CE01 - Terre fluide et solide

Impact of tailings on Environment: example of transfers of elements from Spoil tips in the North of France – ENTRESOL

Spoil tips of the North of France – a regional symbol, an UNESCO world heritage, an ecological niche … and a source of pollutions? Focus on the ANR ENTRESOL project

The spoil tips of the North of France are, on the one hand, a wonderful opportunity for the development of an unique and original ecosystem (colonization by pioneer species) that must be protected, but on the other hand, a potential source of pollutants for the surface environment (leaching and alteration of spoil tips). To know if this balance is broken, transfers must be defined and quantified, and the environmental risk assessed. This is what the ANR ENTRESOL is working on.

Main issues raised and general objectives

Since the beginnings of the industrial period, the exploitation of the underground resources has been constantly expanding, in line with the ever-increasing demand of mankind to satisfy their needs and comfort. This exploitation of the subsoil has strong impacts on the environment notably with the generation of large amounts of waste. These “geological” wastes can represent immense amount of material, piled up to form large hills, significantly modifying the landscape and called tailings. These tailing piles are often stored in direct contact with the atmosphere, meteorological waters and the biosphere, thereby submitted to weathering and leaching of their constituted material. Accordingly, potential pollutant/toxic elements can be transferred from tailings to their direct environment causing a risk for the human health and the ecosystems. With the spreading of mining areas and the extraction of new ores, the risk of pollution by tailings is a worldwide problem.<br />So, to face up to these social, economic and environmental issues of tailings/environment interactions, the processes of release and transfer of pollutants must be accurately understood to quantify their impact and to minimize it.<br />With regard to coal mining in particular, despite a number of environmental hazards associated with this type of mines, and environmental issues due to extraction and dumping of voluminous amounts of discarded mine tailings (spontaneous combustion, landslides, contaminant mobilization, leaching of elements, …), a lot of coal spoil tips around the world have just recently started to be investigated or else are poorly studied.<br />In France, this problematic is particularly present in old mining regions and quarries, notably in the North of France which holds one of the largest coal mining area in the country. Although the spoil tips - tailings formed by the residue and wastes of the past coal exploitation - are considered as sterile, we can observe the formation of neo-soil and the colonization by pioneer species at their surface. This makes them original and unique ecosystems in the region which must be protected. But beyond the originality of this kind of systems and the apparent obvious benefit for biodiversity, spoil tips can also be responsible for transfer of potential pollutant/toxic elements originating from the parent geological material (trace metal elements, sulfur, organic compounds, etc.), and transported via surface and subsurface runoff.<br />This project aims to characterize the spoil tips/water/surrounding environment interaction and transfers, to establish the flow balance of mobile elements, and to identify potential risks of pollution.

The main originality of the project resides in multidisciplinary approach to understand processes and fluxes of individual entire spoil tip bodies. Research in this project will cover 4 different spoil tips with specific characteristics and include geological, geophysical, hydrological, ecological and biogeochemical approaches. Several sampling, measuring and analytical protocols are specially developed for this project and have never been applied before on this type of geological object which are spoil tips. Thus, to characterize the weathering processes of the geological material, nanoscale compositional analyses will be conducted, GPR (Ground Penetrating Radar) will be deployed to study the spoil tips internal hydrological system, and lab leaching tests will be used to establish links between element release from geological material and neo-soil and the in situ composition of surface runoff and percolating waters.

XRD analyses of unaltered black schists sampled on the Ostricourt and the Sabatier Nord spoil tips show a mineralogical homogeneity of the samples: mainly quartz, phyllosilicates (mainly micas of illite and muscovite type, kaolinite and chlorite), and iron oxides. We also note the presence of interstratified, especially for the samples of the Sabatier Nord spoil tip. Observations in optical microscopy and SEM confirm these results, also attesting to the presence of titanium oxides, pyrite and rare alkaline feldspars. During the observation of the samples, witnesses of fluid circulation could be observed: the presence of veins of automorphic quartz, neoformed kaolinite and calcite.
XRD analyses of the alteration fronts (alteration crusting at the surface of schist) show that they consist of quartz and phyllosilicates (mainly illite/muscovite micas), as well as jarosite, completely absent from the unaltered schist. More detailed observations and analyses at the SEM showed the presence of other minerals in the weathering front and especially a chemistry very different from unaltered schist. Indeed, the unaltered schist is composed mainly of Si, associated with quartz, Al and K contained in phyllosilicates, while the weathering front is composed essentially of Fe, S and K, neoformed phyllosilicates and quartz detached from schist. On some parts of the alteration front, the Fe/S ratio is homogeneous, on others, it is variable at the micrometric scale: we then clearly identify a chemical zonation, with two superimposed layers one richer in S and K, and the other richer in Fe.
Ultrathin section, 10 µm x 5 µm x 100 nm (cut with a FIB Focused Ion Beam), cut from a schist sample with a chemically zoned of the alteration front, were observed under a transmission electron microscope (TEM) and analyzed at the synchrotron by STXM-XANES (December, 2021). It was shown that these two layers constituting the alteration front have a thickness of about 10 µm, the one enriched in Fe is composed of oxides and clays, the one enriched in S and K is constituted of a mixture of jarosite more or less well crystallized, oxides and residues of quartz and detrital clays. All these observations confirm that Fe and S can be easily transferred from schist to the environment, as is observable in mine acid drainage, but that two distinct mechanisms – and which need to be identified – are at work.

The perspectives of the ENTRESOL project are based on three research axes (geological, pedological and geochemical): to characterize the parent geological material and the mineral weathering processes (characterization of the source of transfers of elements), to investigate the soil formation and to define the role of neo-soil in transfers of elements, to determine and to quantify transfers of elements to surface environment. Combining results of these three research axes, we will be able to help in development of strategies for a better management of spoil tips and their ecosystems. Although centered on spoil tips in the North of France, results will be transposable to similar systems all over the world (Wallonia (Belgium), Russia, England, …).

P. Claisse, F. Bourdelle, E. Lloret (2021). Impact of tailings on Environment: example of the weathering of schist constituting spoil tips in the North of France. Affiche. RST 2021, Lyon, Novembre 2021.

The exploitation of the subsoil to satisfy human needs and comfort has strong impacts on the environment notably with the generation of large amounts of waste. These “geological” wastes can represent immense amount of material, piled up to form large hills, significantly modifying the landscape and called tailings. These tailing piles are often stored in direct contact with the atmosphere, meteorological waters and the biosphere, thereby submitted to weathering and leaching of their constituted material. Accordingly, potential pollutant/toxic elements can be transferred from tailings to their direct environment causing a risk for the human health and the ecosystems. So, to face up to these social, economic and environmental issues of tailings/environment interactions, the processes of release and transfer of pollutants must be accurately understood to quantify their impact and to minimize it.
In France, this problematic is particularly present in old mining regions and quarries, notably in the North of France which holds one of the largest coal mining area in the country. Although the spoil tips - tailings formed by the residue and wastes of the past coal exploitation - are considered as sterile, we can observe the formation of neo-soil and the colonization by pioneer species at their surface. This makes them original and unique ecosystems in the region which must be protected. But beyond the originality of this kind of systems and the apparent obvious benefit for biodiversity, spoil tips can also be responsible for transfer of potential pollutant/toxic elements originating from the parent geological material (trace metal elements, sulfur, organic compounds, etc.), and transported via surface and subsurface runoff.
This project aims to characterize the spoil tips/water/surrounding environment interaction and transfers, to establish the flow balance of mobile elements, and to identify potential risks of pollution. Thus, we plan to explore three research axes: (1) A geological axis, to characterize the parent geological material and the mineral weathering processes (characterization of the source of transfers of elements), (2) a pedological axis, to investigate the soil formation and to define the role of neo-soil in transfers of elements, and (3) a geochemical axis, to determine and quantify transfers of elements to surface environment. Combining results of these three research axes, we will be able to help in development of strategies for a better management of spoil tips and their ecosystems. Although centered on spoil tips in the North of France, results will be transposable to similar systems all over the world (Wallonia (Belgium), Russia, England, …).

To carry out this project, research will include geological, geophysical, hydrological, ecological and biogeochemical approaches. Several sampling, measuring and analytical protocols are specially developed for this project and have never been applied before on this type of geological object which are spoil tips. Thus, to characterize the weathering processes of the geological material, nanoscale compositional analyses will be conducted (STXM-XANES for the redox state of Fe), GPR (Ground Penetrating Radar) will be deployed to study the spoil tips internal hydrological system, and lab leaching tests will be used to establish links between element release from geological material and neo-soil and the in situ composition of surface runoff and percolating waters. This project draws on the expertise of the participants in mineralogy, biogeochemistry, hydrology and geophysics.

Project coordinator

Madame Emily LLORET (LABORATOIRE GÉNIE CIVIL ET GÉO-ENVIRONNEMENT - LILLE NORD DE France)

The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.

Partner

LGCgE LABORATOIRE GÉNIE CIVIL ET GÉO-ENVIRONNEMENT - LILLE NORD DE France

Help of the ANR 220,853 euros
Beginning and duration of the scientific project: December 2020 - 48 Months

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