Trace element exposure in soils and vegetation of the Gatumba Mining District, Rwanda Master thesis Technical University Braunschweig, Germany Institute of Environmental Science Department of Soil science and Soil physics Advisor: Prof. Dr. Rolf Nieder Second advisor: Prof. Dr. Harald Biester 16.01.2012 Submitted by: Inga Paulmann Matriculation number: 4033080
I Table of contents 0 Abstract...1 1 Introduction...2 2 Background...3 2.1 Coltan...3 2.2 Trace elements involved...3 3 Material and methods...7 3.1 Investigation area...7 3.1.1 Climate of the Gatumba Mining District...8 3.1.2 Geology of the Gatumba Mining District...8 3.1.3 Soils of the Gatumba Mining District...10 3.1.4 Agricultural land use and mining activities in the Gatumba Mining District...10 3.2 Sampling and sample preparation...10 3.2.1 Soil samples...10 3.2.2 Plant samples...11 3.3 Analyses of soil samples...11 3.3.1 Soil acidity...11 3.3.2 Texture...12 3.3.3 Cation exchange capacity...13 3.3.4 Carbon and nitrogen contents...15 3.3.5 Potassium, phosphorus and boron contents...15 3.3.6 Trace element contents...16 3.4 Analyses of plant samples...17 3.4.1 Carbon and nitrogen contents...18 3.4.2 Potassium, phosphorus and boron contents...18 3.4.3 Trace element contents...18 3.4.4 Bioconcentration factors...19 4 Results...20
II 4.1 Quality of soil sample measurements...20 4.2 Reference soil groups, soil properties and trace element contents...21 4.3 Results of chemical analyses according to soil groups...38 4.3.1 Soil acidity...39 4.3.2 Cation exchange capacity...41 4.3.3 Carbon and nitrogen contents...42 4.3.4 Potassium, phosphorus and boron contents...44 4.3.5 Trace element contents...47 5 Quality of plant sample measurements...55 5.1 Plant species and trace element contents...56 5.2 Results of chemical analyses of plants on different soil groups...63 5.2.1 Carbon and nitrogen contents...63 5.2.2 Potassium, phosphorus and boron contents...64 5.2.3 Nutrient bioconcentration factors...66 5.2.4 Carbon and nitrogen contents in samples of dry and rainy season...67 5.2.5 Potassium, phosphorus and boron contents in samples of dry and rainy season...69 5.2.6 Trace element contents...71 5.2.7 Trace element bioconcentration factors of plant samples...77 5.2.8 Relocation of trace elements within plants...78 5.2.9 Trace element contents in samples of dry and rainy season...81 6 Discussion...82 6.1 Soil samples...82 6.1.1 Soil properties, carbon and nutrient contents...82 6.1.2 Trace element contents...85 6.2 Plant samples...90 6.2.1 Carbon and nutrient contents...90 6.2.2 Trace element contents...91 7 Conclusions and Outlook...97
III 8 References...101 Appendix...104 List of Figures Figure 1: Location of the investigation area Gatumba Mining District (red circle), Rwanda...7 Figure 2: Soil sampling locations of Technosols, Fluvisols, Gatumba and Ruhanga Reference soils...8 Figure 3: Geological map of the Gatumba area (Dawaele et al. 2008, modified after unpublished map from Somirwa)...9 Figure 4: Technosol, Ruhanga mining...22 Figure 5: Technosol, Ruhanga mining site...23 Figure 6: Technosol overlying a Gleysol, Ruhanga mining site...24 Figure 7: Technosol, Ruhanga mining site...25 Figure 8: Technosol in Gatumba village 26 Figure 9: Cambic Fluvisol downstream of Ruhanga mining site...27 Figure 10: Cambic Fluvisol at the Nyabarongo River...28 Figure 11: Fluvisol of the Nyabarongo River flood plain..29 Figure 12: Colluvium overlying Fluvisol downstream of Ruhanga mining site.30 Figure 13: Fluvisol downstream of Ruhanga mining site.31 Figure 14: Cambisol in the northeastern part of Gatumba village 32
IV Figure 15: Umbric Leptosol in the northeastern part of Gatumba village..33 Figure 16: Vertic Umbrisol in northeastern part of the Gatumba village 34 Figure 17: Acrisol at the fringe of the Ruhanga mine...35 Figure 18: Acrisol at the slope toe..36 Figure 19: Cambisol on schist at the upper part of the hill..37 Figure 20: Soil acidity in soil groups (P1-16, T: Technosols, F: Fluvisols, R.G.: Reference soils Gatumba area, R.R.: Reference soils Ruhanga area) 39 Figure 21: Soil acidity in nine soil profiles (P1-5 and P10-13) of the dry season (dry s.) and rainy season (rainy s.).40 Figure 22: Cation exchange capacities in soil groups (P1-16)..41 Figure 23: Carbon contents in soil groups (P1-16) 42 Figure 24: Nitrogen contents in soil groups (P1-16)..43 Figure 25: Potassium contents in soil groups (P1-16)..44 Figure 26: Phosphorus contents in soil groups (P1-16)...45 Figure 27: Boron contents in soil groups (P1-16)..46 Figure 28: Trace element contents in Technosols (P1-4, 6)..47
V Figure 29: Trace element contents in Fluvisols (P1-5 and P10-13).48 Figure 30: Trace element contents in Reference soils of the Gatumba area (P7-9)..48 Figure 31: Trace element contents in Reference soils of the Ruhanga area (P14-16).49 Figure 32: Chromium contents in soil groups (P1-16)..51 Figure 33: Lithium contents in soil groups (P1-16)...51 Figure 34: Rubidium contents in soil groups (P1-16)...52 Figure 35: Topsoil trace element contents in nine soil profiles of the dry and rainy season (P1-5 and P10-13) 53 Figure 36: Subsoil trace element contents in nine soil profiles of the dry and rainy season (P1-5 and P10-13) 54 Figure 37: Technosol (P2) cultivated with Ipomoea batatas...61 Figure 38: Fluvisol (P5) cultivated with Manihot esculenta. 61 Figure 39: Nephrolepis spec. near Technosol P6..61 Figure 40: Polygonum pulchrum..61 Figure 41: Acrisol (P14) with the medicinal plant Phytolacca dodecandra 62 Figure 42: Psidium guajava...62 Figure 43: Carbon contents in plant groups on corresponding soil groups (P1-16, T.: Technosols, F.: Fluvisols, R.G.: Reference soils Gatumba, R.R.: Reference soils Ruhanga).62 Figure 44: Nitrogen contents in plant groups on corresponding soil groups (P1-16).63 Figure 45: Potassium contents in plant groups on corresponding soil groups (P1-16).64 Figure 46: Phosphorus contents in plant groups on corresponding soil groups (P1-16)..64
VI Figure 47: Boron contents in plant groups on corresponding soil groups (P1-16)..65 Figure 48: Carbon contents in plants of the dry season (dry s.) and rainy season (rainy s.), sampled on soil profiles P1-5 and P10-14...67 Figure 49: Nitrogen contents in plants of the dry and rainy season, sampled on soil profiles P1-5 and P10-14...67 Figure 50: Potassium contents in plants of the dry and rainy season, sampled on soil profiles P1-5 and P10-14...68 Figure 51: Phosphorus contents in plants of the dry and rainy season, sampled on soil profiles P1-5 and P10-14 69 Figure 52: Boron contents in plants of the dry and rainy season, sampled on soil profiles P1-5 and P10-14..69 Figure 53: Trace element contents in plants of the dry season on Technosols (P1-4, 6) 70 Figure 54: Trace element contents in plants of the dry season on Fluvisols (P5, 10-13).71 Figure 55: Trace element contents in plants of the dry season on Gatumba Reference soils (P7-9) 71 Figure 56: Trace element contents in plants of the dry season on Ruhanga Reference soils (P14-16)...72
VII Figure 57: Lithium contents in plant groups on corresponding soil groups (P1-16) 73 Figure 58: Rubidium contents in plant groups on corresponding soil groups (P1-16)...74 Figure 59: Zinc contents in plant groups on corresponding soil groups (P1-16).75 Figure 60: Trace element contents in plant roots (on soil profiles P1 and 5) 77 Figure 61: Trace element contents in plant stems (on soil profiles P1-5, P10-14)..77 Figure 62: Trace element contents in plant leaves (on soil profiles P1-5, P10-14).78 Figure 63: Trace element contents in plant fruits (on soil profiles P2-4, P11-13) 78 Figure 64: Trace element contents in plants, sampled in dry and rainy season (on soil profiles P1-5, P10-14).79 List of Tables Table 1: Mean chromium and nickel contents in plant samples and contamination values (Ref 1, G: Reference plant sample ground with Janke&Kunkel grinder; Ref 1, K: Reference plant sample cut with ceramic knife)...19
VIII Table 2: Mean recovery rates of trace element contents in certified standards...20 Table 3: Properties of the Technosol horizons...22 Table 4: Trace element contents in the Technosol horizon [ppm]...22 Table 5: Properties of Technosol horizons...23 Table 6: Trace element contents in Technosol horizons [ppm]...23 Table 7: Properties of the Technosol and Gleysol horizons...24 Table 8: Trace element contents in the Technosol and Gleysol horizons [ppm]...24 Table 9: Properties of the Technosol horizon...25 Table 10: Trace element contents in the Technosol horizon [ppm]...25 Table 11: Properties in the Technosol horizons...26 Table 12: Trace element contents in the Technosol horizons [ppm]...26 Table 13: Properties of the Fluvisol horizons...27 Table 14: Trace element contents in the Fluvisol horizons [ppm]...27 Table 15: Properties of the Cambic Fluvisol horizons...28 Table 16: Mean trace element contents in the Cambic Fluvisol horizons [ppm]...29 Table 17: Properties of the Fluvisol horizons...30 Table 18: Trace element contents in the Fluvisol horizons [ppm]...30 Table 19: Properties of the Fluvisol horizons...31 Table 20: Trace element contents in the Fluvisol horizons [ppm]...31 Table 21: Properties of the Fluvisol horizons...32 Table 22: Trace element contents in the Fluvisol horizons [ppm]...32 Table 23: Properties of the Cambisol horizons...33 Table 24: Trace element contents in the Cambisol horizons [ppm]...33 Table 25: Properties of the Umbric Leptosol horizon...34 Table 26: Trace element content in the Umbric Leptosol horizon [ppm]...34 Table 27: Properties of the Vertic Umbrisol horizons...35 Table 28: Trace element contents in the Vertic Umbrisol horizons [ppm]...35 Table 29: Properties of the Acrisol horizons...36
IX Table 30: Trace element contents in the Acrisol horizons [ppm]...36 Table 31: Properties of the Acrisol horizons...37 Table 32: Trace element contents in the Acrisol horizons [ppm]...37 Table 33: Properties of the Cambisol horizons...38 Table 34: Trace element contents in the Cambisol horizons [ppm]...38 Table 35: Mean recovery rates of trace element contents in certified standards...55 Table 36: Trace element contents and guidance values in stem and root vegetables [ppm].56 Table 37: Trace element contents and guidance values in leafy vegetables and herbs [ppm]...58 Table 38: Trace element contents and guidance values in fruits [ppm]...58 Table 39: Trace element contents and guidance values in animal feed [ppm]...59 Table 40: Trace element contents in wild plants [ppm]...61 Table 41: Mean, minimum and maximum nutrient bioconcentration factors of plant samples67 Table 42: Mean, minimum and maximum trace element bioconcentration factors of plant samples...77
Trace element exposure in soils and vegetation of the Gatumba Mining District, Rwanda 1 0 Abstract Soils and vegetation of the Gatumba Mining District (GMD), located in Rwanda, are potential hosts of deleterious trace element accumulation due to open-cast coltan mining of columbitetantalite and cassiterite minerals, derived from rare-metal tantalum pegmatites. Toxic trace elements associated with coltan mine spoils are for example arsenic, lead and uranium. Since mine spoils are cultivated with agricultural crops, the local population may be exposed to deleterious trace elements entering the food chain. Accumulation of such elements is in particular expected for Technosols on mine spoil, slope-water influenced Gleysols and Fluvisols located in the mining district. In the dry season (August 2010) and rainy season (March 2011), horizons (up to three replicates) of Technosols on pegmatite mine spoil, directly influenced by mining activities, of Fluvisols indirectly influenced and of Reference soils developed on dolerite and sandstone/schist, not affected by mining activities, were sampled. On each sampling site, plant species were sampled as well (fifty species altogether, up to six species on a soil profile). The soil samples were analysed on general properties like soil acidity (ph), cation exchange capacity (CEC), texture, carbon and nutrient contents of nitrogen, potassium, phosphorus and boron. Plant samples were analysed on carbon and nitrogen contents. The focus of this study was the analysis of potentially available trace element contents in soil and plant samples, which were extracted with aqua regia and nitric acid. The extracts were measured with the ICP-MS (inductively coupled plasma mass spectrometer) of the Institute of Environmental Geology of the Technical University Braunschweig, Germany. The investigated soils generally were low in ph, CEC, soil organic matter and nutrient contents, except for the Reference soils Umbric Leptosol and Vertic Umbrisol. Trace element contents were below critical values and in range of normal soil contents. With few exceptions resulting from uncertainties on which guidance value for edibles to use and the calculation of guidance values from fresh plant matter for dry matter weight, trace element contents of plants were also below critical limits and in range of normal plant contents. A trend for higher trace element contents in soils and plants grown on soils influenced by mining compared to indirectly influenced or non-affected soils was not observed. Due to the pegmatite parent rock within mining areas, a natural geogenic enrichment of lithium and rubidium in Technosols was identified. In general the geological composition of bedrock was reflected in measurement results. There is no exposure of the local population to deleterious trace element accumulation on investigated GMD sites, due to trace element contents in plants and soils below critical values. However, further research should focus on neighbouring areas or especially on hydrothermal zones in the bedrock, where an enrichment of the elements arsenic and uranium for example is assumed.