characterization of heavy metal contamination from their

Journal of Water and Environment Technology, Vol. 8, No.2, 2010

Address correspondence to Shuzo Tanaka, Asian Center for Environmental Research (ACER), Meisei University, Email: [email protected] Received January 26, 2010, Accepted April 1, 2010.

- 111 -

Characterization of Heavy Metal Contamination from their Spatial Distributions in Sediment of an Urban Lake of Hanoi, Vietnam Tetsuro KIKUCHI*, Huynh Trung HAI**, Shuzo TANAKA* *Asian Center for Environmental Research (ACER), Meisei University, 2-1-1, Hodokubo, Hino, Tokyo 191-8506, Japan

**Institute for Environmental Science and Technology (INEST), Hanoi University of Technology, 301-C10, 1, Dai Co Viet Street, Hanoi, Vietnam

ABSTRACT Ho Tay (West Lake) in Hanoi, Vietnam receives wastewater from the city center and the surrounding residential areas, which can cause both eutrophication and enrichment of toxic heavy metals in the lake ecosystem. The aim of this study is to evaluate the recent trends of metal contamination in this lake from their spatial (horizontal and vertical) distributions in the lake sediments. Sediment cores with up to 70 cm in depth were sampled from four locations in the lake and analyzed for heavy metals (Cd, Cr, Cu, Mn, Ni, Pb and Zn) including a metalloid (As) and total organic carbon (TOC). High concentrations of the metals (except for Mn) and TOC have accumulated in sediment at the site where an inlet of sewage from the city center was located nearby. Increasing trends of the metal contents in the sediment profile toward the surface at the sites distant from the sewage inlets imply that the loads of these metals into this lake have been continuously increasing. In addition, Pb isotopic ratios in sediment profile could be used as an indicator of anthropogenic Pb pollution in the lake. Keywords: heavy metals, lake sediment, spatial distribution

INTRODUCTION Municipal wastewater is not only the pollution source of organic matter and nutrients, but also that of heavy metals to water environment. For example, in Lake Teganuma, one of the most hypereutrophic lakes in Japan, high concentrations of heavy metals, especially Zn, had accumulated in the surface sediment, and the accumulations were most evident near the inlets of two main inflow rivers with urbanized watersheds (Saeki and Okazaki, 1993; Saeki et al., 1993). Elevated concentrations of toxic metals in natural water could have adverse effects not only on living organisms in the ecosystem but also on people ingesting fishery products. In addition, heavy metals in sediment could leach out and then contaminate the surrounding environment if the contaminated sediment is dredged and oxidized without control. Ho Tay (West Lake) is located in the north of central Hanoi, the capital city of Vietnam, and is the largest lake in the Red River (Hong River) Delta. This lake had originally been a segment of the course of the Red River, but is now a semi-enclosed water body with only small inflow and outflow channels (International Lake Environment Committee, 2001).


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In the urban area of Hanoi, domestic and industrial wastewater and stormwater runoff are discharged together without treatment into rivers and lakes through the combined sewers. It has been reported that the total wastewater generated and BOD pollution load in the catchment area of Ho Tay (4.1 km2) were 3200 m3/day and 1067 kg/day, respectively (Japan International Cooperation Agency, 2000). This wastewater load has caused moderate eutrophication in this lake (International Lake Environment Committee, 2001). As for heavy metal pollution in the water environment in Vietnam, it has been reported that high concentrations of Hg and Cr were frequently detected in the effluents from paper industry, and Cr(Ⅵ) was also in the wastewater from metal processing industry (Ishigaki and Chieu, 2003). However, no detailed study on the levels of heavy metal contamination in the inflowing wastewater, lake water and sediment of Ho Tay has been conducted. A portion of heavy metals discharged into a lake will be deposited on the bottom sediment, and they are considered to hardly migrate further within the sediment profile unless significant physical mixings of the sediment (e.g., dredging) occur, except for the elements which exhibit more than one oxidation state (e.g., As, Fe, Mn) (Boyle, 2001). This property of heavy metals enables the vertical distributions of the metals in lake sediment to be applied as an indicator of the “time trend” of heavy metal loads into the lake. Lake water and sediment samples (up to about 70 cm in depth) were collected, as well as the inflowing sewage, from several locations in Ho Tay from 2005 to 2007, and were analyzed for their heavy metal concentrations. The objectives of this paper are ⅰ) to assess the current condition of heavy metal contamination in the water and sediment of Ho Tay by comparing with their background levels in the world and those in other urban lakes, and ⅱ) to describe the recent trends of metal pollution in this lake from their vertical distributions in the sediments. As for Pb, analysis of Pb isotopic ratios, i.e., 207Pb/206Pb and 208Pb/206Pb, in sediment was also employed to obtain supporting information on the degree of anthropogenic Pb pollution. It is known that Pb isotopic compositions of Pb ore, the raw material of industrial Pb products, and coal vary with their locality, and Pb isotopic ratios in environmental samples can be utilized as the fingerprint of anthropogenic Pb pollution (Komárek et al., 2008). The findings in this study will provide useful information in the evaluation of heavy metal pollution in the urban lakes of other developing countries which receive municipal sewages. MATERIALS AND METHODS Site description A map of Ho Tay, including the locations of sampling stations and sewage inlets, is shown in Fig. 1. The surface area and mean depth of this lake are 4.13 km2 and 1.7 m, respectively (International Lake Environment Committee, 2001). This lake is surrounded by residential areas on the western shore and the central part of Hanoi on the southern shore. The eastern shore has been developed as a tourist spot and large hotels have been built. Fishery has also been conducted in this lake.


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During the survey in November 2007, two large sewage inlets into this lake were found. One of them (Sw1) is located on the south-eastern edge of the lake and the sewage is derived from the city center, while the other (Sw2) is located on the western shore and the sewage is from the neighboring residential area. The lake water around Sw1 turned blackish and had a septic odor. No significant discharge of wastewater from factories was observed. Sampling methods Sampling of lake water was conducted at stations S1 to S4 in October 2005 (end of rainy season), January 2006 (dry season) and June 2006 (beginning of rainy season). Lake water at the proximity of Sw2 was also collected in November 2007 and was used as a substitute for the sewage from Sw2, because the water level of the lake was so elevated at that time that the sewage water could not be taken directly from the inlet. Surface water was collected using a vandone-type water sampler. All the samples were collected in polyethylene bottles, kept at 4 ºC in a cooler box while being taken to the laboratory and then stored in a refrigerator until analysis. Surface sediment (0-10 cm in depth) was sampled at each station at the same time as water sampling using an Ekman-Birge grab. In addition, sediment core samples were collected at S1 to S3 in October 2005 and at S4 in June 2006. A single core sample was taken at each station from S1 to S3, whereas triplicate core samples were taken at S4. The core samples were cut at 3-cm interval from the surface. The surface sediment samples and each section of core samples were placed in polyethylene bottles and stored in a freezer. The sediment samples were then thawed and freeze-dried, and chemical analyses were performed for the freeze-dried samples. Polyethylene bottles used for sample storage were pre-cleaned with dilute HNO3 and then rinsed with Milli-Q water.

HoTay Red River

N

(a)

HoTay Red River

N

(a)

S1

S2

S3S4

City center

N

Residentialarea

Sw1

Sw2

(b)

0.68 km

S1

S2

S3S4

City center

NN

Residentialarea

Sw1

Sw2

(b)

0.68 km

Fig. 1 - A map showing the location of Ho Tay in Hanoi (a), and a map showing the locations of sampling stations (S1-S4) and sewage inlets (Sw1 and Sw2) (b).


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Chemical analysis of sample The pH of water samples was measured using a portable pH meter at each sampling station. In order to determine the total concentrations of heavy metals in a water sample, 50 mL of the sample was taken in a Teflon beaker and digested with 5 mL of HNO3 (60% (v/v)) on a hot plate at 95 ºC for 4 hours. After digestion, the sample was transferred into a plastic volumetric flask and adjusted to 50 mL with Milli-Q water. The sample was finally filtered through a syringe filter of 0.45-μm pore size (Whatman, Florham Park, NJ, USA) and analyzed for heavy metals (Cd, Cr, Cu, Mn, Ni, Pb and Zn) including a metalloid (As) using an ICP-MS equipped with octopole reaction system (7500c, Agilent Technologies, Santa Clara, CA, USA). Helium or H2 was used as the reaction gas. Internal standard and recovery for each element in the ICP-MS analysis are summarized in Table 1. In order to determine heavy metal contents in a sediment sample, 0.1 g of the freeze-dried sample was placed in a Teflon beaker and successively digested with 5 mL of HNO3 (60% (v/v)) at 120 ºC for 40 minutes, 1 mL of HClO4 (60% (w/w)) and 3 mL of HF (50% (w/w)) at 160 ºC for 1 hour, and 5 mL of HNO3 (60% (v/v)) at 120 ºC for 40 minutes. After cooling, the digest was transferred into a plastic volumetric flask and adjusted to 50 mL with Milli-Q water. The sample was filtered through a syringe filter of 0.45-μm pore size (ADVANTEC DISMIC-25HP, Toyo Roshi, Tokyo, Japan) and analyzed for heavy metals using an ICP-MS (7500c, Agilent Technologies, Santa Clara, CA, USA). Reaction gas, internal standard and recovery for individual element in the ICP-MS analysis are shown in Table 1. Total organic carbon (TOC) content in the sediment sample was analyzed using a TOC analyzer (TOC-VCPH, Shimadzu, Kyoto, Japan) equipped with a solid sample module (SSM-5000A, Shimadzu, Kyoto, Japan). To obtain supporting information on the degree of anthropogenic Pb contamination in sediment, Pb isotopic ratios in the sediment samples were also analyzed. Sample solution for Pb isotopic analysis was prepared by the same method as the case of the determination of heavy metal contents in sediment samples as described above. Two kinds of Pb isotopic ratios, i.e., 207Pb/206Pb and 208Pb/206Pb, in the sample were determined using an ICP-MS (7500c, Agilent Technologies, Santa Clara, CA, USA).

Table 1 - Mass number, internal standard and recovery for each element in the ICP-MS analysis.

Mean S.D. n Mean S.D. n Reaction gasAs 75 Ga 74.1 1.7 3 70.8 15.7 33 HeCd 111 In 77.8 1.7 3 101 16 31 HeCr 52 Ga / Sc a 83.3 0.7 3 87.7 3.6 33 HeCu 63 Ga 89.5 1.4 3 115 18 33 HeMn 55 Ga / Sc a 89.9 1.2 3 85.9 2.9 33 H2

Ni 60 Ga 114.4 27.6 3 95.4 6.9 33 HePb 205 / 208 a Tl 108.3 7.9 3 105 17 39 H2

Zn 66 Ga 70.1 19.2 3 99.3 11.9 33 He* Internal standard. m /z : 45 (Sc), 71 (Ga), 115 (In), 205 (Tl).† Water: NMIJ CRM7202-a ‘River Water’; Sediment: NMIJ CRM7303-a ‘Lake Sediment’a water / sediment

Element m /z ISTD* Sediment†Recovery (% for the certified value)

Water†


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Helium was used as the reaction gas. The Pb isotopic ratios of the samples were corrected by those of the standard reference material (SRM 981) ‘Common Lead Isotopic Standard’ (National Institute of Standard and Technology, Gaithersburg, MD) measured in the same batch. The percentages of the analyzed values for SRM 981 to the certified ones in the present study were 99.36 ± 0.87% and 99.83 ± 0.84% for 207Pb/206Pb and 208Pb/206Pb, respectively (n = 24 for each isotopic ratio). Beakers, volumetric flasks and bottles used for chemical analyses were pre-cleaned with dilute HNO3 and then rinsed with Milli-Q water. Nitric and perchloric acids (Wako, Osaka, Japan) used for the digestion of water and sediment samples in the determination of metal contents were for the analysis of poisonous metals, and HF (Wako, Osaka, Japan) was of special grade. On the other hand, the ‘Ultrapur-100’ grade HNO3 and HF (Kanto Chemical, Tokyo, Japan) and the ‘TAMAPURE-AA-100’ grade HClO4 (Tama Chemicals, Kanagawa, Japan) were used for the digestion of sediment samples in Pb isotopic analysis. RESULTS AND DISCUSSION Heavy metal concentrations in lake water and inflow sewage The concentrations of heavy metals in lake water of Ho Tay and the inflowing sewage from Sw2 are shown in Fig. 2. Concentrations of Cd in all the water samples in this study were lower than the detection limit (< 0.1 μg/L). The differences in metal concentrations in the lake water among the sampling stations (S1 to S4) were generally small, suggesting that they are uniformly distributed in water of the whole lake. Concentrations of all the metals analyzed were lower than the maximum allowable concentrations adopted in the Vietnamese Surface Water Quality Standards (TCVN 5942-1995; for the water as source of domestic water supply), although As, Mn and Ni in all the sampling time and all the other elements except for Pb in October 2005 were higher than the world’s average values for freshwater (Bowen, 1979) by 1.10-39.4 times. However, concentrations of heavy metals in the sewage from Sw2 were generally higher than those in the lake water by 1.17-9.2 times, with some exceptions. Although this result is only for a single sample, it might support the sewage from the surrounding urban area as the major source of the metals into this lake. As shown in Table 2, the lake water at each station was alkaline at all times of sampling in the present study. Alkaline pH in lake water would promote the transformation of heavy metal cations into suspended forms and their depositions onto bottom sediment, because in the presence of free oxygen insoluble metal carbonates and hydroxides become stable at pH values higher than 8 (Förstner, 1981). Sorption of metal cations onto suspended solids would be also enhanced at higher pH. The pH value was higher in summer (June 2006) than in winter (January 2006) as seen in Table 2.


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0.0

5.0

10.0

15.0

20.0

25.0

As Cr Cu Ni Pb Zn

Con

cent

rati

on (μ

g L

-1)

Lake water (October 2005)

Lake water (January 2006)

Lake water (June 2006)

Sw2 (November 2007)

0

20

40

60

80

100

120

Mn

Con

cent

rati

on (μ

g L

-1)

0.0

5.0

10.0

15.0

20.0

25.0

As Cr Cu Ni Pb Zn

Con

cent

rati

on (μ

g L

-1)

Lake water (October 2005)

Lake water (January 2006)

Lake water (June 2006)

Sw2 (November 2007)

0

20

40

60

80

100

120

Mn

Con

cent

rati

on (μ

g L

-1)

Fig. 2 - Concentrations of heavy metals in lake water of Ho Tay and the inflowing

sewage from the surrounding residential area (Sw2). An error bar represents the standard deviation of the values for four sampling stations (S1 to S4).

S1 S2 S3 S4Temperature October 2005 29.4 30.2 30.5 30.0

(℃) January 2006 16.3 16.0 16.0 16.1June 2006 33.3 33.4 33.0 32.5

pH October 2005 8.60 8.80 9.06 8.73January 2006 8.67 8.30 8.10 7.43

June 2006 8.98 9.16 9.20 9.40

StationDate

Heavy metal concentrations in surface sediment The concentrations of heavy metals in the surface sediment of Ho Tay are summarized in Table 3. The world averages for sediment (Bowen, 1979) and the values for the surface sediments of Lake Teganuma and Moshui Lake, urban lakes in Japan and China, respectively, with similar geographical characteristics to Ho Tay (e.g., surface area: 6.5 km2 and 3.14 km2; average depth: 0.86 m and 1.2 m for Lake Teganuma and Moshui Lake, respectively) (Saeki et al., 1993; Chiyo, 2004; Liu et al., 2008) are also listed for comparison. In general, metal contents in the surface sediment at each station decreased in the following order: S4 > S2 > S1 > S3. Concentrations of heavy metals in sediment at S4, where a sewage inlet from the city center is located nearby, were higher than those at the other stations. This was similar to the case in Lake Teganuma where the accumulations of heavy metals in sediment were most evident near the inlets of two main inflow rivers (Saeki et al., 1993). At less polluted stations (S1 to S3), concentrations of As, Cd, Cr, Cu, Pb and Zn in surface sediment were consistent with or higher than the world averages, while Mn and Ni were lower. At the most polluted station (S4), Cd, Cu, Mn, Ni, Pb and Zn concentrations were lower than the values detected in Lake Teganuma and Moshui Lake, although Cr concentration was still higher than those in the two lakes. It has been reported that Cr is frequently detected in high concentration in the wastewater from paper and metal processing industries in Vietnam (Ishigaki and Chieu, 2003). Concentrations of Cd, Ni and Pb in the surface sediment of Ho Tay tended to be higher in dry season (January 2006) than in rainy season (October 2005 and June 2006),

Table 2 - Temperature and pH of the surface water of Ho Tay.


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although such seasonal variations were not clear for other metals (data not shown). Metal contents in surface sediment can be affected by various factors, including additional depositions of the metals onto sediment surface, resuspension of fine sediment particles enriched with the metals, and affinities of the metal ions with the suspended solids.

Vertical distributions of heavy metals and TOC in lake sediment The vertical distributions of heavy metals and TOC in sediment at each station of Ho Tay are illustrated in Fig. 3. A higher concentration of TOC was detected throughout the sediment profile (0-66 cm in depth) at S4 as compared to the other stations, which suggests that the polluted mud derived from the sewage from the city center had been deposited up to at least 70 cm thick at S4. It has been also reported that mud deposits 50-80 cm in thickness were found on the flat bottom of this lake (International Lake Environment Committee, 2001). The heavy metals analyzed are generally classified into the following two groups with regard to the distribution pattern in sediment profile: Group Ⅰ- Cd, Cr, Cu, Ni, Pb and Zn, and Group Ⅱ- As and Mn. Concentrations of the metals in Group Ⅰ in the sediment profile at S4 (near the sewage inlet from the city center) from the surface down to 30-50 cm were higher than those in the surface sediment at the other stations. Especially, accumulations of Cd, Pb and Zn in sediment at S4 were substantially higher than those at S2 (near the sewage inlet from the residential area). This implies that the sources of these metals deposited in sediment at S4 are not only domestic wastewater, which would be the main source for S2, but also industrial effluents and stormwater runoff. Our previous survey on heavy metal pollution in river sediment of Hanoi also revealed that extremely high concentrations of Cd and Zn were deposited in the river sediment where a factory of plastics was located nearby (Kikuchi et al., 2009). At S4, concentrations of these metals increased almost linearly in the profile from the deepest layer (63-66 cm) up to 21 cm,

World Lake Moshuiaverage a Teganumab Lake c

As 7.7 –

Cd 0.17 1.95Cr 72 86.1 70 - 100Cu 33 203 135 - 170Mn 770 1360Ni 52 89.9 56 - 68Pb 19 113 140 - 160Zn

S1

7.59 (1.89)0.36 (0.30)

78.4 (29.5)49.4 (6.8)

513 (21)40.5 (16.1)55.2 (8.4)

108 (9)

S2

8.93 (2.23)0.25 (0.05)

93.5 (26.8)56.9 (4.4)

515 (33)49.4 (23.1)61.5 (7.5)

150 (32)

S4

11.7 (5.17)0.71 (0.26)

109 (27)73.0 (6.5)

563 (19)51.5 (17.6)88.2 (15.3)

256 (38) 95 935 380 - 440* Mean value (standard deviation in parenthesis) for three sampling events (October 2005, and January and June

2006) except for As and Mn, which are the means for two sampling events (October 2005 and June 2006). World average for sediment (Bowen, 1979). Surface sediment in Lake Teganuma, Japan (the highest value) (Saeki et al., 1993). Surface sediment in Moshui Lake, China taken at the proximity of an outfall of municipal sewage and surface runoff. (Concentration range within the upper 10 cm is indicated.) (Liu et al., 2008)

–: No data

a

S3

6.26 (0.89)0.34 (0.23)

58.2 (23.4)33.5 (8.0)

512 (98)30.8 (10.2)44.1 (4.4)

102 (26)

Station

–

–

bc

–

World Lake Moshuiaverage a Teganumab Lake c

As 7.7 –

Cd 0.17 1.95Cr 72 86.1 70 - 100Cu 33 203 135 - 170Mn 770 1360Ni 52 89.9 56 - 68Pb 19 113 140 - 160Zn

S1

7.59 (1.89)0.36 (0.30)

78.4 (29.5)49.4 (6.8)

513 (21)40.5 (16.1)55.2 (8.4)

108 (9)

S1

7.59 (1.89)0.36 (0.30)

78.4 (29.5)49.4 (6.8)

513 (21)40.5 (16.1)55.2 (8.4)

108 (9)

S2

8.93 (2.23)0.25 (0.05)

93.5 (26.8)56.9 (4.4)

515 (33)49.4 (23.1)61.5 (7.5)

150 (32)

S2

8.93 (2.23)0.25 (0.05)

93.5 (26.8)56.9 (4.4)

515 (33)49.4 (23.1)61.5 (7.5)

150 (32)

S4

11.7 (5.17)0.71 (0.26)

109 (27)73.0 (6.5)

563 (19)51.5 (17.6)88.2 (15.3)

256 (38)

S4

11.7 (5.17)0.71 (0.26)

109 (27)73.0 (6.5)

563 (19)51.5 (17.6)88.2 (15.3)

256 (38) 95 935 380 - 440* Mean value (standard deviation in parenthesis) for three sampling events (October 2005, and January and June

2006) except for As and Mn, which are the means for two sampling events (October 2005 and June 2006). World average for sediment (Bowen, 1979). Surface sediment in Lake Teganuma, Japan (the highest value) (Saeki et al., 1993). Surface sediment in Moshui Lake, China taken at the proximity of an outfall of municipal sewage and surface runoff. (Concentration range within the upper 10 cm is indicated.) (Liu et al., 2008)

–: No data

a

S3

6.26 (0.89)0.34 (0.23)

58.2 (23.4)33.5 (8.0)

512 (98)30.8 (10.2)44.1 (4.4)

102 (26)

S3

6.26 (0.89)0.34 (0.23)

58.2 (23.4)33.5 (8.0)

512 (98)30.8 (10.2)44.1 (4.4)

102 (26)

Station

–

–

bc

–

Table 3 - Concentrations of heavy metals in the surface sediment of Ho Tay (mg kg-1 dry weight).*


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and they were maintained at almost the same levels or slightly increased toward the surface in the upper layer. At the other stations, they generally increased consistently in the profile from around 25 cm up to the surface layer. Assuming that the vertical distributions of these metals in sediment at S3, which is distant from the two sewage inlets (Sw1 and Sw2), reflect the general trends of the metal loads into this lake, the metal loads suggest to have been continuously increasing. Especially, concentrations of Cr, Cu and Ni drastically increased in their vertical distributions from 25 cm toward the surface, suggesting that the amount of these metals used in and discharged from the lake basin has been rapidly increasing.


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Fig. 3 - Vertical distributions of heavy metals and TOC in sediment at each station of Ho

Tay (DW: dry weight). An error bar represents the standard deviation of the values for triplicate core samples.

0

10

20

30

40

50

60

70

0.0 5.0 10.0 15.0 20.0 25.0 30.0

As concentration (mg kg-1DW)D

epth

(cm

)

As

□ S1▲ S2● S3◆ S4

0

10

20

30

40

50

60

70

0.00 0.25 0.50 0.75 1.00 1.25 1.50

Cd concentration (mg kg-1DW)

Dep

th (

cm)

Cd

0

10

20

30

40

50

60

70

0 25 50 75 100 125 150 175 200

Cr concentration (mg kg-1DW)

Dep

th (

cm)

Cr

0

10

20

30

40

50

60

70

0 20 40 60 80 100 120

Cu concentration (mg kg-1DW)

Dep

th (

cm)

Cu

0

10

20

30

40

50

60

70

0 500 1000 1500 2000 2500

Mn concentration (mg kg-1DW)

Dep

th (

cm)

Mn

0

10

20

30

40

50

60

70

0.0 10.0 20.0 30.0 40.0 50.0 60.0 70.0

Ni concentration (mg kg-1DW)

Dep

th (

cm)

Ni

0

10

20

30

40

50

60

70

0 20 40 60 80 100 120 140

Pb concentration (mg kg-1DW)

Dep

th (

cm)

Pb

0

10

20

30

40

50

60

70

0 100 200 300 400 500

Zn concentration (mg kg-1DW)

Dep

th (

cm)

Zn

0

10

20

30

40

50

60

70

0.0 20.0 40.0 60.0 80.0

TOC (g kg-1DW)

Dep

th (

cm)

TOC


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As for the metals in Group Ⅱ, the peaks of As concentration in the sediment profile at S2 and S4 appeared at around 20 cm and 35 cm in depth, respectively, whereas its content increased linearly toward the surface in other stations. On the other hand, a significant accumulation of Mn in the sediment profile was observed at about 25 cm in depth at S1 to S3, although it was concentrated at around 65 cm at S4. It is generally recognized that the oxidation states and behaviors of As and Mn are closely related to the redox condition in sediment. (Arsenic is known to be well adsorbed onto Fe hydroxides, which are vulnerable to redox state in sediment.) However, the relationship between the distributions of As and Mn and that of redox potential (ORP) in the sediment profiles of this lake could not be elucidated in the present study because measurement of ORP was not conducted at the time of sampling. The vertical distributions of two kinds of Pb isotopic ratios, i.e., 206Pb/207Pb and 208Pb/206Pb, in sediment at each station of Ho Tay are described in Fig. 4. For station S4, only one of the triplicate core samples was analyzed for Pb isotopic composition. No clear tendency was recognized for the vertical distribution of 206Pb/207Pb at each station, although the value at S4 seemed to be lower than those at the other stations (Fig. 4 (a)). There was also no characteristic pattern in the vertical distribution of 208Pb/206Pb at each station, but the value clearly decreased in the following order: S4 > S2 ≒ S1 > S3 (Fig. 4 (b)). Neither of the two Pb isotopic ratios was significantly correlated with the Pb concentration at each station. The relationship between 206Pb/207Pb and 208Pb/206Pb in the sediment profile at each station is shown in Fig. 5. The two kinds of Pb isotopic ratios at S1, S2 and S4 were significantly negatively correlated with each other (r = -0.842, p < 0.01). As the degree of anthropogenic pollution progressed, the 206Pb/207Pb value tended to decrease whereas the 208Pb/206Pb increased. On the other hand, the relationship between the two Pb isotopic ratios at S3 was different from that at the other stations, which suggests that the deposition of anthropogenic Pb in the sediment has not been significant at this station. This would be supported by the fact that at S3 no clear increasing trend in Pb content in the sediment profile toward the upper layer, which was seen at the other stations, was recognized (Fig. 3). Thus, the relationship between the two Pb isotopic ratios could be used as an indicator of the degree of anthropogenic Pb contamination in sediment. The Pb in the inflowing sewages into Ho Tay is considered to be mainly derived from industrial effluents and stormwater runoff generated in the catchment area. Factories manufacturing or using battery, inorganic chemicals, electric wires and solder could be the source of Pb through their wastewater (Japan Environmental Sanitation Center, 2005). Vehicle exhaust, filler materials in tires and paint for road markings have been reported to be the major sources of Pb in stormwater runoff (Makepeace et al., 1995; Ozaki et al., 2004). In addition, the use of leaded gasoline, which has been banned since July 2001 in Vietnam (Vietnam Association for Conservation of Nature and Environment, 2004), could have been one of the major sources of Pb deposited in sediment profile via atmospheric deposition and inflow of roadside dust with stormwater runoff (Duzgoren-Aydin, 2007; Komárek et al., 2008). It is thus needed to know the origins and the isotopic compositions of Pb contained in these materials which have been produced or used in Hanoi to evaluate the sources of Pb in the sewages more precisely.


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CONCLUSIONS Water, inflowing sewage and sediment samples had been collected from Ho Tay, an urban lake in Hanoi, and analyzed for their heavy metal concentrations in order to assess the current situation and recent trend of metal contamination in this lake. The results obtained in this study are as follows: ⅰ) Concentrations of all the metals analyzed in lake water were lower than the maximum allowable limits adopted in the surface water quality standards of Vietnam.

0

10

20

30

40

50

60

70

1.16 1.18 1.20 1.22

206Pb / 207PbD

epth

(cm

)

(a)

2.05 2.07 2.09 2.11 2.13

208Pb / 206Pb

S1

S2

S3

S4 (b)

Fig. 4 - Vertical distributions of Pb isotopic ratios in sediment at each station of Ho Tay: 206Pb/207Pb (a) and 208Pb/206Pb (b).

2.050

2.060

2.070

2.080

2.090

2.100

2.110

2.120

2.130

1.160 1.170 1.180 1.190 1.200 1.210206Pb / 207Pb

208 P

b / 20

6 Pb

S1

S2

S3

S4

Fig. 5 - Relationship between 206Pb/207Pb and 208Pb/206Pb in sediment profile at each station of Ho Tay.


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However, the metal concentrations in an inflowing sewage were higher than those in the lake water by 1.17-9.2 times, which might imply that the sewage from the surrounding urban area would be the major source of the metals because this lake is a semi-enclosed system without any other possible sources such as rivers and industrial wastewater. ⅱ) Concentrations of heavy metals in surface sediment at the site, where the sewage inlet from the city center is located nearby, were higher than those at the other sites. Chromium content in surface sediment of this lake was higher than those of two eutrophic lakes in Asia, although the concentrations of other metals were smaller. ⅲ) Higher concentrations of heavy metals (except for Mn) and TOC had accumulated in sediment from the surface to deeper layers at the site near the sewage inlet from the city center as compared to those at the other sites. Especially, accumulations of Cd, Pb and Zn in sediment at this site were substantially higher than those near the sewage inlet from the residential area, which suggests that the sources of these metals at the former site are not only domestic wastewater but also industrial effluents and stormwater runoff. Metal contents in sediment profiles at the other sites also generally increased consistently toward the surface layer, which implies that the metal loads into this lake have been continuously increasing. ⅳ) The two kinds of Pb isotopic ratios, i.e., 207Pb/206Pb and 208Pb/206Pb, in sediment profiles were significantly negatively correlated with each other: as the degree of anthropogenic pollution progressed, the 206Pb/207Pb value tended to decrease whereas the 208Pb/206Pb increased. This relationship could be used as an indicator of the degree of anthropogenic Pb contamination in sediment. In conclusion, the loads of heavy metals into this lake from inflowing municipal sewage have been continuously increasing, and the metals have been mainly accumulated in sediment around the sewage inlets. Thus, treatments of the inflowing wastewater are necessary to be practiced at first to prevent the progress of metal pollution in this lake. ACKNOWLEDGEMENT We thank Ms. Tran La Minh, Mr. Ha Vinh Hung, Dr. Nguyen Pham Hong Lien, Ms. Nguyen Thi Van and Mr. Le Dao in INEST for their assistance in the sampling period. We are also very grateful to Dr. Takuma Furuichi of Mitsubishi Chemical Co., Ltd., and Mr. Haruki Sagami and Mr. Kazuo Taku of ACER for their cooperation in the collection and chemical analyses of samples. REFERENCES Bowen H.J.M. (1979). Environmental Chemistry of the Elements. Academic Press,

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characterization of heavy metal contamination from their

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