Science of the Total Environment 332 (2004) 167 – 182
www.elsevier.com/locate/scitotenv
Concentration and fate of trace metals in Mekong River Delta
R.M. Cenci *, J.-M. Martin
Institute for Environment and Sustainability, Joint Research Centre, via Fermi, 21020 Ispra (VA), Italy
Received 25 August 2003; accepted 13 January 2004
Abstract
In the Mekong River delta and its associated coastal zone trace elements concentrations (Cd, Cu, Ni and Pb) were
measured in the dissolved phase (DP) during dry (March 1997) and wet (October 1997) seasons. As, Co, Cr, Ni, Pb, and Al
were also measured in suspended matter (SM) and total and organic carbon, trace elements (Cd, Cr, Cu, Hg, Mn, Ni, Pb, Zn)
and macro elements in superficial sediments (S). Trace metal concentrations in DP and SM during the contrasting
hydrological conditions were generally found within the range observed for uncontaminated environment. The average DP
concentrations (nM) in the river for March and October are: Cd 0.03 and 0.09, Cu 15 and 14, Ni 7.8 and 8.4, Pb 0.51 and
0.50, respectively. In general there is no significant difference between the concentrations observed during dry and wet
season. The evolution of the DP trace metal concentration in the surface water within the salinity gradient suggests no
noticeable exchange between the particulate and dissolved phase. This result is in good agreement with those observed in
most plume structures studied so far. The average concentrations in the SM (Ag/g) (March, October) at the river end-member
are: As (24; 11), Co (17; 9), Cr (49; 29), Ni (32; 18), Pb (42; 19) and Al (113 000; 67 000), respectively. All trace elements
show higher concentrations in March than in October, with an average increase of two times. This is essentially related to
grain size effect since smaller particles were supplied during dry season. These differences are not reflected in the mixing
zone, which integrates the seasonal variations. The concentration of major elements (C total, C organic, Si, Al, Ca, K, Fe, Mg,
Ti), trace elements (Pb, Zn, Cu, Ni, Mn, Cr, Cd, Hg) in superficial sediments, show similar values during the two seasons and
does not show any important variation with depth, indicating either a very fast sedimentation rate and/or the absence of any
significant contamination.
D 2004 Elsevier B.V. All rights reserved.
Keywords: Trace metals; Dissolved phase; Suspended matter; Sediments; Estuary; Mekong
1. Introduction
The Mekong River is the longest river in Southeast Asia with 4800 km and flows through China,
Thailand, Laos, Cambodia and Vietnam. Water discharge averages are 14 000 m3 s 1 and solid discharge 160 millions tons per year. The drainage basin
* Corresponding author.
0048-9697/$ - see front matter D 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.scitotenv.2004.01.018
of the Mekong River covers 795 000 km2 and the
delta area 39 000 km2 which belongs to the Vietnamese territory.
The Mekong Delta begins in Phnom Penh and
ends at the Vietnam coast from Vung Tau to Cape
Camau. It comprises two main arteries, which are
sub-divided into eight smaller branches. The main
arteries are the Bassac River (two branches with a
discharge of 43.9% in the rainy season in September
and 49% in the dry season March) and the Tien
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R.M. Cenci, J.-M. Martin / Science of the Total Environment 332 (2004) 167–182
Fig. 1. Sampling stations in the Mekong Delta and its associated Coastal Zone (from Vung Tau to Cape Camau), March and October
1997.
River (six branches with a discharge of 56.1% in the
rainy season and 51% in the dry season) (Hungspreugs
et al., 1998).
The climate in the Mekong Delta is regulated by
two seasons: the dry season lasts from November to
March, when winds blow mainly from NE, while the
wet season begins in May and ends in September.
During the wet season approximately 85% of the
annual precipitation occurs and the major wind blows
from SW.
Stanfield and Garrett (1997) stated that a large part
of the Mekong river water discharge is essential to
balance the fresh water budget of the Gulf of Thailand. The river discharge to the sea creates an extensive plume reaching 100 000 km2.
Tidal time-series measurements were performed at
some stations located at the river mouth. During the
sampling period, the amplitude of the semi-diurnal
tidal wave measured at Vung Tau (approx. 17 km of
Station 4) was approximately 2.5 m. The salinity
Table 1
Measurements of dissolved trace metals in reference materials (National Council of Canada and Reference Material)
CASS-2 recommended values
This study
SLRS-3 recommended values
This study
Cd
pM
Cu
nM
Ni
nM
Pb
pM
169 F 35
168 F 24
115 F 18
103 F 16
10.6 F 0.6
11.1 F 0.6
21.2 F 1.1
22.1 F 2.1
5.07 F 0.6
5.02 F 0.7
14.1 F 1.3
13.8 F 1.8
92 F 29
81 F 17
328 F 33
341 F 40
R.M. Cenci, J.-M. Martin / Science of the Total Environment 332 (2004) 167–182
169
Table 2
Measurements of particulate trace metals in reference materials
MESS-1
recommended values
This study
Al (%)
As (Ag/g)
Co (Ag/g)
Cr (Ag/g)
Ni (Ag/g)
Pb (Ag/g)
5.2 F 0.2
10.6 F 1.2
10.8 F 1.9
71 F 11
29.5 F 2.7
34 F 6.1
4.6 F 0.5
9.5 F 1.5
10.1 F 2.2
80 F 10
30.4 F 6.1
40 F 7
intrusion in the Mekong Delta area has important
economical and environmental implications. During
the dry season, salinity intrusion reaches up to the
middle part of the delta and prevents its irrigation. A
large area, approximately 2.1 million ha of the
Mekong Delta zone, is affected by saline intrusion
from 1 to 6 months (Nguyen, 1998) of the year.
The Mekong Delta is a major agricultural region
of Vietnam, in fact the large amount of chemical
fertilisers which are used represents a diffuse
source of contaminants and nutrients to the Delta.
Fertiliser runoff has the potential to increase the
occurrence of algal blooms and to promote the
growth of aquatic vegetation. In addition, the rivers
and the dense channel network represent important
waterways for fluvial transport. In many locations
with high population density, human waste causes
water contamination. In addition some industrial
activity introduces organic matter, trace metals
and organic pollutants to the Delta that may affect
water quality.
The aim of this study was to evaluate the level and
fate of trace metals (TM) in dissolved phase (DP), and
suspended matter (SM) and superficial sediments (S)
in the river Mekong, its delta and the adjacent coastal
zone.
2. Materials and methods
2.1. Sampling, pre-treatment and analysis
Samples were taken according to salinity at the
various stations shown in Fig. 1. The water salinity
was measured in-situ using a SeaBird conductivitytemperature-depth (CTD) profiler Seacat 19. During
the dry season water samples were collected (from 8th
to 14th March 1997) at 16 Stations. The sampling
took place both in the river, its delta and the adjacent
coastal zone. Samples of 2 –7 l of water were taken
manually, with a long 5 m bamboo cane, at a depth of
approximately 30 cm below surface using ‘precleaned’ polyethylene bottles.
From 4th to 14th October 1997, samples were
collected at 32 stations using a Teflon pump
connected to a 10 m long Fluor-polymer tube of high
purity. The samples were taken at a depth of at least 1
m below the surface. Considering the importance of
Fig. 2. Suspended Matter (mg/l) vs. salinity in March and October 1997.
170
R.M. Cenci, J.-M. Martin / Science of the Total Environment 332 (2004) 167–182
Table 3
Salinity and trace metal concentrations (nM) in the dissolved phase
(March 1997)
Station
Salinity
Cd
Cu
Ni
Pb
16
1
2
15
14
11
10
12
13
6
7
4
5
8
9
0.02
0.03
0.24
0.9
10.4
28.8
29.4
30.9
31.9
32.4
32.8
33.5
33.5
33.5
33.6
0.03
15.1
14.8
15.1
14.8
12.8
8.6
8.5
5.9
5.8
6.4
5.5
2.5
4.4
3.1
2.9
7.2
8.0
8.5
7.7
0.36
0.67
0.03
0.09
0.13
0.07
0.03
0.04
0.21
0.04
0.01
0.01
3.9
3.6
3.4
4.8
4.3
3.7
3.7
2.6
2.3
0.51
0.48
0.30
0.24
0.13
0.3
0.3
discharge, no significant turbidity and salinity differences were observed between 0.3 and 1 m. Water
samples were filtered immediately after collection
under a laminar air flow clean bench, using acid
cleaned Millipore 0.45 Am filters and a hand-vacuum
pumping system. Filtrates were transferred into acidcleaned polyethylene bottles and immediately acidified to pH 2 with HNO3 (Merck Suprapur).
The bottles with the filtered and acidified water
were protected by pre-cleaned double plastic bags and
stored in an ice-box until analysis. The filters were
kept in pre-cleaned Petri boxes and used for the
analysis of particulate trace elements.
All analyses were performed in a clean laboratory
(Class-100). For water samples with salinity higher
than 0.5, dissolved TM were extracted using 8hydroxyquinoline (Sturgeon et al., 1981), and the
filtrates were analysed by inductively coupled plasma-mass spectrometry (ICP-MS) and flame-less
graphite furnace atomic absorption spectroscopy
(GFASS) (Boyle and Edmond, 1977). For the water
samples with salinity lower than 0.5, the analyses
were made directly with the same instrumentation.
Salinity was measured in aliquot samples by conductivity using a WTW LF91 conductimeter.
All blank controls showed insignificant levels as
compared to sample concentration. The accuracy,
precision and reproducibility have been tested using
the SLRS-3 and CASS-2 dissolved trace element
standards (riverine water and seawater reference ma-
terial supplied by National Research Council of
Canada).
The suspended matter filters were dried in a
ventilated oven at 45 jC for 2 days. All the elements,
except As, were measured by AAS after dissolution in
HNO3 – HF – HCLO4 (Merck Suprapur) using a microwave oven. As concentrations were measured by
ICP – MS. To test the accuracy of the analysis the
standard MESS-1 from the National Research Council
of Canada with a matrix comparable to that of the
samples was used. Results are shown in Tables 1 and
2. For dissolved and particulate elements there is a
fairly good agreement between reference values and
the concentrations found in this study.
Forty-five sediment samples were collected using a
grab, while two of them were sampled using a gravity
corer during the March survey at Stations 8 and 12.
Table 4
Salinity and trace metal concentrations (nM) in the dissolved phase
(October 1997)
Station
Salinity
Cd
Cu
Ni
Pb
15
16
17
18
19
20
24
26
27
25
23
28
30
4
21
14
12
11
29
8
9
31
6
22
7
5
13
2
3
1
0.05
0.05
0.05
0.05
0.05
1.8
1.8
4.1
6.4
8.8
10.2
13
14.7
15
15
16.4
28.1
18.2
18.2
18.9
18.6
22.7
23.1
25.8
25.8
25.9
28
29.8
29.8
31.5
0.02
0.08
0.21
0.03
0.10
0.10
0.13
0.08
0.04
0.08
0.12
0.06
0.09
0.04
0.07
0.45
0.09
0.19
0.08
0.12
0.12
0.05
0.07
0.16
0.11
0.21
0.09
0.13
0.09
0.23
11.7
23.8
10.6
5.3
15
7.3
7
7.3
8.8
7.7
7
12.1
6.1
9
6.1
7
13.9
7.8
3.9
4.1
5.1
6.6
7.7
7.4
6.5
4.1
4.6
5.1
8.2
2.9
8.7
6.6
4.6
0.51
0.67
0.79
0.37
0.32
0.18
0.19
0.48
0.09
0.08
0.18
0.11
0.11
0.48
0.06
0.13
0.30
0.23
0.10
0.09
0.07
0.13
0.28
0.13
0.24
0.33
0.19
0.30
0.13
0.52
10.7
10.6
10.2
9.8
9.9
10
8.9
7.4
6.8
10.9
9.1
7.3
5.95
8.9
7.6
5.8
7.3
4.9
3.1
3.0
4.4
6.7
5.4
6
4.9
15.1
R.M. Cenci, J.-M. Martin / Science of the Total Environment 332 (2004) 167–182
171
Fig. 3. Trace metal concentrations in the dissolved phase vs. salinity results obtained in March 1997.
The core lengths were 58 cm and 70 cm, respectively. They were sliced each centimeter using a
nylon yarn. The sediment samples were dried at 45
jC to prevent element losses by volatilisation,
ground and homogenised in an automatic agate
mortar.
The analysis of trace and major elements was
performed by X-ray fluorescence (XRF) and AAS
Fig. 4. Trace metal concentrations in the dissolved phase vs. salinity (October 1997).
172
R.M. Cenci, J.-M. Martin / Science of the Total Environment 332 (2004) 167–182
Table 5
Comparison of trace element concentrations (nM) in the dissolved
phase in the Mekong river and in some world rivers
Table 7
Trace element concentrations (Ag/g) in SM (October 1997)
Station
Salinity
As
Co
Cr
Ni
Pb
Al
River
Cd
Cu
Mekong (Mar)
Mekong (Oct)
Changjiang (1)
Huanghe (2)
Pearl (3)
Yenissei (4)
Lena (4)
Ob (4),(5)
Seine (6)
Rhone (7)
Scheldt (8)
Amazon (9)
Mississippi (10)
Orinoco (11)
Danube (12, 2)
World aver. (13)
0.03
0.09
0.035
0.03
0.94
0.011
0.03 – 0.07
0.006
0.132
0.27
0.11
0.06
0.12
0.04
0.12
0.09
15
14
27
12
17.8
29.5
9.4
29.1
48
35
3.3
24
23
18.9
36
23
15
16
17
19
20
24
26
27
25
23
28
30
4
21
14
29
11
9
8
31
6
7
22
5
12
13
2
1
0.05
0.05
0.05
0.05
1.8
1.8
4.1
6.4
8.8
10.2
13
14.7
15
15
16.4
18.2
18.2
18.6
18.9
22.7
23.1
25.8
25.8
25.9
28.1
28
29.8
31.5
18
19
35
30
25
29
27
26
27
9
12
7
16
8
16
9
22
10
11
2
35
5
4
7
2
6
4
3
14
14
24
24
17
16
16
15
15
5
7
4
22
5
11
4
15
6
6
1
24
2
2
4
1
2.4
2
2
80
88
158
115
91
105
113
114
112
35
51
42
104
34
76
33
98
35
37
9
134
16
14
24
27
20
15
11
37
41
77
61
49
60
61
69
62
24
30
24
63
22
35
24
50
23
28
8
76
18
9
18
7
12
12
8
33
34
60
56
42
50
44
46
47
19
28
19
31
16
37
18
31
23
20
8
59
9
18
14
4
10
7
5
75 400
120 000
145 000
118 000
97 000
101 000
116 000
147 000
97 300
33 000
50 500
34 900
102 000
27 800
64 300
33 200
79 600
20 800
27 700
9300
129 000
11 200
10 300
21 200
16 000
13 000
11 000
8200
Ni
Pb
7.8
8.4
2.5
0.51
0.50
0.25
0.18
0.24
0.03
0.08 – 0.35
0.08
0.47
0.42
1
20.3
9.4
0.08
22.9
11.8
27
1.1
5
23
3.4
15
8.5
0.48
0.08
0.15
(1) Elbaz-Poulichet et al. (1987, 1988), Edmond et al. (1985) and
Shiller and Boyle (1991); (2) Huang et al. (1988) and ElbazPoulichet (1988); (3) Trincherini (unpublished) (4) Watras et al.
(1995); (5) Dai and Martin (1995); (6) Chiffoleau et al. (1994);
(7) Elbaz-Poulichet et al. (1996); (8) Zwolsman and van Eck
(1999); (9) Boyle et al. (1982); (10) Trefry et al. (1986); (11)
Eisma et al. (1978); (12) Guieu et al. (1998); (13) Martin and
Windom (1991).
after dissolution in HNO3 – HF – HClO4 of sediment
samples using a microwave oven (Loring and Rantala,
1990). Carbon concentrations were estimated by CHN
elemental analyser. The total Hg concentrations were
Table 6
Trace element concentrations (Ag/g) in SM (March 1997)
Station
Salinity
As
Co
Cr
Ni
Pb
16
1
2
15
14
11
10
12
13
6
7
4
5
8
9
0.02
0.03
0.24
0.9
10.4
28.8
29.4
30.9
31.9
32.4
32.8
33.5
33.5
33.5
33.6
5
10
6
12
15
5
7
7
11
13
16
31
10
25
37
36
10
41
23
5
9
21
20
35
40
36
40
26
45
111
126
52
134
103
58
9
13
11
22
23
35
41
28
63
92
107
30
123
65
14
10
16
16
20
25
21
48
15
49
59
58
36
84
43
17
30
21
49
54
57
17
73
64
17
Al
19 400
73 000
59 000
80 000
56 000
89 200
25 300
49 000
135 700
113 300
26 900
125 400
71 700
21 800
determined in solid samples with an atomic absorption
spectrophotometer (AMA 254) (Cossa et al., 2002).
The relative standard deviations based on 10 replicates of the European reference material CRM 280 (a
Table 8
Average concentrations of SM and Al in the suspended matter
(SM)
As
Co
Cr
Ni
Pb
Al
River
March
*Elem/Al
October
*Elem/Al
24
2.1
11
1.6
17
1.5
9
1.3
49
4.3
29
4.3
32
2.8
18
2.6
42
3.7
19
2.8
113 000
Delta
March
*Elem/Al
October
*Elem/Al
36
0.5
14
0.3
20
0.3
8.8
0.2
64
1
57
1.1
52
0.8
34
1.1
38
0.6
26
0.6
64 000
67 000
53 000
0.5
*Element concentration (Ag/g) 104/Al concentration (Ag/g).
R.M. Cenci, J.-M. Martin / Science of the Total Environment 332 (2004) 167–182
Fig. 5. Trace metal concentrations in suspended matter vs. Al content in March 1997.
173
174
R.M. Cenci, J.-M. Martin / Science of the Total Environment 332 (2004) 167–182
Fig. 6. Trace metal concentrations in suspended matter vs. Al content in October 1997.
R.M. Cenci, J.-M. Martin / Science of the Total Environment 332 (2004) 167–182
175
Table 9
Comparison of the trace element average concentrations of SM in the Mekong river and in the major world rivers (Ag/g)
River
As
Co
Mekong (March)
Mekong (Oct)
Changjiang (1, 2)
Huanghe (2)
Pearl (3)
Lena (4)
Yenisey (5)
Ob (5)
Mackenzie (6)
Rhone (7)
Garonne (6, 8)
Dordogne (8)
Scheldt (9)
Amazon (6, 10)
Mississippi (11)
Seine (12)
Danube (6, 13)
Orinoco (14)
World aver. (15)
23.8
11
7.6
11
37.6
17
9
8.1
5.3
14.6
7.4
5
Cr
49
29
121.3
40
14
14
8.5
39
255
9.5 – 16.1
41
21
110 – 285
193
72
25.6
10
20
100
70
Ni
Pb
Al
32
18
58.8
40
59.7
31
29
35
22
51.3
55.2
44
35-62
105
50.5
21
106
34
89
42
19
50
17
95.1
23
6.8
23
24
43
47.6
35.2
63 – 207
21 – 105
39
110
84
23
35
113 000
67 000
108 000
84 000
107 000
74 000
78 000
69 000
118 000
77 300
88 000
108 000
60 100
113 000
94 000
(1) Huang and Zhang (1990).
(2) Huang et al. (1992).
(3) Trincherini (personal communication).
(4) Martin et al. (1994).
(5) Dai and Martin (1995) and Konovalov and Ivanova (1970).
(6) Martin and Meybeck (1979).
(7) Elbaz-Poulichet et al. (1996).
(8) Kraepiel et al. (1997).
(9) Zwolsman and van Eck (1999).
(10) Gibbs (1977) and Irion (1976).
(11) Presley et al. (1980), Trefry et al. (1986) and Trefry and Presley (1976).
(12) Chiffoleau et al. (1994).
(13) Guieu et al. (1998).
(14) Eisma et al. (1978).
(15) Martin and Windom (1991).
lake sediment with a chemical composition similar to
those measured in the Mekong delta) were 1.5% for
Al, K, and Hg, 1.2% for Cr, 1.9% for Cu, 0.4% for Fe,
Mn, Mg and Ti 2.5% for Ni, 2.7% for Pb and 3.6% for
Cd and Zn.
3. Results and discussion
3.1. General parameters
At the upstream station at Can Tho there was
entirely river water in both seasons, while the river
mouth stations had a salinity of approximately 10 in
October and a salinity of approximately 16 in March.
In the river water the pH was 6.9 in October and 7.8 in
March and offshore 8.1 and 8.2. The temperature was
higher in October than in March (29.3 and 28.4 jC in
the river water and 29.1 and 26 jC in the open sea)
(Landman et al., 1998).
In March the salinity intrusion was observed more
than 30 km upstream, in October fresh water was
found throughout the whole river. At the offshore
stations in March salinity was 32.5 and 33.7 throughout the water column with a difference of less than 1
between surface and bottom water as observed at
station 23 (Landman et al., 1998). In October salinity
varied between 6.8 and 32.3.
176
R.M. Cenci, J.-M. Martin / Science of the Total Environment 332 (2004) 167–182
Fig. 7. Trace element concentrations (Ag/g) vs. SM (mg/l) (March 1997).
Fig. 8. Trace element concentrations (Ag/g) vs. SM (mg/l) (October 1997).
Fig. 9. Kd vs. salinity (March 1997).
R.M. Cenci, J.-M. Martin / Science of the Total Environment 332 (2004) 167–182
177
Fig. 10. Kd vs. salinity (October 1997).
In the river and at the coastal station near the river
mouth, the SM concentrations exhibited variations
related to changes in river discharge. Maximum SM
values up to 1.25 g/l were found in March in the area
around the river mouth (Stations 15 and 14). SM
concentration decreased with increasing salinity (Fig.
2), which is more pronounced during dry season.
There is no turbidity maximum as observed in macrotidal estuaries.
3.2. Trace metal in the dissolved phase
Dissolved TM concentrations are given in Table 3
(March 1997) and Table 4 (October 1997).
The comparison of the Mekong river TM concentrations with major world rivers does not show any
significant difference (Table 5). The Mekong is in the
range of ‘unpolluted’ river water. There is no systematic difference between dry and wet season samples,
Table 10
Average concentration of organic and total carbon, major elements and trace elements in superficial sediments and 2 cores sediments in the
Mekong river and coastal zone
%
*March
*October
Ctot
%
*March
*October
Core no 8
Core no 12
Si
mg kg 1
*March
*October
Core no 8
Core no 12
**Uncontaminated coastal zone
***Max. conc. Acceptable
****Non polluted
****Moderately Polluted
Pb
37
35
38
44
6 – 30
450
< 40
40 – 60
1
1.34
Corg
0.43
0.55
Al
29
27
25
27
Ca
6.9
7.2
K
2
2.8
4.8
1.1
Zn
144
138
139
166
Cu
47
53
41
50
410
< 90
90 – 200
390
< 25
25 – 50
*Superficial sediment.
**Bowen, 1979.
***Volterra and Maffiotti, 1997.****Giesy and Hoke, 1990.
Fe
1.4
1.2
1.4
1.8
Ni
31
35
25
37
< 20
20 – 50
Mg
3.4
2.9
2.7
4
Mn
662
815
591
855
< 300
300 – 500
1.2
1.2
1.5
1.2
Cr
98
88
80
75
260
< 25
25 – 75
Ti
0.35
0.27
0.3
0.6
Cd
0.4
0.5
0.3
0.4
0.1 – 0.6
5.1
Hg
0.034
0.028
0.022
0.064
0.03 – 0.04
0.41
< 0.1
178
R.M. Cenci, J.-M. Martin / Science of the Total Environment 332 (2004) 167–182
Fig. 11. Concentration profile of total Hg, total C, Cu and Pb in the sediment core of station 12.
which indicates that the ‘dilution’ effect observed in
some polluted rivers in Europe (Elbaz-Poulichet et al.,
1996) is not applicable in this area. The similar values
during the two seasons confirm the absence of signif-
icant sources of contamination in this part of the
Mekong.
Figs. 3 and 4 show the dissolved TM concentrations vs. salinity in March and October in the Mekong
R.M. Cenci, J.-M. Martin / Science of the Total Environment 332 (2004) 167–182
Delta. When river water mixes with seawater, various
physico– chemical processes occur which can induce
some solid – liquid interactions between the dissolved
and particulate phases modifying the elemental riverine flux to the sea. However, trace metals can exhibit
contrasting behaviours in different categories of
estuaries.
3.2.1. Cadmium
Comans and Van Dijk (1988) have shown that Cd
desorption should be expected in estuaries due to
chloride and sulfate complexation, and ionic strength
effects. Cadmium mobilisation is well documented in
estuaries: in the Amazon plume (Boyle et al., 1982),
in the Changjiang (Edmond et al., 1985; Elbaz-Poulichet et al., 1987), in the Gironde and Huanghe
estuaries (Elbaz-Poulichet et al., 1987; Kraepiel et
al., 1997), in the Mississippi estuary (Shiller and
Boyle, 1991) and in the lower part of the Scheldt
estuary (Duinker et al., 1982; Salomons and Kerdijk,
1986). In the Mekong delta the results show that the
river and seawater concentrations are not significantly
different for the considered elements. Cadmium
behaves in a conservative way and does not seem to
be affected by estuarine mixing (March 1997) although the results obtained in October, unfortunately
based upon a single sample, may indicate some
mobilisation.
3.2.2. Copper
Copper can exhibit contrasting behaviour. It is
conservative in the Amazon (Boyle et al., 1982), the
Mississippi (Shiller and Boyle, 1991), and the Changjiang (Edmond et al., 1985) surface plumes, but a nonconservative behaviour has been observed in small
estuaries (Windom et al., 1983). In the Mekong Delta,
Cu distribution is conservative within the whole mixing zone and in both seasons.
3.2.3. Nickel
Nickel is also conservative in the Amazon plume
(Boyle et al., 1982), but mobilized from particles at
low salinity in the Changjiang (Edmond et al., 1985).
However, scavenging by resuspended solids or flocculating hydrated iron oxides has been suggested to be
responsible for its removal in the low chlorinity zone
of the Thaı̈ rivers (Windom et al., 1988). Like Cu, Ni
179
behaves in a conservative way in the Mekong Delta
for both seasons.
3.2.4. Lead
The estuarine behaviour of this element is not well
documented in the literature, but it is generally assumed to present an overall conservative behaviour
(Martin and Windom, 1991) although some removal
is found in the Gironde estuary (Elbaz-Poulichet et al.,
1984), in the Gota river estuary (Danielsson et al.,
1983), in the Savannah river estuary (Windom et al.,
1985), and in the Bang Pakong estuary (Windom et
al., 1988). Despite some scattering in the concentrations, there are no significant deviations from a
conservative distribution in the Mekong Delta.
In summary, the results obtained in this study of
the surface plume, have not shown any consistent
evidence of a non-conservative distribution of the
dissolved SM investigated with a possible exception
for Cd in October 1997. This overall conservative
behaviour in the Mekong Delta is likely to result from
the lack of an important SM accumulation in the
surface waters, following a rapid sinking of the
terrigenous particle which does not allow to see any
adsorption/desorption of TM.
3.3. Trace metals in suspended matter
Tables 6 and 7 show trace element concentration in
SM (March 1997 and October 1997).
The element concentrations in river suspended
matter are different during the two investigation
periods: the concentrations are higher during the dry
season (March) than in the wet season (October).
As shown in Figs. 5 and 6, there is a linear
relationship between TM and Al. This clearly demonstrates that the observed variations are related to a
grain-size effect, smaller particles being supplied
during the low water discharged period. Those small
particles are essentially representing the clay mineral
fraction, which is naturally enriched in aluminium.
Once normalised to Al (i.e. TM) the concentration
became very closed (Table 8).
Average concentrations are in good agreement with
some major world rivers (Table 9). Which confirms
the similarity of the Mekong River with major world
rivers as already observed for the dissolved phase.
180
R.M. Cenci, J.-M. Martin / Science of the Total Environment 332 (2004) 167–182
The only striking feature is the increase of the TM
concentration in coastal waters during the March
period. This could be linked to the increased biological production in this area, as observed in several
European estuaries (Elbaz-Poulichet, 1988)
corresponding to the decreasing turbidity, which was
limiting the primary production within the estuary.
By plotting TM concentration vs. SM concentration (Figs. 7 and 8) there is a clear decrease in March
corresponding to the occurrence of smaller particles
enriched in aluminium silicate and/or to a high
biological productivity (see above for high salinity
samples).
4. Distribution coefficient (Kd)
The low reactivity of TM in the surface plume of
the Mekong Delta can be further assessed using the
distribution coefficient (Kd) which is the ratio of the
concentration of the labile reactive fraction of the
particulate TM (Cpl) in a given mass of particles, to
the dissolved concentration (Cfd) in the same mass of
filtrate: Kd = Cpl/Cfd in a given environment. This Kd
should be constant for a given composition of the
suspended particles and of the solution (if a steadystate equilibrium is achieved). Conversely, any change
of the particle surface reactivity and/or of the solution
properties may result in Kd changes (Figs. 9 and 10).
Since TM labile fractions were not measured in the
Mekong Delta, we computed an the total distribution
coefficient, which is operationally defined as the total
particulate TM concentration (residual fraction, labile + residual) divided by the total dissolved TM concentration. As expected, there are no significant
variation between the river end-member and the
marine part of the sampling area despite some scattering for the Pb distribution coefficient, obviously
linked to the larger uncertainty in the dissolved
concentration analyses.
In March (Fig. 9) Kd reflects the enrichments
observed for total TM concentration in SM with a
significant increase at the marine end-member. In
October (Fig. 10) Ni remains rather concentrated
in the mixing zone, while Pb exhibits rather
scattered concentrations, which are likely related to
a greater uncertainty of the dissolved concentration
measurements.
However, this total distribution coefficient is poorly representative of the exchange processes between
the particulate and the dissolved phases, since the
particulate labile fraction in the plumes which can be
exchange with the dissolves one. In addition the
dissolved concentration incorporates a significant
amount of colloidal material (Martin et al., 1995),
which might physically considered with the particulate fraction concentration.
4.1. Trace metals in sediments
Average concentration for TM, major elements and
organic carbon are given in Table 10.
Coastal sediments of the South China Sea represent
the ultimate sink for Mekong particulates. The different climatic conditions, i.e. dry and wet seasons seem
to have no significant influence on the element
concentration in the sediments. The concentration of
major elements, trace elements and organic carbon
show that sedimentation is quite uniform during the
two seasons. The results of several trace elements
show that industrial activities, either in the watershed
or discharged from the coastal city, have little or no
influence on the concentrations, which remain inferior
or similar to the background values proposed for
uncontaminated sediments (Giesy and Hoke, 1990).
The content of all elements in surface sediments
does not exceed baseline levels typical of pristine
sediments, and their variations can be caused by
natural anomalies of geochemical composition of the
sedimentary material.
The concentrations found in the two cores are
comparable to values of the superficial sediments,
corresponding to a very fast sedimentation rate and/
or to the absence of any significant contamination.
The concentrations of Hg and total C in core n 12
show qualitatively similar trends (Fig. 11), which
could indicate the association of Hg with the organic
fraction (Wallschlaeger et al., 1998; Helland, 2001.
5. Conclusions
This work presents the first data on dissolved,
particulate and sediment trace element concentrations
in the Mekong River and in its mixing zone during
two different seasons.
R.M. Cenci, J.-M. Martin / Science of the Total Environment 332 (2004) 167–182
A comparison with other large world rivers showed
a small or negligible anthropogenic input of TM in the
dissolved and particulate phases. This low anthropogenic contribution is confirmed by element concentrations measured in the sediments. The element
concentrations in the suspended matter are in linear
correlation with aluminium, indicating that SM concentration variations are mainly controlled by the clay
mineral abundance. As a general conclusion, no
important contaminated areas were identified in the
Mekong river and its delta, which in turn does not
significantly influence values in the China Sea. With
regards to the mixing zone, a conservative behaviour
of dissolved TM is observed between river water and
sea water.
Acknowledgements
The authors would like to acknowledge the
contributions made by Vietnamese colleagues during
the two cruises, in particular Prof. Phan Van Ninh. We
also greatly acknowledge Captains and their crews of
the Research Vessels Minh Hai 0102 BTS and HQ
626. The analytical support of Pier Trincherini, Bruno
Paracchini and Pietro Barbero is very much appreciated. The work has been supported by the European
Commission (INCO Program Contract n. 12057-9607 A2SPISP B and the JRC). Finally, many thanks to
Nguyen Kim Dan, for the efficient co-ordination of
the project.
References
Boyle EA, Edmond JM. Determination of Cu, Ni, and Cd in seawater by APDC chelate coprecipitation and flameless atomic absorption spectrophotometry. Anal Chim Acta 1977;91:189 – 97.
Boyle EA, Huested SS, Grand B. The chemical mass-balance of the
Amazon plume; II, copper, nickel ancd cadmium. Deep Sea Res
1982;29:1355 – 64.
Bowen HJM. Environmental chemistry of the elements Academic
Press, 1979.
Chiffoleau JF, Cossa D, Auger D, Truquet I. Trace metal distribution, partition and fluxes in the Seine estuary (France) in low
discharge regime. Marine Chem 1994;47:145 – 58.
Comans RNJ, Van Dijk CPJ. Role of complexation processes in Cd
mobilization during estuarine mixing. Nature 1988;336:151 – 4.
Cossa D, Coquery M, Naklé K, Caisse D. Dosage du mercure total
181
et du monométhylmercure dans les organismes et les sédiments
marins. Editions Ifremer France 2002; p. 29.
Dai M, Martin JM. First data on trace metal level and behaviour in
two major Arctic river-estuarine systems (Ob and Yenissei) and
in the adjacent Kara Sea, Russia. Earth Planet Sci Lett 1995;
131:127 – 41.
Danielsson LG, Magnusson B, Westerlund S, Zhang K. Trace metals in the Göta River estuary. Estuarine Coas Marine Sci 1983;
17:73 – 85.
Duinker JC, Nolting RF, Michel D. Effects of salinity, pH and redox
conditions on the behaviour of Cd, Zn, Ni and Mn in the Scheldt
estuary. Thalassia Jugoslavia 1982;18:191 – 202.
Edmond JM, Spivack A, Grant BC, Hu MH, Chen ZX, Chen S,
Zhong XS. Chemical Dynamics of the Changjiang estuary. Sediment dynamics of the Changjiang estuary and the adjacent East
China Sea. Cont Shelf Res 1985;4(1 – 2):17 – 34.
Eisma D, Van der Gaast SJ, Martin JM, Thomas AJ. Suspended
matter and bottom deposits of the Orinoco Delta: turbidity, mineralogy, and elementary composition. Neth J Sea Res 1978;12:
224 – 51.
Elbaz-Poulichet F, Hollinger P, Huang WW, Martin JM. Lead cycling in estuaries illustrated by the Gironde estuary France.
Nature 1984;308:409 – 14.
Elbaz-Poulichet F, Martin JM, Huang WW, Zhu JX. Dissolved Cd
behaviour in some selected French and Chinese estuaries. Consequences on Cd supply to the ocean. Marine Chem
1987;22:125 – 36.
Elbaz-Poulichet F. Apport fluviatiles de Pb, Cd et Cu à l’océan;
comparison avec l’apport atmosphérique. Thèse de Doctorat-èsSciences, Univ. Pierre et Marie Curie, Paris 1988.
Elbaz-Poulichet F, Huang WW, Martin JM, Zhong XM, Zhu JX.
in: Guo Hui Y. et al., (Eds) Biogeochemical behaviour of dissolved trace elements in the Changjiang estuary. Biogeochemical study of the Changjiang estuary and its adjacent coastal
waters of the East China Sea. Proceedings of the International
Symposium Hangzhou, March 1988 China Ocean Press 1990;
p. 293 – 311.
Elbaz-Poulichet F, Garnier JM, Guan DM, Martin JM, Thomas AJ.
The conservative behaviour of trace metals (Cd, Cu, Ni and Pb)
and As in the surface plume of stratified estuaries: Example
of the Rhone River (France). Estuarine Coas Shelf Sci 1996;
42:289 – 310.
Gibbs R. Transport phases of transition metals in the Amazon and
Yukon rivers. Geol Soc Am Bull 1977;88:829 – 43.
Giesy JP, Hoke RA. Freshwater sediment quality criteria: toxicity
bioassessment. In: Baudo R, Giesy J, Muntau H, editors. Sediments: chemistry and toxicity of in-place pollutants. Chelsea:
Lewis, 1990. p. 265 – 348.
Guieu C, Martin JM, Tankere SPC, Mousty F, Trincherini P,
Bazot M, Dai MH. On trace metal geochemistry in the Danube River and Western Black Sea. Estuarine Coas Shelf Sci
1998;47:471 – 85.
Helland A. The importance of selective transport and sedimentation
in trend monitoring of metals in sediments. An example from
the Glomma Estuary, East Norway. Water Air Soil Pollut 2001;
126:339 – 61.
Huang WW, Martin JM, Seyler P, Zhang J, Zhong XM. Distribution
182
R.M. Cenci, J.-M. Martin / Science of the Total Environment 332 (2004) 167–182
and behaviour of arsenic in the Huanghe (Yellow River) estuary
and Bohai Sea. Marine Chem 1988;25:75 – 91.
Huang WW, Zhang J. Effect of particle size on transition metal
concentration in the Changjiang and the Huanghe, China. Sci
Total Environ 1990;94:187 – 207.
Huang WW, Zhang J, Zhu ZH. Particulate element inventory of
Huanghe (Yellow River): a large, high-turbidity river. Geochim
Cosmochim Acta 1992;56:3669 – 80.
Hungspreugs M., Utoomprurkporn W, Snidvongs A, Ratanachomgkiat S. A comparative study of trace metal contamination
in the Mekong Delta and the Chao Phraya Estuary. Proceedings
of the International Workshop on the Mekong Delta. Chiang
Rai, Thailand 1998; p. 150 – 68.
Irion G. Mineralogische und geochemische Untersuchungen an der
pelitischen Fraction amazonischer uberdoden Sedimente. Biogeographica 1976;7:7 – 25.
Konovalov GS, Ivanova AA. River discharge of microelements
from the territory of the USSR to the sea basins. Oceanol
1970;10(4):482 – 8.
Kraepiel AML, Chiffoleau JF, Martin JM, Morel FMM. Geochemistry of trace metals in the Gironde estuary. Geochim Cosmochim Acta 1997;61:1421 – 36.
Landman G, Hutflis V, Ittekkot V. Distribution and behaviour of
suspended matter and sediments in the Mekong River and the
adjacent sea. Proceedings of the International Workshop on the
Mekong Delta. Chiang Rai, Thailand 1998; p. 100 – 15.
Loring DH, Rantala RTT. Techniques in marine environmental sciences sediments and suspended particulate matter: total and
partial methods of digestion, vol. 9. Copenhagen: ICES, 1990.
p. 1 – 14.
Martin JM, Meybeck M. Elemental mass-balance of material carried by major world rivers. Marine Chem. 1979;173 – 206.
Martin JM, Windom HL. Present and future roles of oceanic margins in regulating marine biogeochemical cycles of trace elements. Ocean Margin process Global Change. 1991;45 – 67.
Martin JM, Huang WW, Yoon YY. Level and fate trace metals in
the lagoon of Venice (Italy). Marine Chem 1994;46:371 – 86.
Martin JM, Dai M, Cauwet G. Significance of colloids in the biogeochemical cycling of organic carbon and trace metals in a
coastal environment; example of the Venice lagoon (Italy). Limnol Oceanogr 1995;40:119 – 31.
Nguyen TT. Water quality problems in the lower Mekong Basin.
Proceedings of the International Workshop on the Mekong Delta. Chiang Rai, Thailand 1998; p. 132 – 8.
Presley BJ, Trefry JH, Shokes RF. Trace metal input to the Mississippi Delta sediment. An historical view. Water Air Soil Pollut 1980;13:481 – 94.
Salomons W, Kerdijk HW. Cadmium in fresh and estuarine waters.
In: Mislin H, Ravera O, editors. Cadmium in the environment.
Stuttgart: Birhauser Verlag, 1986. p. 144.
Shiller AM, Boyle EA. Trace element in Mississippi river delta
outflow region: behaviour at low discharge. Geochim Cosmochim Acta 1991;55:3241 – 51.
Stanfield K, Garrett C. Implication of the salt and heat budgets of
the Gulf of Thailand. J Marine Res 1997;53:935 – 63.
Sturgeon RE, Berman SS, Willie SN, Desaulniers JAH. Preconcentration of trace elements from seawater with silica-immobilized
8-hidroxyquinoline. Anal Chem 1981;53:2340 – 77.
Trefry J.H., Presley B.J. Heavy metal transport from the Mississippi
River to the Gulf of Mexico. in: Windom H.L., Duce R.A,
(Eds.), Marine Pollutants Transfer, Lexington Book, Lexington,
Mass. 1976; p. 39 – 76.
Trefry J.H., Nelson T.A., Trocine R.P., Metz S., Welter T. Trace
metal fluxes through the Mississippi river delta system. in: Kullenberg G., (Ed.), Contaminants through the Coastal Zone.
Rapp. Procès-Verbaux. Comm. Int. Exploit. Mer Médit.
(CIESM) 1986; 186: p. 77 – 288.
Trincherini P. European Commission. JRC, IES Ispra (VA) Italy.
Volterra L., Maffiotti A. Criteri per la determinazione della qualità e
del possibile smaltimento di sedimenti marini e fluviali. L’esempio di alcune normative internazionali. Analisi e recupero dei
sedimenti marini. Pitagora Editrice Bologna 1997; p. 29 – 44.
Watras CJ, Morrison KA, Blomm NS. Mercury in remote Rocky
Mountain lakes of Glacier Natural Park, Montana, in comparison with other temperate North American regions. Can J Fish
Aquat Sci 1995;52:1220 – 8.
Wallschlaeger D, Desai MVM, Spengler M, Windmoeller CC,
Wilken RD. How humic substances dominate mercury geochemistry in contaminated floodplain soils and sediments. J
Environ Qual 1998;27:1044 – 54.
Windom HL, Wallace G, Smith R, Dudek N, Maeda M, Dulmage
F, Storti F. Behaviour of Cu in Southeastern United States Estuaries. Marine Chem 1983;12:183 – 93.
Windom HL, Smith RJ, Maeda M. The geochemistry of lead in
rivers estuaries and continental shelf of the southern United
States. Marine Chem 1985;17:43 – 56.
Windom HL, Smith R, Rawlinson C, Hungspreuchs M, Dharmvajni
S, Wattayakorn G. Trace metal transport in a tropical estuary.
Marine Chem 1988;24:293 – 309.
Zwolsman JJG, van Eck GIM. Geochemistry of the major elements
and trace metals in suspended matter of the Scheldt estuary
southwest Netherlands. Marine Chem 1999;66:91 – 111.