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Total Mercury (Hg) concentrations were determined in soil, river sediments and six (6) species of fish from the River Pra Basin in southwestern Ghana by Cold Vapour Atomic Absorption Spectrometry. Mercury concentration (microg g(-1)) ranged from 0.042 to 0.145 for soil: from 0.390 to 0.707 for sediments and from <0.001 to 0.370 for fish. All the fish samples had Hg concentration below the World Health Organisation (WHO) permissible limit of 0.5 microg g(-1) whereas all the sediment samples had levels higher than the US-EPA value of 0.2 microg g(-1). The results obtained from this study showed that fish from River Pra Basin are unlikely to constitute any significant mercury exposure to the public through consumption. No apparent trend of increasing mercury concentration along the main river as it flows downward toward the sea was observed.
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Total Mercury in Fish, Sediments and Soil from the River Pra
Basin, Southwestern Ghana
S. O. B. Oppong R. B. Voegborlo
S. E. Agorku A. A. Adimado
Received: 4 February 2010 / Accepted: 14 June 2010 / Published online: 29 June 2010
ÓSpringer Science+Business Media, LLC 2010
Abstract Total Mercury (Hg) concentrations were
determined in soil, river sediments and six (6) species of
fish from the River Pra Basin in southwestern Ghana by
Cold Vapour Atomic Absorption Spectrometry. Mercury
concentration (lgg
-1
) ranged from 0.042 to 0.145 for soil:
from 0.390 to 0.707 for sediments and from \0.001 to
0.370 for fish. All the fish samples had Hg concentration
below the World Health Organisation (WHO) permissible
limit of 0.5 lgg
-1
whereas all the sediment samples had
levels higher than the US-EPA value of 0.2 lgg
-1
.
The results obtained from this study showed that fish from
River Pra Basin are unlikely to constitute any significant
mercury exposure to the public through consumption.
No apparent trend of increasing mercury concentration
along the main river as it flows downward toward the sea
was observed.
Keywords Mercury Fish Soil Sediment
River Pra basin Ghana
Mercury is a highly toxic metal and its contamination of
the environment has become a major issue of global con-
cern. Mercury is a natural component of the earth crust, and
it does not have any role in the human body. Mercury
comes from a range of natural sources such as volcanoes,
soils, mercury rich geological zones and forest fires, as well
as from fresh water, lakes, rivers and the ocean (Renzoni
et al. 1998). Increased use of mercury in gold recovery in
mining operations in many developing countries in recent
years has raised world concern over the release of this toxic
metal into the environment. Previous experiences of human
mercury poisoning in Manamata Bay in Japan (Irukayama
1977) and in Iraq (Bakir et al. 1973) indicate the dangers
associated with mercury contamination. Recent research
activities on the environmental impact of mercury from
gold mining have concentrated mainly on Latin America,
particularly the Brazilian Amazon. As a result, several
research papers and scientific reports have been published
on mercury contamination and health effects in the Ama-
zon (Pfeiffer and Larceda 1988; Larceda and Salomons
1991; Nriagu et al. 1992; Akagi et al. 1995) Some work,
however, has been undertaken to study environmental
mercury contamination in Ghana where an increasing large
number of people are engaged in artisanal gold mining
using mercury amalgamation technique. Studies on
assessment of mercury levels in water, sediments, soil,
food crops, fish and some human tissues have been carried
out in different areas including River Ankobra and Pra
basins. (Amonoo-Neizer et al. 1996; Adimado and Baah
2002; Golow and Adzei 2003; Bannerman et al. 2003;
Bonzongo et al. 2003; Babut et al. 2003; Golow and
Mingle 2002; Donkor et al. 2006). But most of these
studies did not attempt to link levels of mercury in the
different compartments to the artisanal gold mining activ-
ities and therefore did not suggest that gold mining activ-
ities contribute mercury to the aquatic environment.
Limited number of samples was also tested. Mercury
released into river systems during gold ore-processing is
likely to be transformed into the highly toxic
S. O. B. Oppong R. B. Voegborlo (&)
S. E. Agorku A. A. Adimado
Department of Chemistry, Kwame Nkrumah University
of Science and Technology, Kumasi, Ghana
e-mail: raybrightv@yahoo.com; rbvoegborlo.sci@knust.edu.gh
Present Address:
S. O. B. Oppong
Department of Maritime Engineering,
Regional Maritime University, Accra, Ghana
123
Bull Environ Contam Toxicol (2010) 85:324–329
DOI 10.1007/s00128-010-0059-0
methylmercury and becomes accumulated through
biomagnification in aquatic food chains, particularly in
fish. In spite of the considerable global concern about
mercury contamination of fish and fishery products, infor-
mation on mercury contamination of fish in freshwaters in
Ghana is scanty. This research sought to determine the
levels of mercury in different fish species, soil and surface
sediments from the River Pra Basin in Southwestern Ghana
and attempt to determine if any trend of increasing mercury
concentrations along the river as it flows downstream could
be established due to contributions from the tributaries
where there is a concentration of small scale mining
activities.
River Pra is one of the main rivers in which intense
fishing activities are carried on. Topographical features
demonstrate that river Pra has its source from the conflu-
ence of river Ofin and river Brim. Dunkwa, a small mining
town, is located along river Ofin just before it enters river
Pra. After this confluence, the river Pra then proceeds
through Awisam, Twifo Praso, Twifo Mampong in the
Central Region and down through Daboase and Beposo in
the Western Region and finally joins the Gulf of Guinea at
Shama also in the Western Region of Ghana (Fig. 1). Since
artisanal gold mining activities using mercury are carried
out close by the rivers it is likely that mercury could be
carried along the river Pra. Aquatic species like fish may be
affected by this contamination and eventually affecting
living organisms that may also feed on these aquatic
species.
Materials and Methods
Fish samples were obtained from River Pra at Awisam,
Twifo Praso, Twifo Mampong, Daboase and Beposo
between August 2005 and January 2006 from fishermen.
Soil and sediments were also collected within the same
period. Surface sediments were collected by the grab
method and soil samples were taken about 200 m away
from the bank of the river in each town. The samples were
placed in clean plastic bags. Fish and sediment samples
were stored on ice in an ice chest. All the samples were
immediately transported to the laboratory. Sediment sam-
ples were sieved through 2 mm mesh, air-dried and
homogenized by grinding using mortar and pestle. Soil
samples were air-dried and sieved through 2 mm sieve.
The fish samples were stored in the freezer after they had
been sorted out and identified at the Theoretical and
Applied Biology Department of the Kwame Nkrumah
University of Science and Technology. They were later
removed and allowed to thaw. The weight and total length
of each fish were measured. A portion of edible muscle
tissues were removed from the dorsal portion and stored in
a freezer until analysis.
Fig. 1 Map of Southwestern
Ghana showing the various
sampling sites
Bull Environ Contam Toxicol (2010) 85:324–329 325
123
All glassware were soaked overnight in a detergent
solution; rinsed and soaked in 10% (v/v) HNO
3
solution
overnight. They were rinsed with distilled water followed
by 0.5% KMnO
4
solution and rinsed again finally with
distilled water before use. All reagents used were of ana-
lytical reagent grade (BDH Chemicals Ltd, Poole,
England) unless otherwise stated. Double distilled water
was used for the preparation of all solutions. Fish, sedi-
ments and soil samples were digested for total mercury
determination by an open flask procedure developed at the
National Institute for Minamata Disease (NIMD) in Japan
by Akagi and Nishimura (1991) and reported earlier
(Voegborlo et al. 2004). Total mercury concentrations were
determined in all the digests by cold vapour atomic
absorption spectrophotometry using an automatic Mercury
Analyzer Model HG-5000 (Sanso Seisakusho Co., Ltd.,
Japan) developed at NIMD. The reducing reagent used in
mercury analysis was 0.5 mL of 10% (w/v) SnCl
2
2H
2
Oin
1 M HCl. Quality assurance samples analyzed included
procedural blanks, replicate samples and post-digestion
spikes. The accuracy of the procedure was determined by
analysis of certified reference material (Dogfish muscle,
DORM-2) from the National Research Council (NCR) in
Canada and Fish Homogenate Certified Reference Material
IAEA-407 from International Atomic Energy Agency,
Vienna. Recovery studies were performed by adding
increasing amounts of mercury chloride standard solution
to samples of two different fish species, which were taken
through the digestion procedure. The resulting solutions
were analyzed for mercury concentration.
Results and Discussion
Total Hg concentrations were determined in soil, sediments
and fish species from River Pra at Awisam, Twifo Praso,
Twifo Mampong, Daboase and Beposo. A total of one
hundred and sixty (160) fish samples covering six (6) dif-
ferent species, five (5) soil and five (5) sediment samples
were analysed. Results of mercury (Hg) concentration,
fresh weight of fish are presented in Table 1. Mean results
for soil and sediments are given in Table 2.
The validity of the method used in this study has been
proved by the agreement between the measured concen-
tration (mean =4.67 lg
-1
,n=3) which lies within the
certified (range of 4.38–4.90 lgg
-1
) concentrations in the
dogfish muscle (DORM-2) and concentrations obtained for
the measured (mean =0.215 lgg
-1
,n=3) was in
agreement with the certified values (0.216–0.228 lgg
-1
)
for IAEA-407.
Recoveries were between 98.08% and 103.8%. Mercury
concentrations in fish from Twifo Praso ranged from
0.040 to 0.083 lgg
-1
wet weight for Labeo coubie
(mean: 0.057), 0.033–0.011 lgg
-1
wet weight for Tilapia
zilii (mean: 0.059), from 0.093 to 0.257 lgg
-1
wet
weight for Chrysichthys sp. (mean: 0.187), from 0.143 to
0.187 lgg
-1
wet weight for Heterobranchus longifillis
(mean: 0.165) and from 0.025 to 0.069 lgg
-1
wet weight
for Alestis imberi (mean: 0.041). The highest Hg concen-
tration was recorded in Chrysichthys sp. (0.257 lgg
-1
wet
weight) and the lowest Hg concentration was recorded in
Alestis imberi (0.025 lgg
-1
wet weight). Burger et al.
(2001) reported site-specific differences in muscle tissue
mercury levels in fish from the Savannah River and found
that mercury concentrations generally reflected trophic
levels. Fish species at high trophic levels showed higher Hg
concentrations whereas those at lower trophic levels
recorded low Hg levels. A similar trend was observed in
this study. Hg concentrations increased with trophic levels.
Fish species have been categorized into numerical trophic
values where top predators were assigned a value of 5 and
above; between 4.00 and 4.99 fish are classified as high
level carnivores; middle level carnivores are assigned 3.00
to 3.99. The omnivores, herbivores or dentritivores are
assigned values between 2.00 and 2.99 (Froese and Pauly
2009). Thus from Twifo Praso, Labeo coubie at trophic
level 2.0 (Froese and Pauly 2009) recorded Hg concen-
tration that ranged between 0.040 and 0.083 lgg
-1
wet
weight (mean: 0.057) whereas Chrysichthys sp., at trophic
level 3.4 (Froese and Pauly 2009) recorded Hg concen-
tration range of 0.092–0.257 lgg
-1
wet weight (mean:
0.187). Lange et al. (1994) also showed that mercury
concentration varies with fresh weight of fish and total
length. High correlation between mercury concentration
and total length and fresh weight of fish are normally
observed among carnivorous species whereas poor corre-
lations are observed among herbivorous species. Among
the fish species studied at Twifo Praso, there was a poor
correlation between Hg concentration and fresh weight of
fish for Labeo coubie (r
2
=0.1076, trophic level =2.0),
Tilapia zilli (r
2
=0.1219, trophic level =2.0), Chrysich-
thys sp.(r
2
=0.1456, trophic level =3.4), Heterobran-
chus longifillis (r
2
=0.3864, trophic level =3.7) and
Alestis imberi (r
2
=0.0042, trophic level =3.2). Mercury
concentrations ranged between 0.018 and 0.057 lgg
-1
(mean: 0.042) wet weight for Alestis imberi from Twifo
Mampong and between below 0.001 and 0.136 lgg
-1
(mean: 0.071) wet weight for Labeo coubie from Awisam.
There was poor correlation between mercury concentration
and total length (r
2
=0.0404) and fresh weight (r
2
=
0.0453) for Alestis imberi. Labeo coubie from Awisam
showed good correlation between mercury concentration
and fresh weight (r
2
=0.697). This is in agreement with
other authors who reported similar findings for carnivorous
species (Lange et al. 1994). Mercury concentrations in the
muscle tissue of fish from Beposo ranged from 0.014 to
326 Bull Environ Contam Toxicol (2010) 85:324–329
123
0.216 lgg
-1
(mean: 0.065) wet weight for Alestis imberi,
from 0.104 to 0.370 lgg
-1
(mean: 0.152) wet weight for
Chrysichthys sp., from 0.035 to 0.043 lgg
-1
(mean:
0.039) wet weight for Labeo coubie and from 0.019 to
0.163 lgg
-1
(mean: 0.052) wet weight for Tilapia multi-
fasciata. Hg concentrations were higher for fish at higher
trophic levels. For samples from Daboase, Hg concentra-
tion ranged from 0.020 to 0.037 lgg
-1
wet weight for
Labeo coubie (mean: 0.029), 0.019–0.041 lgg
-1
wet
weight for Tilapia zilli (mean: 0.028), 0.013–0.042 lgg
-1
wet weight for Tilapia multifasciata (mean: 0.026) and
0.069–0.315 lgg
-1
wet weight for Chrysichthys sp.
(mean: 0.165). There was good correlation between Hg
concentration and fresh weight (r
2
=0.8038) and total
length (r
2
=0.8449) for Tilapia zilli. There was however,
poor correlation between Hg concentration and total length
and fresh weight for Labeo coubie,Tilapia multifasciata
and Chrysichthys sp. The mean Hg concentration recorded
for river sediments ranged from 0.390 to 0.707 lgg
-1
with
the highest being recorded at Daboase and the least at
Twifo Praso whereas for soils the levels ranged from 0.042
to 0.145 lgg
-1
with the highest being recorded at Beposo
and the lowest at Twifo Praso.
In an earlier study of mercury levels in sediments and
soil within the river Pra basin, lower concentrations were
reported (Donkor et al. 2006). For sediments a range of
0.007 to 0.057 lgg
-1
(mean =0.026) was reported for
the lower Pra basin and a range of 0.013 to 0.023 lgg
-1
(mean =0.018) for the upper Pra basin. For soil, mercury
levels ranged 0.003–0.202 (mean =0.076) and 0.012–
0.034 (mean =0.025) for the lower and upper Pra basins
respectively. The difference could be attributed to differ-
ent sampling periods, seasons, locations with respect to
the mining areas, volume and flux of waterways and
particle size distribution of the sediments (Larceda and
Salomons 1991). When our results were compared with
levels reported for elsewhere, differences were observed.
The levels obtained for sediment from the Brazilian
Madeira river (0.030–0.350 lgg
-1
; mean =0.130) and
from Brazilian Tapajos river (0.170–0.430 lgg
-1
;
mean =0.290) were lower than values reported in this
study. Far higher values were however reported for sedi-
ments from Philippines Mindanao island (0.920–66.470;
mean =21.030). On the other hand mercury levels
reported for soil in this study were lower than levels
reported for Brazil Madeira river (0.270–0.540 lgg
-1
;
mean =0.390) and for Brazil Tapajos river (0.180–
0.360 lgg
-1
). Recent mercury accumulation rate in sed-
iments is reported to increase with concentrations in fish.
Hunter et al. (1987) reported that more than 90% of the
surface sediments of the Onondaga Lake contained mer-
cury concentrations greater than 0.1 ppm. In the same
year, mercury levels in fish were also found to exceed
0.5 ppm, the maximum permissible levels established by
the World Health Organisation (WHO).
Table 1 Total mercury concentrations (lgg
-1
) in muscle tissue of fish samples from River Pra Basin, Southwestern Ghana
Species Sample site Sample
size (n)
Fresh weight
range (g)
Mean
weight (g)
Hg concentration
range
Mean Hg
concentration
Labeo coubie Twifo Praso 17 9.0–379.2 204.9 0.040–0.083 0.057
Awisam 19 108.1–373.3 244.6 \0.001–0.136 0.071
Beposo 4 137.0–500.6 340.0 0.035–0.043 0.039
Daboase 5 126.1–950.7 391.7 0.020–0.037 0.029
Tilapia zilli Twifo Praso 11 43.3–602.5 359.3 0.033–0.109 0.059
Daboase 5 77.4–201.5 124.7 0.019–0.041 0.028
Tilapia multifaciata Beposo 5 113.4–290.4 181.7 0.019–0.163 0.053
Daboase 10 96.3–184.3 138.2 0.013–0.046 0.026
Chrysichthys sp Twifo Praso 6 161.7–427.5 262.2 0.092–0.257 0.187
Beposo 12 126.7–270.1 201.9 0.104–0.370 0.153
Daboase 10 69.7–660.2 291.6 0.069–0.315 0.166
Heterobranchus longifillis Twifo Praso 3 253.3–301.4 279.3 0.142–0.187 0.165
Alestes imberi Twifo Praso 32 139.3–431.1 235.4 0.025–0.069 0.041
Twifo Mampong 15 173.5–385.3 242.6 0.018–0.057 0.042
Beposo 6 191.5–440.4 258.3 0.014–0.216 0.065
Table 2 Mean mercury concentrations (lgg
-1
) in sediments and
soil from River Pra Basin
Samples Sampling Sites
Awisam Twifo
Praso
Twifo
mampong
Daboase Beposo
Sediment 0.507 0.390 0.500 0.707 0.575
Soil 0.078 0.042 0.079 0.075 0.145
Bull Environ Contam Toxicol (2010) 85:324–329 327
123
The levels obtained for fish by Hunter et al. (1987) are
higher than the levels recorded in the fish species under this
study even though high levels were obtained for the sedi-
ments. Mercury concentrations in bed sediments are not
necessarily correlated with concentrations in fish tissues
(Rose et al. 1999). The results of this study appeared to
follow a similar trend as poor correlation was observed
between mercury concentration in fish and sediments at all
the sampling sites. The relationship between mercury
concentrations of fish and sediments vary as a function of
factors that affect sediment methylation rates and mercury
bioavailability. Although some studies have shown that
sediments can be a sink for mercury (Rudd and Turner
1983; Sorensen et al. 1990), mercury accumulation by fish
depends on the combined effect of the abundance of
available inorganic mercury in sediments/water column,
trophic interaction and the rate at which microflora trans-
form mercury into methylmercury in addition to the spe-
cies-specific accumulation and seasonal variations (Jackson
1990). That mercury concentrations in soil from similar
areas were far lower than concentrations in sediments
suggests that atmospheric deposition was not significant.
River Pra has three main tributaries namely, River
Anum, River Oda and Fumso River. Small scale artisanal
gold mining activities are widespread along these rivers.
The low mercury concentration in all the species studied in
River Pra can be ascribed to low input of Hg into the
environment from artisanal gold mining which involves the
use of Hg; and factors affecting methylation of mercury
were probably not favourable. Human activities such as
agriculture that employs the use of mercurial compounds
could be minimal. An attempt to determine any trend of
mercury accumulation along the main river Pra will require
the use of an indicator obtained at all the sampling loca-
tions. But, no specie of fish was obtained at all the sam-
pling locations. Labeo coubie was available at four out of
the five locations and was therefore used as the indicator.
Even though generally, good correlation between mercury
concentrations and fish fresh weight was not obtained,
mercury concentration was normalized in order to reduce
any effect of variable size composition of the samples.
Labeo coubie did not show any consistent trend along
the main river as it flows downward toward the
sea (Awisam =0.290 lg; Twifo Praso =0.277 lg;
Daboasi =0.073 lg; Beposo =0.115 lg). Similarly, no
trend was exhibited for the sediments. All the fish samples
from the five locations along the Pra River recorded mer-
cury concentrations below the World Health Organisation’s
threshold value of 0.5 lgg
-1
. The results obtained in this
study therefore showed that fish from River Pra does not
constitute any significant methylmercury exposure to the
public through fish consumption from the studied areas.
Whereas the mean values for soil in this study were below
the US-EPA value of 0.2 lgg
-1
, the mean values for
sediment exceeded it.
Acknowledgments The authors would like to thank Mr. Munir
Abdullah Dawood of Pure and Applied Biology Department, Faculty
of Biosciences, Kwame Nkrumah University of Science and Tech-
nology for identifying the fish species.
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... According to the FAO [44], fish accounts for as much as 60 percent of animal protein in the average Ghanaian diet, and 22.4 percent of household food expenditures. Studies in Ghana have shown a mean Hg concentration up to 0.187 µ g/g fish muscle tissue, depending on fish species and the location where the fish is caught [45,46]. These concentrations are well below the European Union recommendation for Hg in fish for human consumption (<0.5 mg/kg). ...
... According to the FAO [44], fish accounts for as much as 60 percent of animal protein in the average Ghanaian diet, and 22.4 percent of household food expenditures. Studies in Ghana have shown a mean Hg concentration up to 0.187 µg/g fish muscle tissue, depending on fish species and the location where the fish is caught [45,46]. These concentrations are well below the European Union recommendation for Hg in fish for human consumption (<0.5 mg/kg). ...
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... Data on mercury concentration in whole small fish as food for humans is scarce in the scientific literature, which limits the basis for comparison. However, scientific reports on mercury concentrations in fish fillet from the Gulf of Guinea and Ghana have reported mercury concentrations of 0.19 mg/kg up to 0.61 mg/kg in fillet of local marine and freshwater fish [34,115,120,127]. The aforementioned studies found higher concentrations of mercury than in the present study, even though the water content was reduced by processing. ...
... The aforementioned studies found higher concentrations of mercury than in the present study, even though the water content was reduced by processing. Due to illegal gold mining, freshwater fish from surrounding watersheds are likely to be contaminated by mercury at higher levels [115,127]. All samples analyzed in this study were below the limit given by the Ghanaian Standards Authority and European Commission of 0.5 mg/kg [107,109]. ...
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... In Ghana, toxins of fungal origin (mushroom toxins and mycotoxins) are another commonly reported causative agent of food poisoning (Apetorgbor et al., 2006;Kortei et al., 2019;Osemwegie et al., 2014) and the rare cases of severe food poisoning can be linked to consumption of other toxins such as phytotoxins and phycotoxins (cyanogenic glycosides which occur in cassava a staple widely consumed in Africa) (Kwaansa-Ansah et al., 2017;Nhassico et al., 2008;Opoku-Nkoom et al., 2013). Furthermore, contamination of aquatic foods (mostly fishes) with methyl mercury has been reported (Gbogbo et al., 2018;Kortei et al., 2020;Oppong et al., 2010). Methyl mercury (methyl derivative) that is formed by bacterial action in an aquatic environment may result in high toxicity (Dolan et al., 2010). ...
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... Fishery products from the Senegalese coasts, that of the Dakar region, are overexploited and marketed locally, in the absence of analyses of mercury and its derivatives [8]. In contrast, in African countries, such as Ghana, the presence of mercury has been assessed in some species of fish and environmental samples [9][10][11][12][13]. ...
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Samples of soils, plantain (Musa paradisiaca), water fern (Ceratopteris cornuta), elephant grass (Pennisetum purpureum), cassava (Manihot esculenta) and mud fish (Heterobranchus bidorsalis) were collected from Obuasi and its environs, which is the most active gold mining town in Ghana. The distribution of mercury and arsenic in these samples from fourteen sampling sites was determined. The annual average surficial soil concentrations of As and Hg from 14 sampling sites have the mean and SD of 12.92 17.48 (range = 2.11–48.87 mg kg–1 d.w.) and 0.93 0.58 (range = 0.29 – 2.52 mg kg–1 d.w.), respectively. The annual average concentrations of As and Hg from plant and grass samples show the mean and SD of 9.05 17.50 (range = 0.49 – 78.71 mg kg–1 d.w.) and 1.85 2.04 (range = 0.12 – 9.68 mg kg–1 d.w.), respectively. Plant/soil concentration ratios of As and Hg showed elevated values for the grass samples, especially from sites within 4 km of the Pompora Treatment Plant. The high bioaccumulation ratios of fern reflected both soil and air sources of pollution. The results substantiated a mercury and arsenic concentration gradient in the area, thereby indicating that the local environment is contaminated by mining activities.