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