Chemosphere 67 (2007) 1794–1801 www.elsevier.com/locate/chemosphere Pollution sources and occurrences of selected persistent organic pollutants (POPs) in sediments of the Mekong River delta, South Vietnam Nguyen Hung Minh a, Tu Binh Minh a, Natsuko Kajiwara a, Tatsuya Kunisue a, Hisato Iwata a, Pham Hung Viet b, Nguyen Phuc Cam Tu c, Bui Cach Tuyen d, Shinsuke Tanabe a,* 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,p -DDT/p,p 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 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 Vietnam The Mekong River basin with an area of nearly 800 thousand square kilometers is an important habitat for * Corresponding author Tel./fax: +81 89 927 8171 E-mail address: shinsuke@agr.ehime-u.ac.jp (S Tanabe) 0045-6535/$ - see front matter Ó 2006 Elsevier Ltd All rights reserved doi:10.1016/j.chemosphere.2006.05.144 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 agriculture areas in the world (MRC, 2002) Rice production is a 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 environment and biodiversity For example, intensive use of persistent organic pollutants (POPs) including organochlorine pesticides (OCPs) and polychlorinated biphenyls N.H Minh et al / Chemosphere 67 (2007) 1794–1801 (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 chemicals are believed to alter normal function of the endocrine and the reproductive systems in humans and wildlife (Colborn et al., 1993; Kelce, 1995; Cheek et al., 1999; Vos et al., 2000) In Vietnam, despite an official ban on the use of OCPs since 1995 (Sinh et al., 1999), there has been continuous 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 breast milk (Minh et al., 2002; Monirith et al., 2003; Minh 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 Subsequently, via sedimentation processes, these chemicals are deposited to the bottom They remain very long in sediment due to their long half-life times (Rawn et al., 2001) From 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 exposed to elevated levels of POPs (Ross and Birnbaum, 2003) Examination of POPs levels in sediment may give basic information on the contamination status, sources 1795 and ecological risks of POPs in the aquatic environments In this study, we collected sediments from different locations along the Mekong River and determined the concentrations of several POPs such as PCBs, DDTs, HCHs (hexachlorocyclohexane isomers), CHLs (chlordane compounds) and HCB (hexachlorobenzene) in order to elucidate their recent contamination status, their usage pattern as well as to decide possible potential pollution sources of these chemicals to the river 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 Tranh De estuary (Fig 1) Sediments named as CC and NKSE were collected near Can Tho city and those named as Hau were collected at other points along Hau River (Table 1) At each site, a grab of cm surface sediment was collected by using Ekman dredge The sediment was well mixed in an aluminum tray and a portion about 200–300 g was put in a clean polyethylene bag and transported 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 mm size, which was used for the chemical analysis 2.2 Analytical methods POPs in sediment were analyzed following the method described by Iwata et al (1994) with some slight Fig Sampling locations in Mekong River, South Vietnam (2003–2004) 1796 N.H Minh et al / Chemosphere 67 (2007) 1794–1801 Table Sampling positions and total organic carbon content of sediments from the Hau River Longtitude TOC (%) 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 Sample ID Latitude 2003 September CC-1 CC-4 CC-7 NK-SE Hau-1 Hau-2 Hau-3 Hau-4 Hau-5 Hau-6 Hau-7 Hau-8 N N N N N N N N N N N N 10°02 19.800 10°02 34.300 10°01 16.000 10°02 08.700 09°44 37.300 09°50 36.600 09°55 53.900 10°02 53.100 10°23 15.000 10°20 17.200 10°11 23.200 10°08 22.500 E E E E E E E E E E E E 105°46 10.800 105°47 06.600 105°46 25.100 105°47 25.200 106°04 11.700 105°59 13.100 105°53 52.900 105°47 46.800 105°26 50.500 105°28 44.400 105°36 43.800 105°40 05.600 1.8 1.3 0.55 1.5 1.0 1.4 1.4 1.3 0.82 1.6 0.86 1.5 2.3 Statistical analysis 2004 May CC-7 Hau-1 Hau-2 Hau-3 Hau-4 Hau-5 Hau-6 Hau-7 Hau-11 Hau-12 N N N N N N N N N N 10°01 12.900 09°55 51.300 09°50 39.400 09°44 35.000 10°02 21.900 10°23 11.800 10°20 21.500 10°11 23.300 10°42 29.700 10°32 52.300 E E E E E E E E E E 105°46 24.000 105°53 47.800 105°59 10.700 106°04 10.200 105°48 12.200 105°26 55.800 105°28 45.200 105°36 43.900 105°07 43.100 105°17 51.800 n.a n.a n.a n.a n.a n.a n.a n.a n.a n.a 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 concentrations 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 Hochiminh city (Minh et al., 2004), suggesting widespread 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 bioaccumulation over the other contaminants In this study, correlation between POPs levels and organic carbon content in the sediment (Table 1) was not observed, probably due to the vast areas investigated and also due to large distance from pollution sources (Iwata et al., 1995) In general, levels of PCBs and DDTs were higher in the rainy season than those in the dry season (Table 2), perhaps 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 collected 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 Vietnam (Table 3), indicating a decreasing temporal trend in the environment In fact, Minh et al (2004) found a 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 Vietnam until 1985 (Sinh et al., 1999) In addition, electrical equipments like transformers containing PCB-contaminated oils were also imported until the mid 1980s (Kannan et al., 1995) Those materials are parts of PCBs sources to the environment, besides releases from heavy weapons used during the Indochina War (Thao et al., 1993) n.a.: not analyzed modifications 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 using an electric shaker (SR-2W model, Taitec Co Ltd.) The soil solution was filtered into a separating funnel containing 600 ml hexanewashed water and 100 ml hexane The funnel was shaken vigorously for 15 and then kept for at least h to separate entirely the aqueous and the hexane layers The aqueous 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 ml under gentle nitrogen stream An equal volume of concentrated H2 SO4 was added to this solution to remove pigment, humic acids and other organic interferences This step was repeated several times until the hexane layer became transparent The solution was further washed three times by hexane-washed water ml of this solution was taken for GPC cleanup, followed by Florisil column chromatography as described previously (Minh et al., 2004) The final solution was treated with activated copper to remove sulfur-containing substances 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 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 differences between groups ðp < 0:05Þ Results and discussion 3.1 Residue levels and contamination pattern N.H Minh et al / Chemosphere 67 (2007) 1794–1801 1797 Table Concentration of OCs (ng/g dry wt.) in sediment collected from the Mekong River, South Vietnam Sample ID PCBs DDTs compounds 0 RCHLs RHCHs HCB p,p -DDE p,p -DDD p,p -DDT RDDTs 2003 September (dry season) CC-1 3.7 CC-4 2.4 CC-7 0.12 NK-SE 0.80 Hau-1 0.12 Hau-2 0.18 Hau-3 0.54 Hau-4 0.60 Hau-5 0.18 Hau-6 0.13 Hau-7 0.15 Hau-8 0.17 1.9 2.1 0.69 0.79 0.022 0.086 0.51 1.3 0.46 0.89 0.53 0.89 0.79 1.9 0.75 0.49 0.011 0.081 0.36 0.32 0.20 0.42 0.32 0.23 0.54 0.98 0.43 0.56 0.011 0.016 0.29 0.22 0.51 0.56 0.15 0.027 3.2 4.3 1.9 1.8 0.043 0.19 1.2 1.9 1.2 1.9 1.0 1.2 0.12 0.18 0.35 0.16 0.025 0.032 0.13 0.049 0.034 0.070 0.056 0.075 0.032