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Reconstructing the history of metal deposition in Singapore lake sediments contributes to understanding the anthropogenic and natural metal deposition in the data-sparse Southeast Asia. To this end, we present a sedimentary record of Pb, Pb isotopes and eleven other metals (Ag, As, Ba, Cd, Co, Cr, Cu, Ni, Tl, U and Zn) from a well-dated sediment core collected near the depocenter of MacRitchie Reservoir in central Singapore. Before the 1900s, the sedimentary Pb concentration was less than 2 mg/kg for both soil and sediment, with a corresponding 206Pb/207Pb of ∼1.20. The Pb concentration increased to 55 mg/kg in the 1990s, and correspondingly the 206Pb/207Pb decreased to less than 1.14. The 206Pb/207Pb in the core top sediment is concordant with the 206Pb/207Pb signal of aerosols in Singapore and other Southeast Asian cities, suggesting that Pb in the reservoir sediment was mainly from atmospheric deposition. Using the Pb concentration in the topmost layer of sediment, the estimated atmospheric Pb flux in Singapore today is ∼1.6 × 10−2 g/m2 yr. The concentrations of eleven other metals preserved in the sediment were also determined. A principal component analysis showed that most of the metals exhibit an increasing trend towards 1990s with a local concentration peak in the mid-20th century.
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Invited paper
A century long sedimentary record of anthropogenic lead (Pb), Pb
isotopes and other trace metals in Singapore
*
Mengli Chen
a
,
b
,
*
, Edward A. Boyle
b
,
c
, Adam D. Switzer
a
,
b
,
d
, Chris Gouramanis
d
a
Asian School of the Environment, Nanyang Technological University, 50 Nanyang Avenue, 639798, Singapore
b
Singapore-MIT Alliance on Research and Technology, Center of Environmental Sensing and Modelling, 1 CREATE Way, #09-03 CREATE Tower, 138602,
Singapore
c
Department of Earth, Atmospheric, and Planetary Sciences, E25-619, Massachusetts Institute of Technology, Cambridge, MA, 02139, USA
d
Earth Observatory of Singapore, Nanyang Technological University, 50 Nanyang Avenue, 639798, Singapore
article info
Article history:
Received 25 November 2015
Received in revised form
19 February 2016
Accepted 20 February 2016
Available online 8 March 2016
Keywords:
Pb
Pb isotopes
Heavy metals
Southeast Asia
Singapore
Sediment
abstract
Reconstructing the history of metal deposition in Singapore lake sediments contributes to understanding
the anthropogenic and natural metal deposition in the data-sparse Southeast Asia. To this end, we
present a sedimentary record of Pb, Pb isotopes and eleven other metals (Ag, As, Ba, Cd, Co, Cr, Cu, Ni, Tl,
U and Zn) from a well-dated sediment core collected near the depocenter of MacRitchie Reservoir in
central Singapore. Before the 1900s, the sedimentary Pb concentration was less than 2 mg/kg for both
soil and sediment, with a corresponding
206
Pb/
207
Pb of ~1.20. The Pb concentration increased to 55 mg/
kg in the 1990s, and correspondingly the
206
Pb/
207
Pb decreased to less than 1.14. The
206
Pb/
207
Pb in the
core top sediment is concordant with the
206
Pb/
207
Pb signal of aerosols in Singapore and other Southeast
Asian cities, suggesting that Pb in the reservoir sediment was mainly from atmospheric deposition. Using
the Pb concentration in the topmost layer of sediment, the estimated atmospheric Pb ux in Singapore
today is ~1.6 10
2
g/m
2
yr. The concentrations of eleven other metals preserved in the sediment were
also determined. A principal component analysis showed that most of the metals exhibit an increasing
trend towards 1990s with a local concentration peak in the mid-20
th
century.
©2016 Published by Elsevier Ltd.
1. Introduction
Anthropogenic lead (Pb) has been an important contaminant in
human history, especially after the Industrial Revolution. The
emission of anthropogenic Pb reached a level greater than 20 times
the global natural Pb emissions in the 1980s (Nriagu,1989), mainly
attributable to usage of leaded gasoline, high-temperature indus-
trial activities and incineration (Kom
arek et al., 2008, Nriagu, 1979).
Anthropogenic Pb has been recorded in many environments,
including the atmosphere (e.g.: Patterson and Settle, 1987; Duce
et al., 1991; Bollh
ofer and Rosman, 2000, 2001; Zhu et al., 2002;
Flegal et al., 2013); terrestrial (e.g.: Boutron et al., 1995; Osterberg
et al., 2008; De Deckker et al., 2010; Kylander et al., 2010; Lee
et al., 2011); aquatic (e.g.: Arnason and Fletcher, 2003; Erel and
Patterson, 1994; Graney et al., 1995; Harrington et al., 1998; Kober
et al., 1999; Li et al., 2000; Eades et al., 2002) and oceanic
(Turekian, 1977; Schaule and Patterson, 1981; Boyle et al., 1986,
2005;Reuer, 2002; Flegal, 1986; Weiss et al., 2003; Kelly et al.,
2009) environments.
Since the 1990s, the Asian contribution of Pb has received
greater attention as the major Pb contributors in 1970s and 1980s
(North America and Western Europe) phased out leaded petrol
earlier than Asian countries. In addition, the increasing emissions of
Pb from coal combustion and other industrial sources have also
received regional attention in Asia as a consequence of the recent
economic boom (e.g.: Díaz-Somoano et al., 2009;Flegal et al., 2013;
Zhang et al., 2010; Zurbrick et al., 2014).
Therefore, the assessment of Asia's contribution of Pb to the
global Pb ux is becoming increasingly important. However,
Southeast Asia remains data-sparse in terms of environmental data
on Pb emissions, despite being the last region in the AsiaePacicto
phase out leaded petrol (Hirota, 2006; USAID, 2009). In addition to
Pb petrol inputs, the rapid economic development in Southeast Asia
has also contributed to a dramatic increase in coal burning
(International Energy Statistics, 2012), exacerbating the emission of
*
This paper has been recommended for acceptance by Charles Wong.
*Corresponding author. Asian School of the Environment, Nanyang Technological
University, 50 Nanyang Avenue, 639798, Singapore.
E-mail address: chen0327@e.ntu.edu.sg (M. Chen).
Contents lists available at ScienceDirect
Environmental Pollution
journal homepage: www.elsevier.com/locate/envpol
http://dx.doi.org/10.1016/j.envpol.2016.02.040
0269-7491/©2016 Published by Elsevier Ltd.
Environmental Pollution 213 (2016) 446e459
Pb and many other metals. For example, the coal consumption in
Malaysia and Indonesia in 2012 was nearly 50 and 30 times the
1980s average (International Energy Statistics, 2012). Also, other
metals could be co-released to the environment, and the concen-
tration of these metals is dependent on which raw materials were
used; which processes were involved in handling the metal-
bearing materials; and which, if any, emission controls were
applied. A few Pb studies have reported Pb contents in Southeast
Asian environments that are higher than other parts of the world.
For example, the Pb concentrations of aerosols in Southeast Asian
cities between 1994 and 1999 were much higher than Europe,
Australia and North America (Bollh
ofer and Rosman, 2000, 2001),
and the Pb/Ca preserved in a coral from southwest of Sumatra,
Indonesia shows an increasing trend of Pb in the coastal waters
since the 1980s (Lee et al., 2014). In addition, the Pb content of li-
chens from the Singapore Straits was found to be in the upper range
of the recorded values around the world (Ng et al., 2006). However,
most of these studies do not shed light on the multi-decadal vari-
ation of Pb in the environment of Southeast Asia or the isotopic
composition of Pb in the environment.
Constraining the long term variation of Pb and other metals in a
variety of environments is important as it allows characterization of
the long-term environmental response to anthropogenic emissions
(e.g: Kelly et al., 2009) and facilitates comparison of metals be-
tween different environments and with different residence times
(Alleman et al., 1999; Hamelin et al., 1997). The Pb isotope ratios
(
206
Pb/
208
Pb and
207
Pb/
208
Pb) and the temporal variability of the
isotopes can differentiate Pb sources, and can be a further step in
evaluating the source of the Pb and its variation through time
(Kom
arek et al., 2008).
One way to investigate the above two aspects is to study the
variation of Pb and Pb isotopes in well-dated lacustrine sediments.
Lacustrine sediments are a sink for metals from the water column
(Foster and Charlesworth, 1996) and can be used as archives of
century-scale Pb variation (Kom
arek et al., 2008). The reliability of
evaluating Pb and other metals from lacustrine records has been
demonstrated from studies carried out in Europe (e.g. Eades et al.,
2002), North America (e.g. Graney et al., 1995; Lima et al., 2005)
and China (e.g. Cheng and Hu, 2010).
In order to assess the anthropogenic Pb input in Southeast Asia
and to evaluate the relative contribution of various Pb sources at
different time periods, we determined the Pb concentration ([Pb])
and Pb isotopes from a
210
Pb-dated sediment core recovered from
the MacRitchie Reservoir in the central catchment region of
Singapore (Fig. 1). The variations of [Pb] and Pb isotopes are
compared with historical Pb emissions from Southeast Asian
countries and the isotopic composition of Pb from potential
sources.
Many metals are co-released during industrial activities (e.g. Cd
and As are released during coal combustion; Ba and Zn are released
during waste incineration, ATSDR, 2007, Salomons and F
orstner,
1984), and some metals (e.g. Ag, Cu, Cr, Ni, Tl) are generally asso-
ciated with electronics industries, which have grown extensively
over time in Singapore. For a more comprehensive story on the
changing release of metals over the last century, the temporal
variability of eleven other metals (Ag, As, Ba, Cd, Co, Cr, Cu, Ni, Tl, U
and Zn) from the sediment are also reported and compared with
the Pb record.
2. Sampling and methodology
2.1. Site information and sampling
Singapore is one of the most developed and densely populated
cities in Southeast Asia (Statistics Singapore, 2014). Additionally, it
houses one of the busiest ports in the world (American Association
of Port Authorities, (2008)). Singapore is a small city-state bordered
to the north and east by Malaysia and to the south by Indonesia.
Singapore is dominated by a monsoonal climate, with northeasterly
winds prevailing between April and September and southwesterly
winds prevailing between November to March (National
Environmental Agency, 2009,Fig. 1a). Despite monsoonal wind
reversals, no clear seasonal variability has been observed in the
concentration of Pb in the aerosols or rain waters recovered from
Singapore (personal communications with Professor Richard
Webster). MacRitchie Reservoir was chosen to investigate the
deposition of Pb (Fig. 1) because it is located in a near pristine
catchment, has a very low dissolved oxygen concentration near the
lake oor, which suggests that little bioturbation of the lake oor
sediment occurs (Chen et al., 1996). The MacRitchie Reservoir is
located in the Central Catchment Nature Reserve of Singapore,
where over 20 km
2
of forested area drains into four reservoirs
(MacRitchie, Upper Seletar, Upper Peirce and Lower Peirce; Public
Utilities Board, 2015). The drainage basin lies on Triassic igneous
granite containing 60e65% feldspar and >30% quartz (Bukit Timah
Granite, Pitts, 1984, Zhao, 1996). The MacRitchie Reservoir was
constructed by damming a creek valley in 1891 (Devi, 2002), and
since then, the catchment of the reservoir has remained relatively
pristine (Public Utilities Board, 2010).
A sediment freeze core was collected on the 6th August 2012
from the southeastern, and deepest section (9 m) of the MacRitchie
Reservoir (Fig. 1c). The coring site was away from the major inlet of
the catchment, and was also sufciently distant from the dam so
that the sediments would not be affected by regular pumping ac-
tivities (Fig. 1d). During sampling, the core was collected by
lowering a freeze-corer to the sedimentewater interface and
pushing into the underlying sediments (Shapiro, 1958). In brief, a
freeze corer consists a hollow rectangular cylinder lled with a
slurry of dry ice and ethanol. While coring, the corer was lowered to
~1 m above the bottom, and released to punch into the sediment
layer by gravity. The dry ice and ethanol quickly froze sediment
surrounding the corer, resulting in a slab of frozen sediment
(including the sedimentewater interface) freezing onto the corer's
surface. The frozen slab of sediment was then carefully detached
from the corer surface and sent back to the laboratory in a cooler
box. During sub-sampling, the frozen core was stored horizontally
at room temperature until soft (~10 min), and the preserved stra-
tigraphy was sub-sampled at 2e8 cm increments (depending on
the sediment density and water content) using a plastic spatula. A
surface water sample was also collected on the day of sampling
using a 1 L trace-metal clean bottle attached to a plastic pole and
dragged across the lake surface. The water sample was ltered
through 0.4
m
m Millipore lters within a few hours of collection
and acidied in a class 100 environment using trace metal clean
reagents and lab-ware.
2.2. Regional on Pb emissions
For a better means of comparing the historical [Pb] recorded in
our core, Pb emissions from Singapore coal burning were estimated
based on the following equation (from Li et al., 2012):
E
coal
¼PP P
percap
C½Pb
coal
EF (1)
Where the E
coal
is the Pb emitted from coal power plants (tons);
the PP was the population (person), P
percap
was the electricity
consumption per capita (kwh/person); C was the amount of coal
used to generate 1 kwh of electricity; [Pb]
coal
was the Pb content in
the coal; and the EF was the emission factor, which was the amount
of Pb released compared to the amount of the amount of Pb
M. Chen et al. / Environmental Pollution 213 (2016) 446e459 447
associated with coal burning. Singapore did not use coal for its
electricity between 1970 and 2010, however, there was a coal po-
wer plant in Singapore that operated between 1927 and early 1970s
(Ramlan, 2014). The 1927e1959 population data came from
Goldewijk et al. (2011) and the 1960 to 1970 population data came
from Statistics Singapore (2014). The earliest data on electricity
consumption per capita for Singapore was recorded in 1971, as
1154.8 kwh/person (UN Data 2014). It is reasonable to expect that
the per capita electricity consumption between the 1920s and
1950s was lower and therefore the P
percap
used in the estimation
only reects an upper limit of the Pb emissions. The constants used
in the coal estimation in this study are 0.472 kg/kWh for C (e.g. Lee
et al., 2014) and 15 mg/kg as the [Pb]
coal
, as an average Pb content in
coal from Australia, Indonesia, China and India (Díaz-Somoano
et al., 2009). The emission factor varies depending on the types
and the efciency of generators, and was assumed to be 80% in this
study (same as in Li et al., 2012 for consistency). The estimated Pb
emission is shown in Fig. 2b.
Fig. 1. Map of MacRitchie Reservoir and its relative location in the central catchment natural reserve of Singapore. The inset maps show: a. Singapore's relative location within
Southeast Asia with the monsoonal wind directions illustrated (see Section 2.1); b. A map of Singapore and its neighboring countries with Central Catchment Nature Reserve (CCNR)
highlight in a square box, the two dots are Jong island with coral record and St James Power Station (STPS, the former coal power station closed in early 1970s) respectively; c. A
close view of CCNR with the name of reservoirs inside the nature reserve are shown in italic letters. Within the MacRitchie Reservoir, the sampling site (black star); major inlet
(black arrow); and the dam (thick black bar) are also illustrated. The satellite image of CCNR is included in the bottom right corner with the nature reserve shown in dark areas and
the residential and/or commercial area shown in light areas; d. An illustration of the reservoir's sedimentology layout, including the major inlet, the delta area near the major inlet,
and the muddy-lake area near the dam. The possible sources of Pb to the reservoir are also illustrated in the gure.
M. Chen et al. / Environmental Pollution 213 (2016) 446e459448
2.3. Determination of particle size and chronology
The particle size distribution of the sediment core was examined
to give an overview of the physical properties of the sediment. For
particle size analysis, HCl and H
2
O
2
were applied to remove the
carbonates and organic components and to disassociate the clays.
The treated samples were analyzed by a laser diffraction particle
size analyzer (Malven Mastersizer 2000) using procedures outlined
in Switzer (2013).
The chronology of the sediment was determined using the
atmospherically-deposited radionuclide
210
Pb (Krishnaswamy
et al., 1971). The total activity of
210
Pb was measured using a Can-
berra Broad Energy Germanium gamma counter at 46.5 keV
(G
aggeler et al., 1976). The corrections for the combined effects of
X-ray self-absorption, counter efciency and sample/counter ge-
ometry were established by rst counting an untreated sample, and
again after mixing the sample with 100 mg of US-DOE CRM 101
standard (a pitchblende-silica mixture based on an aged uranium
deposit that had come into radiochemical equilibrium for all of the
short-lived daughters). The signal enhancement for
210
Pb in the
spiked mixture gave the product of counting geometry and self-
absorption. Similarly, the supported
210
Pb was estimated from its
long-lived
226
Ra parent by counting its short-lived daughter
214
Pb at 351.9 keV for the unspiked and spiked samples to correct
for counting geometry and minor self-absorption. The procedure,
assumptions and data reduction in this study is similar to that
described by Appleby et al. (1986), except that we used an empty-
sample container for blanks rather than anti-coincidence back-
ground correction.
In order to convert the
210
Pb data into estimated dates, the
sedimentation rate was determined by plotting the unsupported
210
Pb activity in a logarithmic scale with depth, as shown by
Tylmann (2004). The
210
Pb derived years are also shown in Fig. 3b.
It should be noted that because the sediments were subsampled at
2e8 cm increments, and because of the uncertainties in the
210
Pb
measurements, the age of a sample estimated from
210
Pb should be
interpreted as an average data over a period (c.a. ~10 years, e.g., a
sample with a
210
Pb age of 1955 should be interpreted as the
1950s).
2.4. Sample preparation and metal analysis
The sediment samples were leached by a combination of ul-
trapure grade 1.75 mol/L HNO
3
-3 mol/L HCl (distilled four times
using a cleaned vycor glass still inside a class 100 clean room).
Samples were leached for 60 min in an ultrasonic bath following
the method described by Graney et al. (1995) and the samples were
left at room temperature for another 24 h to complete the reaction.
This method effectively extracts the metals adsorbed on the par-
ticle surface without dissolving the mineral grains. Therefore, the
leachate represents the anthropogenic fraction of the metals with
some of the mineral-bound metals. The recovery of metal using this
method was proven as [Pb] in the leachate were indistinguishable
from 6 mol/L HCl, 7 mol/L HNO
3
or Aqua Regia digestions (Graney
et al., 1995). After acid leaching, the samples were centrifuged
and the supernatant was extracted. The supernatant was then
ltered through a 0.4
m
m membrane using a syringe and diluted for
metal analysis.
To determine the [Pb] of the sediments, both isotope dilution
and internal standard methods were applied. For isotope dilution, a
known amount of
204
Pb enriched spike (Oak Ridge National Labo-
ratories) was added to each sample and the [Pb] in the sample was
calculated from the measured
204
Pb/
208
Pb ratio (Wu and Boyle,
1997). For the internal standard method, a known amount of in-
dium standard (In) was added to each sample and standard (SPEX
CertiPrep), and the [Pb] in the sample was calculated by comparing
the Pb/In in the sample and the standard (Vanhaecke et al., 1992).
The accuracy of the data was cross-checked by measuring the same
sample using both isotope dilution and internal standard methods,
which also showed a consistent positive relationship suggesting a
negligible offset between the two methods (see supplementary
material).
The metal concentrations of silver ([Ag]), arsenic ([As]), barium
([Ba]), cadmium ([Cd]), cobalt ([Co]), chromium ([Cr]), Copper
([Cu]), nickel ([Ni]), thallium ([Tl]), uranium ([U]) and zinc ([Zn])
were determined applying a quadrupole inductively-coupled
plasma mass spectrometry (Q-ICP-MS,VG PlasmaQuad 2þ) using
the internal In standard method described above.
The iron concentrations ([Fe]) in the sediments were also
analyzed by a Q-ICP-MS (PerkinElmer ELAN DRC-e) following the
same sample preparation method. The [Fe] was calculated by
comparing the signal intensity with a series of standards with
various concentrations. The [Fe] in the sediment was used to
normalize other metals to provide a more objective view on the
variability of metals in the sediment (Supplementary Material S7).
Trace metal clean plasticware (leached from brand new vials
using ultrapure acid and rinsed 5x with 4x distilled deionized
water), ultrapure acid and 4x distilled deionized water were used
throughout the sample preparation process. The procedural blanks
were measured and ranged from 0.04% to 4.1% of sample concen-
tration and were corrected for the reported concentration.
Fig. 2. Estimated historical Pb emissions from gasoline usage and coal combustion
including Singapore (white open diamonds), Malaysia (grey lled triangles), Thailand
(black lled squares, Lee et al., 2014) and Indonesia (white open circles, Lee et al.,
2014). Dotted lines are for the years that data are unavailable. (a) from vehicle gaso-
line combustion. The decline of Indonesian emission in 2006 illustrates the announced
total phase out of leaded gasoline. (b) from power generation-based coal combustion.
The decline of Singapore emission in 1971 illustrates the closure of Singapore's only
coal power plant.
M. Chen et al. / Environmental Pollution 213 (2016) 446e459 449
Pb isotope ratios in the sediment were measured by multi-
collector plasma mass spectrometry (G/V IsoProbe). To prepare
the samples for isotope analysis, an ion exchange column (Eichrom
AG-1X8 chloride form, 200e400 mesh) was employed to extract Pb
from the sample matrices (Reuer et al., 2003), which typically re-
covers 99.98% of the Pb from the sample. To ensure the precision
and accuracy of the Pb isotope measurement, a number of pre-
cautionary measures were undertaken following the methods
described in Reuer et al. (2003) and Boyle et al. (2012), including
the addition of a thallium spike to each sample and standard to
constantly monitor the mass fraction related to the instrument,
monitoring the
202
Hg signal in each sample to correct the isobaric
interference of
204
Pb by
204
Hg, correcting the possible contamina-
tion from two column procedural blanks, monitoring the instru-
mental acid blanks at the beginning and the end of the day and
every 10 samples to correct the ICP-MS background signal related
to either the acid or the instrument, correcting the tailing errors by
measuring the monoisotopic element bismuth at half masses, and
calibrating the Pb isotopes measured in each sample by normali-
zation to NBS 981 standard reference material (Baker et al., 2004)
measured during the same day. The NBS-981 standard reference
material is used during the correction procedure, and an internal
lab standard (BAB3deg) measured to ensure the accuracy of the
NBS-981 standard. Measuring the BAB3deg standard calibrated to
NBS-981 gave a 2 relative standard deviation precision of
206
Pb/
207
Pb of 160 ppm (n ¼42),
208
Pb/
207
Pb of 163 ppm (n ¼42).
The [Pb] in the water was measured by isotope dilution after a
single-batch resin adsorption separation (Lee et al., 2011). In brief,
an aliquot of cleaned Nitrilotriacetic resin was added to the sample
(nal concentration ~2400 beads/1.3 mL water sample) at pH ¼5.3.
After four days of resin uptake, the resins beads were rinsed three
times using distilled deionized water and then leached in 0.1 mol/L
HNO
3
. The leachate was analyzed for [Pb] using Q-ICP-MS. The
procedural blanks averaged 3.7 10
4
m
g/L and was ~3% of our
sample concentration.
2.5. Principal component analysis on 11 metals in the sediments
A principal component analysis (PCA) was carried out on the
concentration of the metals in the sediment to investigate the po-
tential correlations among the metals. Eleven metals (Ag, As, Ba, Cd,
Co, Cr, Ni, Pb, Tl, U and Zn), were included in the PCA. Because the
metals in the sediment vary at different scales (i.e., Ag varies from
0.008 mg/kg to 0.228 mg/kg nd Zn varies from 2.5 mg/kg to
122.5 mg/kg), the concentration data for each metal was stan-
dardized by subtracting its mean and then dividing by its standard
deviation to provide equal weight for all the elements. The stan-
dardized data were analyzed by the PCA function in Matlab
(Jackson, 1991), which returned the eigenvectors, the eigenvalues
and the sample scores. Cu was not included in the PCA as the highly
skewed variation could potentially overwhelm the variation of
other metals. Instead, the variation of Cu is discussed separately.
3. Results
3.1. Sediment core description and
210
Pb chronology
Sediments in the upper part of the core (0e8 cm) were a slurry
of silty clays, afrming the collection of the sedimentewater
interface during the freeze core collection. Below the upper 8 cm
lies denser dark grey silty clays. There was a distinct color change to
reddish yellow at 33 cm that extended to the base of the core and
coincided with an increase in sand content to a sandy silt.
The
210
Pb data was t with a constant sedimentation model of
0.29 cm/yr (Fig. 3b). As such, the
210
Pb date at 34 cm was ca. 1895. It
is likely that the color change at 33 cm marked the damming of the
creek valley and the formation of the reservoir in the year 1891,
which is in good agreement with the
210
Pb date. Using the date of
sampling (year 2012) and the date of reservoir's formation (year
1891) as the two end-member constraints, the
210
Pb dates charac-
terize the chronology of the sediments reasonably well (Fig. 3b).
Fig. 3. The illustration of: (a)The picture of the sediment core. The top, bottom of the core and the change in color during reservoir's construction were marked. (b) Unsupported
210
Pb (black diamonds) in Bq/kg of dry sample plotted at the mid-depth of each subsample. The dotted curve is the 0.29 cm/yr t using a constant sedimentation rate model (see
text). The vertical axis on the left shows the depth of the sediment and the axis on the right shows the converted calendar years from the
210
Pb activities. (c) the sediment
composition change throughout the core, includes clay (light grey); silt (dark grey) and sand (white).
M. Chen et al. / Environmental Pollution 213 (2016) 446e459450
After considering the sampling resolution and uncertainties in
210
Pb measurements and sedimentation rates, we suggest the age
of each sample as an interval of time shown in Fig. 3. The period
each sample represents is listed in the supplementary material.
Samples were interpreted within the timeframe of each period
instead of an exact
210
Pb date (e.g. the 18e21 cm sample with
210
Pb year of 1955 would represent 1950e1960 and so was inter-
preted as 1950s).
The deeper portion of the core was classied as a paleo-soil
because the material was formed before the reservoir's construc-
tion. And as a result, the
210
Pb-derived dates are unlikely to be
applicable to the paleo-soil as they were formed in a different
depositional setting. The paleo-soil samples were all considered as
before 1895without a specic date. Based on the measured
210
Pb
activity, the calculated calendar years are shown in Fig. 3c (and the
representing periods were shown in the supplementary material).
3.2. Variation of Pb and Pb isotopes in the sediment
The [Pb] in the sediment core is shown in Fig. 4a. The general
concentration of Pb was less than or equal to 55 mg/kg, which was
lower than the mid-range concentrations from various guidelines
around the world (82e530 mg/kg dry weight, above the concen-
tration that would likely cause adverse effects on benthic organ-
isms, Burton, 2002). The dissolved Pb in MacRitchie Reservoir water
was measured as (1.1 ±0.07) 10
2
m
g/L, which was two orders of
magnitude lower than the legal safe level of Pb in the drinking
water (10
m
g/L, Attorney General's Chambers Singapore, 2008).
Before 1895, the [Pb] in the soils were 1.2 ±0.6 mg/kg (Fig. 4a).
The sample dated as 1903 was the rst sample after the reservoir
was constructed, showing a concentration of 1.2 mg/kg, compara-
ble to the paleo-soil. The [Pb] started to increase in the 1920s and
reached a peak of 53 mg/kg in the 1950s. After the 1950s peak, the
concentration of Pb decreased to 41 mg/kg in the 1960s, remained
stable over the next two decades (roughly 1960e1984), and then
nally increased to 55 mg/kg in the 1990s. Due to the decadal
resolution sampling, events occurring in 2000s are not resolvable.
The temporal variation of
206
Pb/
207
Pb and
208
Pb/
207
Pb are
shown in Fig. 4b. In the paleo-soil samples prior to 1895, the three
206
Pb/
207
Pb data points were similar and >1.215. Since 1895,
206
Pb/
207
Pb decreased from >1.215 to ~1.140. The decreasing trend
was consistent throughout the core.
3.3. Concentration of other metals in the sediments
The temporal variations of the other 11 metals (Ag, As, Ba, Cd, Co,
Cr, Cu, Ni, Tl, U and Zn) in the sediments are shown in Fig. 5. Since
both the bulk metal concentrations and the metal-to-Fe (Me/Fe)
ratio clearly show similar patterns for each of the metals
throughout the core (Supplementary Material S7), we relate the
high metal concentrations at the top of the core to anthropogenic
inputs. In the sediment, most of the metals had their lowest con-
centrations at the bottom of the core (prior to 1895), and highest
concentrations in 1990s. The concentrations of Ag, As, Ba, Cd, Co, Cr,
Cu, Ni, Tl, U and Zn in the 1990s were signicantly higher than
during the 1900s. Other than the high concentrations in the 1990s,
the variability of the metals are compared using PCA (Fig. 6). The
rst principal component (PC) explains 77% of the metal variance in
the core and the second PC explains 18% of the variance (Table 2).
Reconstructing the time variability of PC1 shows that each metal
concentration increased from the 1890se1950s, decreased during
the 1960s and increased again from the 1970se1990s (Fig. 6b). PC2
shows that the concentration increased in the 1910s, plateaued
from the 1920se1980s and then decreased sharply in the 1990s
(Fig. 6b). This reconstruction of PCs closely matches the measured
changes of each metal in Fig. 5.
By plotting rst PC against the second PC, the metals can be
divided into three different groups (see Fig. 6a): The rst group of
elements are positively correlated to PC1 at similar loadings and
weakly correlated to PC2. This group includes Pb, Ba, Cd, Ag, As and
Co. The second group positively correlates to PC1 but negatively
correlates to PC2. This group includes Zn, Ni and Cr. The third group
positively correlates to PC1 and PC2. This group includes U and Tl.
The increase of Cu was the largest among all the metals (from
less than 1 mg/kg to more than 300 mg/kg). Additionally, almost all
the increase happened between the 1980s and 1990s. For all these
reasons, Cu was treated as a fourth group and is discussed
separately.
4. Discussion
4.1. Delivery of Pb to the MacRitchie Reservoir sediments
The Pb recorded in the sediment could be deposited through
three pathways:
direct dumping of Pb into the reservoir (e.g. Kersten et al., 1997
showed this in coastal waters, but a similar process could
happen in reservoirs);
catchment input via river inow or overland ow (e.g. Kober
et al., 1999); or,
atmospheric deposition via rainfall or dust deposition (e.g.
Kom
arek et al., 2008).
It is improbable that Pb was directly discharged into the reser-
voir since the reservoir has maintained a source of freshwater to the
local community (Devi, 2002) and it has been carefully protected.
Catchment input was not likely a major source either as it remained
nearly pristine (Public Utilities Board, 2010) with no industries
within the catchment. It is possible that the atmospheric deposited
Pb in the catchment could be incorporated in soil particles and
subsequently mobilized into sediment by weathering (Eades et al.,
Fig. 4. (a) The bulk Pb concentration in the sediments (grey lled diamonds) compare
to the Pb/Ca in the Jong island coral (open diamonds, Lee et al., 2014). The GPS location
for the coral is 112054.2500 N, 10347011.1700E. (b) the
206
Pb/
207
Pb ratio (grey lled cir-
cles) and
208
Pb/
207
Pb (open triangles) in the sediment. Note that the
206
Pb/
207
Pb ratios
for natural Pb is ~1.20 and for local aerosol is ~1.14. The error bars of Pb isotopes are
within the size of each point.
M. Chen et al. / Environmental Pollution 213 (2016) 446e459 451
Fig. 5. The concentrations in mg/kg of all the metals (Pb, As, Ag, Ba, Cd, Co, Cr, Ni, Zn, Tl, U, Ba and Cu) measured in the sediment are plotted.
M. Chen et al. / Environmental Pollution 213 (2016) 446e459452
2002; Kober et al., 1999). In this case the weathered soil particles
should contain both geogenic Pb and atmospherically-deposited
Pb. However, the catchment input does not introduce additional
sources (other than atmospheric deposits) into the system. By
excluding the other possibilities, atmospheric deposition was likely
the primary cause of Pb variability to the MacRitchie Reservoir
sediments.
Atmospheric delivery time should not result in an observable lag
between Pb emission and subsequent deposition into sediment as
the residence time of Pb in the lower troposphere is ~7 days (Poet
et al., 1972). The residence time of Pb in MacRitchie Reservoir could
potentially result in some lag, which has not yet been investigated.
However it is reasonable to expect that the residence time to be less
than a few years because a shallow reservoir with a high sedi-
mentation rate should scavenge Pb faster than an oligotrophic
surface ocean (the average residence time of Pb in surface oceans is
~2 yrs, Bacon et al., 1976,Nozaki et al.,1976). As a result, the delivery
time of Pb within the atmosphere and reservoir should be within a
Fig. 6. The plots show the results from principal component analysis. The rst and second principal components explain ~95% of the total variations in the core. (a) The PCA bi-plot
showing PC1 versus PC2. The directions and expressing powers of each metal in the PC space are expressed in vectors. (b) The rst 2 eigenvectors (PCs) from the principal
component analysis. Horizontal axis is a dimensionless unit to express the variation of the principal component; and the vertical axis is the calendar year.
Table 1
Lead concentrations and isotope ratios in MacRitchie bottom core compare to natural soils and sediments.
Location Methodology [Pb] range (mg/
kg)
206/207 208/207 208/206 Source
Soil
Macritchie reservoir (Singapore) HNO3eHCl leach 1.451e2.560 1.214
e1.215
2.506
e2.507
2.063
e2.065
this study
Rural south central Ontario (Canada) HNO3 leach ~6e7 1.31e1.32 Watmough and Hutchinson
(2004)
Pearl River Delta (China) HNO3 þHCLO4 leach 7.7e54.7 1.195 2.499 2.091 Wong et al. (2002)
Jerusalem (Israel) HNO3 þHCLO4 þHF leach 17.0e19.0 1.206
e1.219
2.051
e2.066
Teutsch et al. (2001)
pristine forest soil (Sweden) HNO3 þHCLO4 leach <0.1 Bindler et al. (1999)
Swedish pre-industrialization forest soil
(Sweden)
HNO3 þHCLO4 leach 2.4e6.7 1.372
e1.455
Bindler et al. (1999)
Swiss unpolutted soil (Switzerland) HNO3 leach 5.4e17.5 1.209 2.472 2.044 Hansmann and Koppel (2000)
Sediments
Macritchie reservoir (Singapore) HNO3eHCl leach 1.911 1.199 2.488 1.019 this study
Lake Constance (Central Europe) HNO3eHCl leach 20.5e21.7 1.197 2.472 2.065 Kober et al. (1999)
Lake Erie and other lakes (Northeastern US) HNO3eHCl leach 11.3e13.3 1.252 2.492 1.990 Graney et al. (1995)
Loch Lomond north (Scotland) HNO3eHCl leach 23e29 1.160 Eades et al. (2002)
Loch Lomond south (Scotland) HNO3eHCl leach 11.0e19.0 1.174 Eades et al. (2002)
Loch Ness (Scotland) HNO3eHCl leach 10.4e13.4 1.215 Eades et al. (2002)
Swedish lakes (Sweden) HNO3 þHCLO4 leach ~2e17 1.28e2.01 Renberg et al. (2002)
Lake Tantar
e (Canada) HNO3 þHCLO4 þHF leach 4.14e9.12 1.161 2.444 2.105 Gallon et al. (2006)
Lake Desp
eriers and other lakes (Canada) HNO3 þHF leach ~5e8 1.221 2.118 Gallon et al. (2005)
Hongfeng Lake (southwest China) HNO3 leach 29.04 1.233 2.478 2.009 Zhao et al. (2011)
Liangzhi Lake (central China) HNO3 þHCLO4 leach 15.5 1.193 2.495 2.091 Lee et al. (2008)
M. Chen et al. / Environmental Pollution 213 (2016) 446e459 453
few years. Compared to our sampling resolution (~10 yrs), this
delivery time is small and the variation in the delivery time to the
core site cannot be resolved.
4.2. The variability of Pb content and isotopic composition in the
20
th
century
The lowest section of the core included three soil samples and
one sediment sample deposited prior to the 1900s. The four sam-
ples had invariant Pb concentrations and isotopic compositions
(1.9 ±0.6 mg/kg and
206
Pb/
207
Pb of 1.215 ±0.001, Fig. 4). The [Pb] in
the samples were the lowest across the core, and are also in the
lower range of the [Pb] found in published pristine/unpolluted soils
and sediments from Europe, North America and China (Table 1).
The
206
Pb/
207
Pb in these samples is in good agreement with re-
ported values in the Asian region, including K-feldspar from Asian
rivers (Bodet and Sch
arer, 2001) and in the surface sediments in the
deep basin of the South China Sea (1.18e1.22; Zhu et al., 2010).
Additionally the
206
Pb/
207
Pb in the sediment samples agree with
the isotopic ratio of the upper continental crust from across the
globe (~1.2; Chow and Patterson, 1962; Hamelin et al., 1990). Thus,
with [Pb] and Pb isotope comparable to natural values, and with
known limited development in Singapore region before the 1900s
(Kwa et al., 2009; Singapore Manufacturing Federation, 2012), it is
reasonable to conclude that the bottom of the core contains mainly
natural, preindustrial Pb.
The [Pb] in the sediment increased 27 fold from the 1900s to the
1990s, which advocates for anthropogenic sources. It is also noted
that a local peak in Pb (53 mg/kg) content appeared in the 1950s,
decreased and remained ~43 mg/kg in the 1960s-1980s and then
increased again to 55 mg/kg in the 1990s. Since lower consumption
of leaded petrol was expected in the 1950s, the high [Pb] in the core
may not be solely caused by automobile emissions. Instead, it may
be attributed to a few local industrial sources described as many
small, unlicensed and backyard type foundries and metal fabri-
cating plants with poor ventilation or housekeeping,a number of
Pb recovery works which recover Pb from used batteries using a
very crude form of furnace,a big metal recovery work that re-
covers ironand the coal-red power station in the same period
(Fig. 1b, Singapore Anti-pollution Unit, 1970e1972). These in-
dustries were asked to install air pollution control facilities by June
1972 (Singapore Anti-Pollution Unit, 1970e1972) and the coal po-
wer plant situated on the south of Singapore was decommissioned
in early 1970s (Ramlan, 2014). The sediment core from MacRitchie
Reservoir preserves a ~20% decline in [Pb] and a signicant
decrease in the concentration of other metals (Ag, As, Cd and Zn)
from the 1950s to the 1980s (Figs. 5 and 6), which was likely due to
the introduction/implementation of the Clean Air Act in 1971 and
the closure of the coal power plant.
The [Pb] in the sediment in the 1990s was 55 mg/kg, which was
the highest throughout the core. The high Pb content in the
sediment likely reects leaded petrol usage in the region. Leaded
petrol was the overwhelming source of Pb during the 1970s and
1980s in North America and Western Europe (Flegal, 1986; Nriagu,
1989; Patterson and Settle, 1987), while in Southeast Asia, leaded
petrol emissions peaked in the 1990s (see Fig. 2). Besides leaded
petrol usage in Singapore, Pb emissions from Indonesia and
Malaysia could be transported to Singapore during the migration of
the monsoon (Fig. 1a). The leaded petrol emission from Singapore
and Malaysia started to decrease in the mid-1980s (Fig. 2a), while
the emission from Indonesia continued to increase until 2006
(Hirota, 2006; Lee et al., 2014,Fig. 2a). The increasing Pb emissions
from the 1970se1990s are in agreement with the increasing Pb
concentrations in the MacRitchie Reservoir sediments during this
period.
To better understand the variation of [Pb] in the Singapore re-
gion, the temporal [Pb] variability in MacRitchie Reservoir sedi-
ments were compared to a 50-year-long coral record from an
undeveloped island in the Singapore Straits (Jong Island, Fig. 1b,
Chen et al., 2015), a sediment core from Nee Soon Swamp (Koh,
2014,Fig. 1d) and a sediment core from Lower Peirce Reservoir
(Ee, 2000,Fig. 1d). Comparing the MacRitchie Reservoir Pb record
to the Jong Island coral Pb record is difcult due to the low reso-
lution of the sedimentary record (3 sediment data points versus 46
coral data points), however, both records show an increasing Pb
concentration from the 1960se1990s (Fig. 4). It was noted that the
coral Pb/Ca started to decrease after 2003, in parallel to the general
phasing out the leaded petrol in Southeast Asia (Chen et al., 2015).
Such a drop in Pb content was not seen in the sediment due to the
lower temporal resolution of the sediment core. Comparing the
MacRitchie Reservoir sediments to the other sedimentary records,
all show an increase in [Pb] towards the top of the respective cores.
The Nee Soon Swamp core preserved an increase in [Pb] from 40 cm
upwards (Koh, 2014), and the Lower Peirce Reservoir sediments
(Fig. 1) showed an increase in [Pb] in the top 10 cm (Ee, 2000). The
comparison of MacRitchie Reservoir sediments to the Nee Soon
Swamp and Lower Peirce Reservoir sediments is also difcult as the
210
Pb-dating for the Nee Soon Swamp and Lower Peirce Reservoir
sediments are not available. However, the general agreement be-
tween the sediment core sites supports the inference that the in-
crease of [Pb] in the sediments was in parallel to the regional
increase of atmospheric Pb sources.
4.3. Sources of Pb in the sediment implied by Pb isotopes
There are four potential sources of Pb into the sediment: leaded
petrol, industrial emissions, incineration and coal burning. For lea-
ded petrol, the
206
Pb/
207
Pb from aerosols across Southeast Asian
cities is1.141 ±0.001 for Kuala Lumpur, 1.127 ±0.001 for Bangkok,
1.15 6 ±0.001 for Ho Chi Minh City and 1.131 ±0.001 for Jakarta in the
1990s, when leaded petrol was still in use (Bollh
ofer and Rosman,
2000). Although Singapore was not included in Bollh
ofer and Ros-
man's study, the Pb isotopes in the MacRitchie Reservoir sediment
from the 1970se1990s (~1.137, Figs. 4 and 7)fallwellwithinthe
range of other measured Southeast Asian city aerosol ratios during
the peak in leaded petrol emissions. The agreement in Pb isotope
data between the sediments and regional aerosols supports the
hypothesis that atmospheric deposition from leaded petrol was the
dominant source of Pb to the sediments in MacRitchie Reservoir.
Industrial sources might also contribute large amounts of Pb to
the environment (e.g. Flegal et al., 2013). Industrial Pb emissions
from mining, smelting, waste incineration and coal combustion
were found to be signicant in many cities (e.g. Ragaini et al., 1977;
Shi et al., 2008; Jackson et al., 2004), especially after the phasing out
of leaded petrol (Díaz-Somoano et al., 2009). In the Singaporean
context, recent aerosol measurements show that
206
Pb/
207
Pb in
Table 2
Eigenvalues and variance contribution in principal component analysis.
Principal compoment Eigenvalue Variance contribution
PC1 8.452 76.84%
PC2 1.961 17.83%
PC3 0.263 2.39%
PC4 0.164 1.49%
PC5 0.083 0.75%
PC6 0.043 0.39%
PC7 0.025 0.23%
PC8 0.006 0.05%
PC9 0.002 0.02%
PC10 0.000 0.00%
PC11 0.000 0.00%
M. Chen et al. / Environmental Pollution 213 (2016) 446e459454
Singapore aerosol was ~1.145 (Lee et al., 2014). Since these aerosols
were measured in 2012 and 2013, 10 years after Singapore
concluded phasing out of leaded petrol, the Pb isotopes in
Singapore aerosols should be interpreted as of predominantly in-
dustrial origin. Unfortunately the
206
Pb/
207
Pb in Singapore aerosol
in 2013 was indistinguishable from the aerosols measured in other
major Southeast Asian cities in the 1990s, when leaded petrol was
still in use. Thus, it is still difcult to differentiate the Pb from in-
dustrial sources from leaded petrol using Pb isotopic measure-
ments. More precise Pb isotope measurements of leaded petrol
could potentially provide a more quantitative solution in differen-
tiating these sources.
Incineration might also contribute Pb to the sediment (e.g:
Nriagu, 1979). The y ash collected from all four incineration plants
from Singapore showed
206
Pb/
207
Pb of 1.148 ±0.005 (Chen et al.,
2015), slightly higher than isotope ratios in the top of the sedi-
ment core (1.137), but comparable to Singapore aerosols (~1.145,
Lee et al., 2014). It was difcult to differentiate the relative contri-
bution of Pb from petrol and incineration as they have very similar
isotopic ratios (Chen et al., 2014). However, the isotopic agreement
between incineration y ash and the sediment suggested that
incineration might be a major source of Pb to the sediment from the
1970s to the 1990s, although the relative contributions of these two
sources could not be determined.
Coal combustion might also be a possible Pb source to the
sediments (Díaz-Somoano et al., 2009). Coal-generated Pb emitted
from Malaysia and Indonesia has increased since the 1970s
(Fig. 2b), in conjunction with the increasing Pb concentration in the
MacRitchie Reservoir sediments. However, the Pb isotope ratios
from Australian and Indonesian coal (the type of coal most
commonly used in the region, COMTRADE, 2012; Hargraves, 1993;
Lucarelli, 2000) has
206
Pb/
207
Pb ~ 1.184 and
208
Pb/
207
Pb ~ 2.477 for
Indonesian coals (Díaz-Somoano et al., 2009), and
206
Pb/
207
Pb
~1.195 and
208
Pb/
207
Pb ~2.473 for Australian coals (Díaz-Somoano
et al., 2009), which is much higher than the ratio in the
sediments deposited between the 1970s and 1990s (~1.140 for
206
Pb/
207
Pb, Fig. 4). Therefore, coal combustion is not a dominant
source of Pb to the MacRitchie Reservoir sediments. However, coal
has been used for electricity generation in Singapore from 1927 to
the 1970s (Ramlan, 2014; Wan and Lau, 2009). During the earlier
period when the petrol consumption was limited, coal combustion
may have been a signicant contributor to the Pb in the sediments.
The sediments deposited during the 1910se1940s had a
206
Pb/
207
Pb of ~1.185 and
208
Pb/
207
Pb of ~2.466, similar to Indo-
nesian and Australian coals (Figs. 4 and 7), implying the burning of
coal as a possible source of Pb in the 1910e1940s period.
It should also be noted that a mix of natural (
206
Pb/
207
Pb ~ 1.215,
208
Pb/
207
Pb ~2.507) and atmospheric Pb (
206
Pb/
207
Pb ~1.138,
208
Pb/
207
Pb ~2.414) could result in an isotopic composition com-
parable to the 1910se1940s sediments (Fig. 4). Although the hy-
pothesis of mixing does not exclude the possible contribution from
Indonesian and Australian coals that have isotope ratios close to the
Pb in MacRitchie Reservoir sediment between the 1910s and 1940s,
the Pb isotopes in the MacRitchie Reservoir sediment showed high
linearity on the triple isotope plot (Fig. 7), which would likely be a
result of mixing between the natural Pb and atmospheric Pb
sources.
4.4. An estimation of recent atmospheric Pb ux into the Singapore
region
Since the atmospheric deposition is the main cause of Pb in
recent MacRitchie Reservoir sediment, the atmospheric Pb depo-
sition could be calculated using the actual aerosol and rainwater
data using the following equation.
Pb Flux ¼ðC
aerosol
vÞ
wet
þðC
rain
dÞ
dry
(2)
The Pb ux is calculated as a sum of wet deposition and dry
deposition in Eq. (2), where C
aerosol
is the atmospheric
Fig. 7. The plot of
206
Pb/
207
Pb ratios against
208
Pb/
207
Pb ratios in the MacRitchie sediments (white open triangles) together with a panel of possible end-members. The Singapore
aerosols (Lee et al., 2014) and incineration y ashes are shown as grey lled circles; the Indonesian coals (Díaz-Somoano et al., 2009) are shown as grey lled squares; the Australian
coals (Díaz-Somoano et al., 2009) are shown as grey lled triangles; the South China Sea (SCS) sediments (Zhu et al., 2010) are shown as small grey lled crosses; the south Chinese
Pb ore are shown as grey open crosses; the Singapore corals are shown as dark grey dots; and the Chinese aerosols, Thai aerosols, Indonesian aerosols are shown as black open
squares; grey open diamonds; black open circles correspondingly. The dashed line illustrates the binary mixing scenario between natural and atmospheric Pb sources.
M. Chen et al. / Environmental Pollution 213 (2016) 446e459 455
concentration of Pb (10 ng/m
3
, measured from 2009 to 2012, pers.
comm. Prof. Webster); vis the settling velocity of particulate matter
in the atmosphere (1 cm/s; Gr
onholm et al., 2007); C
rain
is the
rainwater Pb concentration (4.7
m
g/L, measured from 2009 to 2012,
pers. comm. Prof. Webster), and dis the annual rainfall in Singapore
(2400 mm/yr; Public Utilities Board, 2011). With the aerosol and
rainwater data, the calculated Pb ux is 1.4 10
2
g/m
2
yr.
Alternatively, the atmospheric Pb ux into Singapore region
could be estimated using the equation modied from Kober et al.
(1999):
Pb Flux ¼SrðC
measured
C
baseline
Þ(3)
where Sis the sedimentation rate, ris the dry bulk density of the
sediment (1.05 10
3
g/mm
3
), and C
measured
and C
baseline
are the
concentrations of Pb in the sediment from measured sample and
baseline value (assumed 1.9 mg/kg, as the average of the four points
from the bottom of the core), respectively. The estimated Pb ux
using the concentration from the top of the core (55 mg/kg) is
1.6 10
2
g/m
2
yr. The annual atmospheric Pb ux estimated from
MacRitchie Reservoir sediment [Pb] agrees well with the calcula-
tions using current atmospheric measurements.
4.5. Variation of other metals compared to Pb
The PCA analysis of the metals grouped the metals by their
covariability (Fig. 6). The rst PC signies an increase towards 1990s
with a local peak in ~1950s. This feature is shown in many elements
and contributes more than 75% of the variance. The second PC
signies the mid-century peak and decrease since 1970s. This
feature is less common as it contributes ~18% of the variance. Based
on the two PCs, the elements were divided into three groups with
each group of metals having very similar loadings on the PCA biplot
(see Section 3.3 and Fig. 6a).
The rst group of metals contains Pb, Ba, Cd, Ag, As and Co. The
variations of these metals can be summarized as increasing con-
centration over the 20
th
century with a decline in the 1970s. One
possible explanation for the increasing trend over the 20
th
century
was that the production of these metals has all increased (e.g.
Foster and Charlesworth,1996) and their use by Singapore industry
has also increased signicantly. Although emissions of different
metals may be the by-product of different industries, on the large
scale all of these metals follow an increasing trend (Foster and
Charlesworth, 1996; Salomons and F
orstner, 1984). Another
possible explanation for the similarity in the PC1 pattern is that
other metals were co-released during Pb emission, for example, Pb,
Cd and As can be released during coal combustion (Salomons and
F
orstner, 1984), and Ba can be released as a result of fossil fuel
combustion and waste incineration (ATSDR, 2007). Similar trace
metal trends would be observed if the metals were released during
similar processes. The decrease in concentration of the metals in
the 1970s could be explained as a result of the introduction of the
Clean Air Act (Singapore Anti-Pollution Unit, 1970e1972). The
decrease in Pb content in the 1970s accompanied by little change in
the Pb isotopic composition could be the result of improved
emission control.
The second group recognized in the PCA includes Zn, Ni and Cr.
These elements recorded little decrease in 1970s followed by a
sharp increase from the 1980se1990s. The decrease in the con-
centration of Group 1 elements in the 1970s was not clearly
observed in this group, because Zn, Ni or Cr were not included in
the emission control guidelines (Singapore Anti-Pollution Unit,
1970e1972). This group signies less harmful metals as Zn and Ni
are essential nutrients to human health (ATSDR, 2012, EPA, 2005;
Cempel and Nikel, 2006), but could increase to toxic levels due to
anthropogenic activity (e.g.: Zayed and Terry, 2003; Cempel and
Nikel, 2006). Zn can be released by various processes, but most
Zn is released by industrial activities (including metal production),
wood combustion and waste incineration (Nriagu, 1979). Ni is
mainly released by oil combustion and industrial activities (Nriagu,
1979). Cr is released by a variety of industrial processes (Zayed and
Terry, 2003), but also from natural sources such as wind-borne soil
particles (Nriagu, 1989). Although various sources could contribute
Zn, Ni and Cr to MacRitchie Reservoir sediments, the sharp increase
in these elements from the 1980se1990s suggests that they may be
linked to urbanization and industrialization of the Singapore
region.
The third group discerned by the PCA analysis included Tl and U,
which recorded an increase in concentration from the 1890se1950s
and a decrease from the 1950se1990s. The decreasing concentra-
tion from the 1950se1990s differs from the behavior of other
metals. The U concentrations in MacRitchie Reservoir sediments
(0.4e2.4 mg/kg) are lower than local granitic rocks (3.0 mg/kg), and
within the range of sedimentary rocks (0.45e2.2 mg/kg, Turekian
and Wedpohl, 1961). It is more reasonable to compare U concen-
trations to sedimentary rocks due to the soluble characteristics of
uranium. It is unclear whether the variation of Tl was caused by
anthropogenic activities since the concentration of Tl varied from
0.01 to 0.23 mg/kg and is within the range of the Earth's mean
crustal concentration of 0.1e
1.7 mg/kg (Peter and Viraraghavan,
2005). Therefore, based on this evidence, U and Tl cannot be
attributed clearly to anthropogenic activities.
Copper had the largest magnitude and most rapid increase of all
of the metals measured from MacRitchie Reservoir sediments
(Fig. 5). Such increases could only be caused by anthropogenic ac-
tivities. Copper sulfate (CuSO
4
) had been used as an algaecide in
reservoirs around the globe for decades (e.g. Hawkins and Grifths,
1987). Such application has remained the most common method
for controlling cyanobacteria because it was effective, economical
and relative safe to human health (Chorus and Bartram, 1999).
CuSO
4
treatment could be a possible source explaining the increase
in Cu in the MacRitchie Reservoir sediments as the same order of
increase has been observed in other reservoirs across the worlds
that have employed CuSO
4
treatment (e.g. AWWA, 1995; Haughey
et al., 2000). Singapore is no longer using CuSO
4
treatment in the
reservoirs (personal communication with PUB), and the Cu con-
centration in Singapore drinking water is rmly within the safe
level recommended by the World Health Organization (Public
Utilities Board, 2014).
5. Conclusions
The temporal variation of Pb and Pb isotopes was reconstructed
from a sediment freeze core retrieved from MacRitchie Reservoir,
located in the Central Catchment Nature Reserve of Singapore. The
core spans the period from 1895 to 2012 as measured by
210
Pb
dating. The variation of Pb was compared with the historical Pb
emissions from neighboring countries, the isotopic composition of
Pb from potential sources, and the variation of eleven other metals
(Ag, As, Ba, Cd, Co, Cr, Cu, Ni, Tl, U and Zn) in the sedimentary re-
cord., From the reconstruction, the following conclusions can be
drawn:
1. The Pb content in the sediment increases from 1.9 mg/kg in the
1910s to 53 mg/kg in the 1950s. From the 1950s to the 1960s, the
Pb in the sediment decreased by 20% (41 mg/kg) and stayed
~42 mg/kg until the 1980s. From the 1980se1990s, the Pb
content increased to 55 mg/kg. The corresponding decrease in
206
Pb/
207
Pb from 1.199 in the 1910s to 1.137 in 1990s, suggested
an atmospheric input of Pb to the reservoir. The recent
M. Chen et al. / Environmental Pollution 213 (2016) 446e459456
estimated atmospheric Pb ux in Singapore region is
~1.6 10
2
g/m
2
yr.
2. The dissolved [Pb] in MacRitchie Reservoir water was
1.1 ±0.07 10
2
m
g/L, far lower than the allowable drinking
water standards. The highest Pb content in MacRitchie Reservoir
sediments were within the range of threshold values of global
sediment guidelines.
3. The
206
Pb/
207
Pb in recent sediments (1970se1990s) was ~1.14,
similar to the aerosol ratios previously reported in several
Southeast Asian countries/cities. The possible sources included
leaded petrol, industrial sources and incineration. The Pb
recorded in the sediment between the 1910s and 1940s had a
206
Pb/
207
Pb of ~1.18 consistent with a combination of natural
and atmospheric Pb sources and/or from the combustion of coal.
4. The Ag, As Ba, Cd, Co, Cr, Ni and Zn content in the sediment have
all increased signicantly from the 1910se1990s. The increase
was parallel to the urbanization and industrialization of
Singapore. The decrease in concentration of Ag, As Ba, Cd, Co and
Pb in the 1970s and 1980s may be a result of stricter emission
controls since 1971. The Tl and U content in the sediment shows
a decrease from the 1950se1990s, but may not be caused by
anthropogenic activities. The Cu content in the sediment shows
a large increase in 1980s, which probably caused by CuSO
4
treatment in the reservoir.
This study signies the importance of identifying Pb and other
metal concentrations and Pb isotopes from terrestrial archives to
reconstruct the history of Pb and trace metals, and provides the
history of metals in the data-sparse South East Asian region.
Acknowledgments
The research described in this project was funded by the
Singapore National Research Foundation (NRF) through the
Singapore-MIT Alliance for Research and Technology (SMART)
Center for Environmental Sensing and Modelling (CENSAM) and by
the Singapore Ministry of Education under the Research Centres of
Excellence initiative (Grant Number M4430139). We thank
Singapore Public Utility Board for granting us permission to collect
the sediment core from the MacRitchie Reservoir.
We also thank Dr. Hans Eikaas and Dr. Ole Larsen from the
Danish Hydrological Institute, Professor Richard Webster and Dr.
Bahareh Khezri from the Chemistry department at Nanayang
Technological University; Professor Alan Ziegler and Professor Bob
Wasson and their students at the National University of Singapore;
Assoc. Prof. Nathalie Goodkin, Dr. Annette Bolton and Dr. Hong-Wei
Chiang from the Earth Observatory of Singapore (EOS) for their
constructive criticism. This work comprises Earth Observatory of
Singapore contribution No. 119.
Appendix A. Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.envpol.2016.02.040
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