elemental depth profiles in marine sediments of singapore coastal waters

Elemental depth pro®les in marine sediments of Singapore coastalwaters

I. Orli�c *, S.M. Tang

Research Center for Nuclear Microscopy, Department of Physics, National University of Singapore, Kent Ridge, Singapore 119 260,

Singapore

Abstract

Thirty-eight core sediment samples were recently collected from di�erent locations of the Singapore coastal region.

The aim of the project was to trace the history of marine pollution in various coastal regions and to determine the

impact of industrial activities. Two nuclear analytical techniques were employed in this study: particle induced X-ray

emission (PIXE), Rutherford backscattering (RBS) as well as X-ray ¯uorescence (XRF). Combined together these

techniques provide an excellent tool to determine elemental concentrations of more than 30 elements with detection

limits as low as few ppm. Our results show that elemental concentrations in most of the regions do not show a sig-

ni®cant variation with depth. However, in regions where industrial and shipping activities are high, for example the Port

of Singapore area and the northern part of Johore Straits, the concentrations of metals like Cr, Ni, Cu, Zn, Sn and Pb

were found to have an obvious decreasing trend with the depth. In these cores, concentrations in the top 10±15 cm were

sometimes ten times higher than the corresponding base line concentrations. Elemental depth pro®les of Ni, Cu, Zn, As,

Sn and Pb and their mean concentrations in various regions are reported and discussed. Ó 1999 Elsevier Science B.V.

All rights reserved.

Keywords: Marine pollution; Metals; Sediments

1. Introduction

Three decades of fast economical growth ofSingapore has resulted in elevated levels of envi-ronmental pollution both in air and marine envi-ronment. To ®nd the degree of inorganic pollutionin marine environment, heavy metal concentrationlevels in sediments are commonly measured. Thisis because, it is well known that the concentrations

of heavy metals are cumulative, i.e. they are re-tained in sediments and do not decompose normigrate signi®cantly with time [1]. Characterizedwith such `memory e�ect', sediments, especiallymarine and lake sediments are a subject of nu-merous studies of heavy metal pollution world-wide.

During the past two years more than 15 sam-pling trips were made and a total of 38 core sam-ples and more than 40 grab samples were collectedfrom di�erent coastal regions of Singapore. Sam-ples were analysed simultaneously for more than30 elements by using nuclear analytical techniques;

Nuclear Instruments and Methods in Physics Research B 150 (1999) 291±297

* Corresponding author. Tel.: +65-874-2962; fax: +65-777-

6126; e-mail: [email protected]

0168-583X/99/$ ± see front matter Ó 1999 Elsevier Science B.V. All rights reserved.

PII: S 0 1 6 8 - 5 8 3 X ( 9 8 ) 0 1 0 4 8 - 9

particle induced X-ray emission (PIXE) andRutherford backscattering (RBS) as well as X-ray¯uorescence spectroscopy (XRF).

The objective of this study was to providebaseline concentrations of as many as possible el-ements in marine sediments of Singapore coastalregion, with special emphasis on toxic metals andtheir concentration depth pro®les as well as re-gional concentration variations.

Our results show that marine sediments fromthe industrial areas such as Port of Singapore andStraits of Johore-East, have signi®cantly higherconcentrations of heavy metals than those col-lected in other regions. Also, core samples takenfrom these locations show much higher concen-trations in top 10±12 cm with an enrichment factorranging between 2 and 5 for Ni, Zn, Sn and Pb,and up to 20 for Cu. A summary of our ®ndings ispresented and discussed in this work.

2. Experimental

2.1. Sampling and analysis

Marine sediment samples from more than 40di�erent locations of Singapore coastal waterswere collected during the period 1994±1996. Mostof the samples were collected by means of a homemade core sampler and some by using a Ponardredge. Approximate sampling locations are indi-cated in the inset of Fig. 1.

The core sampler used in this study has a totallength of 55 cm and an inner diameter of 5 cm.More details can be found elsewhere [2]. Coresamples collected with this sampler had a typicallength of 15±40 cm with the mean length of 23 � 7cm, which corresponds to approximately 50±60years on a time scale (for the estimated depositionrate is 0.3±0.5 cm/year). Using a specially designedguillotine, each core was divided immediately afterthe collection into 2-cm slices. The central portionof each slice was put into 100 ml polyethylenebottles. Hence, from each core 7±20 sub-sampleswere typically obtained. Samples were dried over-night at the temperature of 70°C, then ground andhomogenized in a mechanical agate mortar for 20±30 min. The powder was then divided into two sub-

samples and pressed into pellets: one with 13-mmdiameter for the PIXE analysis and the other with25-mm diameter for the XRF analysis ± more de-tails can be found in our previous publications [3,4].

2.2. Analytical techniques

The principal analytical technique used in thisstudy was PIXE. Simultaneously with PIXE, RBSspectra were recorded to facilitate PIXE quanti-tative analysis. In such a way, concentrations ofmore than 25 elements were simultaneously de-termined in each sample. Additionally, XRF withtube and Am241 annular source excitation was usedto determine the concentrations of elements forwhich PIXE shows poor sensitivity (elements withatomic numbers between 40 and 60). Combinedtogether, all three techniques enabled us to deter-mine concentrations of more than 30 elements.Details of the experimental setup are given else-where [3,5]. PIXE quantitative analysis was per-formed by means of the computer code TTPIXAN[6] and Gupix [7] and XRF by means of the pro-gram COREX [8].

3. Results

3.1. Introduction

All cores are divided into seven groups accord-ing to their origin: West Johore Straits (WJS; cores

Fig. 1. Enrichment factors calculated for Cu, Zn, Pb and Sn for

all 38 core samples. In the inserted map of Singapore sampling

points are indicated.

292 I. Orli�c, S.M. Tang / Nucl. Instr. and Meth. in Phys. Res. B 150 (1999) 291±297

1±3), West End (WSE; cores 4±9), Port of Singa-pore (POS; cores 10±22), Sentosa and Marina Bay(SMB; cores 23±28), East Coast (EAC; cores 29±31), Changi Point (CP; cores 32±34) and East JohorStraits (EJS; cores 35±38). Average concentrationsof all measured elements are calculated from thetop 8 cm of each core and for all regions. Top 8 cmis chosen as this depth corresponds to the period ofapproximately 25±30 years, i.e. to the time whenindustrial activities picked up in Singapore. Resultsfor several individual cores are already reported inour previous work [2]. In this study the meanconcentrations obtained for several metals like Cr,Mn, Ni, Cu, Zn, As, Sn and Pb are given in Table 1.One can immediately notice that the concentra-tions of Cu, Zn, Pb and Sn are signi®cantly ele-vated in three regions, i.e. Port of Singapore andJohore Straits ± East and West. These are all in-dustrial areas. Port of Singapore is especially verybusy area with many shipyards, re®neries andvarious industries. In several cores collected in theTuas Bay and the nearby Gull Bay concentrationsof Cu and Zn are reaching 400±700 ppm, respec-tively, and the concentrations of Pb and Sn exceed120 ppm in several sites.

In the last two rows of Table 1 the overall meanand the baseline concentrations are given. Theoverall mean concentrations are calculated by av-eraging concentrations of all measured cores (38cores). For calculation of the baseline concentra-tions, sampling locations with the highest con-

centrations have been excluded from thecalculation (all together 21 cores from WJS, POSand EJS regions were excluded).

3.2. Enrichment factors

Baseline concentrations are used to estimateenrichment factors for di�erent regions. In thiswork, enrichment factor is de®ned as the ratiobetween the mean concentration found in the top 8cm of a speci®c core and the corresponding base-line concentration. Results are graphically pre-sented in Fig 1 for Cu, Zn, Pb and Sn, and for allsampling sites. The copper concentrations in coresfrom the Port of Singapore (cores 10±21) exceedthe baseline concentrations by more than 20 times,and in NE Johore Straits (cores 35±38) by four to®ve times. The concentration of copper in otherregions is not signi®cantly higher than the baselinevalues. Zinc, lead and tin are elevated in Port ofSingapore only 2±8 times. Cr, Mn, Ni and As arealmost uniformly distributed and therefore notshown in the ®gure ± see Table 1.

The concentrations of major and minor ele-ments do not show signi®cant regional variations.For example, the mean concentrations of Si and Alare found to be 26 � 3% and 10.5 � 2%, respec-tively. The exceptions are only a few locations (35,38 and 25) where exceptionally high concentra-tions of Cl were found (up to 16%). Minorelements like Fe, K and Ti are also quite uniformly

Table 1

Mean elemental concentrations and standard deviations of Cr, Mn, Ni, Cu, Zn, As, Pb and Sn in seven coastal regions of Singaporea

Region Cr Mn Ni Cu Zn As Pb Sn

WJS 69 � 6 390 � 251 17 � 2 44 � 11 179 � 28 27 � 10 36 � 17 26 � 5

WSE 48 � 24 211 � 39 10 � 5 13 � 8 81 � 19 19 � 10 24 � 4 14 � 8

POS 68 � 25 170 � 52 20 � 6 242 � 138 451 � 195 22 � 5 88 � 34 71 � 43

SMB 64 � 21 238 � 65 15 � 3 30 � 13 103 � 45 17 � 4 38 � 21 27 � 21

EAC 64 � 4 272 � 5 23 � 2 17 � 4 88 � 12 15 � 2 39 � 17 20 � 7

CPT 62 � 3 468 � 152 21 � 1 16 � 2 96 � 6 19 � 6 34 � 4 24 � 5

EJS 59 � 16 303 � 64 20 � 7 67 � 24 212 � 66 19 � 5 47 � 10 34 � 1

Overall

mean

62 � 7 293 � 104 18 � 4 61 � 82 173 � 132 20 � 4 44 � 21 31 � 19

Baseline

Conc.

59 � 7 298 � 101 18 � 5 29 � 22 116 � 54 18 � 2 36 � 8 24 � 8

a In the last two rows overall mean and baseline concentrations are given. For details see text. All concentrations are expressed in ppm.

All values are Avg � Std.

I. Orli�c, S.M. Tang / Nucl. Instr. and Meth. in Phys. Res. B 150 (1999) 291±297 293

distributed in all regions. Sulfur on the other handshows signi®cant increase in two areas: Straits ofJohore-East (36, 37, 38) and Sungei Jurong Canal(18, 19) ± in these locations S concentrations isapproaching 2% which is three to four times thebaseline concentration. As expected, the concen-tration of calcium is much higher in samples thatwere obtained o�shore, in the vicinity of coral is-lands (cores 6, 7, 8 and 9).

3.3. Concentration depth pro®les ± Tuas Bay

As Tuas Bay (TB) seems to be the most pol-luted area, six core samples were taken in that bay.Approximate sampling locations are indicated inFig. 2. The mean concentrations for all cores aregiven in Table 1 and shown as enrichment factorsin Fig. 1. It is interesting to note that the meanconcentrations of most of the metals in TB dem-onstrate signi®cant lateral variation. The obser-vation is illustrated in Fig. 2 for Ni, Cu, Zn andPb. The data are presented as a function of theapproximate relative distance from the mouth ofthe Tuas Bay (see Fig. 2). Cores TB5 and TB6 aretaken at the mouth of Tuas Bay, TB1 in the mid-dle, TB7 and TB8 further in the Bay and TB9 atthe end of the Bay. Concentrations are clearlymuch higher at the mouth and the end of the bayand lower towards the middle. The ®rst and thelast data points (TBRC and SJA) are added onlyas references- both with nearly baseline concen-trations of all metals.

Not only that the mean concentrations of allheavy metals are signi®cantly elevated in this bay,they also vary as a function of depth, in some,although not all, locations. Concentration depthpro®les for Cu, Zn and Pb are shown in Fig. 3 forseveral TB cores. It is apparent that for some ofthe sampling sites deposition of sediments in thepast 20±30 years was disturbed ± probably eitherby intense tra�c of large ships and/or constructionactivities. This is true especially for areas near theshore with shallow waters. On the other hand, coreTB1, taken from the middle of Tuas Bay (at thedepth of approximately 10 m) shows the mostdistinct depth pro®les for all measured metals. Theconcentrations of metals in cores TB5, TB6 andTB9 are almost constant till the depth of about 20

cm and then decrease gradually. However, theconcentrations of metals in TB1 core abruptlydrops at the depth of about 10 cm to their baselinevalues. Very similar pro®les for TB1 sample areobtained for Cr, Mn, Ni, As and Sn. Similarconcentration depth pro®les are obtained for sev-eral other cores from the Port of Singapore region.

3.4. Concentration depth pro®les ± Johore Straits-East, and Changi Point

As mentioned before (see Table 1) slightly ele-vated metal concentrations are found in all corescollected in the Johore Straits i.e. in cores 35, 36,

Fig. 2. Top: Concentrations of Ni, Cu, Zn, Pb and Sn in dif-

ferent regions of Tuas Bay. Bottom: the map of the Tuas Bay

with indicated sampling locations.


37, and 38. However, the depth pro®les in thesecores are not well pronounced as in the TB cores.As an example, the concentration depth pro®les ofCu, Zn, Sn and Pb obtained for the core #38 areshown in Fig. 4. It is interesting to note that lead,even though it has relatively high concentrations in

Fig. 4. (a) Concentration depth pro®les for Cu, Zn, Pb and Sn

for core #38; (b) depth pro®les of Cu, Zn and Pb for core #33.

Cu concentration is clearly decreasing with the depth indicating

small but measurable in¯ux of Cu from the nearby yacht club.

Fig. 3. Concentration depth pro®les for Cu, Zn and Pb for

several cores from the Tuas Bay. Cores taken near the shore

have quite disturbed depth pro®les for all elements (TB9, TB6,

TB7). On the other hand, cores from deeper water (TB1) show

very distinct pro®les. Core TBRC is taken from the outside of

the Tuas Bay and serves only as the reference.


this region (50±60 ppm) does not show signi®cantdecrease of concentration with depth in any ofJohore Straits cores.

The region of the NE Johore Straits is also anindustrial region where a number of power plants,shipyards, and other industries are located on bothsides of straits. However, this region is not as busyas the Port of Singapore, and this has resulted inmuch lower pollution levels.

At the Changi Point the biggest sailing club ofSingapore is located. Therefore, elevated concen-trations of Cu, Zn and Sn were expected due to thewashout of the antifouling paints used by theyachtsman to protect their vessels from algae andbarnacles. This was the reason that this area wasthoroughly screened and 3 core samples as well as26 grab samples were taken in this region. How-ever, the mean concentrations of all metals of in-terest (Ni, Cu, Zn, As, Pb and Sn) were found tohave one of the lowest levels of all regions. Clearlythe impact of sport yachts is not as big as ofcommercial vessels. On the other hand, by in-spection of depth pro®les it was found that theconcentration of Cu in the core #33 shows muchhigher concentration in top layers ± see Fig. 4.While the concentration in the top layers is morethan 20 ppm the concentrations in the bottomlayers are dropping to approximately 10 ppm. Forcomparison, the depth pro®les of Zn and Pb arealso given, but as it can be seen they do not showany signi®cant variation in depth distribution.

3.5. Elemental correlation

To identify pollution source(s) all data arestatistically evaluated. The results of correlationtests clearly show that there is a strong correlationbetween Mn, Fe, Cu, Zn, Pb, Sn and several othermetals. Also, sulfur seems to be very well corre-lated with most of the metals. A few examples ofcorrelation graphs are shown in Fig. 5 for the TB1core and for the following pairs of elements: (a)sulfur with Pb, Sn, Cu and Zn, and (b) copper withPb and Sn. Remarkably good correlation is ob-tained for this sample. Other cores show similarlygood correlation for the same group of elements,obviously indicating a common source. One of themost likely source of pollution is the antifouling

paint used in the shipping industry. Such paintsare used to cover the underwater parts of the vesselto protect them from the algae and barnacles.These paints are based on organic solvents mixedwith highly concentrated toxic metals such as Cu,Zn and Sn. In the case of commercial vessels thetoxic properties of paints are further `enhanced' byadding more copper and even more toxic tin. Theanalysis of several samples of antifouling paintsshows that the concentrations of Cu and Zn aretypically between 15±30%, and that of tin is about5% (these values vary considerably for di�erentbrands of paints). This is certainly not the solesource of pollution in the industrial regions ofSingapore, but we believe that it is the source withthe highest impact.

Fig. 5. (a) Correlation of sulfur with Pb, Sn, Zn and Cu, and

(b) correlation of copper with Pb and Sn. Copper has similarly

good correlation with Zn as well.


4. Conclusions

The combination of PIXE and RBS analyticaltechniques with the XRF technique enables si-multaneous determination of concentrations ofmore than 30 elements in marine sediments withdetection limits of 1±50 lg/g. All methods requireminimal sample preparation, and are fast and ac-curate to approximately 5±20%, depending on el-emental concentrations and atomic number of theelement.

The results of our study shows that the heavymetal pollution in coastal regions of Singapore isstill relatively low. In most of the regions, recentincrease in concentrations of metals like Cu, Ni,Zn, As and Pb is hardly noticeable. The only tworegions a�ected by industrialization are Port ofSingapore and the northern parts of the Straits ofJohore. The concentrations of heavy metals likeCu, Zn, Sn in these regions are increased 2±20times relative to the baseline concentrations.

Acknowledgements

The authors greatly appreciate help that theyhave received during this work, which was a partof ASEAN-Canada Cooperative Programme on

Marine Science sponsored by the Canadian In-ternational Development Agency. Authors arealso grateful to Mr. Choo Thiam Fook for oper-ating the accelerator, and Mr. C.W. Tan for hiscontribution in the fabrication of core sampler.

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elemental depth profiles in marine sediments of singapore coastal waters

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