HEAVY METAL LEVELS IN MARINE SEDIMENTS OF SINGAPORE
B.P.L. GOH and LM. Cl-IOU
Reef Ecology Laboratory, Department of Zoology, National University of Singapore,
Kent Ridge, Singapore 05] 1
Abstract. Marine environmental levels of the metals copper (Cu), zinc (Zn). lead (Pb) and cadmium (Cd) were
measured from sediments collected around 20 coastal locations around Singapore, over a 2-year period. Sediment-
size analysis was conducted on sediment samples, and Atomic Absorption Spectrophotometry was used in the
analyses of sediment heavy metal concentrations. The levels of heavy metals in marine sediment was largely
dependant on sediment particle size, as illustrated by the correlation of sediment size with Multidimensional
Scaling (MDS) conﬁgurations of sediment metal concentrations, In addition, the proximity to shipping activity, and
the release of anti—fouling paint from boats also inﬂuence heavy metal concentrations in marine sediments of
Heavy metals are increasingly being introduced into the environment as contaminants and
pollutants, by-products of industry and human civilization (Patin, 1982; Nriagu & Pacyna,
1988). As a result of this, programmes to regularly monitor the levels of these contaminants
and pollutants in the environment are important in the environmental management of any
industrialised or developing country (Phillips & Yim, 1981), in order to control anthropogenic
sources of pollution and prevent them from rising to levels detrimental to human beings.
Several techniques have been developed to monitor the level of heavy metal contamination
in the marine environment. The most simple, is the use of levels of metals measured from
water, soil or sediment, and the tissue of marine biota as indicators of environmental health.
To complement in situ studies of metal levels in water, heavy metal analyses of sediment and
biota are usually also carried out as the latter two tend to accumulate metals, and give the
additional advantage of indicating contamination levels over time (Bender et al., 1972:
Goldberg er at-., 1978; Luoma 1990).
A few short-term studies on heavy metal pollution in Singapore‘s coastal waters were carried
out in the 1970's by researchers from various tertiary institutes (Chai, 1975; Yap, 1975; Chai
& Wong, 1976; Goh, 1976; Chung, 1979; Rahman et al., 1979). More recently, a six month
study of heavy metal levels in the Singapore River was undertaken (Sin et a1., 1991)., To date,
however, no regular monitoring programme for heavy metals in Singapore's marine
With rapid development and industrialization, the threat of environmental pollution also
increases. Although strict legislation governing the discharge of industrial effluents exists
under the Water Pollution Control and Drainage Act of Singapore (Republic of Singapore
Government Gazette. 1971, 1976), a systematic and comprehensive pollutionimonitoring
programme is also necessary to ensure that pollutants fall within environmentally’ safe limits.
This study was undertaken as part of a project to monitor the heavy metal levels of the
coastal environment of Singapore on a regular basis, over a period of two years. This paper
describes the sediment analysis component of the project and reports the levels of copper
(Cu), zinc (Zn). lead (Pb) and cadmium (Cd) observed in the coastal sediments of Singapore.
2. Materials and Methods
A total of 20 locations comprising mainland coastal and offshore areas encompassing various
ecosystems, and ranging from sites close to various industries (e.g. oil refineries and
shipyards) to sites used for aquaculture activities (Table 1), were sampled regularly every
three months over a period of two years (Fig. 1). Seven sediment sample collections were
made, ﬁom December 1990 to July 1992.
The 20 locations studied, descriptions of acitivities and habitat £333 '
Location Code Activity ‘ Habitat type
Bedok BD Recreational , shipping anchorage Marine mud
Changi CG Reclaimed land. no industries Sandy shore
Kallang Basin KB Transport vessels. shipping channel Dredged mud
Kranji KJ Light industries. e.g. plywood store Mangroves, mudﬂat
Labrador LB Oil pipeline, ship repair, shipping channel Rocky shore
Punggol PG Boatels, other industries Mudﬂat
Pasir Panjang PP Transport jetty. container terminal Dredged sand
Sembawang SB Shipyard Mudﬂat
Sarimbun SR Recreational, no industries Mangroves
Tuas TU Reclaimed land, factories Sandy shore
OFFSHORE LOCATIONS _
Pulau (P.) I-lantu HT Recreational boats. oil reﬁnery in vicinity Coral reef
P. long JG Recreational, oil storage in vicinity Coral reef
Merhau MB Petrol-chemical. pharrnaccuticai factory Marine mud
Rafﬂes Lighthouse RL Recreational. no industries Coral reef
St. John's Island S} Recreational. no industries Corals, mud/rocky
P. Sakeng SK Fishing village, transportjetty Marine mud
Sentosa Lagoon SL Reclaimed beach, recreational Sandy shore
Sentosa North SN Dredged shipping channel. ship repair Rocky shore
P. Semakau SM Earth spoil dump site in vicinity. recreational Coral reef
P. Ubin UB Transport it-.t_ty, guaculture farming Marine mud
The preparation for sample collection and analyses was carried out following the protocol
set out by Kremling et al. (1983). _
At offshore locations, sediments were collected using an Ekman grab. In order to avoid
contamination from the metal sides of the grab, grabbed sediments were subsamplecl from the
centre of the grab using a plastic spatula. At mainland coastal locations, surface sediments
were collected at low tide, using a plastic spatula. Sediments were stored in pre-acid washed
polyethylene containers at a temperature of -20°C until analyses.
""-.?;"..‘.."“4> MB <13
""“”B”“°"‘ Sm’? 0 Sample locations
H11: g§. \ e
P"m""'Z3 st»/1'0 gig]: .' wmwim
Pulau Sudong SJ
Map of Singapore shomng sample locations (BD = Bedok, CG= Changi, KB = Kallang Basin, K} = Kranji,
LB fiabrador. PG = Punggol. PP = Pasir ‘Panjang. SB = Sembawang. SR = Sarimbun. TU = ruas,
HT - Pulau (R) Hantu. JG = P. long, MB = Memau Beacon, R}. = Raﬁles Lighthouse, s3 = St. John‘s lslana‘.
SK = P. Sakeng, SL = Sentosa Lagoon, SN = Semosa North, SM = P. Semakau, UB = P. Ubin)_
DIGESTION AND ANALYSES
Sediment samples were thawed at room temperature prior to analyses. Wet weights of
between 1 to 20 of material were placed into porcelain crucibles and dried in an oven at 100°C
until their dry weights were constant (approximately 3 days).
Digestion of samples was carried out using a modiﬁcation of the method described by Sinex
er al. (1980). Suprapure nitric acid (Merck) was used together with deionised water in the
procedures. Samples were digested in porcelain crucibles using 8ml of 65% nitric acid with
continuous heating on a hot plate at 75°C for approximately 3 hours. Solutions were
evaporated to almost dry, twice redissolved in 10% nitric acid at 65°C, and ﬁltered into 50ml
volumetric ﬂasks through Whatman ﬁlter paper (no. 42). Crucibles were ﬁnally double—rinsed
with 5ml deionised water and the ﬁnal solutions made up to 50ml using 10% nitric acid.
Sediment solutions were directly analyzed for the heavy metals copper, zinc, lead and
cadmium using the Hitachi Polarised Atomic Absorption Spectrophotometer (AAS) with
Zeeman background correction. Two replicate analyses were made for each sediment sample.
Particle size analysis was also carried out on sediment samples employing the method
recommended by Buchanan (1984).
Colmcr concemmtioﬂ (}’§.g" dr)’ Wt.) Comm °‘"'°°""a“°" 9'33.‘ dry W”
Mean results of Eanicle size analyses of sediments (rriglicate)
Location Code Sand fraction ill / clay fraction
63 - 500nm ("/o) < 63um (%
Bedok BD 73.6 26.4
Changi CG 90.2 9.8
Kallang Basin KB 77.7 22.3
Kranji K] 75.} _ 24.9
Labrador LB 73.8 26.2
Punggol PG 93.7 6.3
Pasir Panjang PP 69.5 30.5
Sembawang SB 857 14.3
Sarimbun SR 72.8 27.2
Tuas TU 73.4 26.6
Pulau (P.) Hantu HT 957 4.3
P. Jong JG 99.2 0.8
Merbau MB 86.4 13.6
Raﬁles Lighthouse RL 99.6 0.4
St. John’s Island SJ 74.8 25.2
P. Sakeng SK 76.8 23.2
Senrosa Lagoon SL 75.5 24.5
Sentosa North SN 70.9 29.1
P. Semakau SM 89.5 10.5
P. Ubin UB 72.9 27.1
:EDEC.'9O DMAR.'9l BJUN.'91 '.-ocr.-91 EJAN.‘92 lAl’R.'92AE.lUAl.—.‘92'
BD CG KB K..l LB PG PP SB SR TU
HT 7 JG MB RI. SJ SK SL SN SM UB
FIGURE 2 A
Copper concentrations in sediments collected from mainland and offshore
locations from December 1990 to July l992. Locations as in Table 1
STATISTICAL TREATMENT OF DA TA
Heavy metal data obtained from AAS analyses of sediment was initially analyzed using a
nonparametric 1-way analyses procedure, the Kruskal-Wallis (Chi—Square) test, to compare
differences between locations. This was performed using SAS software.
Statistically signiﬁcant data was represented graphically using Mutidirnensional Scaling
(MDS), a nonparametric multivariate statistical ordination method (Kruskal & Wish, 1978;
Clarke & Green, 1988). l\/DDS was applied to non-standardized data, with double square root
transformations of metal concentrations to analyze differences in metal concentrations
between locations. Resulting scatter plots placed data points in two dimensions, with distances
between points following a rank order as close as possible to that obtained from a similarity
matrix calculated between locations using the Bray-Curtis measure of similarity (Bray &
Curtis, 1957). The extent to which the two-dimensional plot ﬁtted the rank similarity matrix
was indicated by a “stress" coefficient (Clark, 1993).
The one-way ANOSIM randomisation test (Clark & Green, 1988), which tests for signiﬁcant
differences between pairs of similarity indices was used to identify signiﬁcant differences
between speciﬁc locations. In addition, the proportion of the silt / clay fraction (%) measured
from all 20 locations were superimposed onto MDS conﬁgurations of metal concentrations
at the 20 locations in order to visually determine any correlations between sediment size and
metal concentration (Field 21 al., 1982).
The presentation of results for metal analyses of sediment has been arbitrarily divided into
' "mainland": and "offshore" locations, for the separate metal species, Cu, Zn, Pb and Cd,
SIZE ANALYSES AND METAL CONCENTKANONS
Mean results of particle size analyses of sediment samples collected at the 20 locations in
1990, 1991 and 1992 indicate that the majority of sediment samples were high in proportion
of the silt/clay fraction, with the exception of long (JG) and Rafﬂes Lighthouse (RL), two
offshore locations which had bottom sediment made up mainly of sand and coral rubble, and
silt/clay fractions of < 1% (Table 2).
Cu levels measured from sediment ranged from l.4ug.g" dry wt. at JG to a high of
1781 .4,-egg" dry wt. at Pulau Ubin (UB, Fig. 2). Levels of Cu were exceptionally high at UB
in all sampling months compared with the other locations, with concentrations exceeding all
other locations by a factor of 10, and ranging from 31.6 to 178l.4ug.g" dry wt. (Fig. 2).
Copper concentrations from all locations statistically analyzed using the nonparametric 1-way
Kruskal-Wallis test yielded significant differences between locations (X1 = 100.1; df = 19; p
Concentrations of Zn were highest in sediments collected from UB, ranging from 94.9 to
281 .3hg.g'§ dry wt. (Fig. 3). The Kruskal—Wallis test performed to compare locations yielded
signiﬁcant differences in the Zn concentrations of sediments collected from the 20 locations
(X7 = 82.1: df= 19: p < 0.0001).
-‘VEDE>CV.’9i} UMAR.'9l BJUN.'9l ".'OCT.‘9l EJAN.'92 IAPR.‘92 E.lUL.'92!
100 i \
Zinc concentrations (;ig,.g" dry wt.)
BD CG KB KJ LB PG PP SB SR TU
Zinc concentration (tIg.g" dry wt.)
HT JG MB RI. SJ SK SL sN SM UB
Zinc concentrations in sediments collected from mainland and offshore ' “ ' "
locations from December 1990 to July 1992. Locations as in Table l
Pb concentrations ranged from a low of 1.4ttg.g" dry wt. at Punggol (PG), to a high of 82.2
;.»g.g" dry wt. at Sembawang (SB, Fig. 4), while Cd concentrations ranged from non-
detectable levels at several locations, to l.6ng.g“ dry wt. of sediment at Labrador (LB, Fig.
5). Applying the nonparametric Kruskal—Wallis test on Pb sediment concentrations from 20
locations gave signiﬁcant differences in metal concentrations at all locations (X = 98 .2; df
= 19; p < 0.0001), while statistical analyses of the Cd data revealed that all locations were not
signiﬁcantly different in Cd sediment concentrations (13 > 0.05).
The sediment metal concentrations of Cu. Zn and Pb obtained from all 7 sampling months
yielded statistically-significant differences between sample locations. These sediment metal
concentrations plotted in two dimensional MDS arrays showed some similarities in patterns.
with locations most different in terms of metal concentrations located furthest apart from each
other (Figs. 6a - c). in the plot of Cu concentrations, three distinct clusters could be visualised.
The locations. RL. CG. JG and P. Hantu (HT) fonned a cluster on the right. UB was located
_ QEML. _.—._ mm m:o_._3ox_ %_:_. Cu Eacauoo EO\_.._ m:o:8o_
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2w :5. gm 3.
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E::_.w,.. .S...E«n ~o..z<~.m. Peas. ..;a..z:_.a..a..~_<2.u ow. .525,
2-D MDS conﬁgurations for double root—transformed sediment concentrations
of a) copper (stress value = 0.04), b) zinc (stress value = 0.07) and C) lead
(stress value = 0.06) at the 20 sample locations. Locations as in Table l
to the extreme left, and all other locations formed a cluster in the centre (Fig. 6a). suggesting
that the locations could be divided into three statistically different groups, in terms of Cu
sediment concentrations. The ANOSIM test results on the double root-transformed Cu
concentrations (nomstandardized) conﬁrmed this, with most pairwise tests comparing
differences between individual locations in each cluster yielding signiﬁcance levels of < 5%.
The MDS plot obtained from double root-transformed Zn data (non-standardized) also
showed some form of clustering, with locations, RL, CG, JG, HT and PG forming a loose
group on the right and all other locations clustered to the left (Fig. 61:). However, the
ANOSIM test carried out on Zn data did not show as clear a distinction between the clusters
observed as did the Cu results, and pairwise tests comparing similarity indices of locations
within each cluster did not always yield statistically signiﬁcant differences (i.e. p < 0.05)
between locations. '
The two-dimensional MDS array for Pb data (double root-transformed, non—standardized)
also resulted in two clusters, with the locations RL, JG, CG, HT and PG concentrated to the
right,vand the other locations plotted on the left (Fig. 6c), suggesting that the two clusters were
statistical different in terms of Pb sediment concentrations. The ANOSIM test comparing
locations within each cluster generally supported this, with most pairwise tests comparing
these two clusters yielding significance levels of< 5%.
All MDS plots for Cu, Zn and Pb in sediment had low stress values (< 0.07). indicating that
the plots ﬁtted the rank similarity matrices (Bray-Curtis) for individual metals relatively well
In order to visually determine possible correlations between sediment size characteristics and
the clusters formed by locations, the mean proportion (%) of the silt/clay fraction (particles
< 63am) obtained from each location were superimposed (as symbols) onto the two-
dimensional MDS ordinations, individually for Cu, Zn and Pb (Figs. 7a-c). The scaling of the
symbols could be distinguished visually, longer lines indicating higher percentages of the
silt/clay fraction. It was observed that some correlation did indeed exist between sediment
particle size, and metal concentration, as locations with low percentages of the silt/clay
fraction (e.g. HT, RL, CG and JG) were clustered together and yielded signiﬁcantly lower
concentrations of the metals Cu, Zn and Pb (Figs. 6a-c; 7a—c).
2-D MDS conﬁgurations for double root-transformed sediment concentrations
of a) copper, b) zinc and c) lead, with lines representing proportion of the
siltfclay fraction (%) superimposed over each location
In the M138 plots, it was observed that Cu, Zn and Pb levels in coastal sediments collected
were lowest at RL, CG, JG and HT, and highest at UB (Figs. 6a-c). Sediment collected from
locations with relatively higher concentrations of Cu, Zn and Pb were also generally high in
their silt/clay fractions, indicating that metal concentrations were somewhat dependent on
particle size characteristics (Figs. 7a—c). Understandably, finer sediments have larger surface
areas onto which metals may bind or become adsorbed (Libes, 1992), In addition, organic
matter is also known to be present in larger quantities in ﬁner sediments onto which metals
may become bound. However, this alone cannot account for the high concentrations of metals
measured in the sediments of some locations. For example, although the silt/clay fraction of
27.1% from UB was one of the highest obtained, this did not correspond in magnitude, to the
exceptionally high concentrations of Cu and Zn analyzed from UB compared with other
locations (Figs. 2, 3 and 5). Such high concentrations may be attributed to transport vessels
that ferry passengers between mainland Singapore and the island of Pulau Ubin. Sediments
were collected directly from a jetty where all transport boats dock. These boats are
particularly old and may be introducing Cu into the environment in the form of the scrapings
from old boat paint. Similarly, a relatively high level of Cu was also observed in the sediment
collected at SB. although the silt/clay fraction was only l4.3% (Table 2). The contamination
of Cu at SB may result from its location. in close proximity to the Sembawang shipyard
(Table 1: Chia et al.. 1988). Anti-fouling paints that leach out from transport vessels are
probably the main source of Cu and Zn in the sediment samples collected from UB and SB.
although other sources like incidental discharges of diesel oil from ships and_boats are also
possible. It is also interesting to note that although the sediments collected from locations PP,
LB and KJ were observed to have relatively higher concentrations of Cu and Zn
corresponding to their higher silt/clay fractions. these locations are in fact also in the vicinity
of heavy shipping activity and industries.
Antifouling paints used‘ on boats in Singapore are known to contain high levels of Cu and
organo-tin elements, with Zn often used as a primer (pers. comm., Hempel Coatings
(Singapore) Pte. Ltd.) These paints are designed to release toxicants like Cu. organo—tins and
even Zn into surrounding water (Eisler, l98l ). While Singapore has strict laws goveming the
treatment and discharge of industrial effluents and sewage (Republic of Singapore
Government Gazette, 1971, 1976), no legislation exists regarding the use of antifouling paints
on ships. _
Previously measured concentrations of Zn, Pb and Cd from sediments of the Singapore
River were generally higher than the results obtained in this study, while the range of Cu
levels were similar (Sin et al.., 1991). Compared with previous surveys in marine
environments, Pb concentrations in sediments close to oil refineries in this study fell within
the same range as sediment Pb concentrations reported in 1979 (Rahman et al., ‘I 979).
In comparing sediment heavy metal levels from this study with other parts of the world, Cu
concentrations in UB were excluded, as the levels obtained were exceptionally high, and
statistically signiﬁcant. In the Southeast Asian region, metal concentrations from sediment
were similar to the higher concentrations of sediment Cu and Zn measured in this study, while-
the range of Pb concentrations were similar (Calvert et al.. l993). Concentrations of heavy
metals reported for the surface sediments of the Sulu and South China seas were 37 —
llO,xg.g" Cu. 24 ~ l43ng.g" Zn and 2 - 25ug.g“ Pb (Calvert et al., 1993). In comparison with
metal concentrations from coastal sediments from the Gulf of Thailand. mean Cu and Zn
levels in Singapore were similar to mean concentrations reported for Thailand (62 — 26g.g"
Cu, 43 - 79g.g“ Zn), while sediment concentrations of Pb and Cd in Thailand were higher (53
- l85g.g" Pb, 0.77 — 1.5 l g.g" Cd; Menasveta & Cheevaparanapiwat, 1981). Mean
concentrations of Cu and Cd from sediments of the coast of the Arabian Sea, Pakistan, were
similar to this study, while Zn and Pb concentrations in Pakistan were lower (Ashraf et a1.,
1992). In a 5-year study of heavy metal loads in the sediment of Bombay Harbour, the
concentrations of metals, Cu, Zn, Pb and Cd that were reported were distinctly higher than
sediment metal levels encountered in Singapore (Patel, et al., 1985). Bombay Harbour was
documented to be a major sink of anthropogenic pollutants from the western peninsular of the
Indian subcontinent (Patel, et aI., i985).
Heavy metals in sediments from other regions of the world were varied in comparison with
this Singapore study. For example, reported Cu and Zn concentrations in surﬁcial sediments
of the Tyrrhenian Sea in Italy were similar to levels measured in this study, while sediment
Pb concentrations in Italy were higher than those reported in this study (Leoni er al., l99l).
The Cu, Zn and Pb concentrations that contaminated sediments from the Tyrrhenian Sea were
attributed to anthropogenic inﬂuences. Trace metal concentrations of Pb and Cd obtained
from Bermuda sediments were reported to be higher than those in this study, while sediment
Cu and Zn concentrations were similar in range (Lyons et al., 1983). The authors also
attributed higher Cu, Zn and Pb concentrations to street runoff and antifouling paints. Coastal
sediments from the Mediterranean Sea in the vicinity of the Kishon river mouth, Haifa Bay
which were known to be highly polluted, were understandably higher in concentrations of the
metals Cu, Zn, Cd and Pb, as compared to this study (Cohen et al., 1993). The concentrations
of Zn, Pb and Cd from sediments in this study corresponded to levels measured from
uncontaminated coastal areas of Brazil, while Cu concentrations in this study were higher
(Pfeiffer et ai.. 1988). Compared with heavy metal levels in sediments from Brazilian coastal
areas known to be contaminated with urban and industrial wastes, however. the
concentrations 'obtained in this study were lower (Pfeiffer er al.. 1988). Sediments from
Blanca Bay, Argentina, subjected to anthropogenic urban and industrial inputs (Pucci. 1988;
Villa, I988), also contained concentrations similar to the upper ranges of heavy metal levels
in this study. In addition, the metal levels measured from sediments in this study were lower
than levels recorded from Manukau and Waitemata Harbours in New Zealand (Glasby et al..
In conclusion, levels of heavy metals in coastal sediments of Singapore are not only
inﬂuenced by sediment particle size, but also by development, in particular, the shipping
industry. Cu and Zn levels in the marine environment of Singapore could most likely be
attributed to the release of anti-fouling paints from boats. Although the environmental
protection laws in Singapore are adequate and marine pollution levels are relatively low, a
regular monitoring programme involving the analyses of pollutants from sediments. in
addition to water is necessary to augment the already strict legislation on the treatment and
discharge of sewage and industrial wastes.
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