Impact of biogeochemical barriers on the fate of zinc (Zn) in the metal-polluted Iglesiente and Arburese mine districts (SW-Sardinia, Italy)

Mining activity is cause of deep landscape morphology modifications and often leaves critical legacy represented by huge volumes of mine wastes and residues of metallurgical processes. These wastes are usually made up by highly reactive materials which lead to the mobilization and dispersion of harmful elements in soils and waters, up to several kilometers from the source of contamination, representing a serious threat for health of humans, animals and environment.

The Iglesiente and Arburese mine districts, in South West Sardinia (Italy), have been interested by intensive mine exploitation lasting for centuries. The mine activity was mainly addressed to zinc (Zn) and lead (Pb) extraction from primary sulphides and secondary non-sulfide mineralizations (calamine ores). After the mine closure (1968−1995), no real actions addressed to mitigate the environmental impact have been adopted; therefore, either surface or underground waters can interact with minerals present in the flooded adits, mining wastes and tailings, causing the mobilization and dispersion of high amounts of contaminants.

Several studies have been carried out to understand the processes ruling the fate of metals, with particular interest for Zn, the most important contaminant of the study areas. Results of hydrological tracer techniques show that the Zn load of the investigated rivers differs up to 3 orders of magnitude, from about 6 kg/day (Rio San Giorgio) up to about 2000 kg/day (Rio Irvi). These differences do not seem to be linked to a substantially different discharge, but appear to be related to the peculiarity of each river in terms of riverbed morphology, vegetal coverage, water flow and sedimentation/erosional rates, that may favor, in the hyporheic zone, the onset and development of processes limiting the metal mobility. For example, scanning electron microscopy (SEM) analyses of core sample collected from the riverbed has shown the presence of metal sulphides (FeS₂, ZnS) attributable to the supergene formation of secondary phases promoted by the microbial sulphate reduction. Moreover, the metal intake/immobilization in roots and stems of plants (e.g. Phragmites australis and Juncus acutus) and in their rizosphere, have been observed.

These processes constitute natural biogeochemical barriers, that can effectively limit the metal dispersion and whose understanding can help in planning effective remediation strategies.

 

The authors acknowledge CESA (E58C16000080003) from RAS and RAS/FBS (F72F16003080002) grants, FP7 ERANETMED2 72094 SUPREME, the Grant of Excellence Departments, MIUR (ARTICOLO 1, COMMI 314 – 337 LEGGE 232/2016),  the CeSAR (Centro Servizi d'Ateneo per la Ricerca) of the University of Cagliari, Italy, for SEM analysis, and the POR FESR Sardegna 2014-2020 (project cluster Top-Down: TESTARE)