1. Mineralogy and heavy metal distribution in soils along a serpentinized/ultramafic transect of the Great Dyke, Zimbabwe
Courage Bangira, C.T. Hallmark and Richard H. Loeppert
Texas A&M University
Soil & Crop Sciences, College Station,TX 77843
Results
Introduction
Soils formed from ultramafic/serpentinized rocks present agronomic, ecological and environmental challenges (Gasser and Dahlgren, 1994). Metal toxicity, plant nutrient deficiencies and the high potential to contaminate surface- and ground-water are perhaps the most important adverse effects on the environment. Due to their diverse chemical composition, serpentines give rise to soils of varied mineralogy and chemistry.
Table 1.Elemental soil composition (0-0.15m) at different sampling locations.
Figure 2.A typical soil profile located on the crest and footslope position.
Objectives
To determine the soil mineralogical and heavy metal composition in soils across a toposequence of the Great Dyke of Zimbabwe.
Composite soil samples were taken from the crest, footslope and backslope at m and m depths.
Analyses were done on 2mm-sieved samples and/or fractionated materials.
Soil pH was determined in 1:1 (w/v) de-ionized water
Instrumental Neutron Activation Analysis used to determine soil elemental composition.
Mineral identification was done using X-ray diffraction methods and FTIR transmittance.
Elemental composition was determined by INAA method.
Methods
Figure 3. XRD pattern for clay minerals on the crestal position.
Figure 4. Infrared spectra of clay mineralogy on the crest position.
Figure 5. XRD pattern of clay minerals on the footslope position.
Figure 6. Infrared spectra of clay mineralogy on the footslope position
Figure 1. A sketch map of Zimbabwe showing the location and aerial view of the study site
Figure 7. XRD pattern of clay minerals on the toeslope position.
Figure 8. Infrared spectra of clay mineralogy on the toeslope position.
Discussion:
Soils contain elevated levels of heavy metals
A significant reduction of metal concentration occured in backslope.
Talc and kaolinite were predominant in crestal position.
Serpentine was abundant in footslope in all soil fractions.
Smectite, talc, kaolinite and vermuculite occurred on the backslope.
Conclusion:
Weathering of serpentine/ultramafic rocks enriched soil with heavy metals.
Clay minerals in footslope are largely authigenic.
High soil moisture in backslope resulted in the transformation and neoformation of clay minerals.
Significantly low metal levels in backslope could be associated with the mobility of redox-sensitive metals.
Reference:
Gasser, U.G.& Dahlgren, R.A Soil Sci. 158:
Marel van der, H.W. & Beutelspacher, H Atlas of Infrared Spectr. Of Clay Min. and Their Admix.
Acknowledgement: Dr W.D. James and Michael Raulerson for Instrumental Neutron Activation Analysis.
Dept. Soil Science & Agric Engineering, UZ.; Chemistry & Soil Research Institute, Zimbabwe for soil transportation logistics