Embed Size (px)
Citation preview
Estuarine, Coastal and Shelf Science 60 (2004) 91e100
www.elsevier.com/locate/ECSS
Low concentration of heavy metals in theYangtze estuarine sediments, China: a diluting setting
Zhongyuan Chena,), Yoshiki Saitob, Yutaka Kanaic, Taoyuan Weid,Luqian Lid, Heshun Yaod, Zhanghua Wangd
aState Key Laboratory of Estuarine and Coastal Research, East China Normal University, Shanghai 200062, ChinabMRE, Geological Survey of Japan, AIST, Central 7, Higashi 1-1-1, Tsukuba, Ibaraki 306-8567, Japan
cRCDME, Geological Survey of Japan, AIST, Central 7, Higashi 1-1-1, Tsukuba, Ibaraki 306-8567, JapandDepartment of Geography, East China Normal University, Shanghai 200062, China
Received 27 February 2003; accepted 30 November 2003
Abstract
An examination of the spatial and temporal distribution of 17 major heavy metals, i.e., Fe, Zn, Ni, Mg, Co, Mn, etc., was made
in the Yangtze estuarine sediments revealed by a number of vibrocores taken from the different sediment zones, including deltafront, prodelta, and deltaeshelf transition. The results obtained from the numerous core samples, which were also measured for Pb-210 and Cs-137, show that: (1) the silty clay comprising the prodelta facies serves as a depositional sink attracting high
concentrations of heavy metals delivered from the river mouth; (2) after being normalized to aluminum (as a proxy for grain size),most heavy metals presented in the prodelta facies have lower concentrations than in the other adjacent sediment zones; (3) alsoafter normalization, concentrations of most heavy metals in the vibrocore sediments tend to increase up-core; and (4) sedimentation
rates in the study area range fromw2.0 to 6.0 cm/a; hence, the vibrocores contain a sedimentary record of metal deposition coveringmore than 50 years. It has previously been assumed that sediments off the river mouth were heavily polluted due to industrializationof the Shanghai metropolitan area, which peaked about 50 years ago. However, the low concentrations of heavy metals in the study
area before and after normalization do not support this assumption. The Yangtze estuary is characterized largely by the tremendousrunoff and ‘unpolluted’ sediments derived from the upper drainage basin to constitute a unique diluted setting, in which the dispersalbehavior of heavy metals from the adjacent industrialized coast is influenced substantially. As a result, the heavy metals in the studyarea are obviously lower than those previously determined along the coast.
� 2004 Elsevier Ltd. All rights reserved.
Keywords: dilution; dispersal pattern; heavy metals; sedimentation rate; Yangtze estuary
1. Introduction
Over the past century, heavy metals have beendischarged into the world rivers and estuaries asa result of the rapid development of industry (Chenet al., 1997; Tam and Wong, 2000; Chan et al., 2001;Chen et al., 2001; Lee and Cundy, 2001; Ruiz, 2001;Santschi et al., 2001; Zhang et al., 2001; Feng et al.,2002; Lin et al., 2002). The delivery processes are closelylinked to those of fine-grained suspended sediments,
) Corresponding author.
E-mail address: [email protected] (Z. Chen).
0272-7714/$ - see front matter � 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.ecss.2003.11.021
acting as effective carriers, onto which heavy metals arebound. When the littoral processes are encountered,large amounts of suspended sediments begin to floccu-late and settle, and distinctive sedimentary patterns areformed in response to the changed conditions of theestuarine environment with regard to hydrodynamicsetting, salinity, water temperature and redox (Fenget al., 1999; Wright and Mason, 1999; Shen and Pan,2001; Warwick, 2001; Gijs et al., 2002).
The metropolis of Shanghai, located on the easterncoast of China (Fig. 1), discharges more than 6.0 milliontonnes of industrial and domestic sewage water into theYangtze estuary daily (Shanghai Drainage Administra-tion Division, 1985). Contamination by heavy metals
92 Z. Chen et al. / Estuarine, Coastal and Shelf Science 60 (2004) 91e100
C1
122°
Shanghai
Legend
10
20
30
50
121° 123° 124° E32° N
31°
Yellow Sea
EastChina Sea
Study area
C
H
I N
A
Yan
gtze
riv
er
Y2
Y1
10 Bathymetry (m)
Y4
C4
C5
C2
Y5Y6
Y8Y7
Y9
A Delta front
CoastlandSediment zone
B ProdeltaC Delta-shelf transitionD Relict sand zone
0 50 km
40
Yang tze River
Vibrocore site
A
B
B C D
Fig. 1. Geographic location of the Yangtze estuary and the vibrocore sites.
has become an issue of great concern with respect toestuarine aquaculture, reclamation, and habitation(Shanghai Drainage Administration Division, 1985;Chen et al., 2001). Heavy metals have accumulated inconsiderable amounts in the tidal flat sediments,especially near the many sewage outlets, where largetracts of coastal land (O1000 km2) have been reclaimedduring the last few decades to accommodate the greatpressure from the densely populated delta region(O600 people/km2) (Shanghai Drainage AdministrationDivision, 1985; Chen et al., 2001). Heavy metals in theYangtze estuary have also attracted attention and socialconcern because they can adsorb onto the coastal plantsand animals, which are considered as an importantcomponent of the food chain and are thus potentiallyharmful to human health (Shanghai Drainage Admin-istration Division, 1985; Wright and Mason, 1999; Gijset al., 2002). The maintenance of a sound estuarineecosystem depends not only on the effective manage-ment of human activities in the region, but also ona better understanding of the dispersal pattern of heavymetals (as well as other pollutants) and their sedimen-tological, biological and chemical interactions at theestuarine interface.
The Yangtze River carries more than 4:86! 108 t/asediment to the coast via its tremendous discharge, ofwhich about 50% is deposited in its estuary and the restis dispersed into the East China Sea (Chen et al., 1988).Recently, studies on heavy metals linked with studies ofsediment dynamics have clearly revealed various distri-bution patterns and dispersal routes along the coast (Xuet al., 1982; Su et al., 1988; Chen et al., 2001; Zhanget al., 2001). However, the evolution of contaminantdistribution in the subaqueous delta sediments due torecent industrialization is little understood. The presentstudy focuses on the spatial and temporal distribution ofheavy metals in the study area in order to highlightdelivery behavior, with reference to estuarine dynamics,and to evaluate the sedimentary record of pollutionfrom human activities on the estuarine ecosystem.
2. Methods
In 2000, six vibrocores (Y4eY9; 2.0e3.0 m long/vibrocore) were obtained from the Yangtze estuary(Fig. 1) from water depths between 10 and 55 m. Intotal, 181 samples were taken at intervals of 5 cm from
93Z. Chen et al. / Estuarine, Coastal and Shelf Science 60 (2004) 91e100
the six vibrocores. Samples were air-dried at roomtemperature (!30 (C) and sieved through a 200-mmsieve to remove root and shell fragments. Samples werethen ground and partially dissolved in HNO3eHClO4,to be comparable with the former studies (cf. Chenet al., 2001). Seventeen major and trace elements, i.e.,Fe, Zn, Ni, Mg, Co, Mn, Al, Ba, As, Pb, Ga, Ti, Cr, Cu,Sr, V, and Li, were measured in the laboratory byinductively coupled plasma atomic emission spectrom-etry (ICP-AES) (Plasma-2000, PerkineElmer, USA).Parallel testing was completed to validate the laboratoryoutput. Both absolute contents of heavy metals (Figs. 2aand 3a) and contents normalized to that of aluminumare reported (Figs. 2b and 3b).
Thirty-six samples were taken from the six vibrocoresand analyzed for grain size using a Coulter LS100Qparticle size analyzer. Samples were chosen fromsediment sections where changes in lithology occurred.In addition, 108 samples were selected from the sixvibrocores for radioisotope analysis (Pb-210 and Cs-137). Four grams of pulverized sample was added toa tube with a cap. The activities of Pb-210 (peak energy:46.5 keV), Pb-214 (352 keV), Cs-137 (661.6 keV), andK-40 (1461 keV) were then measured by gamma-rayspectrometry by using well-type Ge detectors (ORTECGWL-140230-S and GWL-120230-S). The peak intensi-ties were corrected by sample arrangement according tothe metals (Kanai, 1993). The excess Pb-210 activity wascalculated by subtracting the activity of Pb-214 fromthat of Pb-210, as it was assumed that the supported Pb-210 was in equilibrium with Ra-226 and Pb-214.Sedimentation rates were calculated for all six vibro-cores on the basis of the Pb-210 and Cs-137 measure-ments. CIC (Constant Initial Concentration) model wasapplied for Pb-210 data treatment of the present study(cf. Robbins and Edgington, 1975).
Additional data incorporated into the present data-base include the heavy metals’ distribution in vibrocoreY1, which was taken from the upper tidal flat ofChongming Island in the Yangtze River mouth (Fig. 1;Chen et al., 2001), and the surface sediment grain-sizedistribution of the Yangtze subaqueous delta (zones A,B, C, and D, Fig. 1; Chen et al., 2000).
3. Observations and results
Yellowish gray silty sand (mean grain sizeZ125e63 mm) dominated in vibrocore Y4 located in sedi-ment zone A (delta front facies; Fig. 1) in the Yangtzesubaqueous delta. Light gray to dark gray silty clay andclayey silt (mean grain sizeZ 8e4 mm) composed thesediments of the vibrocores Y5, Y6, Y7, and Y8, whichwere located in sediment zone B (prodelta facies). Graysilty clay (mean grain sizeZ 8e4 mm) interbedded withfine sand (mean grain sizeZ 125 mm) overwhelmingly
dominated in vibrocore Y9 located in sediment zone C(deltaeshelf transition).
The absolute contents of most of the heavy metalsstudied (Fe, Zn, Ni, Mg, Co, Mn, Al, Ba, As, Pb, Ti, Cr,Cu, Sr, and V) taken from the surface samples of the sixvibrocores tend to increase seaward from zone A to zoneB (Fig. 2a), but they decrease in zone C. Ga and Li seemexceptional, because their concentrations increase sea-ward from zone A to zone C. However, after normal-ization to Al as proxy for grain size (Windom et al.,1983), most heavy metals of the surface samples tend todecrease in zone B. In addition, the normalized heavymetal contents in zones A and C are clearly higher thanthose in zone B, although Cu seems not to follow thistrend, but decreases from zone A to zone C (Fig. 2b).
The heavy metal concentrations in the verticalsediment profiles of the six vibrocores show severaldistribution patterns on the basis of varied sedimenta-tion rates. After examining thoroughly, vibrocore Y7with suitable sedimentation rate (discussed below) wasselected as being representative of the down-core profilepattern for the subaqueous Yangtze delta. In most cases,heavy metal contents increase gradually upward fromthe core bottom, for example, Mg, Zn, Cu, Co, Ti, Cr,and V (Fig. 3a). Ni, Mn, and Fe contents also changedgradually, but some fluctuation is observed. The changesin Al, Sr, Ba, Ga, As, and Pb contents are subtle, and insome cases they even decrease upward. After normali-zation to Al, those that increased upward include Mg,Zn, Cu, Co, Ti, Ni, Mn, Sr, Cr, Fe, V, and Li. Ba and Pbdo not change visibly, and Ga and As decrease slightlyupwards (Fig. 3b).
The isotope analysis of vibrocores Y4eY9 indicatesthat the sedimentation rates are higher in zone B,ranging from 2.0 to 6.6 cm/a in cores Y5, Y6, and Y7(Fig. 4), although the rate is lower in Y8 (0.8e2.1 cm/a)located on the seaward margin of zone B, contiguouswith zone C, which was defined as the transitionbetween the subaqueous delta and continental shelf(Fig. 1; Chen et al., 2000). Sedimentation rates on Y4and Y9 were not determinable (explanation givenbelow). The sedimentation rates calculated in thepresent study were based on: (1) the identification ofthe Cs-137 peak, which usually occurred in the lowerpart of most vibrocores in the study area and which hasbeen identified worldwide with fallout from nuclearweapons testing that took place in 1963 (Peirson, 1971;Katsuragi, 1983; Katsuragi and Aoyama, 1986); and (2)the recognition of an exponential trend in Pb-210activity in the vibrocore sediments. Of note, thesedimentation rate in some of the vibrocores could bedetermined only from individual vibrocore sections, ifthe radioisotope content was not well recorded throughthe entire vibrocore sediment section (Fig. 4). In thiscase, sedimentation rates in vibrocores Y4 and Y9 werenot reachable.
94 Z. Chen et al. / Estuarine, Coastal and Shelf Science 60 (2004) 91e100
0
5
10
15
20
25
As,
Ba,
Ga,
Pb
0
10000
20000
30000
40000
50000
60000
70000
80000
Al
As
Ga
Pb
Y1 Y4 Y5 Y6 Y7 Y8 Y9
Al
Ba200
250
300Ba
0
20
40
60
80
100
Cr
Sr
Cu
Li
V
Ti
Y1 Y4 Y5 Y6 Y7 Y8 Y9
Cr,
Cu,
Li,
Sr,
V
0
500
1000
1500
2000
2500
Ti
0
10000
20000
30000
40000
50000
60000
Fe
Mg
0
20
40
60
80
100
120
Zn
Ni
Y1 Y4 Y5 Y6 Y7 Y8 Y9
Fe, M
g
Co,
Ni,
Zn
Co
Mn600
400
Mn
800
A B C
0
0.5
1
1.5
2
2.5
3
Mg
Ni
Fe
Zn
Li
Co
0
0.004
0.008
0.012
0.016
0.02
Mn
Y1 Y4 Y5 Y6 Y7 Y8 Y9
Co,
Fe,
Li,
Mg,
Ni,
Zn
Mn
0
0.4
0.8
1.2
1.6
2
V
Sr
Cr
Pb
Y1 Y4 Y5 Y6 Y7 Y8 Y90
0.01
0.02
0.03
0.04Ti
Ti
Cr,
Pb,
Sr,
V
0
0.2
0.4
0.6
0.8
1
AsGa
Cu
Y1 Y4 Y5 Y6 Y7 Y8 Y9 0
0.001
0.002
0.003
0.004
0.005
0.006
Ba
Ba
As,
Cu,
Ga
A B C
a: Absolute content (ppm) b: Normalized by Al
Fig. 2. (a) Heavy metal concentrations (absolute) of the surface samples from the six vibrocores. (b) Heavy metal contents of the surface samples after
normalization to aluminum. A, B and C imply different sediment zones as indicated in Fig. 1.
4. Discussion
The distribution of sediments (zones AeC) of theYangtze subaqueous delta in association with sedimentnature recorded in the vibrocores of the present studyhighlights the correlation between grain size and heavymetal concentrations. Our previous study shows thatzone A is the delta front facies, consisting in large partof fine sand and sandy silt at a water depth of !10 m;zone B is the prodelta facies, consisting primarily of siltyclay and clayey silt at water depths of 10e50 m; andzone C is the deltaeshelf transition facies, consisting ofmixed clay, silt, and fine sand at water depths of50e60 m (Chen et al., 2000). The sand in the latter faciesis derived from the late Pleistocene relict sand (zone D,Fig. 1) reworked by submarine tidal currents. Without
doubt, zone B, which has the finest grain size, serves asa depositional sink in the Yangtze estuary, attracting thehighest concentrations of heavy metals (Figs. 2 and 3;see Xu et al., 1982; Wang et al., 2001).
Normalization to Al can eliminate the grain-sizeeffect, which helps evaluate quantitatively environmen-tal contamination due to intensified anthropogenicsources (Windom et al., 1983; Zhang et al., 1988;Santschi et al., 2001). The results show that the contentsof most elements in the samples from the six vibrocorestend to increase upward from the vibrocore bottom(normalized values) (Fig. 3b). Compared with thosedetermined along the Yangtze coast, the heavy metalcontents in the study area both before and afternormalization are lower in general (Tables 1 and 2).Zn, Mn, Fe, As, and Pb, which are considered to be the
Fig. 3. (a s after normalization to aluminum. Note that the horizon at 3.0 m
depth wa
95
Z.Chen
etal./
Estu
arin
e,Coasta
landShelf
Scien
ce60(2004)91e
100
) Vertical distribution of heavy metal concentrations (absolute) in vibrocore Y7. (b) Variations in heavy metal content
s dated to 1950 on the basis of the observed sedimentation rates; triangleZ ppm; solid ovalZ element! 104/Al.
Fig places if there is the declining trend in Pb-210ex(Bq/g).
D.L
96
Z.Chen
etal./
Estu
arin
e,Coasta
landShelf
Scien
ce60(2004)91e
100
. 4. Sedimentation rates of the vibrocores determined on the basis of Cs-137 and Pb-210 measurements. Exponential curve is marked at
.Z detection limit of the gamma spectrometer for Cs-137, shown by a dashed line.
Table 1
Absolute contents of h
Position Sam
m)
Ba
(ppm)
Fe
(ppm)
V
(ppm)
Li
(ppm)
Ga
(ppm)
As
(ppm)
Pb
(ppm)
Data
sources
Y1 2 9 e 29,078.0 e e e 9.8 22.2Yang, pers. comm.
Y2 2 9 e 38,435.0 e e e 13.6 29.6
C1 3 2 e 34,956.0 e e e 11.8 19.7
Chen et al., 2001C2 4 3 e 38,680.0 e e e 24.8 14.6
C4 5 8 e 37,023.0 e e e 22.0 11.5
C5 9 4 e 49,607.0 e e e 54.4 15.4
Delta plain 11 4 e e e e e e 10.4
Lu and Tang, 1998Delta plain 11 1 e e e e e e 10.6
Middle tidal flat 8 7 e e e e e e 9.7
Middle tidal flat 8 9 e e e e e e 8.2
Tidal flat 38 3 e e e e e e 40.2
Liu et al., 2000Upper tidal flat ? 1 e e e e e e 56.2
Middle tidal flat ? 9 e e e e e e 41.1
Lower tidal flat ? 1 e e e e e e 43.8
Tidal flat 11 e e e e e e 54.5
Xu et al., 1997Tidal flat 13 e e e e e e 13.7
Tidal flat 1 1 e e e e e e 234.9
Y4 3 8 210.7 25,481.2 65.7 35.3 15.9 11.8 16.5
From author
Y5 5 9 209.1 34,509.7 67.1 41.7 13.0 10.2 15.4
Y6 5 1 303.2 33,945.9 62.8 50.1 20.3 13.7 21.2
Y7 5 4 283.3 46,929.8 80.7 45.9 18.4 13.4 19.4
Y8 5 4 283.3 46,929.8 80.7 45.9 18.4 13.4 19.4
Y9 1 1 231.1 26,978.4 68.6 57.5 16.3 10.9 18.2
97
Z.Chen
etal./
Estu
arin
e,Coasta
landShelf
Scien
ce60(2004)91e
100
eavy metals in vibrocore Y7 and from other studies of the Yangtze estuary, as cited
ple Depth
(cm)
Mg
(ppm)
Zn
(ppm)
Cu
(ppm)
Co
(ppm)
Ti
(ppm)
Ni
(ppm)
Al
(ppm)
Mn
(ppm)
Sr
(ppm)
Cr
(pp
!40 9509.0 71.1 21.4 e e 26.3 35,742.5 494.5 e 16.
!40 11,713.5 585.8 37.3 e e 37.7 60,400.0 755.6 e 21.
!15 14,030.0 92.1 30.1 e e 29.5 54,460.0 e e 48.
!39 15,100.0 65.7 26.5 e e 25.4 46,303.0 e e 43.
!50 13,953.0 47.5 30.0 e e 26.3 37,753.0 e e 39.
!85 18,183.0 62.7 32.0 e e 28.0 70,293.0 e e 43.
!20 e 51.5 14.1 20.5 e 25.9 e 436.3 e 20.
20e40 e 53.6 17.8 20.9 e 26.6 e 438.3 e 23.
!20 e 45.3 17.3 19.7 e 21.5 e 447.2 e 22.
20e40 e 48.3 12.2 17.4 e 19.8 e 339.8 e 26.
Surface e 145.8 76.9 e e e e e e 70.
Surface e 238.8 84.3 e e e e e e 92.
Surface e 130.6 75.8 e e e e e e 68.
Surface e 154.9 77.8 e e e e e e 70.
Surface e 102.3 42.8 e e e e e e e
Surface e 91.5 14.2 e e e e e e eSurface e 550.9 45.4 e e e e e e 65.
!50 11,642.5 65.9 33.2 13.9 1824.9 28.6 40,531.2 511.3 79.9 15.
!50 13,148.9 80.3 25.4 16.3 1826.0 36.2 51,447.5 668.8 73.5 11.
!50 10,825.7 79.9 42.9 14.3 1619.9 36.9 75,238.6 582.2 85.9 12.
!50 15,620.1 107.2 30.8 18.1 1762.3 42.1 74,810.9 694.4 92.2 18.
!50 15,620.1 107.2 30.8 18.1 1762.3 42.1 74,810.9 694.4 92.2 18.
!10 13,117.9 76.6 21.6 16.4 1307.2 38.2 52,964.4 574.6 68.5 10.
Table 2
Heavy metal contents of her studies of the Yangtze estuary, as cited
Position Sampl
Al
Fe!
104/Al
V!
104/Al
Li!
104/Al
Ga!
104/Al
As!
104/Al
Pb!
104/Al
Al
(ppm)
Data
sources
Y1 2 8135.4 e e e 2.7 6.2 35,742.5 Yang,
pers. comm.Y2 2 6363.4 e e e 2.3 4.9 60,400.0
C1 3 6418.7 e e e 2.2 3.6 54,460.0
Chen et al.,
2001
C2 4 8353.7 e e e 5.4 3.2 46,303.0
C4 5 9806.6 e e e 5.8 3.1 37,753.0
C5 9 7057.2 e e e 7.7 2.2 70,293.0
Delta plain 11 e e e e e e e
Lu and
Tang, 1998
Delta plain 11 e e e e e e e
Middle tidal flat 8 e e e e e e e
Middle tidal flat 8 e e e e e e e
Tidal flat 38 e e e e e e eLiu et al.,
2000
Upper tidal flat ? e e e e e e e
Middle tidal flat ? e e e e e e e
Lower tidal flat ? e e e e e e e
Tidal flat 11 e e e e e e e
Xu et al.,
1997
Tidal flat 13 e e e e e e eTidal flat 1 e e e e e e e
Y4 3 6286.8 16.2 8.7 3.9 2.9 4.1 40,531.2
From
author
Y5 5 6707.8 13.0 8.1 2.5 1.9 2.9 51,447.5
Y6 5 4511.8 8.4 6.7 2.7 1.8 2.8 75,238.6
Y7 5 6273.1 10.8 6.1 2.5 1.8 2.6 74,810.9
Y8 5 6273.1 10.8 6.1 2.5 1.8 2.6 74,810.9
Y9 1 5093.7 12.9 10.8 3.1 2.1 3.4 52,964.4
98
Z.Chen
etal./
Estu
arin
e,Coasta
landShelf
Scien
ce60(2004)91e
100
core Y7 after normalization to Al (as a proxy for grain size) and normalized values from ot
e Depth
(cm)
Mg!
104/Al
Zn!
104/Al
Cu!
104/Al
Co!
104/Al
Ti!
104/Al
Ni!
104/Al
Mn!
104/Al
Sr!
104/Al
Cr!
104/Al
Ba!
104/
!40 2660.4 19.9 5.9 e e 7.4 138.4 e 4.7 e
!40 1939.3 96.9 6.2 e e 6.2 125.1 e 3.6 e
!15 2576.2 16.9 5.5 e e 5.4 e e 8.9 e
!39 3261.1 14.2 5.7 e e 5.5 e e 9.4 e
!50 3695.9 12.6 7.9 e e 6.9 e e 10.5 e!85 2586.7 8.9 4.6 e e 3.9 e e 6.2 e
!20 e e e e e e e e e e
20e40 e e e e e e e e e e
!20 e e e - e e e e e e
20e40 e e e e e e e e e e
Surface e e e e e e e e e eSurface e e e e e e e e e e
Surface e e e e e e e e e e
Surface e e e e e e e e e e
Surface e e e e e e e e e e
Surface e e e e e e e e e eSurface e e e e e e e e e e
!50 2872.5 16.3 8.2 3.4 450.3 7.1 126.2 19.7 3.9 51.9
!50 2555.8 15.6 4.9 3.2 354.9 7.0 129.9 14.3 2.3 40.6
!50 1438.9 10.6 5.7 1.9 215.3 4.9 77.4 11.4 1.6 40.3
!50 2087.9 14.3 4.1 2.4 235.6 5.6 92.8 12.3 2.5 37.9
!50 2087.9 14.3 4.1 2.4 235.6 5.6 92.8 12.3 2.5 37.9
!50 2476.7 14.5 4.1 3.1 246.8 7.2 108.5 12.9 1.9 43.6
99Z. Chen et al. / Estuarine, Coastal and Shelf Science 60 (2004) 91e100
major industrial contaminants in the Yangtze estuary,are even lower than the national standard (ShanghaiDrainage Administration Division, 1985). Furthermore,heavy metal concentrations in the Yangtze estuary arealso lower than those of other large estuaries of theworld (Zhang et al. 1988; Gerritse et al., 1998; Palanqueset al., 1998; Vallius, 1999).
The above phenomenon can be explained by whatis called the ‘‘diluted setting’’ of the Yangtze estuary,in which a great amount of the freshwater flow(925! 109 m3/y) and associated unpolluted suspendedsediment (4:86! 108 t/a) discharges into the estuaryannually. On entering into the estuary, heavy metalswould readily adsorb to the fine-grained sedimentparticles, mostly as a clay fraction (!4 mm) that iseasily driven further seaward as estuarine plumes atopthe salt wedge (Chen et al., 1988; Chen et al., 2001). Theprocesses of adhering involve physical, chemical andbiological interactions, which would effectively trans-form heavy metals from aqueous to solid status or viceversa (Wright and Mason, 1999; Lee and Cundy, 2001;Zhang et al., 2001; Feng et al., 2002). The unique dilutedsetting lessens the concentrations of the industrialcontaminants that are expelled from the many coastalpollution sources, primarily located around Shanghai.From the previous studies, a high concentration ofheavy metals would be expected in the subaqueousYangtze delta sediment, taking into account the largeamount of industrial inputs into the estuary (cf. Linet al., 2002). Contaminants that have adhered to thefine-grained particles delivered from the Yangtze coast,except for those that accumulate in the study area, arefurther dispersed to the East China Sea, where theyincrease the hazardous likelihood of marine resources(Lin et al., 2002).
The incremental trend of heavy metals in thevibrocore sediment profile after normalization, asrepresented by that observed in core Y7, reflectsa subaqueous environment that has become increasinglypolluted with the rapid development of industry on thecoast over the past 50 years (Fig. 3b). The many sewageoutlets located along the Yangtze coast expel millions oftonnes of untreated industrial and domestic sewagewater into the estuary daily. As a result, economic andsocial issues have emerged with respect to fisheries, thequality of reclamation, and diseases diffused through thefood chain in the Yangtze estuary (Shanghai DrainageAdministration Division, 1985). It is worth mentioningthat the present problem of raw sewage discharge isbeing largely addressed by the establishment of a sys-tematic network of pipelines throughout the Shanghairegion that are linked to many effective sewage-watertreatment stations.
The radioisotope analysis of core Y7 indicates anaverage sedimentation rate of 6.0 cm/a. Using a linearextrapolation, this verifies that the vibrocore horizon at
3.0 m depth was deposited in 1950. By examining thesedimentation rates ranging fromw2.0 to 6.0 cm/a of thevibrocores (Y5, Y6, Y7) in the study area (Fig. 4), it isunderstood that the sediments of vibrocores 2.0e3.0 mlong can be dated back to at least 50 years ago.Therefore, vibrocores of the present study containa record of heavy metal concentrations associated withindustrial development in the area, which peaked after1950 (Shanghai Drainage Administration Division,1985). The sedimentation rate of Y4 was not determin-able, probably due to higher sedimentation and relativecoarser sediment in zone A (delta front facies; Chenet al., 2000)dconditions that usually cannot preservea sufficient radioisotope content. Also, vibrocore Y9located on the subaqueous delta margin did not result ina good record of radioisotope activity over the past 50years, primarily due to extremely low sedimentationrate.
5. Summary
The Yangtze estuary is a huge sedimentary depo-center. The numerous heavy metals discharged along theYangtze coast, primarily from the Shanghai Metropol-itan area, have been carried into the estuary byadsorption onto fine-grained suspended sediments,resulting in serious pollution of the densely populatedcoastal environment and ecosystem. However, the heavymetals examined in the present study are present inlower concentrations in the Yangtze subaqueous deltathan along the coast, and in some other estuaries aroundthe world. This unusual distribution is explained by thediluted setting of the Yangtze estuary, into which morethan 900 billion cubic meters of freshwater andassociated suspended sediment from the Yangtze Riverare discharged annually as freshwater plumes beingdriven away further seaward. Heavy metals carried bythe ‘unpolluted’ fluvial suspended sediment are the maincourse of dilution.
Acknowledgements
Appreciation should be given to three anonymousreviewers, who largely shaped the manuscript andupgraded the scientific value. The authors are alsoindebted to the No. 1 Marine Geological Survey,Shanghai, for the coring. The China National NaturalScience Foundation (40206005), Asia-Pacific Networkfor Global Change Research and Global Change Systemfor Analysis, Research and Training (2003-12), theShanghai Priority Academic Discipline, and GlobalEnvironment Research, Ministry of the Environmentof Japan supported this research project.
100 Z. Chen et al. / Estuarine, Coastal and Shelf Science 60 (2004) 91e100
References
Chan, L.S., Ng, S.L., Davis, A.M., Yim, W.W.S., Yeung, C.H., 2001.
Magnetic properties and heavy-metal contents of contaminated
seabed sediments of Penny’s Bay, Hong Kong. Marine Pollution
Bulletin 42, 569e583.
Chen, J.Y., Shen, H.T., Yu, C.X., 1988. Processes of Dynamics and
Geomorphology of the Changjiang Estuary. Shanghai Scientific
and Technological Publisher, Shanghai, 454 pp. (in Chinese, with
English summary).
Chen, Z.T., Xu, L., Hong, H.S., 1997. Heavy metals at the
sedimentewater interface of estuary: a review. Advances in Earth
Sciences 12 (5), 434e439 (in Chinese, with English summary).
Chen, Z., Song, B.P., Wang, Z.H., Cai, Y.L., 2000. Late Quaternary
evolution of the subaqueous Yangtze delta: stratigraphy, sedimen-
tation, palynology and deformation. Marine Geology 162,
423e441.
Chen, Z., Kostaschuk, R.M., Yang, M., 2001. Cases and solutions:
heavy metals on tidal flats in the Yangtze Estuary, China.
Environmental Geology 40 (6), 742e749.
Feng, H., Cochran, J.K., Hirschberg, J.D., 1999. 234Th and 7Be as
tracers for the sources of particles to the turbidity maximum of the
Hudson River estuary. Estuarine, Coastal and Shelf Science 49,
629e645.
Feng, H., Cochran, K., Hirschberg, J.D., 2002. Transport and source
of metal contaminants over the course of tidal cycle in the turbidity
maximum zone of the Hudson river Estuary. Water Research 36
(3), 733e743.
Gerritse, R.G., Wallbrink, P.J., Murray, A.S., 1998. Accumulation of
phosphorus and heavy metals in the SwaneCanning Estuary,
Western Australia. Estuarine, Coastal and Shelf Science 47,
165e179.
Gijs, D.J., Nicolas, B., Filip, M.G.T., Marv, G.V., Frederik, K.H.,
2002. Heavy metal contents (Cd, Cu, Zn) in spiders (Pirata
piraticus) living in intertidal sediments of the river Scheldt estuary
(Belgium) as affected by substrate characteristics. The Science of
the Total Environment 289, 71e81.
Kanai, Y., 1993. Well-type Ge detector for measuring small amount of
environmental samples. Radioisotopes 42, 169e172.
Katsuragi, Y., 1983. A study of 90Sr fallout in Japan. Papers in
Meteorology and Geophysics 33, 277e291.
Katsuragi, Y., Aoyama, M., 1986. Seasonal variation of Sr-90 fallout
in Japan through the end of 1983. Papers in Meteorology and
Geophysics 37, 15e36.
Lee, S.V., Cundy, A.B., 2001. Heavy metal contamination and mixing
processes in sediments from the Humber Estuary, Eastern England.
Estuarine, Coastal and Shelf Science 53, 619e636.Lin, S., Hsieh, I-Jy., Huang, C.H., 2002. Influence of the Yangtze
River and grain size of the spatial variation of heavy metals and
organic carbon in the East China Sea continental shelf sediment.
Chemical Geology 182, 377e394.Liu, L., Xu, S.Y., Chen, Z.L., Yu, J., 2000. Spatial distribution of
heavy metals in the sediments of Shanghai tidal flat and
environmental assessment. Shanghai Geology 1, 1e3 (in Chinese,
with English summary).
Lu, J.J., Tang, Y.W., 1998. Distribution and migration of heavy metals
on east tidal flat ecology system in Chongming Island. In: Lu, J.
(Ed.), Wetland Conservation and Research, China. East China
Normal University Press, Shanghai, pp. 259e272 (in Chinese, with
English summary).
Peirson, D.H., 1971. Worldwide deposition of long-lived fission
products from nuclear explosions. Nature 234, 79e80.
Palanques, A., Sanchez-Cabeza, J.A., Masque, P., Leon, L., 1998.
Historical record of heavy metals in a highly contaminated
Mediterranean deposit: the Besos Prodelta. Marine Chemistry 61,
209e217.
Robbins, J.A., Edgington, D.N., 1975. Determination of recent
sedimentation rates in Lake Michigan using Pb-210 and Cs-137.
Geochimica et Cosmochimica Acta 39, 285e304.
Ruiz, F., 2001. Trace metals in estuarine sediments from the south-
western Spanish coast. Marine Pollution Bulletin 42, 482e490.
Santschi, P.H., Presley, B.J., Garcia-Romero, T.L.B., Baskaran, M.,
2001. Historical contamination of PAHs, PCBs, DDTs, and heavy
metals in Mississippi River Delta, Galveston Bay and Tampa Bay
sediment cores. Marine Environmental Research 52, 51e79.Shanghai Drainage Administration Division, 1985. The Setting of
Shanghai Sewage Water Discharge. East China Normal University
Press, Shanghai, 334 pp. (in Chinese).
Shen, H.T., Pan, D.A., 2001. Material Flux of the Changjiang Estuary.
China Ocean Press, Beijing, 76 pp. (in Chinese).
Su, H.J., Lu, W.C., Chen, X.H., Wu, L., Chen, B.L., 1988. Study on
chemical morphology of heavy metals (Al, Ge, Cu, Zn) in the
surface sediments of the Yangtze estuary. Marine Science Bulletin
7, 22e30 (in Chinese, with English summary).
Tam, N.F.Y., Wong, Y.S., 2000. Spatial variation of heavy metals in
surface sediments of Hong Kong mangrove swamps. Environmen-
tal Pollution 110 (2), 195e205.
Vallius, H., 1999. Anthropogenically derived heavy metals in recent
sediments of the Gulf of Finland, Baltic Sea. Chemosphere 38,
945e962.Wang, Z.H., Chen, Z., Li, L.Q., Wei, T.Y., 2001. Temporal and spatial
distribution of trace elements in Yangtze estuary, China: signifi-
cance of diluted setting. Chinese Science Bulletin 46 (Suppl.),
65e73.Warwick, R.M., 2001. Evidence for the effects of metal contamination
on the intertidal macrobenthic assemblages of the Fal estuary.
Marine Pollution Bulletin 42, 145e148.
Windom, H.L., Sipipot, S., Chanporgsang, A., Smith, R.G., Hung-
sprengs, M., 1983. Trace metal composition and accumulation
rates of sediments in the upper Gulf of Thailand. Estuarine,
Coastal and Shelf Science 19, 133e142.Wright, P., Mason, C.F., 1999. Spatial and seasonal variation in heavy
metal in the sediments and biota of two adjacent estuaries, the
Orwell and the Stour, in the eastern England. The Science of the
Total Environment 226, 139e156.Xu, K.S., Huang, S.L., Wu, L.Q., 1982. Heavy metals distribution in
the sediment of Changjiang estuary, its relationship to environ-
ment. Acta Oceanologica Sinica 4, 440e448 (in Chinese, with
English summary).
Xu, S.Y., Tao, J., Chen, Z.L., Chen, Z., Lu, Q.R., 1997. Dynamic
accumulation of heavy metals in tidal flat sediments of Shanghai.
Oceanologia Limnologia Sinica 28 (5), 509e515 (in Chinese, with
English summary).
Zhang, J., Huang, W.W., Martin, J.M., 1988. Trace metals distribu-
tion in Huanghe estuarine sediments. Estuarine, Coastal and Shelf
Science 26, 499e516.Zhang, W., Yu, L., Hutchinson, S.M., Xu, S., Chen, Z., Gao, X., 2001.
China’s Yangtze Estuary: I. Geomorphic influence on heavy metal
accumulation in intertidal sediments. Geomorphology 41 (2e3)
248 pp.
