Environmental-geochemical Surveying and Atmospheric Modeling of Dust Fallout from Tailings Dam in the Rosh Pinah Area, Namibia

Vladimír Majer, Bohdan Kříbek, Jan Pašava, Vratislav Pecina

Czech Geological Survey, Geologická 6, 152 00 Prague 5, Czech Republic

Frederik Kamona, Benjamin Mapani

University of Namibia, Faculty of Earth Science, Department of Geology, Private Bag 13301, Windhoek, Namibia

Abstract. As revealed by environmental-geochemical survey, soils and pasture grass in the area of the Rosh Pinah lead and zinc deposit, Namibia, are affected to various degrees by industrial pollution. The dust blown out from the tailings dam appears to be the major environmental threat in the Rosh Pinah town. Gastric availability of toxic metals in dust particles is high and increases (in % of total content of metals in dust) in the range Fe (6%) → Cr (6%) → As (9%) → Cu (23%) → Zn (39 %) → Pb (86%). The degree of regional industrial pollution of topsoil can be best evaluated using the Coefficient of Industrial Pollution (CIP) based on actual and median concentrations of the given contaminants. Atmospheric modeling revealed that while the whole surface of the tailings dam was kept dry in the past, the amount of dust in the Rosh Pinah town amounted at 40 to 300 μg/m3 of normalized air. The present moistening of the large part of the tailings dam surface has resulted in a substantial decrease in dust, down to 20–100 μm/m3 of normalized air. ………

Keywords. Heavy metals, soil pollution, environment, Rosh Pinah, Namibia, tailings

1 Introduction

The strata-bound zinc and lead mineralization of the Rosh Pinah deposit is hosted by silicified black shale, feldspathic arenites and altered carbonate rocks of the late Precambrian Rosh Pinah Formation, which forms a part of the Port Nolloth Zone (van Vuuren 1981, 1986; Frimmel et al. 2004). The Rosh Pinah mine has been in operation for 29 years. Tailings from the ore treatment plant are transported through piping and distributed over the tailings dam. The dam surface covering an area of 60 hectares was kept dry in the past. Since 1995, the surface of dam has been divided into a system of retention cells. Regular moistening of their surface reduces the degree of drifting of dust. Tailings contain relatively high concentrations of zinc (14,000–16,000 ppm), lead (400–1400 ppm), copper (970–1160 ppm), arsenic (57–148 ppm) and mercury (0.12–0.22 ppm)….

2 Methods

Regional environmental geochemical survey of soils and various species of grass at Rosh Pinah was carried out using the methodology recommended for the regional geochemical mapping by the FOREGS Geochemistry Working Group (Salminen et al. 1998). The mapping was done at a scale of 1:25,000 and covered a total area of 320 km2. The surveyed area was divided into square basic cells 4 by 4 km in size. Each cell was characterized by at least one sampling point, where composite samples of the surface soil horizon (topsoil) were collected from the depth of 0.0–0.05 m, after removal of plant remains. Samples of subsurface soil from the depth of 0.8–0.9 m were collected as reference samples…………………...

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Figure 1. A contour map of the Coefficient of Industrial Pollution (CIP) in the surface soil layer in the Rosh Pinah area. The CIP scale is based on statistical characteristics of the data set. Sampling points are shown by black dots.

3 Extent of soil contamination

Mechanical weathering prevails over chemical weathering in the Rosh Pinah area due to the arid climatic conditions. Therefore, soils (calcaric fluvisols) are developed only on alluvial sediments of river valleys. The pH of soils lies in alkaline range and the soils are characteristic of enhanced content of sodium and high salinity. The shape of the contaminated soil area (dust fallout contamination halo) is elongated towards northwest, parallel to the prevailing summer wind direction. Northerly and northwesterly winds in the winter spread the dust over a larger area, particularly in the broader surroundings of the tailings dam. Generally, most of the dust is transported by wind to uninhabited high mountain areas. The concentrations of total sulfur in the uppermost part of the soil horizon in the close vicinity of the tailings dam reach 12 wt%, whereas the contents of arsenic reach up to 225 ppm, mercury 1.14 ppm, lead 43,030 ppm and zinc 93,060 ppm. The degree of contamination of the mapped area can be best characterized by the Coefficient of Industrial Contamination (CIP), which is a sum of concentrations of selected metals in the surface soil layer (in topsoil) at individual sampling sites divided by the sum of medians of the same metals in the whole mapped area (Kříbek et al. 2003, 2006):…………………………………………

4Surface vs. subsurface soil concept

In order to distinguish natural concentrations of metals from those ascribed to contamination by dust fallout, soils from two depth horizons were sampled: first, from the surface soil, which is most affected by dust fallout, and, secondly, from a reference soil horizon at a depth of 0.8–0.9 m. ………………………………………………

Table 1.Median values of metal concentrations in surface soils and subsurface soils in the Rosh Pinah area. Concentrations of iron and manganese in wt%, other metals in ppm.

Element / Topsoil / Subsurface soil
Cr / 32 / 27
Cu / 27 / 20
Pb / 146 / 12
Zn / 425 / 44
As / 2.72 / 1.84
Hg / 0.043 / < 0.005
Fe / 1.67 / 1.76
Mn / 0.043 / 0.029

5Gastric availability of metals in dust fallout

Based on the results of leaching experiments in acidified glycine, the availability of metals (in % of total content metals in soils extracted with aqua regia) increases in the range: Fe (6 %) → Cr (6 %) →As (9 %) → Cu (23 %) → Zn (39 %) → Cd (57 %) → Mn (65 %) → Pb (86 %). ………………………………………………………

7Results of meteorological modeling

The results of meteorological modeling of dust fallout, taking into account the specific density of dust particles, their grain size, meteorological data and local morphology, are basically consistent with the results ofenvironmental-geochemical mapping. Modeling of dust fallout from the tailings dam was performed in two variants: (1) for a dry surface of the tailings pond, which corresponds to the situation in the past, and (2) for the present situation, where approximately two-thirds of the tailings dam surface are moistened. The results revealed that while the whole surface of the tailings dam was dry in the past, the amount of dust in the Rosh Pinah town corresponded to 40 to 300 μg/m3 of normalized air. The present moistening of the large part of the tailings dam has resulted in a decrease in dust, down to 20–100 μg/m3of normalized air (Fig. 2). ……………………………..

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Figure 2. Fields of annual dust fallout data at Rosh Pinah, Southern Namibia. Situation before moistening of the tailings dam surface (A), and the present situation with moistening of the tailing dam surface (B).

7Conclusions

As revealed by the present study, the tailings dam of the Rosh Pinah deposit, constructed at an infavorable site without knowledge of local meteorological conditions, is a source of dust fallout which contaminates a large part of the Rosh Pinah town. The results of the meteorological modeling are principally in agreement with the field mapping, both confirming the effect of moistening of the tailings surface as a tool towards the reduction of dust fallout in the town. …………………

Acknowledgements

This study was carried out within the framework of the IGCP/SIDA Project No. 594, Assessment of the impact of mining and mineral processing on the environment and human health in Africa. The work was supported by the Czech Science Foundation (project GA ČR 205/08/0321). ………………………………………….

References

Czech ministerial regulation No. 117/1987 Coll. on highest permissible content of heterogeneous substances in fodder: Ministry of Agriculture of the Czech Republic

Davis A, Ruby MV, Bergstrom PD (1992) Bioavailability of arsenic and lead in soils from the Butte, Montana: Environmental Science & Technology 27: 1415–1425

Davis A, Ruby MV, Bloom M, Schoof R, Freeman G, Bergstom PD (1996) Mineralogic constraints on the bioavailability of arsenic in smelter-impacted soils: Environmental Science & Technology 30: 392–399

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