Environmental gradients, soils and mining activities

What are environmental gradients?

Mine tailings are evidence of the exploitation of metal bodies that took place until the last century. These tailings may require remediation because they represent a significant quantity of contaminated materials that can be dispersed in the environment.

The presence of these mine tailings forms gradients related to the presence of metals in the soils, often from upstream (close to mining tunnels, strong contamination) to downstream (lesser contamination). Along these gradients, significant changes in vegetation composition are observed. In addition, along these gradients, and related to the amount of metals in the soil, other factors important for plant development may vary, such as the amount of nutrients present for plant growth, or stoniness and erosion. This part of the project focuses on characterizing the set of physical and chemical parameters of the environment that vary along these gradients.

Gradient of metal presence in soils and associated vegetation change, Chichoué area. From left to right, the contamination decreases to an area not impacted by mining activity (right).
The pink flowered species is Armeria muelleri.

Objectives of this research axis:

The objective of this research axis is to characterize the environmental gradients that influence plant development, the strategies they implement to adapt to them, and ultimately the interactions between plants.

The study of environmental gradients includes the characterization of:

  1. the total metal and metalloid concentrations within the study areas
  2. The environmental availability of metals (potentially transferred from the the soil to the soil solution via physicochemical desorption processes), and the environmental bioavailability (potentially taken up by a living organism via physiological processes)
  3. The other soil and physical parameters: texture, organic matter, water content, nutrients
  4. The topography and micro-relief
  5. The weather conditions, which differ greatly between the study areas at low and high altitudes (1000 to 2000 m). The two main parameters monitored are temperature and humidity (air and soil).

 

Main researchers :

Gaël Bellenfant,  BRGM, Research engineer and post-mining officer, g.bellenfant_at_brgm.fr

Valérie Laperche, BRGM, Research engineer expert in mineralogy and X-Ray Fluorescence measurements, v.laperche_at_brgm.fr

Valérie Sappin-Didier, INRAe, Research fellow, valerie.sappin-didier_at_inrae.fr

Christophe Nguyen, INRAe, Senior scientist, christophe.nguyen_at_inrae.fr

Jérémie Melleton, BRGM, Geologist and expert in gitology, j.melleton_at_brgm.fr

 

Main achievements:

In summer 2020, each study area was subjected to numerous measurements using a portable X-ray fluorescence spectrometer (a method for measuring the mass concentrations of chemical elements) and soil samples were taken for laboratory analysis of total element content, pedogeochemical parameters, and the environmental availability of metals.

Chichoué area: Mine tailings and geological formations
(Photo : Jérémie Melleton)

A geological synthesis was also carried out in June 2020 in order to place the sites in their geological context and to help distinguish between the presence of metals related to former mining activity and spontaneous concentrations (related to natural outcrops).

 

During the project, we characterized the studied gradients, both in relation to contamination and the environmental availability of metals, as well as the more general evolution of other physical and chemical parameters. Pollution characterization on the site was carried out on over 300 soils through direct measurements (1,084 pXRF readings across 7 sites, i.e., 140 to 198 measurements per site, and 16 XRD analyses) and laboratory analyses of pedo-geochemical parameters (particle size distribution, carbonate content, organic carbon, pH, phosphates, CEC, exchangeable cations, mineralization, trace metal elements) and mineralogical components (quartz, amphibole, calcite, goethite, muscovite, galena, talc, chlorite, hydrozincite, feldspar, sphalerite, smectite), as well as geological data (lead and zinc vein-type ore embedded in carbonate rocks). Pollution distribution maps were produced accordingly. Contamination levels at any point were inferred through interpolation of measured values from sampling locations. This work allowed for precise contextualization of observation and experimental zones used by all project partners.

The mechanisms governing the environmental availability of metals/metalloids were studied and modeled using the measured pedo-geochemical parameters. For instance, Cd, Pb, and Zn concentrations weakly bound to soil particles are mainly controlled by soil pH. For more strongly bound forms, availability is governed by both competition among elements and the abundance of sorption sites. This work showed that, on the Sentein site—regardless of pollution intensity, substrate origin (soils developed on schist or carbonate rocks), and thus the metal-bearing mineral phases—metal extraction using acetic acid is a good indicator of environmental metal availability across all soils. Adjusting pollution levels according to the ecotoxicological toxicity of contaminants revealed that the ecological risk is extremely high for all studied soils with respect to Cd, Zn, and Pb. Notably for Cd, whose ecotoxicological risk is approximately 2 and 20 times higher than that of Pb and Zn, respectively.

In addition to soil analyses, digital surface models (DSM) and digital terrain models (DTM) were generated from 30 datasets (collected at the beginning and end of the growing season, using visible and near-infrared imagery, lidar, and with varying resolutions—lidar: 60–80 points per m², 10 cm resolution; photogrammetry: 2–3 mm and 2 cm). These datasets enabled the production of maps representing geomorphological gradients (distance from slope crest, solar exposure, shading, distance to thalweg, scree cones, bare rock, fractures, etc.) and morphometric features (slope, aspect, vertical curvature in the steepest direction), as well as reference vegetation maps (habitat zoning maps).

Illustration of the realisations: left: total Zn contamination map of one study site; right: overview of the different study site at high elevation.