Journal of Environmental Protection, 2011, 2, 1295-1302
doi :1 0.4236/ jep.2011. 210149 Published Online December 2011 (
Copyright © 2011 SciRes. JEP
Evaluation of Sources and Options for Possible
Clean up of Anthropogenic Mercury
Contamination in the Ankobra River Basin in
South Western Ghana
Thomas Mba Akabzaa, Sandow Mark Yidana*
Department of Earth Science, University of Ghana, Legon, Ghana.
E-mail: *, *
Received October 9th, 20 11; revised November 11th, 2011; accepted December 4th, 2011.
The study assesses the spatial distribution and sources of mercury contamination in the Ankobra River Basin in south-
western Ghana and discusses possible remediation options and challenges. Eighty-two (82) samples of water and
streambed sediments from areas of active and historic artisanal mining and historic mine spoil from large-scale mining
were analysed for their total mercury content using cold vapour Atomic Fluorescence Spectrometry (CV-AAS). The
highest Hg concentrations were recorded from historic mine tailings, legacy of large scale mines in the area, which
averaged 795 ppb but ranged from 80 ppb to 2500 ppb. Concentrations in streambed sediments averaged 139 ppb, but
ranged from 63 ppb to 270 ppb. Water, expectedly, gave the lowest Hg concentrations with a mean value of 1.5 ppb, but
ranged from below detection to 8 ppb. Areas worked by artisanal miners and historic tailings dumps at Bondaye and
Prestea recorded the highest mercury values. These high mercury concentration sites constitute potential sources of
major mercury pollution in the area and therefore require major and urgent clean up to mitigate any major health risks.
However, any remediation strategy would require further and detailed study of the contaminated sites and an evalua-
tion of known remediation techniques to achieve maximum results.
Keywords: Ankobra Basin, Mercury Contamination, Clean-Up, Old Tailings, Sediments, Water
1. Introduction
Mercury pollution is a very serious environmental pro-
blem because elevated concentrations are toxic to living
organisms with the degree of toxicity varying with its
chemical form. Organic mercury, particularly methyl mer-
cury is said to be more than 1000 times as toxic as ele-
mental mercury. The effects of mercury on humans are
well documented. It causes damages to the central nerv-
ous system and is especially toxic to the foetus [1,2].
Mercury in the environment comes from both natural
and anthropogenic sources. Naturally, cinnabar (HgS) is
practically the only source of the metal, while the most
important anthropogenic sources of mercury pollution
are from the mining industry [3].
Commercial deposits of cinnabar have not be reported
in Ghana, although stream channel sampling have re-
vealed a promising mercury dispersion zone in the valley
of Tromia in Brong Ahafo region of the country [4]. On
the whole, no significant natural sources of the metal are
known in Ghana as efforts at other areas in the country
have so far not indicated mercury concentrations signifi-
cantly above its crustal abundance of 30 ppb. However,
studies in traditional mining centres in Ghana such as
Obuasi and Tarkwa have reported elevated mercury lev-
els in stream water, sediments, fish, and plants [5,6]. Most
of these studies a ssig ned the activitie s of artisa nal miners,
who use mercury for gold recovery, as the primary sour-
ce of mercury dispersion in Ghana. The present study are a
has over 500 years of artisanal activ ities. Apart from these
artisanal miners, mercury was pervasively used in large-
scale mining in Ghana until the early 1990s when it was
officially prohibited in large-scale mining. During that
period, over 15 large-scale gold mines in the Tarkwa mi-
ning district, many of them abandoned today, used mer-
cury for gold recovery. The area has seen over 100 years
of large-scale gold mining [7]. These mines left on their
Evaluation of Sources and Options for Possible Clean up of Anthropogenic Mercury Contamination 1296 in the Ankobra River Basin in South Western Ghana
trail scattered mercury-laced tailing dumps. For instance,
over hundred years of gold extraction using mercury in
the three underground mines at Prestea, Tuapim and Bon-
daye, about 65 km from Tarkwa, has generated three vast
expanse of tailings dumps, covering a total area of about
30 km2, while similar operations at the Tarkwa under-
ground mine has also ge nerated tailings covering over 16
km2 of land space.
Stud ies ha ve sho wn t hat the A nkob ra Ri ver Ba sin, whic h
takes drainage from these mining anthropogenic activi-
ties, and is indeed itself being mined by both legal and
illegal artisanal miners, has poor abundance of fish and
invertebrates because of heavy metal pollution. Other stu-
dies have reported widespread augmented mercury levels
in food crops such as cassava, cocoyam and in fish in the
area [6].
There has not been any systematic study to assess the
extent of Hg contamination in the area. In fact, the im-
pacts and extent of mercury pollution in Ghana are not
yet understood. This exploratory data clearly shows that
the need to clean up merc ury in the area is a delaye d pri-
ority. This paper, which explores the problem of mercury
pollution in the Ankobra River Basin and its mosaic of
tributaries, has provided data that justifies the urgent call
for the clean up of the contaminated sites. Various clean
up options have b een discussed.
2. The Study Area
The study area is in the Wassa West District of the West-
ern Region of Ghana (Figure 1), and is a major mining
hub in Ghana, defined by several major mining towns
such as Nsuta, Tarkwa, Damang, Abosso, Bogoso, and
Prestea. The area is one of the few remaining corridors of
Ghana’s tropical rain forest that is fast depleting by hu-
man activity, including mining. Annual rainfall data for
the area indicate minimum and maximum values of 1449
mm and 2608 mm respectively, while annual tempera-
tures range from 25˚C to 29 ˚C [8].
The Ankobra River and its mosaic of tributaries (Fig-
ure 2) receive various mine-impacted water from large-
scale gold mines at Prestea, Bogoso, Tarkwa and Da-
mang, as well as small-scale, often illegal mining opera-
tions, within the area.
The gold occurs in lenses of sulphide-bearing quartz
veins, mainly in carbonaceous phyllite, as disseminated
sulphides in the metavolcanics, or as oxidised derivatives
of the two types in the Birimian rocks and in conglomer-
itic horizons as free milling gold in the Tarkwaian. It is
the oxidised ores and the free milling varieties that are
most amenable to dressing with mercury.
3. Materials and Methods
A total of 70 water samples from streams, boreholes, and
hand dug wells; 6 samples each of stream sediment from
channel-bed alluvium and material from old tailings dum-
ps, respectively, were collected and their locations noted
with a Global Positioning System (GPS) and recorded on
a sample location map (Figure 2). Sample collec tion, pre -
servation, storage, preparation and quality control/quality
assurance (QC/QA) protocols followed those outlined by
Figure 1. A map of Ghana showing the study area.
Copyright © 2011 SciRes. JEP
Evaluation of Sources and Options for Possible Clean up of Anthropogenic Mercury Contamination 1297
in the Ankobra River Basin in South Western Ghana
Figure 2. Mercury conta minated sites identified for po ssi ble clean-up planning.
Eppinger et al. [9], APHA [10], Bloom and Fitzgerald
[11] and Barcelona et al. [12]. Water samples were acidi-
fied with pure nitric acid (HNO3) to pH less than 2 to
prevent possible reaction of the dissolved species [13]
and total merc ury deter mined by cold vapour Ato mic Fl uo-
rescence Spectrometry (CV-AAS) and expressed in parts
per billion (ppb) with detection limit of 1 ppb.
Plastic scoop was used to sample stream sediments
and tailings material into p olyethylene sampling bags and
tightly secured. The samples were oven-dried at 40˚C
and sieved to 80 meshes. Total nitric acid extraction was
then applied to this fraction to obtain a leachate, which
was then analysed for Hg using the same cold vapour
Atomic Fluorescence Spectrometry.
Analyses were performed at the Water Research Insti-
tute (WRI) of the Council for Scientific Research (CSIR),
with duplicat es of select samp les anal ysed at the SGS Com-
mercial laboratory and the Ecological Laboratory (Ecolab)
of the University of Ghana for quality assurance and con-
trol purpos es.
4. Results
The results of mercury concentrations in the various me-
dia are presented as summarised descriptive statistics in
Table 1. T he concentration of total mercury in stream se-
diments and mine tailings are presented in Tables 2 and
3, respective ly. Th e sa mpl es e xhibit var iable total Hg mer -
cury concentrations ranging from 33 ppb - 25,000 ppb in
old mine tailing dumps, 63 ppb to 270 pbb in streambed
sediments and from <1 ppb to 8 ppb for water samples
(Table 1).
4.1. Hg in Mine Tailings and Streambed
Mine tailings and stream sediments have mercury con-
centrations of over 10 - 100 fold the levels in water. The
highest Hg concentrations were recorded from historic
mine tailings with average concentration of 795 pbb.
Table 1. Summary descripti ve statisti cs of Hg c once ntrations in various media i n ppb.
Sampling Media Mean Media n Standard Deviation Minimum Maximum Count
Water 1.5 1 1.25 <1 8 70
Streambed sediments 139 100 87.65 63 270 6
Old Mine Tailing Dumps 795 72 1154.67 33 2500 6
Copyright © 2011 SciRes. JEP
Evaluation of Sources and Options for Possible Clean up of Anthropogenic Mercury Contamination 1298 in the Ankobra River Basin in South Western Ghana
Table 2. Conce ntration of mercury in vari ous mine s poil and stream sediments .
Sample Description Hg (ppb)
Old mine tailing dumps
PGRTD01 Old Tailings at Prestea, now being mined by PSGL 2500
SSPGRB01 Sediments from stream ta kin g seepa ge from tailings at Bondaye 97
SSGFL01 Soil from rehabilitated waste rock site 43
TSGFL01 Tailings from abandoned underground mine 80
PGRTD02 Old tailings being re-mined by Bondaye 2050
TSGFL04 Ancient tailin gs from Tarkwa und erground mine 33
Streambed Sediments
SSA01 Sediments from the Ankobra river at point where drainage from PSGL Operations enter it 230
TSPGRB01Sediments from stream draining old tailings at Bondaye 270
GMCTS01 Stream sediments from the Kawere stream near Nsuta 103
SSPGRB03 Stream draining ancient tailings dump near Bonda ye 63
SS1 Stream sediments from Bonsa Stream 63
SSTGL-6 Stream sediment s from Bedibewuo stream near Tarkwa 70
Material taken from tailings dumps in Bondaye and Pre-
stea underground mines gave the highest mercury values
of 2025 ppb and 2500 ppb respectively (Table 2) and
area A in Figure 2.
Merc ury le vels i n strea m sedi ments a re ge neral ly lo w-
er than those from the tailings dumps. The highest Hg
values of 270 ppb and 230 ppb were obtained from stream
sedi ments samp les ta ken fro m a tr ibutar y of t he Anko bra
draining the old tailing dump at Bondaye and the Anko-
bra River itself respectively. Augmented mercury con-
centrations ranging from 63 ppb to 103 ppb were ob-
tained from sediment samples at the Bonsa River, Be-
dibewuo stream, and the Kawere, where active artisanal
mining activities are brisk (Table 2).
4.2. Water Samples
Water samples showed the lowest mercury concentra-
tions among the three media sampled, and ranged ge-
nerally from below detection to 8 ppb, with high concen-
trations coincident with areas of small-scale mining ac-
tivity, such as streams in the Bogoso area, including the
Ankobra River. All samples from streams with artisanal
mining activities gave Hg concentrations of at least 1 ppb.
High merc ury val ues wer e a lso obs erved from wa ter fro m
streams that drain the old tailings dumps at Prestea and
Tarkwa, with the highest value of 8 ppb recorded in a
stream that takes drainage fro m the Bonda ye old tailings
dump which gave the very high Hg concentration of 2050
4.3. Discussion
The data presented can be considered as one of the first
steps to understanding the sources and problem of mer-
cury contamination in the Ankobra River Basin. Water
exhibited much lower Hg concentrations than tailings
and sediments. Hg concentration in water from many
streams far exceeded the World Health Organization
(WHO) maximum guide limit of 1.0 ppb [14]. These ele-
vated mercury concentrations occur in streams in areas
with intense small-scale mining activity and from strea-
ms carrying drainage from the old tailings dumps with
high Hg levels. The most pervasive contamination of Hg
in stre am water s ge ner all y come fr om t he A nkobr a Ri ver
and a mosaic of tributaries around the Bogoso area where
artisanal mining activities are most profound, par- ticu-
larly around the village of Dumasi and its satellite com-
munities (Figure 2). However, the single highest mer-
cury concentration in water came from a stream draining
old tailings dumps at Bondaye with elevated mercury le-
These observations are not surprising as mercury is the
chemical used in gold e xtraction by t hese traditional small
miner s. All sa mpl es take n fro m str ea ms whe re t hes e s ma ll
miner s oper ate gave high mer cury val ues. So me of thes e
miners amalgamate their concentrate right in the streams
leading to the spilling of the Hg into the streams (Figure
3). In fact the low level of mercury in even stream waters
with active artisanal mining activities is not surprising.
Hem [15] has pointed out that mercury in water opened
to the atmosphere is like ly to be much lower owing to its
tendency to escape via vaporization. It may also form
chloride and hydroxide complexes depending on pH and
total chloride concentration.
Sediments from streams and old tailings dumps have
Hg concentrations far in excess of the maximum allow-
ble concentration of 10 ppb of Hg in these media that a
Copyright © 2011 SciRes. JEP
Evaluation of Sources and Options for Possible Clean up of Anthropogenic Mercury Contamination 1299
in the Ankobra River Basin in South Western Ghana
Table 3. Su mmary of remediation meth ods.
Removal Dredging and pu mping out
contaminated materials Well-tested and effective
Expensive, lengthy process;
disposal sites can leak and
re-release contaminant; significant
disturbance of the environment
Expensiv e equipment; mu st be
monito r e d perio di cal l y and
followed by either treatment
and/or burial and containment
in other location
Physical treatment (i.e. sorting) Good for large q uantities
of sediment (20 - 40
Doe s not work wi th high silt, clay
content soils/sediments
Therm al treatment res ulting in
Mercury compounds are
highly volatile at low
Causes more atmospheric mercury
Ch emical treatment on site or
off site
Mercury reacts with
other c om pounds and
can be made biologically
Adding foreign chemicals into an
ecosystem can be dan g erous when
you aren’t sure of the out come
These methods often a re best
applied of site in contaminated
medium that has been removed
Physical barriers placed on site
to contain the contaminant so it
can no longer spread through
th e ecosystem
Well-tested and effective
High cost, barriers are of
questionable permanence,
unkn own unintended ecological
effects fr o m the destruction of the
benthic ecosystem
Barriers can be top, bottom or
lateral side barriers; sometimes
barrie r s can be nat ur al and not
man made
Microbial Act ion Using microbes that can
demethylate mercury to clean
contaminated mediums
Effective in sludges,
wastewater and
controlled environments
like the lab oratory
Not proven for on site remediation Forms the basis of
Various techniques using plants
to remove mercury from the
environment or immobilizing it
within th e environment;
methods include degradation,
extraction, containment or a
combo of all three
Cost effective, less
intrusive than other
meth ods, pollution
captured can be recycled
and r eused in stead of
mining more
Plants grow slow and results take a
while, mercury captured in the
plant may be available to wildlife
feeding on the plant, not well
tested, envir onment must be
suitable t o the acc umula ting plant
Best for sites with low to
medium levels of
Water Quality
Manipulating the water quality
such as oxygen content or pH to
ensure methyl mercury
prod uction will not occ ur
Cost effective, less
intrusive, not highly
Mani p ul a tions ne e d to be
monitored, and may likely need
manipulati on often; ma y disrupt
some ecosy funcstem tioning
Mos t easily implemented by
current water managing
Source: (Adopted from Fuller [3]).
would require immediate remedial action [16]. Mercury
concentrations in these media are about 5 to more than
500 times the maximum allowable limits. The results de-
monstrate that Hg use by artisanal miners and Hg in old
tailings dumps of large scale mining activities constitute
the two major anthropogenic sources of mercury con-
tamination in the area.
The two extremely high anomalous Hg values in the
Copyright © 2011 SciRes. JEP
Evaluation of Sources and Options for Possible Clean up of Anthropogenic Mercury Contamination 1300 in the Ankobra River Basin in South Western Ghana
Figure 3. Artisanal miners wining gold from the bed of the
Bonsa River using mercury.
Bondaye and Preseta mine ancient tailings (Area A in
Figure 2) are related to historic Hg use in gold process-
ing. From the inception of large-scale mining in the area
from 1897 to the early 1990s, Hg was the chemical of
choice for gold extraction b y all co mmercial gold miners
in the area. Small-scale miners are presently reworking
the tailings dumps illegally, sending uncontrolled sedi-
ments and leachate containing Hg into the Ankobra and
its tributaries.
4.4. Containing Widespread Contamination in
the Bas in
The relatively low level of Hg concentration in water-
compared to concentration in sediments and mine tail-
ings is not surprising. The results are indeed consistent
with observation from studies elsewhere, where it has
long been recognised that Hg is more efficiently trans-
ferred from water to bottom sediments [17-19]. Such
rapid transfer is possibly through mechanisms such as
sorption on suspended or bed load material, clay and
organic material [20]. This is supported by the study of
Hg speciation in bottom sediments of water reservoirs,
which has shown that physicall y sorbed admixture is the
predominant form of Hg occurrence in bottom sediments
[17]. Such forms of Hg exhibit weak bonds and can eas-
ily be extracted from the sediments under conditions of
rainstorms resulting in seasonal flushing of doses of Hg
into water [20]. The high Hg co ncentrations in strea mbed
alluvium and old mine tailings in the area constitute ma-
jor sources of Hg contamination in surface waters.
Varekam et al. [18] have pointed out that such high Hg
contaminant sources present serious environmental prob-
lems as erosion of soils with high mercur y loading bring s
these mercury bearing particles into the aquatic system.
Earlier studies have indicated augmented mercury con-
centrations in fish and plant species from the study area
[6]. One possible pathway for fish and plant uptake of Hg
would be transfer fro m sediments through methyla- tion,
which ensures a more bioavailable form of Hg [21]. It is
also known that invertebrates that ingest large quan- ti-
ties of sediments may accumulate higher concentrations
of Hg toxin than species that ingest leaf material [21].
The dangers posed by high mercury concentrations in
sediments have equally been echoed by Hem [15] who
pointed out that mercury ingested from sediments gets
concentrated in the successive biological species along
aquatic food chains so that fish that live in mildly con-
tami nated envir on ment ma y conta in too muc h merc ury to
be used safely for food.
These mercury-laced dumps and sediments pose real
threat to the general environment in the area, as these
sources are capable of re-supplying mobile mercury to
streams in the area during rainstorms. Currently the old
tailings dumps have been evaluated to contain consider-
able gold resources, and ever since this disclosure, thou-
sands of illegal small-scale miners have besieged these
dumps. Their activities are sending considerable tailings
into the drainage system.
There is the urgent need to put intervention measures
in place to clean up the area to prevent widespread mer-
cury contamination in the wider Ankobra basin that ser-
ves numerous communities. Any efforts to contain the
contamination and possibly carry out remediation of
mercury endemic sites would require careful planning. In
the first place, it would require either curtailing the work
of small-scale miners using mercury or providing the
miners with technical support and education to use mer-
cury retorts. However these efforts at ensuring that these
miners use retorts in the past has been quite fruitless.
4.5. Options for Remediation and Challenges
Mercury pollution is an acclaimed human health hazard
and the degree of its toxicity is dependant on its speci-
ation. When converted to methyl mercury by bacteria in
stream water and sediments, its toxicity could be aug-
mented up to a thousand fold that of elemental mercury
due to its facilitated ability to cross cell membranes and
interact in biological systems [1]. Consequently, reme-
diation of mercury contaminated sites, where they are
located close or have the potential to impact watersheds,
is gaining urgent priority in water quality management
The impacts and extent of mercur y pollution in Gha na
are not fully understood. Remediation strategy for these
contaminated sites would be challenging, as remediation
can only be effective if there is a thorough understanding
of the spatial scale of the pollution and the chemical
forms of the mercury. This exploratory work has pro-
Copyright © 2011 SciRes. JEP
Evaluation of Sources and Options for Possible Clean up of Anthropogenic Mercury Contamination 1301
in the Ankobra River Basin in South Western Ghana
vided useful information on the sources and scale of mer-
cury contamination in the study area. The contaminated
sites are sho wn in Figure 3 as A, B, C, D, and E. Sites A
and C are ancient tailings dumps at Prestea and Tarkwa,
respectively. Site B is the Ankobra River near Dumasi
while sites D and E are streams being worked by small
scale miners.
However, the information cannot be said to be ade-
quate for full-scale remediation of the problem as this
would require thorough understanding of the scale of po-
llution, the chemical forms of the mercury in the conta-
minated media, the range of mercury-relevant sites pa-
rameters, that usually vary from location to location due
to changes in temperature, pH, flow rate, solvent types,
etc. [3].
The current levels of data is therefore limited for any
full scale clean up measure, but provide the basis for fur-
ther work to provide the needed information for cleaning
the contaminated sites. According to the USEPA [19] six
main categories of remediation techniques have been wi-
dely used-removal of the contaminated media; treatment
of the medium, immobilisation of the contaminant, mi-
crobial remediation, phytoremediaiton and water quality
management. The merits and demerits of each of these
techniques have been summ arised by Ful l er [3] (Table 3).
Any strategy to deco nta minate the ide nti fied site s would
require an evaluation of these techniques on the basis of
their potential to succeed. Since the Ankobra Basin is ex-
tensive, it will probably require multiple re mediation me-
thods at the different sites. This will probably require de-
tailed individualised response plans that require more
detailed information for the different sites in the basin.
Such a remediation strategy is necessary and urgently
needed to clean the Ankobra River and its tributaries and
the scattered mercury-laced old tailings dumps identified
in this study to prevent a pluming ecological disaster in
the area.
4.6. Securing Pr oc essing A lt er nati ve s t o Me rcury
in the Artisanal Mining Sector
The remediation must address mercury sinks and pro-
duction pathways as the most critical targets for cleaning.
In this particular study area, the challenges presented by
contaminated old tailings site could be overcome once
barriers are erected to prevent dispersal of material from
the contaminated tailings while remediation takes place.
However, it might be more challenging to address this
problem with respect to areas of small scale miners, par-
ticularly, illegal miners. These migratory miners serve as
constant supply of fresh mercury to streams in the area
and unless their activities are curtailed, any remediation
measure would be a wasteful exercise.
It is clear that efforts to stop the operations of these ar-
tisanal miners will be a fruitless venture, under current
constrained opportunities for alternative sources of live-
lihood in the country. The other option is to examine and
possibly implement alternative processing technologies
other than mercury. Alternatives to mercury amalgama-
tion such as, gravity concentration, cyanidation and froth
flotation have successfully replaced mercury in the large-
scale mining sector. However the adoption of such tech-
nologies to the artisanal mining sector would equally be
as problematic as mercury for several reasons including
cost considerations and capacity constraints. Gravity con-
centration presents a better opportunity for success.
However the nature of occurrence of the gold in typical
artisanal work sites in Ghana makes the application of
gravity concentration unattractive. Most of the gold is too
fine and gr avi t y co nc e ntr a tio n would not b e a n effe c - tive
These problems not withstanding, it is an important
challenge for efforts to be geared towards the develop-
ment of alternative processing technology for the arti-
sanal mining sector. Until an alternative to mercury is
found, any efforts any to clean up contaminated sites might
only constitut e a tempor a ry solut ion.
5. Conclusions and Recommendations
The application of mercury in gold ore dressing in small-
scale mining and historic application of mercury in gold
extraction by large-scale miners have resulted in con-
tamination of portions of the Ankobra Basin, including
scattered mercury-laced old tailings dumps at Bondaye,
Prestea and Tarkwa. These mercury contaminated tai-
lings dumps and streambed alluvium present serious en-
vironmental challenges as these serve as reservoirs for
the supply of Hg into water bodies through erosion. The
regulatory authorities would have to disco urage the direct
discharge of untreated mine drainage into streams, par-
ticularly the Bonsa and Ankobra Rivers, which serve as
sources of domestic water supplies for the area. Urgent
measures are needed to clean up delineated mercury con-
tamination in the area. However, this would require a
more detailed study of the sites and a thorough evalua-
tion of available remediation options and their effective-
Since small-scale mining is an unavoidable livelihood
activity, efforts should be made to explore the use of
mercury free extraction technology in the sector.
Efforts must be made to develop alternative processing
technologies to replace mercury in the artisanal mining
6. Acknowledgements
The authors wish to express our sincere gratitude to the
Copyright © 2011 SciRes. JEP
Evaluation of Sources and Options for Possible Clean up of Anthropogenic Mercury Contamination
in the Ankobra River Basin in South Western Ghana
Copyright © 2011 SciRes. JEP
DANIDA-ENRECA Hydrogeological Project for provi-
ding the financial support for this study.
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