environmental-geochemical surveying and atmospheric modeling of dust fallout from tailings dam in the rosh pinah area, namibia vladimír maj

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…………………...

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):…………………………………………
4 Surface 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
5 Gastric 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 %).
………………………………………………………
7 Results 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 of environmental-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/m3 of normalized air (Fig. 2).
……………………………..
F
igure 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).
7 Conclusions
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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