Evaluation of seepage and acid generation potential from evaporation
ponds, Iron Duke Pyrite Mine, Mazowe Valley, Zimbabwe
Seedwel Ravengaia, Richard Owenb and David Lovec,d
a
b
Geology Department, University of Zimbabwe, PO Box MP167, Mount Pleasant, Harare, Zimbabwe
Mineral Resources Centre, University of Zimbabwe, PO Box MP167, Mount Pleasant, Harare, Zimbabwe
c
WaterNet, PO Box MP600, Mount Pleasant, Harare, Zimbabwe
Abstract
Iron Duke Pyrite Mine lies in the Mazowe Valley of northern Zimbabwe. Several urban areas
and commercial farmers are major water users in the catchment. 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.
The mine disposes of its waste water via evaporation ponds. Some of the water in the ponds
evaporates and some is lost through seepage to groundwater. Results of a water budget
analysis of the ponds showed that 160.5m3 per day of acidic effluent with a pH of 2 and
elevated levels of iron and sulphate was being lost through seepage.
As the wastewater evaporates, the secondary minerals melanterite and hexahydrite precipitate.
The solid material in the pond was found to contain 20% iron and 14% sulphate, which is far

Corresponding author: S. Ravengai: Email addresses: seedwel@yahoo.com. tel./fax. +263-4-303557
more than was found dissolved within the pond water. Despite this, the pond water is
undersaturated with respect to both iron and sulphate. Acid generation tests on the solid
material in the pond indicate a minimum of 540,000 moles and a maximum of 1,610,309
moles of acid are generated. The variation can be related to exposure to oxygen: material near
the edges of the pond is more exposed to oxygen and has already reacted further than material
from the centre of the pond; accordingly less acid can be generated. The acidity generated by
the pond is due to the unreacted pyrite that is found in fine particles suspended in mine
waters.
Based on these results, between 20 and 60 metric tonnes of lime are required for complete
neutralisation of the sediments in the pond. Although the ponds were decommissioned after
the conclusion of this study, it is necessary to prevent formation of further acid mine drainage
from existing sediments in the evaporation pond. This could be done by the use of reactive
covers, whose compositions affect the chemistry of infiltrating water. A good reactive cover
could be constructed from lime, overlain by topsoil rich in organic material. This should be
followed by revegetation.
Keywords: Acid mine drainage, groundwater pollution, pyrite mine, water balance
1. Introduction
1.1 Acid Mine Drainage
2
Where the mineral deposit which is being mined is a sulphide mineral., the disposal of large
volumes of waste (principally as tailings) has the potential for generating acid mine drainage
by the following processes:
2 FeS 2( s )  7O2  2 H 2 O(l )  2 FeSO4  2 H 2 SO4
(1)
4 FeSO4 13O2 ( g )  2 H 2 SO4  2 Fe2 ( SO4 ) 3  2 H 2 O(l )
(2)
Fe2 ( SO4 ) 3  6 H 2 O(l )  4 Fe(OH ) 3( s )  3H 2 SO4
(3)
Acidic drainage conditions, coupled with metal pollution, can persist for decades and can
result in extensive environmental degradation if not properly managed (Ashton et al., 2001;
Love and Hallbauer, 1998; Kambole, 2003). Iron Duke Mine (IDM) is a case in point: mining
operations have been in progress since 1914, well before the advent of strict legal and
corporate environmental controls. In this paper, the volume and nature of seepage and the acid
generation potential of the ponds are quantified.
1.2. Study Area
IDM is located in north-east Zimbabwe, approximately 45km north of Harare and 15 km east
of Mazowe (Fig. 1). It produced gold until 1940 and now produces pyrite for sulphuric acid
manufacture. The mine lies at the southern end of a gorge cut by the Yellow Jacket River
through the south-west – north-east trending Iron Mask Range and is surrounded by
commercial farms. The Yellow Jacket River flows north through the range to meet the
Mazowe River, which flows along the western edge of the range. The urban areas of Bindura,
3
Goromonzi, Shamva, Marondera, Murehwa and Mutoko all draw water from within the
Mazowe Valley. Commercial farmers are major water users. 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.
(Fig 1. Location of Iron Duke Mine)
The geology of the area has been described by Baglow (1993) and Chitiyo (2000). The pyrite
mineralisation developed at IDM is stratiform and occurs in the Archaean metasediments and
metavolcanics of the Iron Mask Formation. These rocks are locally succeeded by Archaean
greenstones of the Arcturus formation and metagreywackes of the Shamvaian Group to the
north-west. The main IDM ore body comprises pyrite with minor traces of other sulphides eg
pyrrhotite and chalcopyrite, and gold. The ore body is hosted by greenstones in the hanging
wall and ferruginous cherts in the footwall.
Water is used in the underground workings for wet drilling and for watering down to decrease
fumes and dust. Additionally, groundwater seeps into the workings via fissures. The fines
generated during drilling and blasting provide a large surface area for chemical reactions to
take place, thus accelerating acid production. The result of this is that mine waste water
pumped from underground is acutely acidic (pH 1). The Ministry of Rural Resources and
Water Development granted IDM an exemption to discharge 170 m3day-1 of wastewater with
a pH 1 from underground workings into the Yellow Jacket River. This exemption expired in
1990, and so the mine then constructed a water treatment plant for the water that comes from
underground workings. Treated water is discharged to evaporation ponds. About 170 m3 of
acidic water is pumped from underground daily.
4
The water treatment plant uses about two tonnes of lime daily to neutralise the acidic
underground water. The plant consists of six tanks fitted with agitators. River water
(approximate pH 7) is mixed with lime in the first two tanks to form a paste, which then flows
into two neutralising tanks where the underground acidic effluent is allowed to flow at
predetermined rates with continuous mixing, from which it overflows into the last two tanks.
When the pH is above 6.5, the effluent is discharged to the either of two of the three
evaporation ponds.
The three ponds are located some 500 m north-east of the shaft and each of these ponds
measures 5400 m2. Ponds 2 and 3 receive about 170m3 of treated effluent daily from the water
treatment plant on a rotational basis. Pond 1 is a sewage pond and receives about 50 m 3 of
effluent daily. The waste water is usually not spread evenly over the pond surface but tends to
accumulate in local depressions, thus reducing their efficiency as evaporation ponds.
1.3 Previous Work
The environmental impact of IDM has been studied with reference to rivers by Magadza and
Masendu (1986) and by Gratwicke (1999a). A biotic water quality index study of the Yellow
Jacket and Mazowe rivers showed that acid mine drainage from the mine degraded the water
quality in both rivers (Gratwicke, 1999b).
However, there has been limited qualitative work done on acid mine drainage at IDM. A
study of the tailings dam and waste rock pile at IDM recorded drainage with pH as low as
0.52, supersaturation with respect to iron and sulphate and high concentrations of arsenic and
5
a number of metals (Williams and Smith, 2000). Much of these metals were later precipitated
as sulphates, principally melanterite, and hydrous oxides in the Yellow Jacket River,
downstream of the tailings dam. The evaporation ponds were not studied.
A preliminary study on the groundwater in the vicinity of the evaporation ponds
(Mandingaisa, 2000) described acidic pollution of the groundwater, probably as a result of
seepage from the evaporation ponds. However, the volume of seepage and the pollutant load
contributed to the groundwater by the evaporation ponds were not quantified.
2. Materials and Methods
2.1 Content of Evaporation Ponds
A sampling grid was established for Evaporation Pond 2 since it is the one that was receiving
mine waste water throughout the time of sampling. This grid allowed for sampling from the
edges and centre of the pond. Seven samples of solid material., each weighing 2 kg, were
taken from the pond at a depth of 10 cm below the pond surface. All the samples were then
dried in an oven at 80oC. The fine-grained samples were crushed to destroy the crumb
structure and were later homogenised. The samples were then put in new clean sample bags
ready for analysis.
The sulphate content was analysed by gravimetric methods of analysis in which 1g of sample
was weighed in a 400ml beaker and 10 ml of hydrochloric acid and 90 ml of water were
added. The mixture was then boiled for ten minutes. It was then filtered with 12.5 cm No 40
6
filter paper and then washed three times with hot water before 15 ml of barium chloride was
added. The solution was allowed to settle and then filtered through No 42 filter paper and was
again washed with hot water. The filter paper was then placed in a previously weighed
porcelain crucible and dried on a hot plate. The sample was finally ignited for thirty minutes
at 800C and weighed. The percentage of sulphate was then calculated.
The percentage of iron was determined as follows: 1g of sample was weighed into a platinum
crucible and 20 ml of 50% HCl and 10 ml HClO4 was added. The mixture was then heated on
a hot plate to dryness. It was then cooled and 20 ml of 1:1 HCl was added for digestion for 10
minutes. The final mixture was then cooled and washed into 100 ml volumetric flask and
introduced to an Inductively Coupled Plasma spectrophotometer.
The iron and sulphate content in the pond water was analysed by Ion Chromatography. The
water sample was first filtered using a 0.45μm filter paper and 2ml of sample was each
introduced into the cation column and anion column to determine the total iron content and
total sulphate content respectively.
Secondary minerals that had precipitated out of the mine water were sampled separately.
These samples were analysed by X-Ray diffraction at the Federal Institute for Geosciences
and Natural Resources (BGR) in Hannover, Germany using a Philips X’pert PW3710
instrument.
2.2 Water Budget
7
A water budget analysis was done in order to quantify seepage. This technique is based on the
following equation:
Inflow = Outflow  Change in Storage
(4)
At the IDM evaporation ponds:
Mine Water Discharge + Rainfall = Evaporation + Seepage  Change in Storage (5)
The amount of evaporation was measured using an evaporation pan of known thermal
coefficient and the change in storage was measured by measuring the change the water level
in Evaporation Pond 2 and the change in that pond’s flooded surface area, and then
calculating the changes in volume of wastewater in Evaporation Pond 2. By determining the
amount of evaporation and change in storage, seepage could then be deduced since inflow
from rainfall and mine discharge was known. There is a potential weakness in the method
since the waste water accumulates in a small portion of the pond, but evaporation would be
also taking place from the wet sediments throughout the pond surface. Thus the decline in the
pond water levels may be partially attributed to evaporation from these moist sediments. This
effect was considered to be negligible due to the extreme fine grained nature of the pond
sediments, which severely reduces the rate of flow of groundwater through these sediments.
2.3 Analysis of Acid Generation Potential
1 g of sample material from the ponds was weighed and placed in a beaker: 30 ml of deionised water was added to the beaker and stirred. The pH of the solution was then recorded
8
and the solution was placed on a hot plate set at 40oC and stirred continuously to ensure that
the solid particles mixed thoroughly with the water. The pH of the solution was monitored
every hour. The procedure was repeated every day for fourteen days – until there were no
further changes in pH. This indicated that chemical equilibrium was achieved. The solution
was then titrated with 0.025 mol dm-3 sodium
hydroxide solution using phenolphthalein indicator and the amount of acid that had been
generated was thus quantified.
3. Results
3.1 Contents of Evaporation Ponds
During the period of study, Evaporation Pond 2 was receiving water from the water treatment
plant. The solid (sediment) material from the ponds was found to contain up to 20 % (200,000
ppm) iron and up to 14 % (140,000 ppm) sulphate.
The water in Evaporation Pond 2 was found to contain 7,845 ppm of iron, 7,167 ppm of
sulphate and a pH of 2.
X-ray diffraction determined that the mineral precipitates from the mine waters were
melanterite (FeSO4.7H2O and FeSO4.9H20), with minor hexahydrite (MgSO4).
3.2 Water Budget
9
(Table 1. Measured inflow, evaporation and change in storage with calculated seepage
for Evaporation Pond 2, IDM)
The water budget analysis shows, based on a pond surface area of 5,400 m2, that there is an
average loss of 32.9 m3 of water to evaporation per day, and an average loss of 160.5 m3 of
water as seepage from Evaporation Pond 2.
3.3 Iron and Sulphate Content of Seepage
The chemistry of the seepage from the evaporation ponds is a function of the chemistry of the
water in the ponds and of leachate from solid material in the ponds. Based on the measured
iron and sulphate content of the pond water, and the calculated seepage of 160.5 m 3 of water
per day, seepage from Evaporation Pond 2 should carry 293 kg of dissolved iron and 284 kg
of dissolved sulphate per day.
3.4 Acid Generation Potential
(Table 2. Measured acid generation potential and calculated total potential acidity of
Evaporation Pond 2, IDM)
Although there is some variation in the acid generation potential within Evaporation Pond 2,
the potential acidity is over 800,000 moles H+, equivalent to over 39 metric tonnes of
sulphuric acid, H2SO4.
4. Discussion
10
4.1 Seepage From The Ponds
Results obtained show that a total volume of mine water discharge into Evaporation Pond 2 is
170.0 m3day-1, and the average total volume of seepage is 160.5 m3day-1. This demonstrates
that the ponds are functioning more as seepage ponds, and only partially as evaporation
ponds. The seepage has a pH of 2 and a chemical load of 293 kg of dissolved iron and 284 kg
of dissolved sulphate per day.
The solubility of iron (II) sulphate in water is 486g l-1 (Weast, 1970), equivalent to 486,000
ppm. This value indicates that the evaporation pond water is undersaturated with respect to
iron and sulphate. The pond water can therefore still dissolve more iron and sulphate from the
solid material thereby increasing the chemical load to the groundwater. Solubility of iron (II)
sulphate in water indicates that an additional quantity of up to 91 g l -1 of iron and 161 g l-1 of
sulphate (from water budget – equation 5) can still be dissolved and form part of seepage.
These figures represent the maximum amount of iron and sulphate that dissolved since the
solubility of any species in water also depends on the presence of other dissolved species
present.
4.2 Acid Generation Potential
The results of the acid generation potential tests make it clear that there is a substantial
quantity of acid generating material in the evaporation pond. The acidity generated by the
pond material is due to unreacted pyrite and other sulphides, found in fine dust particles
11
suspended in the mine waste waters. Such particles are generated during drilling and blasting
underground and end up in the ponds as the water is pumped from the mine to the ponds.
Samples that were collected from the middle of the pond generated more acid than those taken
from the edges of the pond. This is to be expected since the samples that are found on the
edges of the pond are more exposed to free oxygen, increasing sulphide oxidation reaction
rates. It therefore means that these particles can stay in their reduced form at the middle of the
pond, mostly covered by water and reducing conditions prevail.
Evaporation Ponds 2 and 3 were decommissioned shortly after this study was concluded and
two new ponds are now in use. However, with continued input of water from rainfall, the old
ponds will continue to generate acid. Over time, the reactive front where sulphide oxidation
and acid mine drainage reactions take place will move down through the solid pile until,
decades hence, all remaining sulphide has reacted. A kinetic study would be able to determine
how much acid would be generated annually and for how many years into the future. Such a
study would also have to model microbial aspects of sulphide mineral oxidation (Edwards et
al., 2000).
Protective low permeability coverings can have some effect in decreasing pollution load to
groundwater (eg Carlsson et. al., in press). An example would be a clay layer across the top of
the decommissioned evaporation ponds. Such a layer, by decreasing water inflow, would
decrease the volume of water reacting with pyrite (see equations 1 and 3), slowing the
reactions. Critically, a decreased water inflow inhibits the transport of dissolved material from
the pond into the subsurface and groundwater.
12
However, the use of reactive coverings are much more effective. An example would be
constructing a layer containing agents such as lime or limestone across the top of the
decommissioned evaporation ponds, overlain by topsoil rich in organic material. Such agents
change the chemistry of water infiltrating into the ponds by raising the pH and adding
dissolved carbonate. This can neutralise acid generated within the evaporation pond. It is also
necessary to raise the pH of infiltrating water sufficiently to precipitate dissolved metals.
These metals would then remain in the pond, instead of being transported to groundwater as
dissolved metals in seepage. As an alternative to extensive use of lime, metals can be removed
from solution by organic anions such as 1,3-benzenediamidoethanethiol (Matlock et al., in
press). The construction of a cover should be followed by revegetation of the top of the pond.
Conclusions
(i)
Evaporation Ponds 2 and 3 at IDM are in fact seepage ponds, with a daily seepage
from Evaporation Pond 2 of 160.5 m3 – equivalent to 94% of inflow volume.
(ii)
Seepage from Evaporation Pond 2 has a pH of 2 and carries a daily load of around 293
kg of dissolved iron and 284 kg of dissolved sulphate – this is a conservative estimate,
since the dissolution of solid material in the pond shall add to the pollution load.
(iii)
The potential acidity of Evaporation Pond 2 is over 800,000 moles H+, equivalent to
over 39 metric tonnes of sulphuric acid, H2SO4.
(iv)
Based on the IDM experience, the advisability of using unlined “evaporation ponds”
for disposal of acidic mine effluent is questionable, since this practice has transferred
the problem of acid mine drainage to surface water to the much longer term problem
of serious groundwater contamination.
13
(v)
Because of their capacity for continuing generation of acid and other pollutants,
decommissioned sulphide-rich dumps and facilities such as IDM’s evaporation ponds
require careful, long-term management, for example by the use of reactive coverings.
(vi)
Detailed prediction of acid generation over time would require a kinetic and
biogeochemical study. This would be able to determine annual acid, iron and sulphate
loads now that Ponds 2 and 3 have been decommissioned.
Acknowledgements
This research was supported by Iron Duke Pyrites (Pvt) Ltd and by a University of Zimbabwe
postgraduate scholarship to Seedwel Ravengai. The community of Iron Duke Mine is thanked
for their hospitality during the fieldwork. Douglas Maguze and Kudzai Musiwa (University of
Zimbabwe Geology Department) are thanked for their assistance in analytical work. The
Federal Institute for Geosciences and Natural Resources (BGR) in Hanover, Germany is
thanked for the XRD analyses. The University of Zimbabwe Faculty of Engineering is
thanked for loan of the evaporation pan. Ian Love (University of Zimbabwe Chemistry
Department) is thanked for valuable discussions. Constructive reviews by Jenny Day and an
anonymous reviewer improved the quality of this paper.
References
Ashton, P., Love, D., Mahachi, H. and Dirks, P., 2001. An Overview of the Impact of Mining
and Mineral Processing Operations on Water Resources and Water Quality in the
14
Zambezi, Limpopo and Olifants Catchments in Southern Africa. Report to the Minerals,
Mining and Sustainable Development Project, Southern Africa, 338p.
Baglow, N., 1993. Geological map of the country around Bindura. Harare: Zimbabwe
Geological Survey.
Carlsson, E. Öhlander, B. and Holström, H in press. Geochemistry of the infiltrating water in
the vadose zone of a remediated tailings impoundment, Kristineberg mine, northern
Sweden. Applied Geochemistry.
Chitiyo, G.A., 2000. Iron Duke mine sulphur reserves and resources. Internal report, Harare:
Iron Duke Pyrites (Pvt) Ltd.
Edwards, K.J., Bond, P.L., Druschel, G.K., McGuire, M.M., Hammers, R.J. and Banfield,
J.F., 2000. Geochemical and biological aspects of sulfide mineral dissolution: lessons
from Iron Mountain, California. Chemical Geology, 169, 383-397.
Fetter, C.W., 1994. Applied Hydrogeology. 3rd edition. New Jersey: Prentice-Hall.
Gratwicke, B., 1999a. An assessment of the water quality in the Yellow Jacket River using
the SASS4 Biomonitoring Technique. BSc Honours Dissertation (unpublished),
Department of Biological Sciences. Harare: University of Zimbabwe.
Gratwicke, B., 1999b. The effect of season on a biotic water quality index: a case study of the
Yellow Jacket and Mazowe Rivers, Zimbabwe. South African Journal of Aquatic
Science, 24, 24-35.
Kambole, M.S., 2003. Managing the water quality of the Kafue River. Physics and Chemistry
of the Earth, 28, 1105-1109.
Love, D. and Hallbauer, D.K., 1998. Characterisation of groundwater contamination in a
dolomitic aquifer: A case study from the Far West Rand. Geocongress 98, Geological
Society of South Africa, Pretoria, South Africa, 10 July 1998. Extended abstracts, 272275
15
Magadza, C.H.D. and Masendu, H., 1986. Some observations on effluent in the Yellow Jacket
stream, Zimbabwe. Zimbabwe Science News, 20, 11-15.
Mandingaisa, O., 2000. Effects of evaporation ponds on groundwater: AMD Disposal At Iron
Duke Mine, Glendale, Zimbabwe. BSc Honours Dissertation (unpublished), Department
of Geology. Harare: University of Zimbabwe.
Matlock, M.M., Howerton, B.S. and Atwood, D.A. in press. Chemical precipitation of heavy
metals from acid mine drainage. Water Research
Weast, R.C., 1970. Handbook of chemistry and physics. Magraw-Hill book company 13th
edition.
Williams, T.M. and Smith, B., 2000. Hydrochemical characterisation of acute acid mine
drainage at Iron Duke mine, Mazoe, Zimbabwe. Environmental Geology, 39, 272-278.
16
Table 1. Measured inflow, evaporation and change in storage with calculated seepage for
Evaporation Pond 2, IDM
Date
Measured
Change in water
Evaporation
Seepage
inflow (m3)
level (mm day-1)
(mm day-1)
(mm day-1)
07/09/2001
170
-18.0
5.0
13.0
08/09/2001
170
-17.5
5.0
12.5
09/09/2001
170
-18.3
5.0
13.5
10/09/2001
170
-29.0
4.0
25.0
11/09/2001
170
-29.0
6.0
23.0
12/09/2001
170
-34.7
6.0
28.7
13/09/2001
170
-48.5
5.0
43.5
14/09/2001
170
-47.0
5.0
42.0
23/09/2001
170
-15.5
8.0
7.5
24/09/2001
170
-47.0
12.0
35.0
170
-30.45
6.1
24.4
Average
17
Table 2. Measured acid generation potential and calculated total potential acidity of
Evaporation Pond 2, IDM
Pond sediment sample
mMol(H)
Total acid generation potential of
number
pond sediments in moles
P2NS50
23.13
540 000.0
P2NS10W
43.75
1 021 621.6
P2CS50W
48.75
1 338 378.4
P2CS10
60.63
1 415 675.7
P2CS10E
68.96
1 610 309.2
P2CS50E
38.75
904 864.9
P2CS50
35.25
823 135.1
Average
45.6
1 064 855.0
18
Fig 1. Location of Iron Duke Mine.
0
19
20km