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Speciation of mercury in water at the bottom of Minamata Bay, Japan
Marine Chemistry 112 (2008) 102–106
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Marine Chemistry j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a r c h e m
Speciation of mercury in water at the bottom of Minamata Bay, Japan Takashi Tomiyasu a,⁎, Akito Matsuyama b, Tomomi Eguchi b, Kohji Marumoto b, Kimihiko Oki c, Hirokatsu Akagi b a b c d
Faculty of Science, Kagoshima University, Korimoto, Kagoshima 890-0065, Japan National Institute for Minamata Disease, Minamata, Kumamoto 867-0008, Japan Kagoshima University Museum, Korimoto, Kagoshima 890-0065, Japan International Mercury Laboratory Inc., Minamata, Kumamoto 867-0034, Japan
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Article history: Received 8 November 2007 Received in revised form 4 July 2008 Accepted 7 July 2008 Available online 12 July 2008 Keywords: Mercury pollution Speciation of mercury in water Marine sediment Minamata Bay Remobilization of mercury
a b s t r a c t Total mercury (T-Hg) and methylmercury (MeHg) concentrations were determined in water at the bottom, in the suspended solid, and in the surface sediment of Minamata Bay to assess the remobilization of mercury from the sediment into the water column. The water and sediment samples were taken from the bottom of the bay using a gravity core sampler at nine locations in October 2002 and six locations in April 2005. The average concentration of T-Hg and the proportion of MeHg in the sediment were 3.71 ± 1.90 mg/kg and 0.27 ± 0.28%, respectively. The water contained 1.80 ± 1.00 ng L− 1 of T-Hg, which was higher than the value reported for the upper-middle depth of Minamata Bay. The results suggest that the sediment is an important source of mercury in the water of Minamata Bay. The percentage of MeHg in the water at the bottom was 50.7 ± 24.6%, also considerably higher than in the upper-middle layer of the water column, suggesting that MeHg may be the predominant form of mercury released from the sediment into the water. The percentage of MeHg was lower in the resuspended sediment than in the surface sediment. The amount of mercury eluted from the sediment into the water was estimated at 0.46 kg and 0.11 kg per year for T-Hg and MeHg, respectively. © 2008 Elsevier B.V. All rights reserved.
1. Introduction From 1932 until 1968, mercury-contaminated effluent was discharged into Minamata Bay from an acetaldehyde-producing factory (Minamata City, 2000). As much as 380–455 t of mercury was used and about 250 t was deposited in Minamata Bay during this period. Total mercury (T-Hg) concentrations as high as 2000 mg kg− 1 were found in sediment near the overflow of the plant (Kitamura et al., 1960). The Minamata Bay Pollution Prevention Project (1977– 1990) disposed of sedimentary sludge containing more than 25 mg kg− 1 of mercury. Through this project, 1,510,000 m3 of polluted sediment was dredged, and the heavily contaminated area near the overflow of the plant was reclaimed (Fig. 1). Just after the project, mercury concentrations of 0.06– 12 mg kg− 1 in the upper layer of sediment were reported ⁎ Corresponding author. Tel.: +81 99 285 8107; fax: +81 99 259 4720. E-mail address: [email protected] (T. Tomiyasu). 0304-4203/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.marchem.2008.07.003
(Kumamoto Prefecture, 1998). However, these values are still about 50 times higher than the background level for this area (Tomiyasu et al., 2006). Several investigations have reported on mercury concentrations in the Yatsushiro Sea sediment (Kitamura et al., 1960; Sakamoto et al., 1995; Kudo et al., 1998; Tomiyasu et al., 2000), and some studies reported the transport of mercury with sediment from Minamata Bay to Yatsushiro Sea (Kumagai and Nishimura, 1978, Yano et al., 2003, Rajar et al., 2004). When contaminated sediment is resuspended and transported, some mercury species may dissolve in the water column (Covelli et al., 1999; Gill et al., 1999). Logar et al. (2001) reported on T–Hg and methylmercury (MeHg) concentrations being present in the water from the upper and middle layers of Minamata Bay, which are values higher than in uncontaminated areas. However, the concentration of mercury in the water just above the sediment has not been previously investigated in Minamata Bay. In this study, T–Hg and MeHg concentrations were determined from water at the bottom, in suspended solids,
T. Tomiyasu et al. / Marine Chemistry 112 (2008) 102–106
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Fisheries Research Laboratory, Kagoshima University. Bottom water samples were taken with surface sediment using a PHLEGER gravity core sampler (Hisanaga Co., Ltd.) at nine locations in October 2002 and six locations in April 2005. The length and inside diameter of the inner tube of the sampler were 60 cm and 3.3 cm, respectively. The water was taken in the 20–40 cm part of the inner tube (170–340 mL) on top of the sediment. Although the upper part of water layer was clear, the lower layer just above the sediment (several centimeters) was turbid, which may include some sediment that was easily resuspended during the sampling process. The clear and turbid layers were put into separate bottles. The uppermost part of the sediment core (surface sediment) was then cut at a thickness of 2 cm and brought back to the laboratory. To determine mercury concentrations, the clear layer of water at the bottom was filtrated with a 0.45-μm Millipore filter. The turbid layer of the bottom water was centrifuged, and the concentration of mercury in the supernatant was determined after filtration with a 0.45-μm Millipore filter. The mercury concentration was also determined in the solid remaining in the centrifuge tube. 2.2. Mercury analysis 2.2.1. Measurement of mercury concentrations in surface sediment and suspended solid The concentration of mercury was measured using the method proposed by Akagi and Nishimura (1991) and modified by Akagi et al. (1995). The precision and accuracy of the technique has been repeatedly verified by interlaboratory calibration exercises (Matsuo et al., 1989; Malm et al., 1995), including an analysis of reference standards (e.g., IAEA 085 and 086). An overview of the analytical procedures is given here. For quantification of the total mercury concentration, a known amount of sample was placed in a 50-mL volumetric flask, to which 1 mL of distilled water, 2 mL of a 1:1 nitric acid-perchloric acid solution, and 5 mL of concentrated sulfuric acid were added. After standing for a few minutes, the volumetric flask was heated on a hot plate at 230 °C for 20 min. After cooling, the digested sample was made up to 50 mL with water and a suitable aliquot of the resulting solution (b20 mL) was analyzed for mercury by cold vapor atomic absorption spectrometry using a semi-
Fig. 1. Map showing the study area. The dredged spoils were placed in the Reclaimed Area.
and in surface sediment to assess the remobilization of mercury from the sediment into the water column. 2. Materials and methods 2.1. Sampling The sampling points are shown in Fig. 1. The positioning of the sampling sites was determined with a Furuno GPS plotter GP-1500 (FCV-663) installed on the research vessel of the
Table 1 Total mercury and methylmercury concentrations in solid samples October 2002
April 2005
Sediment St
M1 M2 M3 M4 M5 F1 F2 F3 O1 ave. s.d.
ss
Sediment
ss
T-Hg
MeHg
Me/T
T-Hg
MeHg
Me/T
T-Hg
MeHg
Me/T
T-Hg
MeHg
Me/T
(mg kg− 1)
(μg kg− 1)
(%)
(mg kg− 1)
(μg kg− 1)
(%)
(mg kg− 1)
(μg kg− 1)
(%)
(mg kg− 1)
(μg kg− 1)
(%)
1.58 2.21 2.47 2.33 4.43 3.93 3.63 6.38 6.56 3.72 1.80
4.6 4.0 6.3 4.4 6.6 28.2 36.7 29.8 6.6 14.1 13.3
0.30 0.18 0.26 0.19 0.16 0.72 1.00 0.47 0.10 0.38 0.30
2.64 2.70 2.53 2.76 4.10 4.16 5.44 6.62 5.41 4.04 1.51
2.4 1.1 6.0 4.0 5.4 18.2 34.2 21.5 7.1 11.1 11.2
0.09 0.04 0.24 0.14 0.13 0.44 0.63 0.33 0.13 0.24 0.19
0.61 1.84 – 6.73 – 3.55 4.61 4.84 – 3.70 2.21
2.6 1.0 – 1.7 – 2.3 1.7 1.1 – 1.7 0.7
0.43 0.05 – 0.02 – 0.07 0.04 0.02 – 0.11 0.16
2.80 2.64 – 4.44 – 4.68 6.99 7.82 – 4.90 2.13
3.7 3.0 – 4.6 – 2.8 2.9 4.8 – 3.6 0.9
0.13 0.11 – 0.10 – 0.06 0.04 0.06 – 0.09 0.04
104
T. Tomiyasu et al. / Marine Chemistry 112 (2008) 102–106
automated mercury analyzer (Model Hg-3500, Sanso, Japan). For the MeHg analysis, sediment samples (ca. 1 g) were shaken with 10 mL of 1 M potassium hydroxide in ethanol for 10 min using a mechanical shaker. The treated sediment was acidified with hydrochloric acid and was bubbled with nitrogen gas for 5 min. The sample was then mixed with 2 mL of 20% hydroxylamine hydrochloride and 2 mL of 20% EDTA and shaken with 5 mL of purified 0.05% dithizone in toluene. The toluene layer was transferred into a 10-mL test tube and washed twice with 3 mL of 1 M sodium hydroxide. The toluene layer was transferred into another test tube, and the MeHg contained in the organic layer was back-extracted with 2 mL of a 5-ppm sodium sulfide solution. After the toluene layer was removed, the aqueous layer was washed with 2 mL of toluene and acidified with 3–4 drops of 1 M hydrochloric acid. The excess sulfide ions were removed by bubbling with nitrogen gas for 5 min, and then MeHg was reextracted into 0.2 mL of purified 0.01% dithizone in toluene. The toluene layer was washed with 1 M sodium hydroxide and with distilled water and then subjected to ECD gas chromatography for the quantification of MeHg. The mercury concentration in solid samples, based on wet weight, was converted to a dry weight value by using the water content, which was measured separately by heating the sample at 110 °C for 6 h. 2.2.2. Determination of mercury in water samples The total concentration of mercury was determined based on the preconcentration of mercury from water samples by extraction of mercury dithizonates (Hg–Dz). In the first step, the simultaneous extraction of Hg2+ and CH3Hg+ dithizonates into toluene was performed. An aliquot of this extract was used to determine the total concentration of mercury. The organic solvent in the aliquot was removed by evaporation in a rotavapour. The remaining Hg–Dz complex was decomposed by acid digestion, and mercury content was measured by CVAAS using SnCl2 reduction. The limit of detection (LOD) was 0.01 ng L− 1 in the 2 L of water sample used. As described above, after removal of an aliquot, the remaining extract was used for the quantification of MeHg. MeHg was stripped from the organic solvent into an aqueous Na2S solution. After the removal of H2S by evaporation, MeHg–Dz was back-extracted into a small volume of organic solvent and detected by gas chromatography coupled with electron capture detection (GCECD). The LOD was 0.02 ng L− 1 in the 2 L of water sample used. The system was calibrated with the use of a standard solution of MeHg prepared in an aqueous cysteine solution. Recoveries obtained on the basis of the standard addition method were nearly quantitative, so no recovery factors were necessary for the calculation of final results. The repeatability of measurements was about 3–5%. 3. Results and discussion 3.1. Mercury concentrations in sediment and suspended solid As shown in Table 1, the total concentration of mercury ranged from 0.61 to 6.73 mg kg− 1 (mean, 3.71 ± 1.90 mg kg− 1) in the surface sediment and from 2.53 to 7.82 mg kg− 1 (mean, 4.38 ± 1.76 mg kg− 1)in the suspended solid. There was no large difference between the sediment and suspended solid concentrations. These values are about 70 times higher than the estimated background concentration (Tomiyasu et al., 2006) in this area. As shown in
Fig. 2. The correlation of mercury concentrations between the sediment and suspended solid [Fukuro Bay (●), Minamata Bay ( ), outside of Minamata Bay (○)]; (a) T–Hg concentration, (b) MeHg concentration, (c) percentage of MeHg. The dashed lines indicate y = x.
T. Tomiyasu et al. / Marine Chemistry 112 (2008) 102–106 Fig. 2(a), when the T–Hg concentration in the suspended solid was plotted against that in sediment, a significant correlation was observed (r = 0.72, p b 0.01). This result suggests that part of the sediment can readily be suspended, and the mercury in the sediment can move with the particles. The MeHg concentration in the sediment and suspended solid was 1.01– 36.7 µg kg− 1 (mean, 9.17 ± 11.86 µg kg− 1) and 1.07–34.2 µg kg− 1 (mean, 8.12 ± 9.26 µg kg− 1), respectively. This corresponds to 0.27 ± 0.28% and 0.18 ± 0.17% of the total concentration of mercury, respectively. The repeatabilities were checked by making duplicate measurements of the samples, which had variabilities that ranged from 0.6 to 10.2% (mean, 5.5%). In 2002, the percentage of MeHg was higher in Fukuro Bay where the sediment was not dredged, but in 2005, the value had dropped and had no significant difference from Minamata Bay. The reason for this is not clear and long-term monitoring accounting for seasonal changes may be required to elucidate the behavior of mercury in the bay. The correlations between the sediment and suspended solid for MeHg concentrations and MeHg percentages are shown in Fig. 2(b) and (c), respectively. Although a linear relation with high correlation coefficients is observed, the slope of the line for the plots of percentages of MeHg in sediment vs. suspended solid, 0.54, is significantly smaller than 1 (Fig. 2(c)). This may suggest that MeHg is being eluted from suspended particles into the water column. 3.2. Mercury concentrations in water samples The results are shown in Table 2. In 2002, three to four samples were taken at each sampling point. The volume of water obtained in the gravity core sampler varied. The mercury concentration in each sample of clear water was measured and tended to increase with a decrease in the volume of the water sample. This suggests that water containing a higher concentration of mercury lies near the bottom. The relative standard deviations in three or four samples of clear water at each station were 10–80% (mean 45%) and 5–46% (mean 27%) for T–Hg and MeHg, respectively. On the other hand, the turbid layer was combined for each sampling point, since the volume obtained in a single sampling was too small to use for measurements. In 2005, although only one sample was taken from one station, special attention was payed to take the same volume, ca. 200 mL. The T–Hg concentrations in the water samples taken in 2002 and 2005 were in the range of 1.3–4.3 ng L− 1 (mean, 2.3 ± 1.0 ng L− 1) and 0.84–1.64 ng L− 1 (mean, 1.0± 0.30 ng L− 1), respectively. These values were higher than the concentration of T–Hg reported in the upper-middle depths of Minamata bay, 0.78± 0.57 ng L− 1 (Logar et al., 2001). The mercury concentration was higher in turbid water than the upper clear water layer. As shown in Fig. 3, the concentration increased near the sediment, which suggests that the sediment is an important source of mercury in water of Minamata Bay. The MeHg concentration in the water samples ranged from 0.32 to 1.65 ng L− 1 (mean, 0.91 ± 0.39 ng L− 1) in 2002 and 0.28 to 0.77 ng L− 1 (mean, 0.53 ± 0.20 ng L− 1 ) in 2005. This was 48.2 ± 25.0% and 54.4 ± 25.8%, respectively, of the T–Hg concentration,. These values are considerably higher than the 0.045 ± 0.018 ng L− 1 and 7.6 ± 3.7% reported for the MeHg concentration and %MeHg in the upper-middle depth of Minamata Bay (Logar et al., 2001). The concentration of MeHg and percentage of T–Hg in turbid water measured in 2002 was 2.23 ± 1.55 ng L− 1 and 24.1%, respectively.
105
The MeHg concentration was higher in the turbid than the clear layer of water at the bottom. This result supports the elution of mercury from the sediment. It should also be considered that the turbid water layer was affected by porewater from the sediment during the sampling with the gravity core sampler. 3.3. Movement of mercury from sediment into water column The percentage of MeHg in the solid phase, 0.09–0.38%, was about two orders of magnitude lower than the 24–54% in the clear or turbid layers of water at the bottom. As described above, the gradation of the T–Hg concentration suggests the elution of mercury from sediment. Bloom and Lasorsa (1999) reported that MeHg was two orders of magnitude less strongly bound to sediment than inorganic mercury. In addition about 80% of all the mercury in pore water near the sediment surface was MeHg. The large difference in percentages of MeHg observed between the solid and water phases in the current study strongly suggests that MeHg is the predominant form of mercury at the elution step. The average T–Hg and MeHg concentrations in sediment are 3.7 mg kg− 1 and 9.2 µg kg− 1, respectively. By considering the slope of 0.54 in Fig. 2(c), 46% of the MeHg in sediment was released into the water column through resuspension. Thus it was roughly estimated that 4.2 µg of MeHg was released from 1 kg of sediment. With the percentage of MeHg in the turbid layer as 24%, the total amount of mercury released from 1 kg of sediment can be estimated at 17.5 µg. Rajar et al. (2004) calculated that the amount of sediment resuspended and transported in Minamata Bay is 26,500 t per year. Thus, the amount of mercury eluted annually would be approximately 0.46 kg Hg/year total and 0.11 kg Hg/year of MeHg. The volume of water in Minamata Bay has been estimated at 2.5 × 107 m3 (Rajar et al., 2004). If Minamata Bay was closed, the mercury concentration in water would reach 19 ng L− 1 of T–Hg and 4.4 ng L− 1 of MeHg based on the amount of mercury eluted from the resuspended sediment. However, the mercury concentration should decrease with dilution through the exchange of seawater with the Yatsushiro Sea and/or by adsorption on particles in the water column. However, MeHg can be readily absorbed by aquatic organisms and accumulate in the aquatic food chain, ultimately reaching significant levels in fish. Sato et al. (1997) reported higher MeHg concentrations in the adductor muscles of mussels collected near the reclaimed area of Minamata Bay. Thus, there is also a need to continue to monitor the levels of mercury in water and to estimate the levels of mercury in organisms from this area. Mason et al. (2006) reported that the MeHg flux is not only due to its release because of a pore water gradient or due to desorption from the solid phase, but potentially due to its in situ production in the upper layers of the sediment. In our study, the amount of mercury desorbed was estimated from the difference in the percentage of MeHg between the surface sediment and suspended solid, thus the in situ production of MeHg could not be taken into consideration. Ogrinc et al. (2007) reported that MeHg production depends on the partitioning of Hg(II) influenced by the organic carbon content in the sediment of the Mediterranean Sea. The reduction of organic carbon in the sediment could increase pore water Hg(II) and enhance bacterial production of MeHg. In our study, the organic matter content was obtained as ignition loss for
Table 2 Mercury concentration in bottom water samples October 2002
April 2005
Clear layer St.
T-Hg
stdev
ng L− 1 M1 M2 M3 M4 M5 F1 F2 F3 O1 ave stdev
1.66 1.34 3.32 1.37 4.27 2.56 1.55 1.96 2.59 2.29 1.00
Turbid layer MeHg
stdev
ng L− 1 0.57 0.37 0.34 0.52 2.14 1.53 0.47 1.49 2.06 – –
n: Number of samples.
1.04 1.08 0.32 0.80 0.51 0.73 0.93 1.16 1.65 0.91 0.39
MeHg/T−Hg
n
% 0.05 0.19 0.13 0.13 0.16 0.33 0.18 0.46 0.46 – –
62.2 80.3 9.7 58.6 11.9 28.7 59.9 59.0 63.9 48.2 25.0
4 3 3 3 3 4 3 3 4 – –
Clear layer
T-Hg
MeHg
MeHg/T-Hg
ng L− 1
ng L− 1
%
9.83 7.84 9.31 7.07 22.29 8.01 7.45 5.63 7.10 9.39 5.00
2.92 4.14 1.19 1.52 5.21 0.97 0.77 2.05 1.29 2.23 1.55
29.7 52.8 12.8 21.5 23.4 12.1 10.3 36.5 18.1 24.1 13.8
n
1 1 1 1 1 1 1 1 1 – –
T-Hg
MeHg
MeHg/T-Hg
ng L− 1
ng L− 1
%
0.89 0.84 – 0.96 – 0.94 1.64 1.04 – 1.05 0.30
0.56 0.74 – 0.34 – 0.48 0.28 0.77 – 0.53 0.20
62.4 87.5 – 35.2 – 50.7 16.9 73.8 – 54.4 25.8
n
1 1 – 1 – 1 1 1 – – –
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T. Tomiyasu et al. / Marine Chemistry 112 (2008) 102–106
Fig. 3. T–Hg and MeHg concentrations in seawater of Minamata Bay.
the suspended solid in 2002 by heating the sample at 375 °C for 4 h. The range of values, 4.1–7.1% (mean 5.4 ±0.9%, n = 9), is about one order of magnitude larger than that for the Mediterranean Sea. Although the higher organic matter content may reduce the methylation potential in Minamata Bay, other factors, such as oxygen levels (Hammerschmidt and Fitzgerald, 2008), redox potential, and acid volatile sulfide (Ouddance et al., 2008), can also influence methylation potential. Estimation of the methylation-demethylation process at the sediment-water interface is the next important step to elucidate the flux of mercury in Minamata Bay.
4. Conclusions A gradation of the T–Hg concentration in the water phase could be clearly seen. The concentration was highest near the sediment and decreased gradually. The MeHg concentration in the water at the bottom was also high, and about 24–54% of all the mercury was MeHg. Therefore, the predominant form of mercury during the elution from sediment into the water column is MeHg. The percentage of MeHg of total mercury was lower in the resuspended sediment than the surface sediment. Therefore, the amount of mercury eluted from the sediment through the resuspension and transport of bottom sediment was estimated to be 0.46 kg and 0.11 kg per year for T–Hg and MeHg, respectively. Acknowledgement This work was supported by Grants-in-Aid (No.13680631, No.15404003, and No. 18404001) for Scientific Research from the Japan Society for the Promotion of Science. References Akagi, H., Nishimura, H., 1991. In: Suzuki, T., Imura, N., Clarkson, T.W. (Eds.), Advances in Mercury Toxicology. Plenum Press, New York, pp. 53–76. Akagi, H., Malm, O., Branches, F.J.P., Kinjo, Y., Kashima, Y., Guimaraes, J.R.D., Oliveira, R.B., Haraguchi, K., Pfeiffer, W.C., Takizawa, Y., Kato, H., 1995. Human exposure to mercury due to goldmining in the Tapajos River
basin, Amazon, Brazil: speciation of mercury in human hair, blood and urine. Water Air Soil Pollut. 80, 85–94. Bloom, N.S., Lasorsa, B.K., 1999. Changes in mercury speciation and the release of methyl mercury as a result of marine sediment dredging activities. Sci. Total Environ. 237, 379–385. Covelli, S., Faganeli, J., Horvat, M., Brambati, A., 1999. Porewater distribution and benthic flux measurements of mercury and methylmercury in the Gulf of Trieste Northern Adriatic Sea. Estuar. Coast. Shelf Sci. 48, 415–428. Gill, G.A., Bloom, N.S., Cappellino, S., Driscoll, C.T., Dobbs, C., McShea, L., Mason, R., Rudd, J.W.M., 1999. Sediment-water fluxes of mercury in Lavaca Bay, Texas. Environ. Sci. Technol. 33, 663–669. Hammerschmidt, C.R., Fitzgerald, W.F., 2008. Sediment-water exchange of methylmercury determined from shipboard benthic flux chambers. Mar. Chem. 109, 86–97. Kitamura, S., Ueda, K., Niino, J., Ujioka, T., Misumi, H., Kakita, T., 1960. Minamata-byo ni kansuru Kagaku-dokubutu Kensaku Seiseki in Japanese. J. Kumamoto Med. Soc. 34, 593–601. Kudo, A., Fujikawa, Y., Miyahara, S., Zheng, J., Takigami, H., Sugahara, M., Muramatsu, T., 1998. Lessons from Minamata mercury pollution, Japan — after a continuous 22 years of observation. Water Sci. Technol. 38, 187–193. Kumagai, M., Nishimura, H., 1978. Mercury distribution in seawater in Minamata Bay and the origin of particulate mercury. J. Oceanogr. Soc. Jpn. 34, 50–56. Kumamoto Prefecture, 1998. Minamata-wan kankyo fukugen jigyou no gaiyou (in Japanease). Logar, M., Horvat, M., Akagi, H., Ando, T., Tomiyasu, T., Fajon, V., 2001. Determination of total mercury and monomethylmercury compound in water samples from Minamata Bay, Japan: an interlaboratory comparative study of different analytical techniques. Appl. Organomet. Chem. 15, 515–526. Mason, R., Kim, E.H., Cornwell, J., Heyes, D., 2006. An examination of the factors influencing the flux of mercury, methylmercury and other constituents from estuarine sediment. Mar. Chem. 102, 96–110. Matsuo, N., Suzuki, T., Akagi, H., 1989. Mercury concentration in organs of contemporary Japanese. Arch. Environ. Health 44, 298–303. Malm, O., Branches, F.J.P., Akagi, H., Castro, M.B., Pfeiffer, W.C., Harada, M., Bastos, W.R., Kato, H., 1995. Mercury and methylmercury in fish and human hair from the Tapajos river basin, Brazil. Sci. Total. Environ. 175, 141–150. Minamata City, 2000. Minamata Desease — the History and Lessons. Ogrinc, N., Monperrus, M., Kotnik, J., Fajon, V., Vidimova, K., Amouroux, D., Kocman, D., Tessier, E., Žižek, S., Horvat, M., 2007. Distribution of mercury and methylmercury in deep-sea surficial sediments of the Mediterranean Sea. Mar. Chem. 107, 31–48. Ouddance, B., Mikac, N., Cundy, A.B., Quillet, L., Fischer, J.C., 2008. A comparative study of mercury distribution and methylation in mudflats from two macrotidal estuaries: The Seine (France) and the Medway (United Kingdom). Appl. Geochem. 23, 618–631. Rajar, R., Zagar, D., Cetina, M., Akagi, H., Yano, S., Tomiyasu, T., Horvat, M., 2004. Application of three-demensional mercury cycling model to coastal seas. Appl. Geochem. 171, 139–155. Sakamoto, H., Tomiyasu, T., Yonehara, N., 1995. The contents and chemical forms of mercury in sediments from Kagoshima Bay, in comparison with Minamata Bay and Yatsushiro Sea, southwestern Japan. Geochem. J. 29, 97–105. Sato, M., Haraguchi, K., Ando, T., Tomiyasu, T., Akagi, H., 1997. Distribution of total mercury and methylmercury in tissues of the mussel Mytilus galloprovincialis. Environ. Sci. 5, 225–237. Tomiyasu, T., Nagano, A., Yonehara, N., Sakamoto, H., Rifardi, Oki, K., Akagi, H., 2000. Mercury contamination in the Yatsushiro Sea, south-western Japan: spatial variations of mercury in sediment. Sci. Total Environ. 257, 121–132. Tomiyasu, T., Matsuyama, A., Eguchi, T., Fuchigami, Y., Oki, K., Horvat, M., Rajar, R., Akagi, H., 2006. Spatial variations of mercury in sediment of Minamata Bay, Japan. Sci. Total Environ. 368, 283–290. Yano, S., Tada, A., Oshikawa, H., Nakamura, T., Akagi, H., Matsuyama, A., Tomiyasu, T., Rajar, R., Horvat, M., 2003. Minamata-wan ni okeru teideidoutai no genchikansoku (in Japanese). Kaigan Kogaku Ronbunshu 50, 1006–1010.
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Marine Chemistry j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a r c h e m
Speciation of mercury in water at the bottom of Minamata Bay, Japan Takashi Tomiyasu a,⁎, Akito Matsuyama b, Tomomi Eguchi b, Kohji Marumoto b, Kimihiko Oki c, Hirokatsu Akagi b a b c d
Faculty of Science, Kagoshima University, Korimoto, Kagoshima 890-0065, Japan National Institute for Minamata Disease, Minamata, Kumamoto 867-0008, Japan Kagoshima University Museum, Korimoto, Kagoshima 890-0065, Japan International Mercury Laboratory Inc., Minamata, Kumamoto 867-0034, Japan
a r t i c l e
i n f o
Article history: Received 8 November 2007 Received in revised form 4 July 2008 Accepted 7 July 2008 Available online 12 July 2008 Keywords: Mercury pollution Speciation of mercury in water Marine sediment Minamata Bay Remobilization of mercury
a b s t r a c t Total mercury (T-Hg) and methylmercury (MeHg) concentrations were determined in water at the bottom, in the suspended solid, and in the surface sediment of Minamata Bay to assess the remobilization of mercury from the sediment into the water column. The water and sediment samples were taken from the bottom of the bay using a gravity core sampler at nine locations in October 2002 and six locations in April 2005. The average concentration of T-Hg and the proportion of MeHg in the sediment were 3.71 ± 1.90 mg/kg and 0.27 ± 0.28%, respectively. The water contained 1.80 ± 1.00 ng L− 1 of T-Hg, which was higher than the value reported for the upper-middle depth of Minamata Bay. The results suggest that the sediment is an important source of mercury in the water of Minamata Bay. The percentage of MeHg in the water at the bottom was 50.7 ± 24.6%, also considerably higher than in the upper-middle layer of the water column, suggesting that MeHg may be the predominant form of mercury released from the sediment into the water. The percentage of MeHg was lower in the resuspended sediment than in the surface sediment. The amount of mercury eluted from the sediment into the water was estimated at 0.46 kg and 0.11 kg per year for T-Hg and MeHg, respectively. © 2008 Elsevier B.V. All rights reserved.
1. Introduction From 1932 until 1968, mercury-contaminated effluent was discharged into Minamata Bay from an acetaldehyde-producing factory (Minamata City, 2000). As much as 380–455 t of mercury was used and about 250 t was deposited in Minamata Bay during this period. Total mercury (T-Hg) concentrations as high as 2000 mg kg− 1 were found in sediment near the overflow of the plant (Kitamura et al., 1960). The Minamata Bay Pollution Prevention Project (1977– 1990) disposed of sedimentary sludge containing more than 25 mg kg− 1 of mercury. Through this project, 1,510,000 m3 of polluted sediment was dredged, and the heavily contaminated area near the overflow of the plant was reclaimed (Fig. 1). Just after the project, mercury concentrations of 0.06– 12 mg kg− 1 in the upper layer of sediment were reported ⁎ Corresponding author. Tel.: +81 99 285 8107; fax: +81 99 259 4720. E-mail address: [email protected] (T. Tomiyasu). 0304-4203/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.marchem.2008.07.003
(Kumamoto Prefecture, 1998). However, these values are still about 50 times higher than the background level for this area (Tomiyasu et al., 2006). Several investigations have reported on mercury concentrations in the Yatsushiro Sea sediment (Kitamura et al., 1960; Sakamoto et al., 1995; Kudo et al., 1998; Tomiyasu et al., 2000), and some studies reported the transport of mercury with sediment from Minamata Bay to Yatsushiro Sea (Kumagai and Nishimura, 1978, Yano et al., 2003, Rajar et al., 2004). When contaminated sediment is resuspended and transported, some mercury species may dissolve in the water column (Covelli et al., 1999; Gill et al., 1999). Logar et al. (2001) reported on T–Hg and methylmercury (MeHg) concentrations being present in the water from the upper and middle layers of Minamata Bay, which are values higher than in uncontaminated areas. However, the concentration of mercury in the water just above the sediment has not been previously investigated in Minamata Bay. In this study, T–Hg and MeHg concentrations were determined from water at the bottom, in suspended solids,
T. Tomiyasu et al. / Marine Chemistry 112 (2008) 102–106
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Fisheries Research Laboratory, Kagoshima University. Bottom water samples were taken with surface sediment using a PHLEGER gravity core sampler (Hisanaga Co., Ltd.) at nine locations in October 2002 and six locations in April 2005. The length and inside diameter of the inner tube of the sampler were 60 cm and 3.3 cm, respectively. The water was taken in the 20–40 cm part of the inner tube (170–340 mL) on top of the sediment. Although the upper part of water layer was clear, the lower layer just above the sediment (several centimeters) was turbid, which may include some sediment that was easily resuspended during the sampling process. The clear and turbid layers were put into separate bottles. The uppermost part of the sediment core (surface sediment) was then cut at a thickness of 2 cm and brought back to the laboratory. To determine mercury concentrations, the clear layer of water at the bottom was filtrated with a 0.45-μm Millipore filter. The turbid layer of the bottom water was centrifuged, and the concentration of mercury in the supernatant was determined after filtration with a 0.45-μm Millipore filter. The mercury concentration was also determined in the solid remaining in the centrifuge tube. 2.2. Mercury analysis 2.2.1. Measurement of mercury concentrations in surface sediment and suspended solid The concentration of mercury was measured using the method proposed by Akagi and Nishimura (1991) and modified by Akagi et al. (1995). The precision and accuracy of the technique has been repeatedly verified by interlaboratory calibration exercises (Matsuo et al., 1989; Malm et al., 1995), including an analysis of reference standards (e.g., IAEA 085 and 086). An overview of the analytical procedures is given here. For quantification of the total mercury concentration, a known amount of sample was placed in a 50-mL volumetric flask, to which 1 mL of distilled water, 2 mL of a 1:1 nitric acid-perchloric acid solution, and 5 mL of concentrated sulfuric acid were added. After standing for a few minutes, the volumetric flask was heated on a hot plate at 230 °C for 20 min. After cooling, the digested sample was made up to 50 mL with water and a suitable aliquot of the resulting solution (b20 mL) was analyzed for mercury by cold vapor atomic absorption spectrometry using a semi-
Fig. 1. Map showing the study area. The dredged spoils were placed in the Reclaimed Area.
and in surface sediment to assess the remobilization of mercury from the sediment into the water column. 2. Materials and methods 2.1. Sampling The sampling points are shown in Fig. 1. The positioning of the sampling sites was determined with a Furuno GPS plotter GP-1500 (FCV-663) installed on the research vessel of the
Table 1 Total mercury and methylmercury concentrations in solid samples October 2002
April 2005
Sediment St
M1 M2 M3 M4 M5 F1 F2 F3 O1 ave. s.d.
ss
Sediment
ss
T-Hg
MeHg
Me/T
T-Hg
MeHg
Me/T
T-Hg
MeHg
Me/T
T-Hg
MeHg
Me/T
(mg kg− 1)
(μg kg− 1)
(%)
(mg kg− 1)
(μg kg− 1)
(%)
(mg kg− 1)
(μg kg− 1)
(%)
(mg kg− 1)
(μg kg− 1)
(%)
1.58 2.21 2.47 2.33 4.43 3.93 3.63 6.38 6.56 3.72 1.80
4.6 4.0 6.3 4.4 6.6 28.2 36.7 29.8 6.6 14.1 13.3
0.30 0.18 0.26 0.19 0.16 0.72 1.00 0.47 0.10 0.38 0.30
2.64 2.70 2.53 2.76 4.10 4.16 5.44 6.62 5.41 4.04 1.51
2.4 1.1 6.0 4.0 5.4 18.2 34.2 21.5 7.1 11.1 11.2
0.09 0.04 0.24 0.14 0.13 0.44 0.63 0.33 0.13 0.24 0.19
0.61 1.84 – 6.73 – 3.55 4.61 4.84 – 3.70 2.21
2.6 1.0 – 1.7 – 2.3 1.7 1.1 – 1.7 0.7
0.43 0.05 – 0.02 – 0.07 0.04 0.02 – 0.11 0.16
2.80 2.64 – 4.44 – 4.68 6.99 7.82 – 4.90 2.13
3.7 3.0 – 4.6 – 2.8 2.9 4.8 – 3.6 0.9
0.13 0.11 – 0.10 – 0.06 0.04 0.06 – 0.09 0.04
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T. Tomiyasu et al. / Marine Chemistry 112 (2008) 102–106
automated mercury analyzer (Model Hg-3500, Sanso, Japan). For the MeHg analysis, sediment samples (ca. 1 g) were shaken with 10 mL of 1 M potassium hydroxide in ethanol for 10 min using a mechanical shaker. The treated sediment was acidified with hydrochloric acid and was bubbled with nitrogen gas for 5 min. The sample was then mixed with 2 mL of 20% hydroxylamine hydrochloride and 2 mL of 20% EDTA and shaken with 5 mL of purified 0.05% dithizone in toluene. The toluene layer was transferred into a 10-mL test tube and washed twice with 3 mL of 1 M sodium hydroxide. The toluene layer was transferred into another test tube, and the MeHg contained in the organic layer was back-extracted with 2 mL of a 5-ppm sodium sulfide solution. After the toluene layer was removed, the aqueous layer was washed with 2 mL of toluene and acidified with 3–4 drops of 1 M hydrochloric acid. The excess sulfide ions were removed by bubbling with nitrogen gas for 5 min, and then MeHg was reextracted into 0.2 mL of purified 0.01% dithizone in toluene. The toluene layer was washed with 1 M sodium hydroxide and with distilled water and then subjected to ECD gas chromatography for the quantification of MeHg. The mercury concentration in solid samples, based on wet weight, was converted to a dry weight value by using the water content, which was measured separately by heating the sample at 110 °C for 6 h. 2.2.2. Determination of mercury in water samples The total concentration of mercury was determined based on the preconcentration of mercury from water samples by extraction of mercury dithizonates (Hg–Dz). In the first step, the simultaneous extraction of Hg2+ and CH3Hg+ dithizonates into toluene was performed. An aliquot of this extract was used to determine the total concentration of mercury. The organic solvent in the aliquot was removed by evaporation in a rotavapour. The remaining Hg–Dz complex was decomposed by acid digestion, and mercury content was measured by CVAAS using SnCl2 reduction. The limit of detection (LOD) was 0.01 ng L− 1 in the 2 L of water sample used. As described above, after removal of an aliquot, the remaining extract was used for the quantification of MeHg. MeHg was stripped from the organic solvent into an aqueous Na2S solution. After the removal of H2S by evaporation, MeHg–Dz was back-extracted into a small volume of organic solvent and detected by gas chromatography coupled with electron capture detection (GCECD). The LOD was 0.02 ng L− 1 in the 2 L of water sample used. The system was calibrated with the use of a standard solution of MeHg prepared in an aqueous cysteine solution. Recoveries obtained on the basis of the standard addition method were nearly quantitative, so no recovery factors were necessary for the calculation of final results. The repeatability of measurements was about 3–5%. 3. Results and discussion 3.1. Mercury concentrations in sediment and suspended solid As shown in Table 1, the total concentration of mercury ranged from 0.61 to 6.73 mg kg− 1 (mean, 3.71 ± 1.90 mg kg− 1) in the surface sediment and from 2.53 to 7.82 mg kg− 1 (mean, 4.38 ± 1.76 mg kg− 1)in the suspended solid. There was no large difference between the sediment and suspended solid concentrations. These values are about 70 times higher than the estimated background concentration (Tomiyasu et al., 2006) in this area. As shown in
Fig. 2. The correlation of mercury concentrations between the sediment and suspended solid [Fukuro Bay (●), Minamata Bay ( ), outside of Minamata Bay (○)]; (a) T–Hg concentration, (b) MeHg concentration, (c) percentage of MeHg. The dashed lines indicate y = x.
T. Tomiyasu et al. / Marine Chemistry 112 (2008) 102–106 Fig. 2(a), when the T–Hg concentration in the suspended solid was plotted against that in sediment, a significant correlation was observed (r = 0.72, p b 0.01). This result suggests that part of the sediment can readily be suspended, and the mercury in the sediment can move with the particles. The MeHg concentration in the sediment and suspended solid was 1.01– 36.7 µg kg− 1 (mean, 9.17 ± 11.86 µg kg− 1) and 1.07–34.2 µg kg− 1 (mean, 8.12 ± 9.26 µg kg− 1), respectively. This corresponds to 0.27 ± 0.28% and 0.18 ± 0.17% of the total concentration of mercury, respectively. The repeatabilities were checked by making duplicate measurements of the samples, which had variabilities that ranged from 0.6 to 10.2% (mean, 5.5%). In 2002, the percentage of MeHg was higher in Fukuro Bay where the sediment was not dredged, but in 2005, the value had dropped and had no significant difference from Minamata Bay. The reason for this is not clear and long-term monitoring accounting for seasonal changes may be required to elucidate the behavior of mercury in the bay. The correlations between the sediment and suspended solid for MeHg concentrations and MeHg percentages are shown in Fig. 2(b) and (c), respectively. Although a linear relation with high correlation coefficients is observed, the slope of the line for the plots of percentages of MeHg in sediment vs. suspended solid, 0.54, is significantly smaller than 1 (Fig. 2(c)). This may suggest that MeHg is being eluted from suspended particles into the water column. 3.2. Mercury concentrations in water samples The results are shown in Table 2. In 2002, three to four samples were taken at each sampling point. The volume of water obtained in the gravity core sampler varied. The mercury concentration in each sample of clear water was measured and tended to increase with a decrease in the volume of the water sample. This suggests that water containing a higher concentration of mercury lies near the bottom. The relative standard deviations in three or four samples of clear water at each station were 10–80% (mean 45%) and 5–46% (mean 27%) for T–Hg and MeHg, respectively. On the other hand, the turbid layer was combined for each sampling point, since the volume obtained in a single sampling was too small to use for measurements. In 2005, although only one sample was taken from one station, special attention was payed to take the same volume, ca. 200 mL. The T–Hg concentrations in the water samples taken in 2002 and 2005 were in the range of 1.3–4.3 ng L− 1 (mean, 2.3 ± 1.0 ng L− 1) and 0.84–1.64 ng L− 1 (mean, 1.0± 0.30 ng L− 1), respectively. These values were higher than the concentration of T–Hg reported in the upper-middle depths of Minamata bay, 0.78± 0.57 ng L− 1 (Logar et al., 2001). The mercury concentration was higher in turbid water than the upper clear water layer. As shown in Fig. 3, the concentration increased near the sediment, which suggests that the sediment is an important source of mercury in water of Minamata Bay. The MeHg concentration in the water samples ranged from 0.32 to 1.65 ng L− 1 (mean, 0.91 ± 0.39 ng L− 1) in 2002 and 0.28 to 0.77 ng L− 1 (mean, 0.53 ± 0.20 ng L− 1 ) in 2005. This was 48.2 ± 25.0% and 54.4 ± 25.8%, respectively, of the T–Hg concentration,. These values are considerably higher than the 0.045 ± 0.018 ng L− 1 and 7.6 ± 3.7% reported for the MeHg concentration and %MeHg in the upper-middle depth of Minamata Bay (Logar et al., 2001). The concentration of MeHg and percentage of T–Hg in turbid water measured in 2002 was 2.23 ± 1.55 ng L− 1 and 24.1%, respectively.
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The MeHg concentration was higher in the turbid than the clear layer of water at the bottom. This result supports the elution of mercury from the sediment. It should also be considered that the turbid water layer was affected by porewater from the sediment during the sampling with the gravity core sampler. 3.3. Movement of mercury from sediment into water column The percentage of MeHg in the solid phase, 0.09–0.38%, was about two orders of magnitude lower than the 24–54% in the clear or turbid layers of water at the bottom. As described above, the gradation of the T–Hg concentration suggests the elution of mercury from sediment. Bloom and Lasorsa (1999) reported that MeHg was two orders of magnitude less strongly bound to sediment than inorganic mercury. In addition about 80% of all the mercury in pore water near the sediment surface was MeHg. The large difference in percentages of MeHg observed between the solid and water phases in the current study strongly suggests that MeHg is the predominant form of mercury at the elution step. The average T–Hg and MeHg concentrations in sediment are 3.7 mg kg− 1 and 9.2 µg kg− 1, respectively. By considering the slope of 0.54 in Fig. 2(c), 46% of the MeHg in sediment was released into the water column through resuspension. Thus it was roughly estimated that 4.2 µg of MeHg was released from 1 kg of sediment. With the percentage of MeHg in the turbid layer as 24%, the total amount of mercury released from 1 kg of sediment can be estimated at 17.5 µg. Rajar et al. (2004) calculated that the amount of sediment resuspended and transported in Minamata Bay is 26,500 t per year. Thus, the amount of mercury eluted annually would be approximately 0.46 kg Hg/year total and 0.11 kg Hg/year of MeHg. The volume of water in Minamata Bay has been estimated at 2.5 × 107 m3 (Rajar et al., 2004). If Minamata Bay was closed, the mercury concentration in water would reach 19 ng L− 1 of T–Hg and 4.4 ng L− 1 of MeHg based on the amount of mercury eluted from the resuspended sediment. However, the mercury concentration should decrease with dilution through the exchange of seawater with the Yatsushiro Sea and/or by adsorption on particles in the water column. However, MeHg can be readily absorbed by aquatic organisms and accumulate in the aquatic food chain, ultimately reaching significant levels in fish. Sato et al. (1997) reported higher MeHg concentrations in the adductor muscles of mussels collected near the reclaimed area of Minamata Bay. Thus, there is also a need to continue to monitor the levels of mercury in water and to estimate the levels of mercury in organisms from this area. Mason et al. (2006) reported that the MeHg flux is not only due to its release because of a pore water gradient or due to desorption from the solid phase, but potentially due to its in situ production in the upper layers of the sediment. In our study, the amount of mercury desorbed was estimated from the difference in the percentage of MeHg between the surface sediment and suspended solid, thus the in situ production of MeHg could not be taken into consideration. Ogrinc et al. (2007) reported that MeHg production depends on the partitioning of Hg(II) influenced by the organic carbon content in the sediment of the Mediterranean Sea. The reduction of organic carbon in the sediment could increase pore water Hg(II) and enhance bacterial production of MeHg. In our study, the organic matter content was obtained as ignition loss for
Table 2 Mercury concentration in bottom water samples October 2002
April 2005
Clear layer St.
T-Hg
stdev
ng L− 1 M1 M2 M3 M4 M5 F1 F2 F3 O1 ave stdev
1.66 1.34 3.32 1.37 4.27 2.56 1.55 1.96 2.59 2.29 1.00
Turbid layer MeHg
stdev
ng L− 1 0.57 0.37 0.34 0.52 2.14 1.53 0.47 1.49 2.06 – –
n: Number of samples.
1.04 1.08 0.32 0.80 0.51 0.73 0.93 1.16 1.65 0.91 0.39
MeHg/T−Hg
n
% 0.05 0.19 0.13 0.13 0.16 0.33 0.18 0.46 0.46 – –
62.2 80.3 9.7 58.6 11.9 28.7 59.9 59.0 63.9 48.2 25.0
4 3 3 3 3 4 3 3 4 – –
Clear layer
T-Hg
MeHg
MeHg/T-Hg
ng L− 1
ng L− 1
%
9.83 7.84 9.31 7.07 22.29 8.01 7.45 5.63 7.10 9.39 5.00
2.92 4.14 1.19 1.52 5.21 0.97 0.77 2.05 1.29 2.23 1.55
29.7 52.8 12.8 21.5 23.4 12.1 10.3 36.5 18.1 24.1 13.8
n
1 1 1 1 1 1 1 1 1 – –
T-Hg
MeHg
MeHg/T-Hg
ng L− 1
ng L− 1
%
0.89 0.84 – 0.96 – 0.94 1.64 1.04 – 1.05 0.30
0.56 0.74 – 0.34 – 0.48 0.28 0.77 – 0.53 0.20
62.4 87.5 – 35.2 – 50.7 16.9 73.8 – 54.4 25.8
n
1 1 – 1 – 1 1 1 – – –
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T. Tomiyasu et al. / Marine Chemistry 112 (2008) 102–106
Fig. 3. T–Hg and MeHg concentrations in seawater of Minamata Bay.
the suspended solid in 2002 by heating the sample at 375 °C for 4 h. The range of values, 4.1–7.1% (mean 5.4 ±0.9%, n = 9), is about one order of magnitude larger than that for the Mediterranean Sea. Although the higher organic matter content may reduce the methylation potential in Minamata Bay, other factors, such as oxygen levels (Hammerschmidt and Fitzgerald, 2008), redox potential, and acid volatile sulfide (Ouddance et al., 2008), can also influence methylation potential. Estimation of the methylation-demethylation process at the sediment-water interface is the next important step to elucidate the flux of mercury in Minamata Bay.
4. Conclusions A gradation of the T–Hg concentration in the water phase could be clearly seen. The concentration was highest near the sediment and decreased gradually. The MeHg concentration in the water at the bottom was also high, and about 24–54% of all the mercury was MeHg. Therefore, the predominant form of mercury during the elution from sediment into the water column is MeHg. The percentage of MeHg of total mercury was lower in the resuspended sediment than the surface sediment. Therefore, the amount of mercury eluted from the sediment through the resuspension and transport of bottom sediment was estimated to be 0.46 kg and 0.11 kg per year for T–Hg and MeHg, respectively. Acknowledgement This work was supported by Grants-in-Aid (No.13680631, No.15404003, and No. 18404001) for Scientific Research from the Japan Society for the Promotion of Science. References Akagi, H., Nishimura, H., 1991. In: Suzuki, T., Imura, N., Clarkson, T.W. (Eds.), Advances in Mercury Toxicology. Plenum Press, New York, pp. 53–76. Akagi, H., Malm, O., Branches, F.J.P., Kinjo, Y., Kashima, Y., Guimaraes, J.R.D., Oliveira, R.B., Haraguchi, K., Pfeiffer, W.C., Takizawa, Y., Kato, H., 1995. Human exposure to mercury due to goldmining in the Tapajos River
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