Pollution sources and occurrences of selected persistentorganic pollutants POPs in sediments of the Mekong River delta, South Vietnam a Center for Marine Environmental Studies, Ehime Uni
Trang 1Pollution sources and occurrences of selected persistent
organic pollutants (POPs) in sediments of the Mekong River
delta, South Vietnam
a
Center for Marine Environmental Studies, Ehime University, Bunkyo-cho 2-5, Matsuyama 790-8577, Japan
b
Hanoi National University, 334 Nguyen Trai Str., Thanh Xuan Distr., Hanoi, Viet Nam
c
Faculty of Agriculture, Ehime University, Tarumi 3-5-7, Matsuyama 790-8566, Japan
d
Nong Lam University, Thu Duc Distr., Hochiminh, Viet Nam
Accepted 26 May 2006 Available online 16 January 2007
Abstract
The Mekong River delta is one of the largest agricultural land in the Southeast Asia It plays a very important role for agriculture and fisheries in South Vietnam However, comprehensive studies on the environmental pollution of persistent organic pollutants (POPs) in Mekong River delta have not been carried out in recent years In this study, we collected sediment samples from the Mekong River to evaluate the contamination and ecological risks caused by several POPs The contamination pattern of POPs was DDT > PCBs > CHLs > HCHs > HCB DDTs are the most abundant pollutants, their concentration ranging from 0.01 to 110 ng/g dry wt, followed
by PCBs (0.039–9.2 ng/g dry wt) DDTs and PCBs concentrations were higher in sediment from adjacent to urban areas than those from rural and agricultural sites, suggesting urban areas as important point sources of DDTs and PCBs to the river Ratio of p,p0-DDT/p,p0 -DDE was lower compared to those previously reported However, some samples still had the ratio higher than 0.5, indicating recent input of DDT into the aquatic environments This result shows that although the magnitude of contamination decreased over time, recent inputs of DDTs to the river still occur Some sediment samples had concentrations of DDT compounds higher than the standards from the Canadian Environmental Quality Guideline, suggesting continuous monitoring for POPs contamination in the Mekong River is necessary
Ó 2006 Elsevier Ltd All rights reserved
Keywords: Vietnam; Mekong River; POPs; Sediment; Pollution source
1 Introduction
The Mekong River is the longest river in southeastern
Asia, which flows a distance of almost 4800 km from China
through Myanmar, Thailand, Laos, Cambodia and
Viet-nam The Mekong River basin with an area of nearly 800
thousand square kilometers is an important habitat for
approximately 60 million people The Mekong River delta (MRD) in South Vietnam, which is inhabited by about 20 million people, is one of the most highly productive
major agronomic activity in MRD contributing half of the rice production in Vietnam On the other hand, economic development in MRD also raised concerns over the envi-ronment and biodiversity For example, intensive use of persistent organic pollutants (POPs) including organochlo-rine pesticides (OCPs) and polychlorinated biphenyls
0045-6535/$ - see front matter Ó 2006 Elsevier Ltd All rights reserved.
doi:10.1016/j.chemosphere.2006.05.144
*
Corresponding author Tel./fax: +81 89 927 8171.
E-mail address: shinsuke@agr.ehime-u.ac.jp (S Tanabe).
www.elsevier.com/locate/chemosphere
Trang 2(PCBs) may have led to their considerable residues in soil,
water and sediment Moreover, relative persistence of such
chemicals together with natural processes like evaporation
and runoffs might enhance their ubiquitous distribution in
the environment and food chains Several of these
chemi-cals are believed to alter normal function of the endocrine
Col-born et al., 1993; Kelce, 1995; Cheek et al., 1999; Vos et al.,
2000)
In Vietnam, despite an official ban on the use of OCPs
evidence on their use throughout the country, particularly
for dichlorodiphenyltrichloroethane (DDT) Moreover,
recent studies carried out in Hanoi and Hochiminh city
showed high levels of DDTs in birds, mussels and human
et al., 2004), suggesting relevant contamination by DDTs
in the local environment High contamination by POPs in
MRD may be expected due to high population density
and the intensive agronomic activities in this region
Despite this fact, no comprehensive study to evaluate the
contamination status caused by POPs in this region has
been carried out recently
Generally, POPs are hydrophobic and therefore, readily
bind to the particle fraction in lake and river waters
Subse-quently, via sedimentation processes, these chemicals are
deposited to the bottom They remain very long in sediment
sediment, they can be taken up and retained in benthic
organisms and consequently biomagnified through aquatic
food chains to higher trophic levels Humans, through
ingestion of contaminated fish and shellfish, may be
basic information on the contamination status, sources
and ecological risks of POPs in the aquatic environments
In this study, we collected sediments from different loca-tions along the Mekong River and determined the concen-trations of several POPs such as PCBs, DDTs, HCHs (hexachlorocyclohexane isomers), CHLs (chlordane com-pounds) and HCB (hexachlorobenzene) in order to eluci-date their recent contamination status, their usage pattern
as well as to decide possible potential pollution sources of these chemicals to the river
2 Materials and methods 2.1 Sample collection Sediment samples were collected in September 2003 and May 2004 from the Hau River – the biggest branch of the Mekong River, which crosses South Vietnam and empties into the East Sea Sampling points were selected along the Hau River from Chau Doc town to Can Tho city
and NKSE were collected near Can Tho city and those named as Hau were collected at other points along Hau
sedi-ment was collected by using Ekman dredge The sedisedi-ment was well mixed in an aluminum tray and a portion about 200–300 g was put in a clean polyethylene bag and trans-ported to our laboratory in boxes packed with gel ice In the laboratory, sediments were dried in room temperature, ground and sieved for a particle fraction of less than 2 mm size, which was used for the chemical analysis
2.2 Analytical methods
Fig 1 Sampling locations in Mekong River, South Vietnam (2003–2004).
Trang 3modifications Approximately 15 g of air-dried sediment
sample were placed in a conical flask containing 15 ml
water 100 ml acetone was then added and the flask was
shaken vigorously for 60 min using an electric shaker
(SR-2W model, Taitec Co Ltd.) The soil solution was
fil-tered into a separating funnel containing 600 ml
hexane-washed water and 100 ml hexane The funnel was shaken
vigorously for 15 min and then kept for at least 8 h to
sep-arate entirely the aqueous and the hexane layers The
aque-ous layer was discarded and the hexane layer was washed
three times with 100 ml water Volume of hexane in the
final solution was measured for calculating the recovery
from initial 100 ml (this recovery value was used as the
correction factor during calculation) The solution was
concentrated to about 10 ml by Kuderna–Danish (KD)
apparatus and further to 5 ml under gentle nitrogen
added to this solution to remove pigment, humic acids
and other organic interferences This step was repeated
sev-eral times until the hexane layer became transparent The
solution was further washed three times by hexane-washed
water 4 ml of this solution was taken for GPC cleanup,
followed by Florisil column chromatography as described
trea-ted with activatrea-ted copper to remove sulfur-containing
sub-stances For this step, several strings of copper wires
activated by HCl were put into the solution and kept for
an hour until no black sulfur soot appeared on the copper
strings The final solution was further concentrated up to
about 20 times the original volume, if necessary, prior to the quantification by GC/ECD Good recovery rates (85%–110%) were obtained for all compounds The results were not corrected for recovery rates
2.3 Statistical analysis The statistical analysis was performed with the StatView statistical software package (SAS Inc., 1998, Version 5) and the Mann–Whitney U test was used to examine statistical
3 Results and discussion 3.1 Residue levels and contamination pattern
In general, the residue pattern of POPs in sediment from the Hau River followed the order: DDTs > PCBs > CHLs P HCHs P HCB However, their concentrations varied among the sampling sites, showing higher concen-trations in sediments close to urban areas such as Can Tho city The pattern found in this study was similar to those recently observed in human breast milk samples from
and dominant contamination by DDTs and PCBs in the environment as well as throughout the food chains The abundance of DDTs and PCBs in Vietnam may be due
to their larger usage as well as higher persistency and bio-accumulation over the other contaminants In this study, correlation between POPs levels and organic carbon
due to the vast areas investigated and also due to large
gen-eral, levels of PCBs and DDTs were higher in the rainy
because more residues of such pollutants are transported from lands into the river by stormwater in rains
Concentration of PCBs varied between 0.039 and 9.2 ng/g dry wt Interestingly, PCBs levels in sediments col-lected near urban areas such as Can Tho, Chau Doc and Long Xuyen were higher than those in sites away from urban areas, indicating metropolitan areas as sources of PCBs pollution to the river However, PCB levels in the present study were approximately five times lower than those in sediments collected in the early 1990s from South
decreasing trend of PCBs in human breast milk in South Vietnam with half-life ranging from 10 to 18 years for various PCB congeners Approximately 30 000 tons of PCB-contaminated industrial oils were imported to Viet-nam until 1985 (Sinh et al., 1999) In addition, electrical equipments like transformers containing
et al., 1995) Those materials are parts of PCBs sources to the environment, besides releases from heavy weapons used
Table 1
Sampling positions and total organic carbon content of sediments from
the Hau River
2003 September
CC-1 N 10°02 0 19.800 E 105°46 0 10.800 1.8
CC-4 N 10°02 0 34.3 00 E 105°47 0 06.6 00 1.3
CC-7 N 10°01 0 16.0 00 E 105°46 0 25.1 00 0.55
NK-SE N 10°02 0 08.7 00 E 105°47 0 25.2 00 1.5
Hau-1 N 09°44 0 37.3 00 E 106°04 0 11.7 00 1.0
Hau-2 N 09°50 0 36.6 00 E 105°59 0 13.1 00 1.4
Hau-3 N 09°55 0 53.9 00 E 105°53 0 52.9 00 1.4
Hau-4 N 10°02 0 53.1 00 E 105°47 0 46.8 00 1.3
Hau-5 N 10°23 0 15.0 00 E 105°26 0 50.5 00 0.82
Hau-6 N 10°20 0 17.2 00 E 105°28 0 44.4 00 1.6
Hau-7 N 10°11 0 23.2 00 E 105°36 0 43.8 00 0.86
Hau-8 N 10°08 0 22.5 00 E 105°40 0 05.6 00 1.5
2004 May
CC-7 N 10°01 0 12.9 00 E 105°46 0 24.0 00 n.a.
Hau-1 N 09°55 0 51.3 00 E 105°53 0 47.8 00 n.a.
Hau-2 N 09°50 0 39.4 00 E 105°59 0 10.7 00 n.a.
Hau-3 N 09°44 0 35.0 00 E 106°04 0 10.2 00 n.a.
Hau-4 N 10°02 0 21.9 00 E 105°48 0 12.2 00 n.a.
Hau-5 N 10°23 0 11.8 00 E 105°26 0 55.8 00 n.a.
Hau-6 N 10°20 0 21.5 00 E 105°28 0 45.2 00 n.a.
Hau-7 N 10°11 0 23.3 00 E 105°36 0 43.9 00 n.a.
Hau-11 N 10°42 0 29.7 00 E 105°07 0 43.1 00 n.a.
Hau-12 N 10°32 0 52.3 00 E 105°17 0 51.8 00 n.a.
n.a.: not analyzed.
Trang 4DDTs residue levels were highly variable among
sam-pling sites, ranging from less than 0.01 to 110 ng/g dry
wt Similar to the distribution of PCBs, concentrations
were higher at sampling sites close to urban areas (e.g
NK-SE, CC-1, CC-4, CC-7) and decreased downstream
DDTs levels from sampling sites near Long Xuyen town
and Can Tho city were one to two orders of magnitude
higher than those from their respective downstream sites,
except the sediment at Hau-1 site in 2004 Sediment
col-lected at this site showed very high DDTs level in the rainy
season of 2004 compared to those in the dry season of 2003
(Table 2) In addition, this sample had particularly high
input of DDT to the river Stormwater might have carried
DDTs from several sources such as agricultural lands or
municipal areas which are sprayed for hygiene purposes
and vector control, into the river
Comparison of DDTs levels in Hau River sediment
with those in previous studies in Vietnam demonstrated a
decreasing trend Although the mean concentration
(arith-metic mean) of DDT in Hau River was 5.4 ng/g dry wt, the
median concentration (geometric mean) was only 1.5 ng/g
dry wt In fact, rather high DDTs residues in samples such
as CC-7 and Hau-1 have strong influence on the mean
value Therefore, the median concentration and the range
of DDTs was used for evaluating their contamination trend
in sediments from Vietnam The range of DDTs in the Mekong River sediments was several times lower than sed-iments collected in 1990 from mangroves of Duyen Hai and
suggests consistent decreasing input of DDTs to the aqua-tic environments of Vietnam
HCHs, CHLs and HCB in the present study were 10–20 times lower compared to DDTs and PCBs, implying less contamination by such chemicals in the Hau River Among those, CHLs levels were slightly higher than HCHs and HCB, ranging from 0.004 to 1.9 ng/g dry wt The present range of CHLs was lower than those found in the
et al., 2001) All studies, however, revealed no clear differ-ences in the levels between rural and urban areas Our results probably indicated relatively low contamination
by CHLs in MRD
HCH concentrations in this study were slightly lower compared to those recently found in the sediments of the
HCHs in the North Vietnam were observed in human
Table 2
Concentration of OCs (ng/g dry wt.) in sediment collected from the Mekong River, South Vietnam
p,p 0 -DDE p,p 0 -DDD p,p 0 -DDT RDDTs
2003 September (dry season)
2004 May (rainy season)
Summary
Range 0.039–9.2 <0.01–15 <0.01–46 <0.01–44 <0.01–110 <0.004–19 <0.02–1.3 <0.006–0.080 RDDTs ¼ p;p 0 -DDE þ p;p 0 -DDD þ p;p 0 -DDT; RCHLs ¼ trans-chlor þ cis-chlor þ trans-nona þ cis-nona; RHCHs ¼ aHCH þ bHCH þ cHCH (when re-sults were less than quantification limits, the limits were insterted to calculate means).
Trang 5breast milk (Minh et al., 2004), suggesting more usage of
HCHs in the North compared to South Vietnam Rather
low levels of HCHs and HCB in sediments in Vietnam
could be due to high average temperature in the region as
enhance their distribution in aqueous and air phase rather
3.2 Geographical comparison of POPs in sediment Generally, PCBs levels in Mekong river sediments are relatively low compared to sediments from other locations
were comparable with those in some parts of the Pearl
Table 3
Comparison of organochlorines in surface sediments from various locations in the worlda
A – Vietnam
B – World
‘‘–’’: Data is not available.
a
Concentration in ng/g dry wt.
b
As alochlor 1254 mixture.
Hau-5 Hau-3 Hau-4 Hau-11 Hau-6 Hau-12 CC-7 Hau-1 Hau-8 Hau-2 Hau-4 Hau-7 CC-1 CC-7 CC-4 Hau-3 Hau-1 Hau-6 NK-SE Hau-5 Sep 2003
May 2004
Hau-5 Hau-3 Hau-4 Hau-11 Hau-6 Hau-12 CC-7 Hau-1 Hau-8 Hau-2 Hau-4 Hau-7 CC-1 CC-7 CC-4 Hau-3 Hau-1 Hau-6 NK-SE Hau-5 Sep 2003
May 2004
DDTs composition (%)
Fig 2 DDTs composition in sediments collected from the Mekong River, South Vietnam in September 2003 and May 2004.
Trang 6coastal areas (Fillmann et al., 2002) However, the levels in
the Mekong River were lower than several other areas in
the Pearl River delta (e.g Daya Bay and Minjiang River,
Table 3) as well as other locations of prominent industrial
activities like the Black Sea, the Yukon Lake in Canada
recent PCBs residues in sediments throughout the South
East Asian region are rather scarce The only
comprehen-sive monitoring of sediments in this region was made
relatively higher levels of PCBs in sediments from Vietnam
compared to other Asian developing countries could be
seen The similar trend was observed in other studies
exam-ining biological samples such as fish, mussels, birds and
Mon-irith et al., 2003; Minh et al., 2004)
In the global comparison, while DDT residue levels in
the sediment from the Hau River are comparable to those
in several locations of the Pearl River delta such as the
Minjiang River and the Lingding Bay, concentrations were
higher than in sediments from most rivers and lakes of
from relatively contaminated areas such as the Alexandria
harbour (Egypt), the Macao harbour (China) and the
Black Sea (Ukraine) have higher DDTs levels than the
present study
3.3 Composition of OCs in sediment samples
Fig 2demonstrates the contribution of three major DDT
(abbreviated to DDT, DDD and DDE, respectively) Except
the sediment Hau-1 collected in 2004, DDE was the most
abundant compound followed by DDT and DDD
Interest-ingly, proportion of DDT was higher in sediments collected
Moreover, in sediment Hau-1, DDT composition was as
high as 80%, clearly indicating fresh residues of DDT to
the river
Ratios of DDT and its metabolites such as DDT/DDE
could be useful to evaluate degradation features of the
et al., 1998) During the last decade, some studies examined
DDTs residues in sediments from various locations of
Viet-nam For an appropriate evaluation of the temporal trend
of DDT in sediments from Vietnam, ratios of DDT and
suggested that in sediments, DDT/ DDE ratios lower than
0.33 could be the result of aged DDTs mixtures in the
envi-ronment, while those higher than 0.5 might indicate recent
inputs More than half of the sediments collected in 1990
var-ious locations of South Vietnam had the ratio above 0.5
(Fig 3) In the present study, some sediment samples like
CC-4, CC-7, NK-SE, Hau-5 Hau-6 and Hau-1 also had a
ratio above 0.5, perhaps also indicating contribution from
recent usage of DDT in MRD However, it should also be
noted that the ratio is influence not only by the transforma-tion kinetic but also by the loss of the substances from the system (e.g through offsite transport) relative to new inputs Besides, in this study, relative contributions of the inputs from recent usage and from old residues in agricul-tural lands have not been elucidated yet Further study
in the soil and air samples may be useful to distinguish the recent and old sources of DDTs in the environment (Bidleman et al., 1998)
3.4 Toxicological assessment The risk assessment was carried out based on the stan-dard of Canadian Environmental Quality Guideline for
such as the interim sediment quality guideline (ISQG) and the probable effect level (PEL) for DDT compounds and PCBs were compared with their respective concentrations
in sediments from the Mekong River as well as sediments
1994; Phuong et al., 1998) Compared to the criteria for DDE, DDD and DDT (ISQG: 1.42, 3.54, 1.19; PEL: 6.75, 8.51, 4.77 in ng/g dry wt, respectively), five sediments in the present study exceed the ISQG value for DDE and two
On the other hand, more than half of the sediments collected from 1990 to 1996 had levels beyond the guidelines for all compounds Regardless of such a difference between sam-pling locations in such surveys, this result probably demon-strates the decreasing levels and less toxicological stress of DDTs on aquatic biota of the Mekong River Nevertheless,
it is probably necessary to continue monitoring the levels of DDTs as some sediment samples had levels exceeding the ISQG values The ISQG and PEL for total PCBs are 34.1
0 1 2 3 4
5
DDT/DDE
6.
14
0 1 2 3 4 5
6.8 14
Fig 3 Ratios of DDT/DDE in sediments collected in various locations in Vietnam from 1990 to 2004 S-VN’04 denotes samples from South Vietnam in 2004 (present study); S-VN’90: South Vietnam in 1990 ( Iwata
et al., 1994 ); S-VN’96: South Vietnam in 1996 ( Phuong et al., 1998 ); N-VN’96: North Vietnam in 1996 ( Nhan et al., 1998 ); N-VN’97: North Vietnam in 1997 ( Nhan et al., 2001 ).
Trang 7and 277 ng/g dry wt, respectively Those guidelines for total
PCBs are much higher than the concentrations observed in
this study, suggesting rather lower toxicological effect
caused by PCBs contamination in the sediment of Mekong
River Besides, considering the fact that so far, no treatment
facility for municipal and industrial discharges is available in
MRD, there should be concerns toward potential pollution
of several other organic contaminants such as antibiotics
and surfactants which are also present in these discharges
Therefore, further studies toward possible adverse effects
of total organic pollutants in the Mekong River sediments
may be necessary to provide more comprehensive risk
assessment
4 Conclusions
This study has demonstrated DDTs and PCBs as two
major organochlorine contaminants in the Mekong River
in South Vietnam Furthermore, urban areas along the river
are apparent major pollution sources of such chemicals to
the aquatic environment Although DDT was officially
phased out in Vietnam, the results provided evidence for
recent inputs of DDTs into the river sediment
Toxicologi-cal assessment suggests that in general, the concentrations
of PCBs are below the Canadian guidelines, while
concen-trations of DDTs in some sediments exceed those levels
More comprehensive studies would be needed in order to
clarify the pathways of inputs of DDT to the aquatic
environment as well as to investigate further
contamina-tion by other organic pollutant groups such as
dioxin-related compounds, polybrominated diphenyl ethers and
surfactants
Acknowledgements This study was supported by a Grant-in-Aid from the Scientific Research on Priority Areas (Project No 13027101) of the Japanese Ministry of Education, Science, Sports, Culture and Technology and by Scientific Research (Project No 12308030) of Japan Society for the Promotion
of Science (JSPS) Financial assistance was also provided
by Research Revolution 2002 (RR 2002) project for Sus-tainable Coexistence of Human, Nature and the Earth (FY 2002) of the MEXT of the Japanese Government; the Core University Program between Japan Society for the Promotion of Science (JSPS) and National Center for Natural Science and Technology, Vietnam (NCST) and
‘‘21st Century COE Program’’ from the Japanese Ministry
of Education, Science, Sports, Culture and Technology The authors also wish to thank Dr A Subramanian (Ehime University) for the critical reading of this manu-script and to thank the staff of Nong Lam University, Hochiminh City, Vietnam for their valuable support dur-ing our sampldur-ing surveys
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