Journal of Environmental Protection, 2011, 2, 720-728
doi:10.4236/jep.2011.26083 Published Online August 2011 (http://www.SciRP.org/journal/jep)
Relationship between Heavy Metal Concentrations
in Bottom Sediments and the Clam, Galatea
Paradoxa (Born 1778) from the Volta Estuary,
Ghana
Hashem A. Madkour1, Kwasi A. Obirikorang2, Steve Amisah2, Fred A. Otchere3,
Daniel Adjei-Boateng2
1National Institute of Oceanography and Fisheries (NIOF), Red Sea Branch, Hurghada, Egypt; 2Department of Fisheries and Water-
shed Management, Faculty of Renewable Natural Resources, Kwame Nkrumah University of Science and Technology (KNUST),
Kumasi, Ghana; 3Binpal Engineering Ltd., Surrey, Canada.
Received January 3rd, 2011; revised May 2nd, 2011; accepted June 7th, 2011.
ABSTRACT
This research was carried out at two locations, Ada and Aveglo at the Volta Estuary, Ghana to evaluate the importance
of sediments as regulatory media in controlling the levels of four metals, Manganese (Mn), Zinc (Zn), Iron (Fe) and
Mercury (Hg) in the tissues of the clam, Galatea paradoxa (Born 1778) and to investigate the possible relationships
between the concentrations of the metals in the sediments and the clams. The clams were categorized into three size
classes as follows: small (25 - 40 mm), medium (41 - 55 mm), and large (above 55 mm). To understand the possible
relationships between the concentrations of the studied metals in the sediments and in the tissues of the three clam size
classes, the monthly concentrations of the studied metals were graphed and subjected to Pearson correlation analyses
(p < 0.05) to identify metal accumulation patterns and determine whether or not positive relationship patterns existed
between the concentrations in the clams and sediment samples. The correlation revealed no simple linear relationships
between the concentrations of four heavy metals in the clam tissues and the sediments at the two sampling stations al-
though some distinct trends were observed. Mn concentrations in the clams and sediments from the two stations showed
some clear positive relationship patterns with some increments in monthly sediment concentrations resulting in incre-
ments in clam tissue concentrations. This relationship though, was not too clear-cut.
Keywords: Heavy Metal, Galatea Paradoxa, Sediments, Volta Estuary
1. Introduction
Sediments are an important sink for a variety of pollut-
ants, particularly heavy metals and may serve as an en-
riched source of such pollutants, especially in estuarine
ecosystems. In some cases, sediments may hold over
99% of the total amount of heavy metals present in an
ecosystem [1] and these metals may be present in the
estuarine system as dissolved species, as free ions or
forming organic complexes with humic and fulvic acids.
Additionally, many metals associate readily with par-
ticulates and become adsorbed or co-precipitated with
carbonates, oxyhydroxides, sulphides and clay minerals.
Consequently, sediments accumulate contaminants and
may act as long-term stores for metals in the environment
[2]. Exposure of sediment-dwelling organisms to metals
may occur via uptake of interstitial waters, ingestion of
sediment particles and via the food chain [3]. The occur-
rence of elevated concentrations of trace metals in sedi-
ments found at the bottom of the water column can be a
good indicator of man-induced pollution rather than
natural enrichment of the sediment by geological weath-
ering [4,5].
Bivalves as sedentary filter feeders usually have the
ability to accumulate high metal concentrations in their
tissues without metabolising them appreciably [6-8].
Bottom sediments are an important source of heavy met-
als to bivalves because heavy metals tend to adhere to

Relationship between Heavy Metal Concentrations in Bottom Sediments and the Clam, Galatea 721
Paradoxa (Born 1778) from the Volta Estuary, Ghana
sediment particles and if bivalves take in these sediment
particles as part of their normal feeding process, the
heavy metals are also ingested [9]. This research was
thus carried out at the Volta Estuary, Ghana to evaluate
the importance of sediments as regulatory media in con-
trolling the levels of four metals, Manganese (Mn), Zinc
(Zn), Iron (Fe) and Mercury (Hg) in the tissues of the
clam, Galatea paradoxa (Born 1778) and to investigate
the possible relationships between the concentrations of
the metals in the sediments and the clams.
2. Methodology
2.1. Study Area
The study was carried out at Ada and Aveglo, both at the
Volta Estuary, Ghana, over an 18-month period, from
March 2008 to August 2009. Ada (Latitude 05°49′18.6 ″
N and 000°38.46′1″E) and Aveglo (05°53′28.2″N and
000°38′24.7″E) represent the southern and northern lim-
its of the most active clam fishing grounds at the Volta
Estuary (Figure 1).
2.2. Study Species
The clam, Galatea paradoxa (Born 1778) is a commer-
cially important bivalve species exploited mainly for its
flesh and is consumed boiled or fried. It is a filter-feeding
organism with a wide distribution extending from the
Gulf of Guinea to the Congo [10]. Limited information
about the prevalence and commercial exploitation of this
clam is available from only a few countries, including
Ghana, Nigeria and Cameroon, despite its extensive dis-
tribution in the wider West African region [11]. In Ghana,
the Volta estuary represents the main fishing grounds of
G. paradoxa. Clam fishing represents a viable source of
income and livelihood for the local people. Furthermore,
it constitutes an important and affordable protein source
to the riparian communities along the Volta estuary and
beyond [12].
Collection and Processing of Samples
Surface Sediments
Riverbed sediment samples were collected on a
monthly interval for 18 months using an Ekman grab at
the two locations from March 2008 to August 2009.
Sediment samples were collected from each sampling
sites according to the standard procedures described in
USEPA’s sediment sampling guide [13] and were kept in
LDPE bottles pre-washed with 10% HC1 and stored in
insulated iced chests for analysis in the laboratory. In the
laboratory, 500g of the sediment subsamples from each
Figure 1. Map showing the clam sampling locations at Ada and Aveglo in the Volta estuary in Ghana.

Relationship between Heavy Metal Concentrations in Bottom Sediments and the Clam, Galatea 722
Paradoxa (Born 1778) from the Volta Estuary, Ghana
sampling location were placed in ceramic mortars for
drying at 80˚C for 48hrs to a constant weight [14]. The
dried samples were then gently disaggregated and 250 g
of each sample stored in 250 ml acid-washed LDPE bot-
tles and kept at 4˚C in a refrigerator for heavy metal and
analyses [13].
Clam Samples
Clam samples were obtained from fishermen’s catch
from the two sampling stations at monthly intervals for
18 months and transported to the laboratory, submerged
in river water, in insulated chests within 12 hours for
processing and storage for heavy metal analyses. In the
laboratory, clam samples were cleansed to remove the
mud and any debris and then washed with double dis-
tilled water. For each site, clams of various sizes were
obtained and grouped into three size classes of 10 indi-
viduals, each based on shell lengths. The categorization
was as follows: small (25 - 40 mm), medium (41 - 55
mm), and large (above 55 mm). The various clam size
classes were purged of ingested organic and inorganic
particles before being analyzed for heavy metal accumu-
lation by keeping each size class in distilled water for a
24-hour depuration. After the depuration process, a ster-
ile stainless steel knife was used to dislodge and remove
the soft tissue of each clam from the shell [15]. The flesh
of each subsample was oven-dried to a constant weight at
60˚C for 48 hours. Each dry clam sample was weighed
on a Sartorius BP 210 S micro balance to the nearest
0.0001g. Individuals of each size class were ground to-
gether into fine powder using a porcelain pestle and
mortar. Homogenized subsamples were stored in airtight,
acid-washed (0.1MHCl) snap-top glass vials for heavy-
metal analyses [16].
Digestio n of the Samples
About 0.5 g of the homogenized clam subsamples and
sediment samples were weighed into a 50 ml digestion
tube and 1 ml of distilled water, 2.0 ml perchloric acid
(HNO3–HClO4) (1:1 vv) and 5.0 ml sulphuric acid
(H2SO4) were added. Each mixture was refluxed at
200˚C for 30 minutes in a clean fume chamber. The
completely digested subsamples were allowed to cool at
room temperature, and the undigested portions were fil-
tered off through a Whatmann Glass Microfibre filter
paper (GF/C) to obtain a clear solution and diluted to 50
ml in volumetric flasks with double distilled water [17,
18].
Determination of Zn, Fe and Mn
Concentrations of Mn, Zn and Fe were determined us-
ing a Buck Scientific Model VGP flame Atomic Absorp-
tion Spectrophotometer (AAS) and the wavelengths and
detection limits for the studied metals are shown in Ta-
ble 1. All tissue and sediment analytical batches were
Table 1. Wavelengths and detection limits for the studied
heavy metals.
Element Slit Wavelength Detection Limit
Manganese 0.7 385.2 0.001
Zinc 0.7 213.9 0.005
Iron 0.7 248.3 0.03
accompanied by blanks at a minimum rate of one blank
per 20 samples. Replicate analyses were conducted on
10% of the samples to evaluate precision of the analytical
techniques. The data were expressed as total concentra-
tion (μg/g dry weight (dw).
Determination of Total Mercury (THg)
The Automatic Mercury Analyzer (Model HG 5000)
equipped with a mercury lamp at a wavelength 253.7 nm
was used for the determination of Total Mercury (THg)
in the clam soft tissue and sediment samples. Responses
were recorded on strip chart recorders as sharp peaks.
The peak heights were used for computation of the total
mercury concentrations in the clam and expressed as
microgram per gram dry weight (μg/g dw). Total mer-
cury concentrations were validated according to standard
procedures described for Mercury Analyzer Model HG
5000 to check for precision and accuracy.
Physicochemical Water Parameters
Monthly measurement of temperature, salinity, pH,
pressure, total dissolved solids (TDS), conductivity and
dissolved oxygen (DO) of the Volta River were taken at
both sites for the period using a Hanna (HI 9028)
multi-parameter probe.
Statistical Analysis
The evaluation of the relationship between the con-
centrations of the four heavy metals in the tissues of the
three clam sizes and the sediment samples was per-
formed using the Pearson correlation (p < 0.05). All de-
scriptive statistics and graphs were executed using the
GraphPad Prism 5 Software.
3. Results
3.1. Physicochemical Parameters of the Volta
Estuary
The physicochemical parameters of the Estuary over the
18-month sampling period were fairly similar at the two
sampling locations. The summary of the results are
shown in Table 2.
3.2. Metal Concentrations in the Tissues of the
Clams
Ada Sampling Station
Manganese (Mn) concentration in the whole soft tissue
of the small-sized clams varied from 73 μg/g in June
2008 to 867 μg/g in July that same year depicting a rela

Relationship between Heavy Metal Concentrations in Bottom Sediments and the Clam, Galatea 723
Paradoxa (Born 1778) from the Volta Estuary, Ghana
Table 2. Physicochemical parameters of the volta estuary at
ada and aveglo.
Sampling
Station Parameter Range Mean ± SD
Ada pH 6.18 - 8.50 6.94 ± 0.52
Temperature (˚C) 27.28 - 29.59 28.60 ± 0.80
Salinity 0.02 - 0.03 0.027±0.005
DO (mg/l) 1.52 - 8.76 4.19 ± 1.93
TDS (mg/l) 27 - 35 30.06 ± 2.65
Conductivity (µS/cm) 52 - 70 60 ± 5.16
Aveglo pH 6.23 - 7.28 6.85 ± 0.27
Temperature (˚C) 27.19 - 29.62 28.68 ± 0.69
Salinity 0.02 - 0.04 0.028±0.005
DO (mg/l) 1.58 - 6.79 3.89 ± 1.80
TDS (mg/l) 27 - 42 31 ± 3.48
Conductivity (µS/cm) 54 - 84 62.83 ± 7.62
tively wider variation in Mn concentrations. The medium-
sized clams recorded Mn values of between 68 μg/g in
May 2008 and 336 μg/g in August 2008. Mn concentra-
tion in the tissues of the large-sized clams ranged from
49 μg/g in June 2008 to 316 μg/g in February, 2009. The
highest concentration of Zn in the tissue of the small-
sized clams (59 μg/g) was recorded December 2008 with
the lowest concentrations of 19 μg/g being recorded in
the month of August of the same year. Concentrations of
13 μg/g and 57 μg/g were recorded as the lowest and
highest concentrations for the medium-sized clams in the
months of March and December, 2008 respectively. In
the large-sized clams, Zn concentrations varied between
16 μg/g in March 2008 and 49 μg/g in December, 2008.
The results obtained for Fe in the tissues of the
small-sized clams indicated a highest value of 484 μg/g
in January 2009 and a lowest value of 103 μg/g in No-
vember, 2008. The medium-sized clams recorded Fe
concentrations ranging from 79 μg/g to 340 μg/g in April
2009 and February 2009 respectively. The large- sized
clams recorded values ranging from a low of 96 μg/g in
June 2009 to a high of 313 μg/g in January 2009. Total
Mercury (THg) concentrations for the small-sized clams
ranged from 0.028 μg/g in April 2008 to 0.042 μg/g in
August 2008. The medium-sized clams recorded a high-
est THg value of 0.049 μg/g in March and September
2008 and a lowest value of 0.035 μg/g in April 2008.
THg concentrations ranged between a low of 0.044 μg/g
and high of 0.059 μg/g in July 2008 and September 2008
in the large-sized clams.
Aveglo Sampling Station
The highest concentration of Mn in the small-sized
clams was observed in February 2009 (201 μg/g) and
lowest in June 2008 (79 μg/g). The medium-sized clams
recorded concentrations varying between a low of 73
μg/g in May and June 2008 and 206 μg/g in March of the
same year. The large-sized clams recorded values rang-
ing from 72 μg/g in November 2008 to 228 μg/g in Au-
gust 2008. Zn concentrations for the small-sized clams
ranged from a lowest value of 25 μg/g in April 2008 to a
highest value of 59 μg/g in December of the same year.
Values of 16 μg/g and 54 μg/g in June and December
2008 respectively were recorded as the lowest and high-
est values for the medium-sized clams. The large-sized
clams recorded Zn concentrations ranging from 16 μg/g
in June 2008 and 48 μg/g in September and December
2008. Results from the monthly analyses of Fe in the
tissues of the small-sized clams indicated a lowest value
of 119 μg/g in October 2008 and a highest value of 427
μg/g in January 2009. The medium-sized clams had Fe
concentrations ranging between a low of 72 μg/g and a
high of 539 μg/g in November 2008 and March 2008
respectively. 79 μg/g and 304 μg/g were the lowest and
highest values recorded in the months of November 2008
and January 2009 respectively for the large-sized clams.
THg concentrations in the tissues of the small-sized
clams ranged from 0.037 μg/g to 0.055 μg/g in May and
March 2008 respectively. Sampled medium-sized clams
recorded values ranging from 0.042 μg/g in July 2008 to
0.056 μg/g in August 2008. The large-sized clams had a
lowest THg concentration of 0.037 μg/g in March 2008
and a highest concentration of 0.074 μg/g in June 2008.
3.3. Metal Concentrations in the Sediment
Samples
Ada Sampling Station
The concentrations of Mn in the sediments sampled
from the Ada sampling station did not vary considerably
over the 18-month period and were generally similar.
The lowest Mn concentration of 110 μg/g was recorded
in November 2008 whiles the highest of 393 μg/g was
recorded in March 2009. Zn concentrations were gener-
ally very low in the Ada sediments with very small varia-
tions in concentrations over the sampling period. Con-
centrations were observed to be lowest in September
2008 where concentrations were in trace amounts below
the detection limit of the AAS. A concentration of 9 μg/g
was recorded in the month of July 2009 as the highest Zn
concentrations during the sampling period. The monthly
Fe concentrations over the sampling period were gener-
ally very high and followed no particular pattern. Con-
centrations varied from a lowest value of 696 μg/g in
August 2008 to 2758 μg/g in March of the same year.
Concentrations of THg ranged from 0.0069 μg/g to
0.0240 μg/g in the months of April and August 2008 re-
spectively.
Aveglo Sampling Station
Mn concentrations in the Aveglo sediments varied
from a low of 100 μg/g in May 2008 to 290 μg/g in Au-

Relationship between Heavy Metal Concentrations in Bottom Sediments and the Clam, Galatea 724
Paradoxa (Born 1778) from the Volta Estuary, Ghana
gust 2008. Concentrations of Zn in the sediments were
very low, similar to the concentrations recorded at Ada
between the months of May and September of 2008. Zn
values ranged from the lowest value 1 μg/g in the months
of October and November 2008 to the highest of 8 μg/g
in the months of May and June 2009. Fe concentrations
were relatively very high with the lowest concentration
of 1114 μg/g recorded in June 2008 and the highest value
of 3476 μg/g in August of 2008. THg concentrations in
the sediments were far lower than the concentrations
recorded in the tissues of the clams of all the three size
classes. The lowest concentration of 0.0078 μg/g was
observed in May 2008 and the highest of 0.0230 μg/g in
June 2008.
3.4. Relationship between Metal Concentrations
in Sediments and Clam Tissues
G. paradoxa are predominantly found at the sediment-
water interface and thus metal contaminants in the sedi-
ments have a potential to significantly influence the
metal concentrations in their tissues.
To understand the possible relationships between
metal concentration in the sediments and accumulation in
the tissues of the three clam size classes as far as Mn, Zn,
Fe and Hg were concerned, the monthly concentrations
of the studied metals were graphed and subjected to
Pearson correlation analyses (p < 0.05) to identify metal
accumulation patterns and determine whether or not
positive relationship patterns existed between the con-
centrations in the clams and sediment samples (Figures 2
and 3).
The correlation analyses revealed no simple linear re-
lationships between the concentrations of four heavy
metals in the clam tissues and the sediments at the two
sampling stations although some distinct trends were
observed. Mn concentrations in the clams and sediments
from the two stations showed some clear positive rela-
tionship patterns with some increments in monthly sedi-
ment concentrations resulting in increments in clam tis-
sue concentrations. This relationship though, was not too
clear-cut.
No defined accumulation patterns were established for
Zn, Fe and Hg concentrations at the two sampling loca-
tions. Zn concentrations in the sediment samples were
consistently low with values over the 18-month sampling
period all falling below 10 μg/g. Fe concentrations on the
other hand were clearly higher in the sediment samples
and showed no distinct link with the concentrations in the
clam tissues.
Results of the Spearman correlation analyses revealed
no positive correlation (p > 0.05) between sediment con-
centrations of the four metal and the concentrations in the
tissues of the three clam size classes.
4. Discussion
It is generally agreed that heavy metal uptake occurs
mainly from water, food and sediment. However, effec-
tiveness of metal uptake from these sources may differ in
relation to ecological needs and metabolism of animals
and concentrations of the heavy metals in water, food
and sediment as well as some other factors such as salin-
ity, temperature and interacting agents [19].
Analyses of the clam and sediment samples revealed
no distinct relationship between heavy metal levels in
clam tissues and sediments in which they thrive. An ob-
servation of the concentrations of all the clam size
classes and sediments from the two stations revealed no
significant relationship patterns, indicating no distinct
trend in metal uptake by the clams as far as the sediment
in/on which they are found is concerned. A possible ex-
planation of this phenomenon could be that heavy metal
accumulation in the clams may not be directly or solely
derived from sediments. Other sources of heavy metals in
bivalve tissues are derived from living or dead suspended
particles and from dissolved metals in the water [20].
According to [20], the release of heavy metal from
sediments and the subsequent availability to living things
is controlled by the behaviour of heavy metals them-
selves, and the physical and chemical condition of the
surrounding environment and this probably explains why
there are no clear relationships between the concentration
of heavy metals in clam tissues, and those in sediment.
For example, although monthly concentrations of Fe in
the sediments from both stations were markedly higher
than Mn concentrations, the monthly concentrations of
the two metals were relatively similar in the tissues of the
clams. This phenomenon could be due to the fact that Fe
is deposited much more quickly but is strongly bound to
the sediments under the estuarine conditions [20]. Fe is,
thus, not readily available to the clams as far as the
sediments are concerned as a heavy metal source. Mn on
the other hand can be said to be released much more eas-
ily from sediments than Fe and thus is more readily
available to the clams.
The relatively consistent monthly concentrations of
Mn, Fe and Zn in whole soft tissues of G. paradoxa may
well represent efficient metabolism and detoxifying
processes that include transportation, transformation,
sequestration and/or excretion of excess metals [21]. The
results further suggest that the levels of contamination of
these metals do not exceed the clam’s capacity of regula-
tion [22,23]. The relatively hgher concentrations of Zn i

Relationship between Heavy Metal Concentrations in Bottom Sediments and the Clam, Galatea
Paradoxa (Born 1778) from the Volta Estuary, Ghana
725
Figure 2. Relationships between Mn, Zn, Fe and Hg concentrations in the tissues of the different clam size classes and sedi-
ments from the Ada sampling station.
in the clam tissues compared to the concentrations in the
sediments suggests a high rate of accumulation by the
clams—a physiological mechanism induced by exposure
or even a high relevance of the water as an additional
source of contamination [24].
Biological variables such as size, sex or changes in

Relationship between Heavy Metal Concentrations in Bottom Sediments and the Clam, Galatea 726
Paradoxa (Born 1778) from the Volta Estuary, Ghana
Figure 3. Relationships between Mn, Zn, Fe and Hg concentrations in the tissues of the different clam size classes and sedi-
ments from the Aveglo sampling station.
tissue composition and reproductive cycle as well as the
season of sampling and the hydrodynamics of the estuary
have to be considered as far as variations in metal con-
centrations are concerned. Seasonal variations have been
reported to be higher in winter/dry than in summer/wet.
These seasonal variations have been related to a great
extent to seasonal changes in flesh weight during devel-
opment of gonadic tissues [18,25-27]. Variability of

Relationship between Heavy Metal Concentrations in Bottom Sediments and the Clam, Galatea 727
Paradoxa (Born 1778) from the Volta Estuary, Ghana
heavy metal concentrations can also be caused by
changes in the physiological conditions of the clams [28,
29] and environmental parameters including temperature,
pH, salinity, oxygen concentrations [30,31]. Except Hg,
all the metals examined in this study are essential for
clams and have intracellular regulatory mechanisms [32]
to keep their concentrations in the clam tissues in equi-
librium.
5. Conclusions
The study revealed no simple linear relationships be-
tween the concentrations of four heavy metals in the
clam tissues and the sediments at the two sampling sta-
tions although some distinct trends were observed. Mn
concentrations in the clams and sediments from the two
stations showed some clear positive relationship patterns
with some increments in monthly sediment concentra-
tions resulting in increments in clam tissue concentra-
tions. This relationship though, was not too clear-cut
possibly indicating that heavy metal accumulation in
clams may not be directly or solely derived from the
sediments but from other sources such as living or dead
suspended particles and from dissolved metals in the
water.
6. Acknowledgements
The authors are grateful to the International Foundation
for Science (IFS) for providing financial support
(A/4421-1) to conduct this research work and the De-
partment of Fisheries and Watershed Management of the
Kwame Nkrumah University of Science and Technology,
Kumasi for logistical support.
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