Combining microscopy with spectroscopic and chemical methods for tracing the origin of atmospheric fallouts from mining sites
Introduction
Soil-derived particulate matter is one of the main component of particles in the atmosphere [1], [2]. Particulate matter (PM) can have a natural origin, like desert dust particles, crustal emission (due to soil erosion), and volcanism, but also anthropic sources as mining or industrial airborne dust [2]. Mining activities in the South of France underwent major development in the 19th century (Zinc rush) [3]. Nowadays, numerous unexploited mining sites are abandoned and left without any specific care for contaminated waste and mining residues [4], [5], [6]. A common feature of mine ores is their heavy-metal composition. Besides the exploited metal, many other metals are present and can be toxic for ecosystems and humans. This is the case for zinc mines, where Zn is generally associated with other metals such as lead, cadmium or arsenic [7], [8], [9]. Large-scale mining activities have produced huge quantities of metal-rich wastes due to poor extraction efficiencies, which were deposited in tailing ponds [10]. The problem of such wastes relates to their capacity to release toxic metals into soils and superficial and groundwater, but also into the atmosphere through re-scattering or saltation processes [11].
The fate and impact of stored mining residues are of special concern since the risk associated with such pollution highly depends on metal concentrations and speciation, which control their toxicity [12]. Indeed, biogeochemical parameters, such as solubility and biological interactions, are specific to each of these elements. For instance, highly insoluble lead species accumulate in the environment, although ageing processes can enhance pollutants' solubility/mobility (e.g. organic lead complexes, lead carbonates, etc.) [13], [14], [15], [16]. However, lead associated to airborne particles can be inhaled or ingested by humans. Children are particularly sensitive to exposure to soil particles due to their prolonged outdoor activities in the ground vicinity [17]. Absorbed lead-rich PM (<10 μm) are easily dissolved in the body [18], thus becoming harmful. Indeed, under gastric conditions (pH < 3), some Pb species are soluble and can therefore easily pass through cell walls and reach blood circulation system [19], [20]. In the case of frequent human contacts with lead-rich particles, adverse effects such as lead poisoning can be observed [21], [22]. Csavina et al. [23] showed that human health problems are often due to mining airborne particles, particularly in Zn and Pb extraction sites. Several studies have reported positive correlations between particles' effects on human health and the distance between ore extraction and inhabited zones [24], [25], [26], [27]. However determining precisely the geographical origin of toxic particles remains a challenge, because of the multiple possible sources in polluted zones.
In the present study, we combined a set of complementary techniques to identify the origin of lead-enriched particles poisoning inhabited zones close to a Zn-mining site in the south of France. We chose eight sampling sites positioned within 3 km around Saint-Laurent le Minier (Gard, France), where the “Avinières Zn-mine” is located. Cases of children poisoned with lead have been reported in this zone [28], [29], questioning the safety of these inhabited zones. The precise origin of this contamination has not yet been clearly identified, since fugitive emissions are provided by eolian transport. These emissions may originate from several potential places in the mining area, including the mine, two ore treatment zones and the tailings. A steep-sided valley, forming a narrow corridor, subjected to winds of variable strengths/directions, characterizes the topography of the study site. This specific topography could limit the source-target distance and may imply airborne contamination due to non-vegetalized rocky slopes.
In this context, the purpose of this study was to investigate dissemination and transportation of lead-enriched airborne particles, PM, around “The Avinières” mine to identify the main source of lead-PM impacting the population. For that, a combination of physical and chemical methods was applied for the analysis of samples of surface soil and atmospheric fallout dusts from the selected locations of the contaminated area, to trace the origin of atmospheric fallouts (Fig. 1).
Samples elemental-composition was evaluated to investigate specificities among sampling sites. Soil particles were analyzed by X-ray diffraction to identify the mineral phases of both soils and fallout particles. Raman microspectrometry (RMS) was used to identify the molecular composition of these particles. This approach was coupled with particle morphology characterization by high-resolution scanning electron microscopy (SEM–EDX) to produce specific fingerprints of all studied sites, which permitted discrimination of the main sources of lead-enriched particles. The physical-chemical fingerprints of all samples were analyzed using a hierarchical clusters analysis (HCA), which confirmed our findings and permitted the clear identification of the origin of harmful Pb rich-PM. This will permit to limit the risk associated with these particles and to constrain the polluted zone to be remediated.
Section snippets
Site description and sampling
Saint-Laurent le Minier (43.930585°N, 3.656328°E, France, Gard) is located in a mining area with numerous Zn-rich ore extraction sites. The geological setting of the studied site is located at the southern end of Massif Central in the contact area between the crystalline and Paleozoic formations of the Cevennes and Causses Mesozoic formations. The sector’s geological history has favored the emergence of mineral veins enriched in Pb and Zn by the circulation of hydrothermal fluids during
Extent of the metal(loid) contamination
Table 1 presents metal(loid) s contents measured in dust samples from soils of the four “source sites” (SiteD i.e S1D, S2D, S3D and S4D). Fe, Pb and Zn are the main elements in all dust samples whereas As and Cd are the lowest. The ore storage site (S3D) is the least concentrated sample for all elements. The other sites (S1D, S2D and S4D) show high contents of As, Cd, Pb and Zn. The fingerprint of S2D is characterized by Zn and Cd concentrations twice higher than the other samples (7% and 0.06%
Conclusion
This study aimed to discriminate the origin of atmospheric metallic fallouts causing a persistent pollution in a steep-sided valley hosting an ancient zinc mine. Several potential sources of metallic particles were identified and analyzed by combining step by step chemical and physical methods to hierarchize sources' contribution to target sites and discriminate the major site responsible for lead re-deposition in habitations area. Deposition fluxes of metals contained in fallouts were measured
Acknowledgements
The authors wish to acknowledge the French Energy and Environment Management Agency (ADEME) for funding this study and to the SYMETAL Project of the CESA program of the French National Research Agency (ANR). We also thank Nathaniel Findling for Rietveld refinement expertise. LTHE is part of Labex OSUG@2020 (ANR-10 LABX56). LASIR is a part of Labex CaPPA (ANR—11-LABX-0005-01).
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2020, ChemosphereCitation Excerpt :Five strategic sites that have been selected for air biomonitoring over an area of 25 km2 in the mining district of La Unión (Fig. 1 and Blondet et al., 2019): the urban area of La Unión, the main mining tailing zone called “Avenque Tailing”, the coastal area corresponding to “Playa del Gorguel” and located near the Mediterranean Sea, an area of soil remediation by organic amendments “Santa Antonieta” and finally the agricultural area (corresponding to an old mining area) of “Las Matildes”. Atmospheric depositions (dry and wet) were also sampled using Owen gauges (NFX43-014 standard, Navel et al., 2015) exposed for 1 year and collected every 6 months (from May 2017 to April 2018). This experiment was entirely described in Blondet et al. (2019).
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2019, Science of the Total EnvironmentCitation Excerpt :The fate of stored mining residues remains a very high concern because the associated risk depends on metal contents but also on the mineral species in presence. Actually, according to abiotic parameters (such as pH, temperature, redox potential, etc.), metal speciation can change and enhances their bioavailability, and then their toxicity for the environment and the human populations (Goix et al., 2014; Navel et al., 2015). The “Sierra Minera” of Cartagena-La Unión corresponds to a large zone of mining activities in the south-east of Spain whose first exploitation by the Carthaginians (Murcia county, n.d.) is the oldest in Europe and dates back to the 3rd century BCE (Gonzalez-Fernandez et al., 2011).