Draining through industrial areas of the Minas Gerais mining state (Brazil), some tributaries of the São Francisco River constitute a potential environmental hazard for this great river and threaten the quality of the regional soils for agriculture and other activities. Extensive geochemistry and mineralogy of sediments, soils and alluvial plains from six selected areas within the Consciência drainage basin close to an important Zn-extraction plant, have been carried out. In this report, detailed mineralogy of those samples and supporting geochemical data are discussed, taking into account their specific climactic and environmental context. Petrographic and electron microprobe characterization of the sand-grained fraction of these materials was complemented by XRD on their finer fraction: the main contaminant minerals are willemite (one of the Zn ores used in the industrial plant) and jarosite, though their contents are quite variable in the studied areas and also with depth; minor amounts of Zn-, Pb-, Cd-, and Mn-bearing mineral phases are also frequent, usually as inclusions in willemite or in polycrystalline clasts, or adsorbed on the finer materials, such as clay minerals and associated Fe-hydroxides. Mineralogical contamination is responsible for high metal contents in the soils and sediments of the areas closer to the plant (e.g. Zn ≫ 2000 mg kg −1 and Cd ≫ 20 mg kg −1 , which are the Intervention Values for Industrial Areas) and the greatest contamination risks are related to the more labile phases that circulate throughout the alluvial plains, the shallow sediments and the stream bed. Monitoring the mineral/chemical contamination and its extent also constitutes a useful basis for future proposals to remediate and recover this industrial area in order to decrease medium-and long-term negative impacts of metal contamination on the local and downstream environments.

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The geochemical composition of rare earth elements (REE) in the bottom sediments of two Dominican reservoirs and in soils from their catchments was studied to identify possible sources of the deposited materials. Knowledge of the origin of the sediments will serve to control the excessive rates of erosion and sedimentation that occur annually due to periodic extreme climatic events that promote excessive silting of the lakes, followed by loss of storage capacity and degradation of water quality. The REE contents of sediments and soils were normalized to the North American Shale Composite (NASC) and the ratio of light/heavy rare earths (LREE/HREE ratio), Ce and Eu anomalies, and some fractionation parameters were determined. The REE patterns are more homogeneous in the sediments, indicating uniform sedimentation in both deposits. The sediment data reflect depletion of REE from the sources, enrichment of light REE (LREE) and some middle REE (MREE), and positive Eu and Ce anomalies. All data were plotted in correlation diagrams between some fractionation parameters of light–middle–heavy REE and anomalies of Ce and Eu. The similarity of the ratios between these parameters in all samples and the overlap of data from soils and rocks on the sediment projection in the diagrams allowed a good discrimination of the main sources of the materials.

In areas contaminated by potentially toxic elements (PTEs), knowledge of processes of metal mobilisation is the basis for the choice of appropriate remediation methodologies. The mobilisation of metals is a function of several factors, and the response to these factors must be well known during the planning of remediation strategies. The activity of an ore metallurgical plant in South-East Brazil resulted in major contamination by several heavy metals. Reversing the contamination’s negative impact required geochemical assessment of the area, including the physicochemical characterisation, quantification, and delimitation of PTEs, and the rating of the solubilisation/mobilisation capacity of these elements. The definition of spatial patterns for PTEs’ distribution allowed the construction of contamination risk maps which work as a tool for the mitigation and control of the contamination plume. The chemical analysis of interstitial water and selective and sequential extraction methodologies showed that elements that occur in the environment in critical concentrations (Zn, Cd, Pb, As) are mostly associated with easily mobilised forms (soluble, exchangeable cations, associated with Mn oxides). Given the great mobility of the contamination plume, any process of removal of contaminated material becomes unfeasible, thus the strategy of remediation for the stream and associated alluvial deposits must be based on methods of in situ decontamination.

A column experiment at a laboratory level was carried out to assess the effect of the application of nanotechnology in the decontamination of soils and alluvial deposits with high levels of potentially toxic elements (PTEs). A suspension of zero-valent iron nanoparticles (nZVI) was injected at three different concentrations in selected samples (two sediments, one soil). For most of the elements, the retention by nZVI was proportional to the concentration of the suspension and the trend was similar. Metals were immobilized by adsorption on the surface layer of the nanoparticles and/or by complexation, co-precipitation, and chemical reduction. By day 60 following injection, the nZVI lost reactivity and the retained species were desorbed and back into the soluble phase. The definition of spatial patterns for PTEs’ distribution allowed for the construction of contamination risk maps using a geostatistical simulation approach. The analysis obtained from the extractable contents of five target elements (Zn, Cu, Cd, Pb, As) was cross-checked with the estimated map network to assess their retention efficiency. Data from the analysis of these elements, in the extractable phase and in the porewater of the sediments/soils, indicate the nZVI injection as a suitable technique for reducing the risk level of PTEs in contaminated Fe-rich tropical environments.