agrochemical and pcbs residues in the mekong river delta,vn

7/28/2019 Agrochemical and PCBs Residues in the Mekong River Delta,VN

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Agrochemical and polychlorobyphenyl (PCB) residues in the Mekong River

delta, Vietnam

Fernando P. Carvalho a,b,*, J.P. Villeneuve b, C. Cattini b, I. Tolosa b, Dao Dinh Thuan c, Dang Duc Nhan d

a Nuclear and Technological Institute, E.N. 10, P-2685-953 Sacavém, Portugalb International Atomic Energy Agency, Marine Environment Laboratories, 4 Quai Antoine 1 er , MC 98000, Monacoc Hanoi University of Mining and Geology, Co Nhue, Tu Liem, Hanoi, Viet Namd Vietnam Atomic Energy Commission, 59, Ly Thuong Kiet, Hanoi, Viet Nam

a r t i c l e i n f o

Keywords:

DDT

PCB

Hexachlorocyclohexane

Endosulfan

Bioaccumulation

Marine molluscs

a b s t r a c t

An environmental survey on pesticide residues and polychlorobyphenyl compounds (PCBs), encompass-

ing more than 70 polar and non-polar compounds quantifiable by the techniques used, was performed in

the Mekong River delta based on analyses of water, sediment and bivalve mollusc samples. Few polar

compounds, such as diazinon and fenotrothion, were detected in water but a high number of non-polar

chlorinated compounds, such as DDT, HCH, endosulfan and PCBs, were detected in sediments and biota.

The highest concentrations measured were of DDT with an average 6.3 ng gÀ1 dry weight (range 0.32–

67 ng gÀ1) in sediments and 38.6 ng gÀ1 (range 5.5–123 ng gÀ1) in molluscs’ soft tissues. Amongst chlori-

nated compounds, DDT concentrations were followed in decreasing order by those of PCB, endosulfan,

hexachlorocyclohexane and chlordane. Residues of organochlorine compounds originate from local usage

of agrochemicals although with a likely contribution also of atmospheric deposition of residues (not mea-

sured) originated elsewhere. Concentrations of PCB and pesticide residues in the aquatic environment of

the Mekong River delta are lower than values reported for other regions of Vietnam and Asia. Neverthe-

less, current concerns about the effects of chlorinated compounds on public health advise improved con-

trol of chemical residue discharges in order to abate environmental contamination.Ó 2008 Elsevier Ltd. All rights reserved.

1. Introduction

The Mekong is the third largest river in Asia and the first in the

South East Asia. It flows across China, Laos, Thailand, Cambodia

and Vietnam, with a catchments’ area of about 800,000 km2, 8%

of which in Vietnam, and annually discharges 475 km3 of water

into the South China Sea (Dudgeon, 1995). In Vietnam, the Mekong

River spreads in a wide delta with ramifications into many chan-

nels draining 20% of the surface of the country. The delta had nine

main sea communicating channels, which gave rise to the other

name of Mekong, i.e., Cuu Long (nine dragons, in Chinese). The del-

ta of the Mekong is a river borne sedimentary formation, with an

average height of 5 m above the sea level, and a surface area of

30,000 km2. The area of the delta is administratively divided in

13 provinces (Long An, Dong Thap, Tien Giang, Ben Tre, Tra Vinh,

Vinh Long, An Giang, Can Tho, Hau Giang, Soc Trang, Bac Lieu, Ca

Mau and Kien Giang) in which live 16.5 million inhabitants.

The Mekong River has a very abundant and diverse aquatic fau-

na. The populations of many fish species migrate up and down the

river and, with floods, spread over the plains into rice paddies,

small ponds, and irrigation and navigation channels. Fish, mollusc

and crustaceans are important diet components to the people liv-

ing in the delta. The annual fish harvest of the lower Mekong basin

was estimated at about 500,000 tonnes (Dudgeon, 1995) and aqua-

culture is a fast growing activity.

Rice is the dominant cereal crop in the delta, growing round the

year in this region. In 1998, the rice growing area in the Mekong

delta has been 3.8 Mha and the rice production yield was 4.19 ton-

nes haÀ1 in the same year. Sugar cane is an industrial crop grown

mainly in Can Tho, Soc Trang, Tra Vinh and Ben Tre, and occupied

anarea of 74.5 kha in1998 (MARD, 1998).Vegetables and fruits are

also grown in the region and the Mekong delta became the first

agriculture zone of Vietnam. The traditional rice growing practice

is seeding and agrochemicals are currently used. Amongst the

compounds used in rice, the most common herbicides and fungi-

cides seem to be 2,4D (2, 4 dichlorophenoxy)acetic acid, atrazine,

propiconazole, captan, and isoprothiolane. Amongst the insecti-

cides the common ones are endosulfan, lindane, chlorpyrifos, diaz-

inon, fenobucarb, and fenvalerate. Other and more persistent

organochlorine pesticides, available at low price in Asian markets,

are likely used also by farmers as crop protection chemicals and for

sanitation purposes, especially to control malaria vectors.

0025-326X/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.doi:10.1016/j.marpolbul.2008.04.042

* Corresponding author. Address: Nuclear and Technological Institute, E.N. 10,

P-2685-953 Sacavém, Portugal. Tel.: +351 219946332; fax: +351 219941995.

E-mail address: [email protected] (F.P. Carvalho).

Marine Pollution Bulletin 56 (2008) 1476–1485

Contents lists available at ScienceDirect

Marine Pollution Bulletin

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / m a r p o l b u l

mailto:[email protected]

http://www.sciencedirect.com/science/journal/0025326X

http://www.elsevier.com/locate/marpolbul

http://www.elsevier.com/locate/marpolbul

http://www.sciencedirect.com/science/journal/0025326X

mailto:[email protected]



Polychlorinated biphenyls (PCBs) are industrial chlorinated hydro-

carbons used as electric insulators in transformers, hydraulic fluids

and paint additives (Waid, 1987). PCBs became widespread con-

taminants and are amongst the persistent, bioaccumulative, and

toxic synthetic chemicals targeted by international conventions

to be phased out (UNEP, 1997). The harmful effects of these chlo-

rinated hydrocarbons on wildlife and on human health are well

established (Colborn et al., 1993; Woolcoot et al., 2001; Taylor

et al., 2003; Isobe et al., 2007).

The amounts of chlorinated hydrocarbons applied in Vietnam

are not accurately known. This paper reports the first field survey

on pesticide and PCB residues carried out in the Mekong delta.

2. Material and methods

The field sampling was performed in mid May 1998, before the

beginning of the rainy season. Water, sediment, and biota samples

were collected in various sites of the Mekong delta, from the bor-

der with Cambodia to the coast of South China Sea ( Fig. 1).

Water samples were collected in 8 out of 16 sampling stations

directly drained into large brown glass bottles previously rinsed

with hexane. The bottles with water samples, as well as the sedi-

ment and biota samples, were kept in the shade in an ice-chest

box used for transportation in the field. Treatment of the samples

was done in the same day of the collection, in the laboratory. The

water samples were filtered through clean (pre-combusted) glass

fibre filters (GF/F, Whatman) using vacuum from a hand-pump. A

volume of 500 ml of filtered water was immediately spiked with

200 ng of chlortion used as internal tracer, swirled, and immedi-

ately drawn through a solid-phase extraction (SPE) column LiChro-

lut EN. After passing the water sample the SPE column was dried

with the air flow, packed in aluminium foil and kept in the refrig-

erator until analysis in the laboratory. This method is described in

detail in Tolosa et al. (1999). Briefly, a wide variety of compounds

of differing hydrophobicity, including organophosphorus, triazines,

carbamates, and phtalimides, the compounds sorbed on the col-

umn were eluted with ethyl acetate. The volume of the eluate is

concentrated to 500 lL and dibenzothiophene added to serve as

co-injected standard. The extracts were analysed in a gas chro-

matograph HP5890 equipped with a flame photometric detector

(FPD) for determination of the organophosphorus compounds,

and with a nitrogen and phosphorus detector (NPD) for determina-

tion of herbicides, using in this case desmetryn as a co-injected

standard to monitor variations in NPD response (Tolosa et al.,

1999). This method was carefully tested for the recovery of 40

compounds from various water types, with very satisfactory ana-

lytical results.

An attempt was made to measure apolar organochlorine pesti-

cides in the filtered water, using the same SPE columns. Extracts

eluted from the column were injected into a GC–MS HP5889 ‘‘En-

gine B” using negative ion chemical ionisation (NICI), and specifi-

cally the following compounds were looked for: hexachloro-

benzene (HCB), lindane, pp0-DDE, pp0-DDT, DDMU, op0-DDE,

op0-DDD, op0-DDT, heptachlor, aldrin, dieldrin, endrin, a-endosul-

fan, b-endosulfan, endosulfan sulphate and PCBs. The same extract

was also screened by GC–MS with electron impact (EI).

Surface sediment samples were collected with a clean stainless

steel spatula directly into hexane rinsed glass jars. Sediment sam-

ples stored in glass jars were kept frozen until analysis. In the lab-

oratory these samples were freeze dried and sieved through a

250lm metal sieve in order to remove the coarse fraction, which

was always less than 2% in weight of sediment collected.

Fig. 1. Location of the sampling sites (1–16) in Mekong River delta, South of Vietnam. The map shows the river arms and the dense network of navigation canals (lines).

F.P. Carvalho et al. / Marine Pollution Bulletin 56 (2008) 1476–1485 1477



Sediment burrowing bivalve molluscs were selected as moni-

toring species due to the fact that they do not undertake up and

down-river migrations and, thus, integrate contamination over

time at the same site. Bivalve molluscs were collected by hand

from the sediment floor of channels and river arms. It was possible

to gather sufficient bivalve molluscs at 13 out of the 16 sampling

sites. These sediment-dwelling molluscs, although collected in

similar biotopes and occupying similar ecological niches, were

not all of the same species. Biological species names are given in

Table 1, with the geographic coordinates recorded with a portable

GPS at the sampling points. From each site, always more than 20

individuals of bivalve molluscs’ species available were dissected

and soft tissues combined in one sample. The samples of bivalve

soft tissues were frozen, freeze dried, and the resulting powder

homogenized.

About 5 g of dry sediment, and a similar amount of bivalve tis-

sues, were used for analysis of chlorinated hydrocarbons, accord-

ing to techniques described in detail elsewhere (Villeneuve and

Cattini 1986; Villeneuve et al., 1999). Briefly, at the beginning of

the analysis 25 ng of 2, 4, 5 trichlorobiphenyl were added to each

sample as an internal standard for control of the chemical recovery

yield. Samples were extracted for 8 h in a Soxhlet apparatus with a

mixture of hexane and dichloromethane (1:1) for sediments and

with only hexane for biological samples, followed by the clean

up of extracts performed on methanol and hexane pre cleaned

Florisil columns. From the Florisil column three fractions were

eluted for separation of PCBs, HCB, DDE, aldrin (1st fraction), toxa-

phene, DDD, DDT, HCHs (2nd fraction), and dieldrin, endrin and

endosulfans (3rd fraction). Elution solvents used were hexane

(1st fraction), hexane/MeCl2 70:30 (2nd fraction) and MeCl2 (3rd

fraction). Chromatographic analyses were performed with a gas

chromatograph HP 5880A, equipped with a capillary column

25 m long, 0.2 mm internal diameter, coated with SE 54 silica

phase, and an electron capture detector (ECD). Chromatographic

conditions used were the following: splitless injector 250 °C,

detector 300 °C, oven 70 °C (2 min) with a ramp of 3 °C/min to

260 °C (20 min). This method is currently used in the laboratoryin the quantification of 30 organochlorine compounds. Confirma-

tory analyses were performed with a mass spectrometer HP5889

‘‘Engine B” using negative ion chemical ionisation (NICI).

Thirteen polychlorobyphenyl congeners (congeners number 44,

49, 52, 101, 105, 118, 128, 138, 149, 153, 170, 180 and 200) were

systematically quantified in sediment and biota samples. PCB re-

sults are shown asP

PCB (the sum of 13 congeners), and as com-

mercial mixtures aroclor 1254 and aroclor 1260.

Solvents used were of high purity pesticide quality (Burdick and

Jackson Labs, Muskegon, MI, USA). Authentic standards of organo-

phosphorous, triazines, and phtalimides of purity >98% were pur-

chased from Riedel-de Haën (Seelze, Germany) and organochlorine

authentic standards, purity >99%, were purchased from Interchim

and Promochem (France).

The quality control of the analytical performance for water sam-

pleswasdonethrough therepeatedtestingof recoveryof compounds

added in known amounts to water samples. Quality assurance for

chlorinated hydrocarbons (Carvalho et al., 1997) was made with ref-

erence materials IAEA-142 (mussel tissue homogenate) and IAEA-

357 (estuarine sediment), co-analysed with each batch of samples.

Concentrations of residues in water samples are expressed in

ng L À1 and in solid samples in ng gÀ1 on sample dry weight. For

sediment and biota samples conversion of concentrations on dry

weight to lipid weight can be made using the concentration of hex-

ane extracted lipids in sample materials (HEOM) provided in Ta-

bles. Whenever a compound was not detected, the limit of

detection (LOD) for the compound is provided (as <LOD value).

3. Results and discussion

3.1. Pesticide residues in water

Amongst all the compounds searched for in filtered water sam-

ples, only diazinon, fenitrothion, nonylphenol and endosulfan sul-

phate cyclic were detected. Diazinon was quantified in 6 out of 8

samples, with concentrations ranging from 3.5 to 42.8 ng L À1, and

non quantifiable traces were found in the other two samples (Table

2). Fenitrothion was quantified in 5 out of 8 samples with concen-

trations ranging from3.3 to 11.9 ng L À1. Traces of a few other com-pounds, e.g. nonylphenol, caffeine, were detected by GC–MS-EI and

only endosulfan sulphate cyclic was detected in samples 6, 8 and 9

by GC–MS-NICI (Table 2). Non detection of other apolar com-

pounds in filtered waters was not surprising due to the small vol-

ume of water samples used and to the low water solubility, and

concurrent high octanol-water partition coefficients (K ow) of these

compounds, with log K ow values ranging from 3 to 6. The theory

and experience shows that these compounds concentrate in the so-

lid phase, particularly in the particulates and lipid fraction of or-

ganic matter, being their concentration in the water phase very

low (Connell, 1994). Taking into account the low detection limit

of this SPE method for a wide range of agrochemicals, 1–4 ng L À1

depending on the compound, it is apparent that usage of these

compounds either has been very low in this region or chemicaldegradation has been very rapid in the conditions of the tropical

environment, as shown before in other regions and through exper-

imental research (Pereira and Rostad, 1990; Lacorte et al., 1995;

Barcelo and Hennion, 1997; Carvalho et al., 1992; Readman et al.,

1993; Kale et al., 1999; Nhan et al., 2002)

3.2. Pesticide residues in sediments and biota

Concentrations of organochlorine pesticides in bottom sedi-

ments are shown in Table 3. The higher concentrations measured

were those of DDT compounds, with DDT displaying an average

6.3 ng gÀ1 (median 1.85 ng gÀ1, range 0.32–67 ng gÀ1, n = 16), fol-

lowed by those of hexacyclochorohexane, HCH averaging

0.14ng g

À1

(median 0.02 ng g

À1

, range 0.003–0.89 ng g

À1

, n = 14),and those of endosulfans with an average 0.073 ng gÀ1 (median

Table 1

Sampling stations in the Mekong River delta and bivalve mollusc species collected for

analysis

Station L atitude N Longitude E S ite description Speciesa

VN 1 10°48.1370 105°20.4840 Hong Ngu, Mekong River 1

VN 2 10°46.5500 105°21.5090 Hong Ngu area, drainage from

rice fields

–

VN 3 10°25.0590 105°38.6300 Cao Lanh, river near ferry harbour 1, 2, 3, 4

VN 4 10°20.3970 105°47.6450 Navigation canal, near houses,

fruit trees, no rice

1, 2

VN 5 10°13.3160 105°57.1010 Vinh Long, canal 1, 3

VN 6 10°05.3800 105°52.2190 Can Tho, canal 2

VN 7 10°03.1320 105°47.3960 Can Tho, river mudflat, near ferry

harbour

–

VN 8 09°49.4310 105°48.5970 Phung Hiep, canal 1, 3

VN 9 09°36.7320 106°06.8180 Long Phu, canal near river mouth 2

VN 10 09°58.1860 106°13.3190 Tra Vinh, canal 5

VN 11 09°41.0520 106°30.4770 Duyen Hai, near the mouth of

Mekong, shrimp farming area

–

VN 12 09°52.7260 106°23.8020 Tra Vinh, canal 2

VN 13 10°21.7720 105°59.7430 Canal 1, 2

VN 14 09°59.6400 106°37.9760 Ba Tri, at the coast, large marine

clams

6

VN 15 10°03.1890 106°34.3370 Canal, zone of rice paddies 2

VN 16 10°08.8870 106°30.3440 Giong Trom, canal 2

a Unionidae Family: 1 – Lanceolaria spp., 3 – Sinanodonta sp.; Corbiculidae

Family: 2 – Corbicula spp.; Amblemidae Family: 4 – Lamprotula quadrangulosa, 5 –Oxynaia micheloti; Veneridae Family: 6 – Meretrix lyrata.

1478 F.P. Carvalho et al. / Marine Pollution Bulletin 56 (2008) 1476–1485



0.023 ng gÀ1, range 0.007–0.49 ng gÀ1, n = 14), and chlordane with

an average 0.021 ng gÀ1 (median 0.017 ng gÀ1, range 0.004–

0.063 ng gÀ1, n = 13). Other organochlorine pesticides, namely

hexachlorobenzene (HCB), heptachlor, and the drins, occurred in

concentrations generally lower than 0.01 ng gÀ1.

In the soft tissues of molluscs (Table 4) the higher concentrations

of organochlorine insecticides were again thoseof DDT, with an aver-

age 38.6 ng gÀ1 (median 18.8 ng gÀ1, range 5.46–123 ng gÀ1, n = 13).

In decreasing order, the concentrations of DDT were followed by

thoseof endosulfan averaging0.51 ng gÀ1 (median 0.46 ng gÀ1, range

0.11–1.3 ng gÀ1, n = 12), chlordanewith anaverage0.38 ng gÀ1 (med-

ian 0.33 ng gÀ1, range 0.046–0.91 ng gÀ1, n = 13), and HCH with an

average 0.25 ng gÀ1 (median 0.21 ng gÀ1, range 0.08–0.61 ng gÀ1,

n = 12). A few other organochlorine insecticides, such as aldrin and

dieldrin, were also present but in concentrations much lower than

those for the compounds mentioned above (Table 4).

The hydrophobic and liphophilic properties of these chlorinated

pesticides have been well documented (Spacie et al., 1995). All

compounds listed in Tables 3 and 4 are known to generally accu-

mulate in the lipid fraction of biological tissues and, furthermore,

are known as environmentally persistent. Once released in the

aquatic environment, compounds such as DDT are rapidly sorbed

onto sediment particles and, later, concentration equilibrium in

the system is maintained by a slow process of exchanges between,

mainly, the sediment pool and biota (Bierman, 1990). Inthecase of

deposit-feeding bivalve molluscs, the bioconcentration of chlori-nated hydrocarbons is likely to take place with the ingestion of

sediment particles followed by digestion and absorption of sedi-

ment associated organic matter. Actually, for compounds such as

DDTs in mussels, oysters, and clams it has been reported for sev-

eral coastal regions that concentrations in biota reflect concentra-

tions of the same compounds in sediments (Sericano et al.; 1990;

Nhan et al., 1998; Carvalho et al., 2002; Chen et al., 2002; Wei

et al., 2006). Plotting the concentration values in bivalve molluscs

on concentrations in sediments, a positive correlation, statistically

significant at P < 0.05 (95% confidence level), was found for DDT,

endosulfan, and PCB (Fig. 2). However, no statistically significant

correlationcould be find (P > 0.60) for chlordanes, lindane, dieldrin,

endrin, and HCB, likely because these were present in very low

concentrations in sediments. The observed concentration of chlori-nated hydrocarbons in molluscs is the balance of accumulation

processes and metabolic degradation of compounds and excretion

of their water soluble metabolites (Di Giulio et al., 1995).

Our results suggest that DDT, specially DDE, and PCB are recal-

citrant to biotransformation and, thus, accumulate in tissues with

little metabolization. On the other hand, compounds such as endo-

sulfan, HCB and lindane are bioaccumulated but seem to be largely

destroyed in the metabolic pathways by enzyme activity (Krishna

and Philip, 2008). Destruction of these OC compounds in the aqua-

tic environment may, thus, largely depend upon their cycling

through biota and metabolization rather than upon photolysis

and hydrolysis in the water column. This interpretation is also

underscored with results from experiments using 14C-labelled

compounds on the degradation of chlorpyrifos, lindane, and DDT(Carvalho et al., 1992; Kale et al., 1999).

The geographic distribution of concentrations determined in

sediments and in molluscs from the Mekong delta clearly shows

that the highest DDT concentrations were measured close to

villages and towns suggesting use of DDT to control mosquito.

Endosulfans were higher in the district of Tra Vinh (total a-,

b-endosulfan and endosulfan sulphate amounted to 0.49 ng gÀ1

in sediment) which might be linked to endosulfan usage in sugar

cane plantation. This is also the same station where dieldrin or

endrin was measured in higher concentrations. In the past, these

cyclodiene pesticides were commonly used in sugar cane planta-

tions in other countries. In general, concentrations of DDT in clams

increased from station 1, close to Cambodia, to the coast, indicating

that the main application of DDT was made in the lower delta, in

Vietnam. Concentrations of PCB in clams were also higher in the

lower delta than at the border with Cambodia.

It may be noticed in all samples that, amongst DDT derivatives,

DDE was the most abundant compound accounting to DDT with a

decimal fraction of 0.38 ± 0.07 in sediments and 0.45 ± 0.10 in mol-

luscs (Tables 4 and 5). This is generally interpreted as an indication

that DDT is not from a recent application (Tolosa et al., 1995; Minh

et al., 2007). However, in the conditions of the tropics it may hap-

pen that at the moment of usage there is already a significant

amount of DDE present in the commercial mixture available. Fur-

thermore, in the aquatic–tropical environment and in molluscs,

DDT is rapidly transformed into DDE, which in turn, has a longer

environmental persistence half-life than the parent compound(Carvalho et al., 1992; Minh et al, 2007).

Occurrence of endosulfan in the Mekong delta sediments, in

concentrations that generally rank immediately after DDT, indi-

cates that usage of more modern and more expensive crop protec-

tion chemicals was accessible to the agriculturalists of this region.

Furthermore, the presence of endosulfan compounds in water, sed-

iments and bivalve mollusc tissues confirm the contention that has

been made already in other regions about the unexpected environ-

mental persistence of this compound (Carvalho et al., 2002).

Results for HCH concentrations in the deltaare of the same order

of magnitude that those reported for China years after the ban on

technical HCH(Li et al., 1998). Worldwide, hexacyclochlorohexanes

has been the most used insecticide and China was a main producer

and a major user of technical HCH. HCH residues were reported al-most everywhere in Asia (Iwata et al., 1994; Monirith et al., 2003).

Local usage and deposition of HCHfollowing atmospheric transport

from other latitudes might have both occurred in the region of the

Mekong delta. However, results from our survey indicated that lin-

dane (pure c HCH) had already replaced technical HCH, at least in-

land, near Vinh Long (e.g., stations VN 4, VN 5), while at the more

coastal stations a- and b-HCH isomers, from technical HCH usage,

were still present in sediments and biota (Tables 4 and 5). Interest-

ingly, in mollusc tissues the three HCH isomers were generally de-

tected (Table 5). In these samples lindane (c HCH) in average

accounted for a decimal fraction of 0.49 ± 0.20 of HCH, while a-

HCH and b-HCH accounted for 0.28 ± 0.17 and 0.27 ± 0.15, respec-

tively. As lindane displays a shorter environmental half-life than

other HCH isomers (Li et al., 1998), our measurements underscorethe interpretation that lindane recently has been more used in

Table 2

Results of analyses of water soluble compounds

Compounds VN 1 VN 2 VN 3 VN 6 VN 8 VN 9 VN 11 VN 13

Diazinon (ng L À1) <1 6.7 3.5 42.8 9.7 10.0 <1 12.9

Fenitrothion (ng L À1) <1 <1 <1 3.3 3.7 3.6 9.1 11.9

Caffeine À À À À À + À À

Nonylphenol À À À + À À À À

Endosulfan sulphate cyclic – – – + + + À +

Nonylphenol and caffeine were detected by GC/MS/EI and endosulfan sulphate cyclic by GC/MS/NICI.




the delta than the technical HCH mixture. Another likely source for

lindane is atmospheric deposition following transport from neigh-

bour countries in SE Asia, such as Malaysia, where lindane is com-

monly used in palm tree plantations (Iwata et al, 1994; Monirith

et al., 2003; Taylor et al., 2003).

3.3. PCBs in sediments and biota

Polychlorobyphenyls (PCBs) were measured in sediment and bi-valve samples (Tables 3 and 4). PCB averaged 0.49 ng gÀ1 (median

0.28, range 0.11–2.0 nggÀ1, n = 16) in sediments and

6.82ng gÀ1(median 5.20, range 1.89–19.4 ng gÀ1, n = 13) in biota.

PCBs determined as commercial mixtures display in sediments

the average concentration of 0.44 ng gÀ1 (median0.19 ng gÀ1, range

0.073–2.0 ng gÀ1, n = 16) for aroclor 1254, and 0.68 ng gÀ1(median

0.42ng gÀ1, range 0.19–2.3 ng gÀ1, n = 16) for aroclor 1260. In clams

the same compounds display average concentrations of 8.5 ng gÀ1

(median 5.6 ng gÀ1, range 1.6–24 ng gÀ1, n = 13) and 3.8ng gÀ1

(median2.6, range 0.38–9.1 ng gÀ1

, respectively). Aroclors are com-plex mixtures of PCBs, with varying degrees of chlorination, used as

Table 3

Concentrations (ng gÀ1 dry weight) of chlorinated hydrocarbons in sediment samples of the Mekong Delta, Vietnam

Compounds Station VN 1 Station VN 2 Station VN 3 Station VN 4 Station VN 5 Station VN 6 Station VN 7 Station VN 8

Dry/wet weight 0.75 0.57 0.64 0.56 0.69 0.56 0.62 0.50

HEOM(mg gÀ1) 0.003 0.13 0.11 0.11 0.082 0.22 0.063 0.21

HCB 0.003 0.013 0.022 0.009 6.9 0.025 0.009 0.013

Lindane <0.001 0.009 0.003 0.89 0.68 0.012 <0.001 0.004

a-HCH <0.002 <0.002 <0.002 <0.002 0.009 <0.002 <0.002 <0.002

b-HCH <0.002 <0.002 <0.002 <0.002 0.004 0.006 <0.002 <0.002RHCH ND 0.009 0.003 0.89 0.69 0.02 ND 0.004

pp0-DDE 0.11 2.0 0.67 0.99 0.86 0.64 0.49 0.66

pp0-DDD 0.056 1.3 0.38 0.43 0.50 0.79 0.48 0.46

pp0-DDT 0.11 0.094 0.12 0.18 1.0 0.080 0.070 0.18

DDMU 0.013 0.95 0.14 0.20 0.19 0.15 0.13 0.18

op0 -DDE 0.019 0.021 0.007 0.016 0.013 0.012 0.007 0.010

op0 -DDD 0.016 0.15 0.052 0.063 0.043 0.15 0.065 0.070

op0 -DDT <0.002 <0.002 <0.002 <0.002 <0.002 <0.002 <0.002 <0.002

RDDT 0.32 4.52 1.37 1.88 2.61 1.82 1.24 1.56

Heptachlor <0.002 <0.002 <0.002 <0.002 <0.002 <0.002 <0.002 <0.002

Aldrin <0.003 <0.003 <0.003 <0.003 <0.003 <0.003 <0.003 <0.003

Dieldrin <0.003 0.005 <0.003 <0.003 <0.003 0.012 0.006 0.005

Endrin <0.015 0.022 0.043 0.014 <0.015 0.026 <0.015 0.017

cis Chlordane <0.002 <0.002 <0.002 <0.002 <0.002 <0.002 <0.002 <0.002

trans Chlordane <0.002 <0.002 <0.002 0.008 0.015 <0.002 0.016 0.005

trans Nonachlor 0.007 <0.003 <0.003 0.005 0.014 0.015 0.019 0.013

RChlordane 0.007 ND ND 0.013 0.029 0.015 0.035 0.018a-Endosulfan <0.002 0.016 0.005 0.004 0.003 0.018 0.005 0.029

b-Endosulfan <0.002 0.004 <0.002 <0.002 <0.002 0.006 <0.002 <0.002

Endosulfan sulphate <0.003 0.039 0.020 0.006 0.004 0.049 0.009 0.026

REndosulfan ND 0.059 0.025 0.01 0.007 0.073 0.014 0.055P

PCB 0.132 0.303 0.184 0.404 0.621 0.275 0.276 0.282

Aroclor 1254 0.088 0.13 0.15 0.19 0.85 0.35 0.19 0.27

Aroclor 1260 0.28 0.40 0.31 0.81 0.81 0.43 0.56 0.34

Compounds Station VN 9 Station VN 10 Station VN 11 Station VN 12 Station VN 13 Station VN 14 Station VN 15 Station VN 16

Dry/wet weight 0.42 0.41 0.44 0.51 0.46 0.56 0.4 0.37

HEOM (mg gÀ1) 0.16 0.21 0.17 0.29 0.31 0.13 0.12 0.30

HCB 0.009 0.013 0.009 0.016 0.013 0.006 0.007 0.016

Lindane 0.004 0.005 0.005 0.015 0.022 0.004 0.006 0.013

a-HCH 0.003 <0.002 <0.002 0.018 0.080 0.007 0.027 0.019

b-HCH <0.002 <0.002 0.008 0.032 0.032 <0.002 0.007 0.013

R HCH 0.007 0.005 0.013 0.065 0.13 0.011 0.040 0.045

pp0-DDE 0.88 0.53 0.17 18 2.3 0.13 2.4 0.91

pp0

-DDD 0.76 0.61 0.16 41 1.7 0.19 2.6 0.79pp0-DDT 0.11 0.40 0.082 3.5 0.19 0.060 0.22 0.22

DDMU 0.23 0.15 0.029 4.2 0.49 0.024 0.43 0.22

op0 -DDE 0.010 0.011 <0.002 0.19 0.059 0.003 0.042 0.022

op0 -DDD 0.083 0.077 0.035 0.53 0.37 0.041 0.55 0.14

op0 -DDT <0.002 <0.002 <0.002 0.069 <0.002 <0.002 <0.002 <0.002

RDDT 2.07 1.78 0.48 67.49 5.11 0.45 6.24 2.30

Heptachlor <0.002 <0.002 <0.002 <0.002 <0.002 <0.002 <0.002 <0.002

Aldrin <0.003 <0.003 <0.003 <0.003 <0.003 <0.003 <0.003 0.011

Dieldrin 0.010 0.013 <0.003 0.007 <0.003 <0.003 <0.003 0.008

Endrin 0.039 0.045 <0.015 0.075 0.057 0.091 0.066 0.12

cis Chlordane <0.002 <0.002 0.005 <0.002 0.038 0.004 <0.002 0.014

trans Chlordane <0.002 <0.002 <0.002 <0.002 0.018 0.022 0.004 0.012

trans Nonachlor 0.005 <0.003 <0.003 0.017 0.007 <0.003 <0.003 0.015

RChlordane 0.005 ND 0.005 0.017 0.063 0.026 0.004 0.041

a-Endosulfan 0.022 0.038 0.015 0.011 0.055 <0.002 0.014 0.011

b-Endosulfan 0.006 0.062 <0.002 <0.002 0.011 <0.002 <0.002 <0.002

Endosulfan sulphate 0.065 0.39 <0.003 0.006 0.073 <0.003 <0.003 0.011REndosulfan 0.093 0.49 0.015 0.017 0.139 ND 0.014 0.022P

PCB 0.354 0.223 0.118 0.985 2.016 0.106 0.169 1.348

Aroclor 1254 0.25 0.094 0.073 1.3 2.0 0.10 0.11 0.83

Aroclor 1260 0.70 0.25 0.19 1.2 1.8 0.20 0.28 2.3

ND: Not detected; HEOM: Hexane extractable organic matter; <0.00 x: less than the limit of detection.




Table 4

Concentrations (ng gÀ1 dry weight) of chlorinated hydrocarbons in bivalve mollusc soft tissues of the Mekong Delta, Vietnam

Compounds Station

VN 1

Station

VN 3

Station

VN 4

Station

VN 5

Station

VN 6

Station

VN 8

Station

VN 9

Station

VN 10

Station

VN 12

Station

VN 13

Dry/wet

weight

0.24 0.22 0.30 0.26 0.17 0.19 0.22 0.13 0.19 0.21

HEOM

(mg gÀ1)

82 72 79 45 82 49 70 77 78 37

HCB 0.10 0.079 0.053 0.045 0.057 0.061 0.19 0.15 0.11 0.068

Lindane 0.27 0.14 0.20 0.042 < 0.005 0.039 0.084 0.16 0.067 0.012

a-HCH 0.053 0.063 0.036 0.020 <0.004 0.038 0.046 0.096 0.036 0.087

b-HCH 0.096 0.011 0.042 0.018 <0.009 <0.009 0.072 <0.009 0.062 0.041

RHCH 0.42 0.21 0.28 0.08 ND 0.08 0.20 0.26 0.17 0.14

pp0 -DDE 7.9 8.5 25 5.7 11 6.9 30 5.9 38 9.0

pp0 -DDD 4.0 5.9 12 3.4 5.5 3.9 31 6.6 67 9.1

pp0 -DDT 2.3 1.1 2.7 0.15 0.63 0.23 1.3 2.3 6.3 0.61

DDMU 0.67 0.77 2.3 0.65 0.91 0.35 4.1 0.91 5.5 0.68

op0-DDE 0.080 0.10 0.28 0.063 0.12 0.052 0.42 0.067 0.43 0.21

op0

-DDD 0.71 0.64 1.2 0.39 0.67 0.50 2.6 0.79 5.8 1.1 op0-DDT 0.23 0.023 0.11 <0.002 <0.002 <0.002 <0.002 <0.002 <0.002 <0.002

RDDT 15.89 17.03 43.59 10.35 18.83 11.93 69.42 16.57 123.03 20.7

Heptachlor <0.007 <0.007 <0.007 <0.007 <0.007 <0.007 <0.007 <0.007 <0.007 <0.007

Aldrin 0.16 0.030 0.10 <0.006 0.031 0.063 0.57 <0.006 1.7 0.026

Dieldrin 0.67 1.4 1.1 0.12 0.44 <0.008 0.82 <0.008 23 <0.008

Endrin 0.65 0.31 <0.053 0.17 <0.053 0.98 2.7 5.1 1.7 0.40

cis Chlordane 0.013 0.043 0.12 0.15 0.19 0.34 0.21 0.047 0.29 0.081

trans

Chlordane

0.022 0.050 0.064 0.037 0.095 0.30 0.084 0.11 0.059 0.065

trans

Nonachlor

0.011 0.057 0.16 0.080 0.29 0.27 0.14 0.17 0.15 0.18

RChlordane 0.046 0.15 0.34 0.27 0.58 0.91 0.43 0.33 0.50 0.33

a-Endosulfan 0.43 0.30 0.46 <0.006 1.3 0.70 0.49 0.66 0.17 0.80

b-Endosulfan <0.007 <0.007 0.033 0.013 <0.007 0.020 <0.007 <0.007 <0.007 0.018

Endossulphate <0.008 0.019 0.024 0.11 <0.008 <0.008 <0.008 <0.008 0.031 <0.008

REndosulfan 0.43 0.32 0.52 0.12 1.3 0.72 0.49 0.66 0.20 0.82

PPCB 4.037 5.011 8.492 5.2 6.226 2.032 9.268 3.442 5.5 16.098

Aroclor 1254 5.1 5.1 5.6 3.1 7.0 2.8 13 6.1 9.7 24

Aroclor 1260 2.3 2.7 8.0 7.1 2.6 2.0 5.4 1.7 1.5 5.8

ND: Not detected; HEOM: Hexane extractable organic matter; <0.00Â: less than the limit of detection.



industrial chemicals. The concentrations of PCBs in sediments are

low in comparison with concentrations measured in industrialized

countries (Dannenberger and Lerz, 1996). The likely source of PCBs

may be the old electric transformers in the citiesand oils used in en-

gines and other machinery on board of boats. Indeed, the water-

ways and fluvial navigation are, respectively, the roads and the

transportation means more used in the Mekong delta area. Repairs

on board are often made on the water and leaks of lubricating oils

are frequent. Weapons used during the Indochina War (1961–

1975) were suggested as another potential source of PCBs in Viet-

nam (Thao et al., 1993) but there is no clear evidence for this. Fur-

thermore, in contrast with the distribution of pesticides, PCBs were

more evenly distributed reinforcing the impression that PCBs may

come from pollution by oils used in boat engines, and thus releasedeverywhere in the canals and river arms.

Chlorobiphenyl congeners (CBs) concentrate also in bivalve tis-

sues. A graphic plot (not shown) of all individual CB concentration

data in bivalves on the concentration of CBs in sediments, showed a

statistically significant positive correlation (R2 = 0.57, n = 169,

P < 0.001). Aninvestigation was made onthe patternof the bioaccu-

mulation of CBs through normalising the molar concentration of

each CB with the molar concentration of CB153, a congener known

to be recalcitrant to metabolic transformation, using concentration

ratios CB x/CB153 (Kannan et al., 1994). A plot of these CB ratios in

bivalves against the CB ratios in sediments did not show, however,

a significant modification of the pattern of CB ratios (Fig. 3). CBs of

Group I (congener n°180) are actually somewhat lower in clams;

CBs of Group II (congeners n°52, 101 and 149) display the same ra-tio in sediments and bivalves; CBs of Group III (congeners n°105,

118, 128, 138 and 170) and Group IV (congeners n°44 and 49) also

display similar ratios in both compartments. These results suggest

that bivalve molluscs do not have enzymatic capability to metabo-

lise the CB congeners and, thus, are passive accumulators of these

contaminants. This is not the case for every aquatic species. Differ-

ent patterns of CB concentration in biota, and thus differential met-

abolisation of CB congeners, have been described for plankton,

gastropod molluscs, fish, seals, whales, etc. (Boon et al. 1989; Khan-

nan et al., 1989). Results of a research carried out in the area of Ha-

noi, North of Vietnam, showed that a common freshwater

gastropod mollusc ( Angulyagra sp.) was able to metabolise some

CB congeners (Nhan et al., 2001). However, metabolisation of CBs

by bivalve molluscs, including mussels Mytilus edulis (Khannanet al., 1995), North Sea clams Macoma baltica, marine clams Meretrix

meretrix (Boon et al., 1989), and other species of marine and fresh-

water clams (this work), either it is not significant or does not occur

at all. Therefore, the current results confirm low degradation of CBs

by bivalves and, as a consequence, CB bioaccumulation and effec-

tive food chain transfer of these compounds to man.

3.4. Comparison with other regions of Vietnam

Table 5 summarizes data available on organochlorine com-

pounds reported for various regions of Vietnam. Concentration of

DDT in sediments of the Mekong delta reported in this study (mean

6.3 ng gÀ1 dry weight) is lower than DDT in sediments of freshwa-

ter canals reported for the metropolitan area of Ho Chi Minh City

(mean 80 ng gÀ1 dry weight) (Phuong et al, 1998). Concentrations

of DDTs in sediments of the Mekong delta are also much lower

than those of DDT in Vietnamese soils, 110 ng gÀ1 (Thao et al.,

1993), and more comparable to concentrations measured in the

Saigon-DongNai river and estuarine sediments at the South of Viet-

nam (Minh et al., 2007). Concentrations of HCHs, chlordanes and

endosulfans in the Mekong delta are also lower than values re-

ported for the Saigon River and Ho Chi Minh City area (Iwata

et al., 1994; Minh et al., 2007), but are comparable to concentra-

tions measured in the Hanoi region and in the Red River valley at

the North of Vietnam (Nhan et al., 1999).

Concentrations of PCBs in Mekong delta sediments, with aver-

age 0.49 ng gÀ1 dry weight, are much lower than concentrations

reported for freshwater canals in Ho Chi Minh City, averaging

223 ng gÀ1 dry weight (Phuong et al. 1998), and for Hanoi canals

with 11 ng gÀ1 dry weight (Nhan et al., 2001). It is interesting to

notice that in the South of Vietnam, PCB concentrations in sedi-

ments decrease from Ho Chi Minh City canals, to the Saigon-Dong-

Nai River system, to the Saigon-DongNai estuary, and are even

lower in the Mekong River delta (Table 5). The high PCB concentra-

tions near large cities were attributed to the uncontrolled waste

discharge by industries and motor repair workshops (Nhan et al.,

2001; Minh et al., 2007).

Organochlorine residue concentrations in clams collected forthis study are moderate for DDTs and PCBs, and low for HCH espe-

cially (Table 5). Values determined for molluscs from the Mekong

delta are comparable to results reported for the North of Vietnam

(Nhan et al., 1998; Nhan et al., 1999). Furthermore, they are com-

parable also to PCB and DDT concentrations in shellfish bought in

the Ho Chi Minh City market (Kannan et al., 1992).

Concentrations determined in sediments and clams (Tables 3

and 4) indicate that Mekong waters flowing from the neighbour

countries into Vietnam carry minor amounts of DDTs, PCBs and

other OCs. In the delta, use of DDT and PCBs increase environmen-

tal contaminant levels, especially near towns and villages. Never-

theless, current levels of DDTs and PCBs in sediments and

molluscs of the Mekong River delta are not as high as reported

for freshwater canals by the metropolitan area of Ho Chi Minh City(Phuong et al., 1998; Minh et al., 2007).

3.5. Comparison with other regions in Asia

The most comprehensive monitoring programme of persistent

organic pollutants in the coastal marine environment of Asia-Paci-

fic region was the International Mussel Watch (Monirith et al.,

2003). However, this programme did not include South of Vietnam

and Mekong delta areas. Results of the Mussel Watch programme

showed widespread contamination of coastal waters in Asia with

DDTs, HCHs, chlordanes and PCBs. Concentrations of DDTs gener-

ally rank the highest amongst those of persistent organic pollu-

tants and the higher values were measured at the coast of China,

Hong-Kong and Vietnam. Values reported for DDTs in these coastalareas are of the same order of magnitude as those measured in the

Fig. 2. Plot of concentrations of organochlorine pesticides in bivalve tissues on

concentrations in sediments. Equations of the best fit lines are: RDDT: log y = 0.-

50(log x) + 1.23; R2 = 0.47; p = 0.009; REndosulfan: log y = 0.40(log x) + 0.16; R2 -

= 0.41; p = 0.03;P

PCB: log y = 0.64(log x) + 1.00; R2 = 0.61; p = 0.002




Mekong delta and reported herein. On the other hand, HCH con-

centrations measured in the Mekong delta were generally lower

than HCH concentrations measured along the coast of China and

India (Chen et al., 2002; Monirith et al., 2003; Yang et al., 2004;

Wei et al., 2006).

4. Conclusions

A few polar herbicides, fungicides and insecticides, namely

diazinon, fenotrothion and endosulfan sulphate cyclic, were de-

tected in water samples from the Mekong River delta. These resi-dues originate in the neighbour rice fields but, in all water

samples the concentration of residues was low, i.e., fairly below

acute toxic levels to aquatic biota (Mayer, 1987; Peterson et al.,

1994).

In most of sediment and biota samples the non-polar hydropho-

bic chlorinated hydrocarbons, especially DDTs, HCHs, endosulfans,

and PCBs, were measured. In particular, the more persistent orga-

nochlorine compounds, such as DDTs, were found everywhere in

the delta. This is explained by the general occupation of this fertile

land and by the dispersal of residues by floods. Some locations dis-

played high concentrations of DDTs, probably due to recent appli-

cation for malaria vector control.

Concentrations of organochlorine residues, such as DDTs andHCHs, in bivalve molluscs from the Mekong delta are comparable

Table 5

Comparison of concentrations (ng gÀ1 dry weight) of organochlorine compounds in various regions of Vietnam

Compounds and samples Number of samples Average Median Min–Max References

RDDT:

Soils (North, Hanoi) 5 115 74 19–330 Thao et al. (1993)

Soils (Centre, Binh Tri) 10 146 10 0.7–1300 Thao et al. (1993)

Soils (South, Ho Chi Minh) 8 56 27 1.9–280 Thao et al. (1993)

Sediments (North, Red River) 16 26 16 7–81 Nhan et al. (1998, 1999, 2001)

Sediments (North, coastal) 13 6 6 3–10.4 Nhan et al. (1999)Sediments (South, Ho Chi Minh, Saigon River) 16 2.6 1 0.4–12 Iwata et al. (1994)

Sediments (South, Ho Chi Minh canals) 11 80 49 1.8–254 Phuong et al. (1998)

Sediments (South, Ho Chi Minh canals) 6 37 – 12–72 Minh et al. (2007)

Sediments (South, Saigon-DongNai River) 9 5.6 – 0.21–23 Minh et al. (2007)

Sediments (South, Saigon-DongNai estuary) 7 1.2 – 0.15–5.4 Minh et al. (2007)

Sediments (South, Mekong delta) 16 6.3 1.8 0.32–67 This work

Molluscs (North, Hanoi) 10 166 69 6–864 Nhan et al. (1998)

Molluscs (North, coastal) 7 18 19 12–24 Nhan et al. (1999)

Molluscs (South, Ho Chi Minh, Saigon River) 1 36 – – Kannan et al. (1992)

Molluscs (North and Centre, coastal) 9 250 – 12–1550 Monirith et al. (2003)

Mollucs (South, Mekong delta) 13 39 19 5.4–123 This work

Fish (carp) (North, Hanoi) 2 192 – – Thao et al. (1993)

Fish (various species; several origins) 16 130 – – Kannan et al. (1992)

RHCH:

Soils (North, Hanoi) 5 16 4 1.3–55 Thao et al. (1993)

Soils (Centre, BinhTri) 10 2.1 0.8 0.09–5.7 Thao et al. (1993)

Soils (South, Ho Chi Minh) 8 1.8 1.7 0.23–4.0 Thao et al. (1993)Sediments (North, Red River) 12 0.8 0.6 0.07–3.1 Nhan et al. (1998, 1999, 2001)

Sediments (North, coastal) 5 8.5 1.8 1.2–34 Nhan et al. (1999)

Sediments (South, Ho Chi Minh canals) 16 114 10 0.4–790 Iwata et al. (1994)

Sediments (South, Ho Chi Minh canals) 6 <0.05 – <0.05 Minh et al. (2007)

Sediments (South, Saigon-DongNai River) 9 0.011 – <0.01–0.03 Minh et al. (2007)

Sediments (South, Saigon-DongNai estuary) 7 0.012 – <0.005–0.022 Minh et al. (2007)

Sediments (South, Mekong delta) 14 0.14 0.02 0.003–0.89 This work

Molluscs (North, Red River) 9 0.4 0.3 0.15–1.16 Nhan et al. (1998)

Molluscs (North, coastal) 8 7.8 1.5 0.7–47 Nhan et al. (1999)

Molluscs (South, Ho Chi Minh) 1 14 – – Kannan et al. (1992)

Molluscs (North and Centre, coastal) 9 0.3 – 0.15–0.50 Monirith et al. (2003)

Molluscs (Mekong delta) 13 0.25 0.21 0.08–0.61 This work

Fish (carp) (North, Hanoi) 2 2.5 – – Thao et al. (1993)


RPCB:

Soils (North, Hanoi) 5 21 17 5.5–39 Thao et al. (1993)

Soils (Centre, Binh Tri) 10 4.2 3.3 0.6–12 Thao et al. (1993)Soils (South, Ho Chi Minh) 8 20 4.1 1.5–130 Thao et al. (1993)

Sediments (North, Red River) 16 11 4.7 0.7–40 Nhan et al. (1998, 1999, 2001)

Sediments (North, coastal) 13 6.6 2.2 0.5–28 Nhan et al. (1999)

Sediments (South, Ho Chi Minh canals) 15 126 7.6 0.2–630 Iwata et al. (1994)

Sediments (South, Ho Chi Minh canals) 11 223 162 nd–590 Phuong et al. (1998)

Sediments (South, Ho Chi Minh canals) 6 81 – 46–150 Minh et al. (2007)

Sediments (South, Saigon-DongNai River) 9 6.8 – 0.33–22 Minh et al. (2007)

Sediments (South, Saigon-DongNai estuary) 7 0.90 – 0.49–1.2 Minh et al. (2007)

Sediments (South, Mekong delta) 16 0.49 0.28 0.11–2.0 This work

Molluscs (North, Red River) 9 22 7.7 5.5–76 Nhan et al. (1998)

Molluscs (North, coastal) 7 14 16 5.1–25 Nhan et al. (1999)

Molluscs (South, Ho Chi Minh) 1 75 – – Kannan et al. (1992)

Molluscs (North and Centre, coastal) 9 7 – 1–17 Monirith et al. (2003)

Molluscs (South, Mekong delta) 13 6.8 5.2 1.9–19 This work

Fish (carp) (North, Hanoi) 1 34 – – Thao et al. (1993)





to residue concentrations reported for molluscs of Red River and

freshwater canals near Hanoi, in the North of Vietnam (Nhan

et al., 1999). Those concentrations are, however, lower than con-

centrations measured in molluscs from the canals of the metropol-

itan area of Ho Chi Minh City and Saigon River at the South of

Vietnam (Phuong et al., 1998) and in mussels collected at the mar-

ine coast (Monirith et al., 2003). PCBs are one interesting case be-

cause they were detected in all samples from the Mekong delta

despite the little industrialization of this region. Nevertheless, in

the aquatic environment of the Mekong delta PCBs are present in

concentrations lower than those measured in the canals by the

Ho Chi Minh City, confirming that the city industries currently

are the main source of PCBs.

Current levels of DDTs and PCBs in bivalve molluscs from thedelta are moderate in comparison with the area of Ho Chi Minh ca-

nals. However the generalized consumption of molluscs, as well as

of other aquatic biota from the region, renders effective the food

chain transfer of contaminants from the aquatic environment to

man and raises the problem of human exposure to these xenobiot-

ics with the ingestion of food (Khannan et al., 1995; Nhan et al.,

1998; Carvalho et al., 2002). Taking into account that fishery prod-

ucts contribute to near half of the Vietnamese dietary intake,

appropriate action by regulatory authorities may be needed to re-

duce contamination levels of the aquatic environment.

Acknowledgements

Support to this researchis gratefullyacknowledged.The Vietnam

Atomic Energy Commission was supported by the IAEA Research

Contract No. 7940/MC. This project was supported by the Swedish

International Development Authority (SIDA) through the IAEA.

Thanks are also due to Mr. Nguyen Xuan Duc, Institute of Oceanog-

raphy, Vietnam Academy of Sciences and Technologies, that identi-

fied the mollusc species. The International Atomic Energy Agency

(IAEA) is grateful forthe support providedto itsMarine Environment

Laboratories by the Government of the Principality of Monaco.

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