Impact of a Base Metal Slimes Dam on Water Systems, Madziwa
Mine, Zimbabwe
K. Lupankwaa, D. Lovea,b, B.S. Mapania and S. Msekac
a
Geology Department, University of Zimbabwe, PO Box MP167, Mount Pleasant, Harare, Zimbabwe
b
c
WaterNet, PO Box MP600, Mount Pleasant, Harare, Zimbabwe
Safety, Health and Environment Department, Bindura Nickel Corporation, PO Box 35, Bindura, Zimbabwe
Abstract
The Mazowe Valley contains several of Zimbabwe’s largest current mining operations, is
densely populated and is also a major agricultural area. The urban areas of Bindura,
Goromonzi, Shamva, Marondera, Murehwa and Mutoko all draw water from within the
Mazowe Valley. Irrigation of commercial crops is also a major water user. Accordingly,
managing the impact of mining operations on water quality in the Mazowe Valley must be a
major priority for sustainable development in this area. Madziwa Mine, 150 kilometres northeast of Harare, is a case in point. Mining took place between 1966 and 2001. The main
sulphides were chalcopyrite, pyrrhotite, pentlandite and pyrite. Waste from the mine's plant
has been disposed of via a tailings dam, the focus of this study.
Surface water samples were collected at twelve sites around the slimes dam and groundwater
samples were collected from six boreholes. The samples were analysed for dissolved metals
using atomic absorption spectrometry and for anions using gravimetry and titration.

Corresponding author: K. Lupankwa: Email address: lupankwa@yahoo.com. tel./fax. +263-4-303557
The surface water chemical analyses showed that acidic effluent with high concentrations of
iron, nickel and sulphate emanates from the tailings dam. Concentrations of metals are lower
after the water has passed through natural wetlands. Chemical analysis of groundwater
showed similarly high levels of acidity, sulphate and metal. These findings show that acid
mine drainage is seeping from the tailings dam.
Efforts are being made to reduce the effects of the acid mine drainage. For surface seepage
from the dumps these efforts include diverting acidic effluent from the dump into natural
wetlands that neutralise the acidity. To reduce drainage into the groundwater efforts are being
made to plant trees with high evaporation rates to minimise the amount of water that can
cause acid mine drainage.
Keywords: Water pollution, Acid mine drainage, Base metal mine, Constructed wetland
1. Introduction
1.1.Base Metal Mining and the Environment
Rocks that contain valuable amounts of base metals often contain sulphide minerals. Many of
the sulphides, e.g. pyrite, are dumped in waste rock dumps with gangue material. Sulphides
containing valuable metals (e.g. pentlandite, which contains nickel) are processed, much of
the metal extracted, and the residue dumped in slimes (tailings) dams. This results in more of
the sulphides being exposed to water and air, resulting in large amounts of acid, sulphate,
metals and metalloids (like copper, nickel, cobalt and arsenic) being released into the
2
surrounding environment (Love and Hallbauer, 1998). The contaminated water, referred to as
acid mine drainage, is often reddish-brown in colour indicating high levels of iron (III) (e.g.
Gandy and Evans, 2002). The main chemical impacts of acid mine drainage on water include
reduction in pH, destruction of the bicarbonate buffering system, and increases in
concentrations of soluble and particulate metals (Ravengai et. al., 2003). This can have severe
impacts to aquatic resources, can stunt terrestrial plant growth, harm wetlands and
contaminate groundwater. The overall impact on the ecosystem is the elimination of species
and simplification of the food chain (Gray, 1997).
At Madziwa mine, located 150 kilometres north-east of Harare (Fig. 1), mining of nickel from
a nickel-copper ore body hosted in sulphide-bearing altered pyroxenites, gabbros and norites
took place from 1966 to 2001. In these rocks, the sulphides, in order of abundance, are
chalcopyrite, pyrrhotite, pentlandite and pyrite. Large amounts of these sulphides were
brought to the surface, where they were piled into rock waste dumps and waste from the plant
was dumped into a slimes dam. Because mining operations commenced long before
environmental controls were properly understood or legally required, the sulphide-bearing
rock waste and slimes were dumped directly on the ground without any impermeable lining at
the base. These dumps now have the potential for generating acid mine drainage and thus
contaminating surface water and groundwater.
A study was undertaken in order to investigate the impact of the slimes dam on the
surrounding surface water and groundwater and to recommend possible remediation methods.
1.2.Site Description
3
Nickel-copper ore was mined from three shafts, namely Sulphide, Amms and Phoebus (Fig.
1). The main sulphides in order of abundance were chalcopyrite (CuFeS2), pyrrhotite (Fe1-xS),
Pentlandite [(Fe, Ni)9S8] and pyrite (FeS). The ore bodies differed in shape, form and size
from one mine to the other, being massive at Sulphide, semi-massive at Amms and
disseminated at Phoebus.
(Fig.1. Location and Environs of Madziwa Mine, Zimbabwe. Phoebus Shaft is 15 km to
the east.)
Waste from the mining activities was discarded at three rock dumps. Dressing of blended ore
from the three Madziwa shafts, and concentrator rejects from Trojan Nickel Mine in Bindura,
was carried out at a plant near Sulphide Shaft. Slimes from this process were dumped at a
single slimes dam. The slimes dam covers an area of approximately 1.4ha and has five
benches and a total height of 36 meters.
On the southern side of the slimes dam, seepage collects in a drain and flows southwards
down a small valley. Seepage from the western toe of the slimes dam collects in a drain,
which flows, along with seepage from the northern side of the slimes dam, into a natural
wetland that is to the north of the slimes dam. Outflow from this wetland is discharged into
the Mudzinge River. Seepage from the eastern side passes through another natural wetland to
the east of the slimes dam. Flow from this wetland is joined after a few kilometres by flow
from the southern drain and other inflow waters, especially during and soon after the rainy
season of November to March, and also flows into Mudzinge River.
4
The Mudzinge River, which thus receives all seepage from the Madziwa slimes dam, drains
into Mfurudzi river which in turn flows into Mazowe River. The Mazowe Valley contains
several of Zimbabwe’s largest current mining operations (nickel, gold, pyrite and industrial
minerals), is densely populated and is also a major agricultural area. The urban areas of
Bindura, Goromonzi, Shamva, Marondera, Murehwa and Mutoko all draw water from within
the Mazowe Valley. Irrigation by commercial agriculture is also a major water user (Ashton
et. al., 2001). Accordingly, managing the impact of mining and minerals processing
operations on water quality in the Mazowe Valley must be a major priority for sustainable
development in this area.
2. Materials and Methods
Surface water samples were collected at 12 sites (see Table 1) around the slimes dam. To
investigate the effect of mine dump on groundwater, six boreholes were sunk around the
slimes dump (see Table 2). Samples were collected just before the rains (October 2001) and
soon after the rainy season (May 2002).
Two water samples were collected from each site on each sampling occasion. The sample for
cation determination was filtered into a 500ml plastic bottle containing ~2ml of concentrated
nitric acid. Whatman filter paper number 542 with a pore size of 2.7µm was used for filtering
water samples in the field. Samples were stored in a refrigerator before analysis. Hanna pH
meter (model H19023C) was used to measure pH, and Conductivity and Total Dissolved
Solids were measured using a multi range conductivity/TDS meter (model HI8733) in the
laboratory. The samples were analysed for dissolved metals using Varian 200HT atomic
absorption spectrometry equipment at the Geology Department, University of Zimbabwe. For
5
the anions, the gravimetric method was used to determine the concentration of sulphate and
titration method was used for carbonate, bicarbonate and chloride at the Tobacco Research
Board, Zimbabwe.
(Table 1. Location of surface water sampling points, Madziwa)
(Table 2. Location of boreholes, Madziwa)
3. Results
3.1.Surface Water
The analytical results of surface water samples for the two sampling occasions are shown in
Table 3.
(Table 3. Results of chemical analysis of surface water, Madziwa)
On the southern side of the dump, yellowish-brown precipitates (yellow boy) were seen on the
bed of the drain and the stream, together with some whitish to pale green efflorescent salts on
the sides of the man-made drain. Efflorescent salts were more extensive after the rainy season
(May 2002) than before the rainy season (October 2001). Results of analysis of the samples
collected from the southern side of the dump showed that:
1. The pH was low in all samples during both sampling periods.
2. Both before and after the rainy season, sulphates, iron and nickel tended to decrease
with distance from the dump, while copper and calcium increased. The values of
6
sulphate, calcium and pH in the water samples were higher after the rains than before
at all the southern toe sample points. Nickel and copper values were lower in samples
taken after the rains than in those taken before the rains.
On the eastern side, yellow boy was observed on the bed upstream of the wetland in both
sampling periods. Downstream of the wetland no yellow boy was observed in either sampling
periods. Efflorescent salts were observed only on tree roots and on some rocks in May 2002
(end of the rain season). Analysis of samples collected on the eastern side of the dump
showed that:
1. pH was low before the effluent entered the wetland but increased significantly
downstream.
2. TDS, calcium, sulphate, nickel and iron concentrations were generally high before the
effluent entered the wetland and decreased downstream of the wetland in both seasons.
TDS values were generally higher for October 2001 compared to May 2002. Sulphate
and calcium were higher in May 2002 than in October 2001. Nickel and iron showed
no systematic changes between the sampled dates.
On the western side the effluent had a dark brownish colour. There were also extensive
efflorescent salts. For samples collected on the western side, analytical results showed that:
1. pH was very low (<3) for both sampling periods.
2. TDS, EC, iron, nickel and calcium were higher in October 2001 than in May 2002.
On the northern side some yellow boy and extensive efflorescent salts were observed in the
seepage from the dump before it entered the wetland. Analytical results showed that in the
effluent from the northern side before entering the wetland,
7
1. pH was near to neutral (~6.7) for both sampling periods.
2. Calcium, sulphate, iron and nickel concentrations were higher in May 2002 than in
October 2001.
With respect to the northern wetland, total dissolved solids, electrical conductivity, iron,
sulphate and nickel were higher in samples from between the slimes dam and the wetland than
downstream of the wetland.
Analysis of samples from the Mudzinge River show that:
1. TDS, EC, Ni and sulphates were higher in sample downstream of the mining area than
upstream of it.
2. Bicarbonates and carbonates were higher upstream of the mine area than downstream
of it.
3.2.Groundwater
The analytical results of groundwater samples for the two sampling occasions are shown in
Table 4.
(Table 4. Results of analysis of chemical groundwater, Madziwa)
The water sampled from all the boreholes was clear to the naked eye. Analytical results from
groundwater samples showed that the transition metals (Ni, Fe, Co and Cu) were generally
less than 1 mg/l. The concentration of calcium and cobalt were higher in May 2002 than in
October 2001 in the groundwater samples while the concentrations of the other metals were
8
lower. Metal concentrations, total dissolved solids and electrical conductivity were much
lower in groundwater samples than in surface water samples.
(Fig. 2. Piper diagram of groundwater chemistry, Madziwa: a) October 2001; b) May
2002)
The piper diagram (Fig. 2) shows that:
1. Water from the eastern borehole (Mw44) and both southern boreholes (Mw51 and 52)
plotted close to the sulphate plus chloride apex.
2. Water from the northern borehole (Mw41) had highest proportions of sodium plus
potassium and sulphate. The other northern borehole (Mw43) had highest proportions
of magnesium.
3. The control borehole (Mw36) had a very high concentration of bicarbonate and
insignificant sulphate concentration (14 mg/l) when compared to the boreholes near
the mine dumps.
4. Discussion
The reddish-brown colour of water in the western drain fits the description (Parker and
Robertson, 2000) of ferrihydrite (Fe5HO8.4H2O) and the yellow-orange colour in the southern
drain fits the description of a mineral referred to as schwertmannite (Fe8O8(OH)6SO4 , an iron
oxyhydroxysulfate). Efflorescent salts accumulate around intermittent seeps and surface
streams due to evaporation of sulphate-rich waters. These dissolve at the onset of the rains
and are major source of dissolved metals in streams after storms.
9
High levels of sulphate, conductivity, nickel and iron in water samples near the mine dumps
are likely to have resulted from the oxidation of sulphides occurring in the tailings dump. It
has been noted that sulphate and electrical conductivity are more reliable indicators of mining
effluents than pH and heavy metals are (Gray 1996). This was attributed to the fact that
sulphate is a conservative anion in mining environments and is less affected by sorption
processes or fluctuations in pH than heavy metals (Gray, 1996). High concentrations of heavy
metals could have resulted from dissolution of the metals in the acidic water of the slimes
dam resulting from oxidation of sulphides.
It has been noted (e.g. Chapman et al., 1983) that sorption could account for the attenuation of
metals in streams affected by acid mine drainage. Metals can adsorb to clays, organic matter,
oxides of manganese, iron and aluminium and to organic ligands that are themselves adsorbed
to the surface (Smith et al., 1993). This could explain the decrease in metals with distance
from the tailings dump on the southern side of the dump.
The other processes that attenuate high levels of metals in acidic mine water include
secondary mineral precipitation and co-precipitation. Sulphates may be adsorbed to hydrous
iron oxides or may co-precipitate with hydrous iron oxide (Kimball, 1999). This could explain
the decrease in sulphate concentrations with distance from the points of discharge.
In wetlands, metals are either precipitated from solution or are taken up by plants. This could
be the reason why there are sharp decreases in metals concentrations after both the northern
and eastern wetlands.
10
The slight differences in metal concentrations between October 2001 and May 2002 could be
attributed to poor rains in the 2001-2002 rainy season. This also explains the presence of
extensive efflorescent salts on the drain walls and ground surfaces in May 2002.
According to Zimbabwean wastewater regulations (MRRWD, 2000), effluent discharged into
the environment is classified into four categories. Effluent in the blue class is considered to be
safe, that in green is low hazard, and yellow is medium hazard whereas that in red is
considered to be high hazard. Concentrations of sulphates above 500 ppm are considered to be
in the red category. The concentrations of sulphates in all surface samples in the mining area
and in the boreholes SP1, SP2, NP1 and EP1 were above 500 ppm and thus the water was in
the red category.
5. Conclusions and Recommendations
Surface and groundwater chemistry showed acidic effluents with high concentrations of iron,
nickel and sulphate emanating from the slimes dam. Where surface water passes through
natural wetlands, there is neutralisation of acidity and decrease in metal and sulphate
concentrations. Efforts by the mining company have resulted in the very acidic effluent from
the western and northern sides of the slimes dam being diverted into the northern wetland via
an artificial drain. This has resulted in this effluent being neutralised. Effluent from the
eastern side of the slimes dam is neutralised by passage through the eastern wetland.
The main effluent that is currently polluting the Mudzinge River is from the southern side. A
possible solution to this contamination is to make a drain that could be used to divert seepage
11
from the southern side into the eastern wetland. Another possibility would be to develop
another wetland in the course of the seep on the southern side.
Ameliorating contamination of groundwater in Madziwa is more complex than handling the
surface water problems because there is no impermeable barrier between the slimes dam and
the ground on which it stands. Therefore acidic effluent can seep directly from the slimes dam
into the ground below and eventually into the aquifer. Efforts can be made to reduce
groundwater contamination by putting lime into the man-made drains to neutralise the acidity.
Another activity being carried out by the mine is planting of trees, including Eucalyptus sp.
(blue gum), around the slimes dam to stabilise the slimes dam so that material does not slump,
and also for aesthetic purposes. Since these trees have a high water consumption rate, they
have the potential to decrease the flow of groundwater away from the slimes dam, thus
potentially decreasing contamination of the surrounding aquifer. An intervention that might
be effective would be the establishment of reactive barriers in the form of lime or limestone
grouts, dug into the soil around the slimes dam. Water seeping from the dam and through the
soil would pass through these grouts and potentially be neutralised, before it seeps into the
aquifer.
Acknowledgements
The research reported in this paper is part of work being carried out by Ms. Lupankwa as part
of M.Phil. studies at the University of Zimbabwe. This research is funded by the Bindura
Nickel Corporation and was initiated by the late Dr. Leonard Chimimba and Mr. Vaughan
Smith (Bindura Nickel Corporation) with Prof. Paul Dirks and Dr. Mia Moers (then at the
University of Zimbabwe). Dr. Dennis Shoko (University of Zimbabwe), Prof. Paul Dirks and
12
Prof. Andy Fourie (University of the Witwatersrand) are thanked for useful discussions.
Constructive reviews by Jenny Day and an anonymous reviewer improved the quality of this
paper.
References
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and Mineral Processing Operations on Water Resources and Water Quality in the Zambezi,
Limpopo and Olifants Catchments in Southern Africa. Report to Minerals, Mining and
Sustainable Development Project, Southern Africa, 338p.
Chapman, B.M., Jones, D.R. and Jung, R.F., 1983. Process controlling metal ion attenuation
in (AMD) streams, Geochimica et Cosmochimica Acta 47: 1957-1973.
Gandy, C.J. and Evans, K.A., 2002. Laboratory and numerical modelling studies of iron
release from a spoil heap in County Durham. In Younger, P.L. and Robins, N.S. (Eds)
Geological Society Special Publications 198: Mine Water Hydrogeology and
Geochemistry. The Geological Society of London, London, pp 205-214.
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ground water. Environmental Geology 27: 358-361.
Gray, N.F., 1997. Environmental impact and remediation of acid mine drainage: A
management problem. Environmental Geology 30: 62-71.
Kimball, B.A., 1999. Seasonal variation in metal concentration in a stream affected by acid
mine drainage, St. Kevin Culch, Colorado. In Plumlee, G.S. and Logsdon, M.J. (Eds) The
environmental geochemistry of Minerals deposits. Part A; Processes Techniques and
Health Issues. Reviews in Economic Geology 6A: 467-478, Society of Economic
Geologists.
13
Love, D. and Hallbauer, D.K., 1998. Characterisation of groundwater contamination in a
dolomitic aquifer: a case study from the Far West Rand. Proceedings of the Geological
Society of South Africa Conference, Pretoria, South Africa, pp 272-275.
MRRWD (Ministry of Rural Resources and Water Development), 2000. Water (waste and
effluent disposal) regulations Statutory Instrument 274 of 2000.
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Environment Foundation Occasional Paper 11.
Ravengai, S., Owen, R.J.S. and Love, D., 2003. Evaluation of seepage and acid generation
potential from evaporation ponds, Iron Duke Pyrite Mine, Mazowe Valley, Zimbabwe.
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Smith, B.J. and Schumacher, J.G., 1993. Surface water and sediment quality in the old lead
belt southeastern Missouri 1988-1989. US Geological Survey Water Resources
Investigations Report 93-4012, 92pp.
14
Table 1. Location of surface water sampling points, Madziwa.
Point
Location
Mw49
Mine stream on south side of slimes dam, where drain trends north-south
Mw48
Mine stream on south side of slimes dam, in between confluence and slimes dump
Mw47
Mine stream on south side of slimes dam, just before confluence
Mw46
Stream receiving flow from mine stream on south side of slimes dam (below
confluence)
Mw53
Seepage from east side of slimes dam, before eastern wetland
Mw54
Downstream of eastern wetland, approximately 1km from slimes dump
Mw55
Downstream of eastern wetland, approximately 500m from slimes dump
Mw45
Seepage from north side of slimes dam, before northern wetland
Mw42
Downstream of northern wetland
Mw40
Western drain, just before wetland
Mw56
Mudzinge River, down stream of all inflows from mine area
Mw37
Control point: Mudzinge River, upstream of all inflows from mine area
15
Table 2. Location of boreholes, Madziwa.
Point
Location
Depth
Mw51
70m south of slimes dam (SP1)
25m
Mw52
130m south of slimes dam (SP2)
30m
Mw43
200m north of slimes dam (NP2)
25m
Mw41
310m north of slimes dam (NP1)
25m
Mw44
130m east of slimes dam (EP1)
25m
MW36
Control point: existing borehole, five
kilometres upstream of the mine
16
Table 3. Results of chemical analysis of surface water, Madziwa
Point
pH
Ec (µS/cm)
Ni(mg/l)
Fe(mg/l)
Ca(mg/l)
SO4(mg/l)
HCO3 (mg/l)
01-Oct May-02 Oct-01 May-02 Oct-01 May-02 Oct-01 May-02 Oct-01 May-02 Oct-01 May-02 Oct-01 May-02
Mw37
8.5
7.3
933
580
0.2
0.5
0.8
0.4
3.8
289.1
Mw40
2.3
2.6
5565
3100
4.9
0.8
10.7
346.7
600.6
Mw42
7.3
7.5
1562
1200
2.2
0.1
0.6
0.5
Mw45
6.6
7
3180
2000
2.0
2.3
4.0
Mw46
2.5
3
4900
3100
14.7
0.9
Mw47
2.4
2.7
4980
3100
15.2
Mw48
2.3
2.6
4840
3100
Mw49
2.9
2.5
5100
Mw53
2.5
3.1
Mw54
8.1
MW55
7.9
16.0
297.5
640.5
746.0
887.2 3521.0
1.5
250.1
89.5
406.9
123.4
636.0
126.2
215.0
4.5
498.5
825.5
501.9 1960.0
126.2
231.8
12.2
11.1
523.6
964.2
718.2 3253.0
<0.01
<0.1
0.9
19.9
104.5
519.5
816.2
786.0 3014.0
<0.01
<0.1
15.7
7.2
6.2
255.2
559.4
937.6
749.6 3214.0
<0.01
<0.1
3100
19.3
8.4
13.8
551.6
434.8
796.7 1298.3 3395.0
<0.01
118.9
4670
2200
2.3
4.8
8.7
12.4
514.3
893.8
535.6 2851.0
<0.01
<0.1
7.9
2880
1300
0.7
0.4
0.7
0.1
312.8
454.1
491.0
556.0
192.3
326.4
7.6
3550
1600
0.9
0.7
0.7
0.1
443.4
547.8
353.4 1644.0
234.4
533.8
17
0.7
MW56
8.2
7.9
1435
1100
2.7
0.7
1.0
0.1
104.9
388.4
110.2
no
result
18
120.2
222.7
Table 4. Results of chemical analysis of groundwater, Madziwa
Point
Ni (mg/l)
Fe (mg/L)
Co (mg/L)
Cu (mg/L)
Na(mg/l)
Ca(mg/l)
Mg(mg/l)
K(mg/l)
Oct 01 May 02 Oct 01 May 02 Oct 01 May 02 Oct 01 May 02 Oct 01 May 02 Oct 01 May 02 Oct 01 May 02 Oct 01 May 02
Mw36
0.05
0.49
0.88
0.32
0.01
0.05
0.05
0.02
64.00
78.88
67.33
Mw41
0.15
0.55
0.78
0.48
0.02
0.12
0.05
0.02
33.70 175.98
Mw43
0.88
0.23
0.92
0.85
0.03
0.13
0.06
0.03
62.30
53.30
15.67 184.60
13.33
4.90
<0.01
10.30
73.33
11.20 211.53
1.80
<0.01
Mw44
0.45
0.55
0.87
0.46
0.02
0.15
0.06
0.03 153.00 191.54 306.90 808.82 261.40 321.30
1.90
<0.01
Mw51
0.65
0.73
0.89
0.91
0.03
0.06
0.06
0.02 159.00 171.60 605.20 517.73 355.60 351.52
1.70
<0.01
Mw52
0.90
0.46
1.41
0.72
0.02
0.19
0.06
0.04 145.30 215.53 532.00 559.94 225.60 235.31
1.00
<0.01
Table 4. Results of chemical analysis of groundwater, Madziwa (continued)
Point
Cl(mg/l)
SO4(mg/l)
CO3(mg/l)
HCO3 (mg/l)
Oct 01 May 02 Oct 01 May 02 Oct 01 May 02 Oct 01 May 02
19
34.00
4.22
Mw36
35.0
14.0
248.4 3551.0
Mw41
11.2
49.0
Mw43
44.6
15.0
Mw44
12.3
Mw51
Mw52
<0.01
-
696.9
15.0
<0.01
258.4
433.1
480.0
10.5
<0.01
126.2
625.3
17.0
263.5 2496.0
10.5
<0.01
246.4
588.2
12.1
15.0
697.7 2384.0
<0.01
<0.01
153.3
274.5
12.1
21.0
373.0 2007.0
<0.01
<0.01
147.3
259.3
32.8
20
Fig.1. Location and Environs of Madziwa Mine, Zimbabwe. Phoebus Shaft is 15 km to the
east.
21
Fig. 2. Piper diagram of groundwater chemistry, Madziwa: a) October 2001; b) May 2002
22