A research report submitted to the Faculty of Science, University of the Witwatersrand in fulfilment of the requirements for the degree of Master of Science, Johannesburg, August, 2014. / Efflorescent minerals are a common feature of the soil surface in seasonal environments where evapotranspiration (ET) exceeds precipitation (P), and are formed by the evaporation of salt solutions from the soil during periods of drying. On the Highveld gold fields, ET exceeds rainfall by approximately two-and-a-half times during the dry season, and soils overlying acid mine drainage and along polluted stream banks can become covered by distinctively coloured mineral efflorescent crusts. Whereas some efflorescent minerals are relatively insoluble and present a negligible environmental hazard (for example, gypsum), others may be readily soluble and contain high concentrations of potentially toxic metals (for example, copiapite, jarosite and uranyl sulphate). During periods of rainfall, such salts are washed further afield and into surface water bodies and act as sources of episodic pollution.
The presence of some efflorescent minerals can be detected from their characteristic reflectance signatures using remote-sensing (RS) of the electromagnetic spectrum. The species of efflorescent minerals present is a useful indication of the spatial extent of sub-surface contamination, and also of the chemical conditions of the substrate, in particular the concentration of total dissolved solids, pH and redox conditions.
The aim of this study was therefore to assess the use of remote-sensing on indicator efflorescent minerals as a cost-effective aid in the spatial mapping of acid rock-drainage polluted soils and water-bodies. This study describes the range of efflorescent crusts identified on different land-use areas and soil classes in a Highveld gold-mining region. Crusts were first measured in-situ under natural sunlight using a portable analytical spectral radiometer (ASD) as well as using X-ray diffraction (XRD). They were then dissolved in deionized water and the resulting salt solutions allowed to evaporate prior to analysis under controlled lighting conditions. Spectra were post-processed and compared with
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geological spectral reference libraries. The salt solutions were also analyzed for metal and sulphate content and the results were used to establish evaporation models from which mineral precipitation could be predicted.
Minerals identified in the visible near-infra red (VNIR) region included iron oxides (hematite and goethite), and the sulphate mineral jarosite. In the short wave infra-red (SWIR) region clay minerals of the smectite group were dominant. Gypsum and Al-Mn-Mg-Na sulphate salts were identified in the SWIR region as mixtures occurring with clay minerals. Minerals identified in the VNIR-SWIR region were all confirmed by X-Ray diffraction (XRD). Upon dissolution, geochemical modeling revealed that gypsum and jarosite are the most common minerals expected to precipitate. The precipitation of gypsum and jarosite indicates persistent acidic conditions after dissolution of mineral salts. Gypsum and jarosite were also accurately identified by hyper-spectral spectroscopy and confirmed by XRD and geochemical modeling. Agreement between spectral interpreted minerals and geochemically precipitated mineral phases demonstrated the ability of hyper-spectral data in detecting efflorescence minerals on the soil surface. Using partial least squares regression (PLSR) combined with bootstrapping, reflectance spectrum was significantly correlated with geochemical variables.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:wits/oai:wiredspace.wits.ac.za:10539/16878 |
| Date | 05 February 2015 |
| Creators | Maya, Mafuza |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | application/pdf |
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