Metals discharge from underground mine drainage of Argentiera Pb-Zn-Ag mine, NW Sardinia, Italy

Mining activities have affected, even in ancient times, numerous areas of the national territory. The footprint of its presence are often evident and sometimes visually impactful, as exploitation of underground georesources implies the modification of the original environmental configurations. These changes can be both superficial and underground and involve a series of hazards that remain even when the activity ends. Among the risks associated with mining work, the pollution of superficial and/or deep aquifers stands out. This study focuses on assessing the metals load discharged from the abandoned mineral district of the Argentiera (NW Sardinia, Italy). During last century, this mine was one of the most important Italian silver ore body, associated with Pb-Zn-Ag-Sb sulphide mineralization. After the Roman open air mining works, the ore exploitation extended into several kilometres of interconnected underground tunnels. Its drainage is discharged through a gallery at 4 m asl along the coast into the Mediterranean Sea. From this artificial spring, currently partially occluded, a small stream characterised by water with a red colour with an abundant precipitation of amorphous Fe(III)-(oxy)hydroxides flows out with a significant metals load. At the sampling site, the physico-chemical parameters (pH, temperature, total dissolved solid) were measured with a portable Hanna HI 991301 probe. Water samples were collected in pre-cleaned bottles after 0.4-μm filtering and fractions for cation analysis were acidified with suprapure-grade 1% v:v of nitric acid 65%. Mine drainage has a temperature higher compared to the annual average air temperature (21.0 °C for water), conductivity (TDS) >4 mS/cm and the pH is <6.0. For metals composition, diluted water were analysed by inductively coupled plasma-optical emission spectrometry (ICP-OES) (Varian 710-ES). Among the major dissolved components, calcium (Ca) is dominated cation, exceeding sodium (Na) concentration. Minor element such as potassium (K), and silicon (Si) were detected. Results show that the mine drainage of Argentiera contains high concentration of iron (Fe), magnesium (Mg), manganese (Mn), chromium (Cr), cadmium (Cd), arsenic (As) and very high of zinc (Zn). All these elements are potentially toxic elements and pose a serious risk of ecosystem contamination. On the other hand, Mn-Cr-Cd and As are recognised as Critical Raw Material by European Commission because of their strategic importance in green-transition technologies. Further investigation should be addressed to understand the environmental impact and to explore the viability of CRM recovering from this Acid Mine Drainage (AMD).