The PhD student Max Giannetta, from the Groundwater and Hydrogeochemistry research group, will […]
The PhD student Max Giannetta, from the Groundwater and Hydrogeochemistry research group, will defend his thesis online on 25th March at 17h.
If you want to attend online, please contact with doctorat.ciencies.terra@ub.edu up to 48 hours before the defence.
Title: A biogeochemical study of an abandoned Pb-Zn mine in the Aran Valley, Spain; applications of natural attenuation of heavy metals via secondary hydrozincite precipitation
Directors: Jordi Cama and Josep M. Soler
Thesis Committee: M. Mercè Corbella, Cristina Domènech and Eva Marguí
Abstract
The Aran Valley in Catalonia (Spain), was the site of large-scale Zn, Pb, Cu, Fe, and Ag mining from the late 19th century until approximately 1950. Although mining activities ceased over 70 years ago, some of the abandoned mining relics (e.g. tunnels, processing facilities, and tailings dumps) exhibit elevated concentrations of metals in the associated water systems, thus posing a health risk in the neighboring ecosystem. In this study, the largest underground zinc mine in the Aran Valley (the Victoria Mine) was chosen as a field site to showcase the processes affecting metal mobility in the environment. This mine is composed of 15 km of tunnel galleries, many of which are flooded today, and is situated beneath a steep mountain slope. Moreover, the lowermost gallery exhibits a continuous output of water, whose composition reflects a flow path through the mine and overlying host rock.
Three sampling campaigns to the mine (2019, 2020, and 2021) provide a spaciotemporal dataset showing the evolution of solute concentrations through the system along a flow path. Rainwater from the upper catchment flows into the host rock above the mine where it dissolves ore materials (primarily Zn (sphalerite) and Fe (pyrite) sulfides with Ni and Cd impurities) causing elevated Zn, Ni, and Cd concentrations where these waters enter the mine. From there, the precipitation of hydrozincite (Zn5(CO3)2(OH)6) along the tunnel gallery where water flows serve as a metal polishing mechanism resulting in significantly diminished metal concentrations (e.g. Zn from 155 to 10 ppm, Ni from 377 to 32 ppb, and Cd from 105 to 22 ppb). Hydrozincite has been shown to be product of biomineralization in other sites. However, by way of DNA sequencing of local communities, microbial batch experiments, morphological comparisons to samples known to be a product of biomineralization and geochemical modeling, it is confirmed that the mineral forms abiotically at the Victoria Mine.
Characterization of the solid samples taken from the mine reveals several different morphologies, Zn zonations in hydrozincite (i.e. purity changes), and small amounts of smithsonite (ZnCO3) and calcite (CaCO3). In general, all solids show layering, which is a result of intermittent precipitation of distinct solid products. These precipitation patterns are likely a result of changing solute concentrations and precipitation rates. The cause of these changes is hypothesized to be rain events, which change the water residence time in both the host rock and in the gallery, thereby altering the water composition.
Solubility experiments and speciation calculations done using the CrunchFlow code demonstrate that hydrozincite here does not have a constant Keq value, but rather a range of values (30.0 < log[Keq] <37.68 at 7°C). This behavior is presumed to be a result of precursor effects, such that amorphous solids with higher solubilities may form before the structured hydrozincite. Notably, all analyzed solids from the collected mine samples show significant amounts of amorphous material (i.e. broad XRD peaks).
A CrunchFlow reactive transport model constructed to capture the processes occurring along the flow path through the catchment (i.e. rainwater infiltration and flow through the host rock → mixing of different gallery source (drip) waters → flow of water through the gallery), successfully reproduces the measured concentrations. Further, the model predicts the formation of hydrozincite, calcite, and smithsonite while maintained their relative proportions consistent with that of the samples. A sensitivity analyses of the most relevant parameter values (e.g. hydrozincite solubility, flow velocity, dispersivity and mineral reactivities) is performed as a way to capture the range of behaviors expected to occur, and thereby predict future changes to metal mobility. The model confirms that changes to flow velocity, which would translate to changes in volumetric discharge through the system caused by rain events, could be the cause of mineral layering and metal zonation in hydrozincite. The model also confirms that the natural metal attenuation process is expected to occur through a wide range of system perturbations. Thus, the abandoned mine poses little risk to the neighboring ecosystem.
