. This difference was attributed to different geochemical conditions, but also to a lack of a biogeochemical barrier like that seen to be acting along and below the riverbed in Rio San Giorgio. Several years of intense vegetation growth in the river bed of Rio San Giorgio created a biogeochemical barrier to metal loading, and the cumulative Zn load there was near 8 kg/day, despite being a drainage with a greater mass of mine wastes to contribute to the load.

Sardinia↗

USGS2016

The role of natural biogeochemical barriers in limiting metal loading to a stream affected by mine drainage

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 lead to an apparent decrease in Zn load up to 60%.

Iglesias↗

USGS2015

Isotopic insights into biological regulation of zinc in contaminated systems

Aquatic organisms use a variety of biogeochemical reactions to regulate essential and non-essential trace metals. Many of these mechanisms can lead to isotopic fractionation, thus measurement of metal isotopes may yield insights into the processes by which organisms respond to metal exposure. We illustrate these concepts with two case studies, one involving an intra- and the other an extra-cellular mechanism of Zn sequestration. In the first study, the mayfly Neocloeon triangulifer was grown in the laboratory, and fed a diet of Zn-doped diatoms at Zn levels exceeding the requirements for normal mayfly life functions. The N. triangulifer larvae consumed the diatoms and retained their Zn isotopic signature. Upon metamorphosis, the subimago life stage lost Zn mass either in the exuvia or by excretion, and the Zn retained was isotopically enriched. Thus, Zn uptake is nonfractionating, but Zn regulation favors the lighter isotope. Thus the Zn remaining in the subimago was isotopically heavier. In the second study, Zn was adsorbed on the cell walls and exopolysaccharide secretions of cyanobacteria, which favored the heavier Zn isotope. Continued adsorption eventually resulted in nucleation and biomineralization of hydrozincite {Zn5(CO3)2(OH)6}. These case studies demonstrate the utility of Zn isotopes to provide insights into how aquatic insects respond to metal exposure.

Procedia Earth and Planetary Science↗