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Spatial distributions of 137Cs and 239+240Pu in surface seawater within the Exclusive Economic Zone of East Coast Peninsular Malaysia
ARTICLE IN PRESS Applied Radiation and Isotopes 68 (2010) 1839–1845
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Spatial distributions of 137Cs and 239 + 240Pu in surface seawater within the Exclusive Economic Zone of East Coast Peninsular Malaysia Zaharudin Ahmad n, Yii Mei-Wo, Ahmad Sanadi Abu Bakar, Hidayah Shahar Radiochemistry and Environmental Group, Malaysian Nuclear Agency (Nuclear Malaysia), Bangi, 43000 Kajang, Malaysia
a r t i c l e in fo
abstract
Article history: Received 5 June 2009 Received in revised form 4 January 2010 Accepted 1 April 2010
The studies of 137Cs and 239 + 240Pu distributions in surface seawater at South China Sea within the Exclusive Economic Zone (EEZ) of Peninsular Malaysia were carried out in June 2008. The analysis results will serve as additional information to the expanded baseline data for Malaysia’s marine environment. Thirty locations from extended study area were identified in the EEZ from which large volumes of surface seawater samples were collected. Different co-precipitation techniques were employed to concentrate cesium and plutonium separately. A known amount of 134Cs and 242Pu tracers were used as yield determinant. The precipitate slurry was collected and oven dried at 60 oC for 1–2 days. Cesium precipitate was fine-ground and counted using gamma-ray spectrometry system at 661.62 keV, while plutonium was separated from other radionuclides using anion exchange, electrodeposited and counted using alpha spectrometry. The activity concentrations of 137Cs and 239 + 240 Pu were in the range of 3.40–5.89 Bq/m3 and 2.3–7.9 mBq/m3, respectively. The 239 + 240Pu/137Cs ratios indicate that there are no new inputs of these radionuclides into the area. & 2010 Elsevier Ltd. All rights reserved.
Keywords: 137 Cs 239 + 240 Pu Seawater East coast Peninsular Malaysia Baseline data
1. Introduction Rapid development of nuclear power industry in the recent years, especially in the Asia Pacific region, has raised some concern to the countries within the area. As of October 1, 2008, there were 439 nuclear power plants in operation, 36 nuclear power plants under construction, 99 were in planning and 232 were proposed to build (World Nuclear Association, 2008). In East and South Asia alone, there are over 109 nuclear power reactors in operation, 18 under construction and a further 110 are planned to build. China and India have established 9 new nuclear power plants in the last 4 years and have 10 more under construction. Japan has the highest number of nuclear power plants in Asia, with 53 in operation (International Atomic Energy Agency, 2001). Nuclear power generation has tremendous benefits in meeting the electricity needs of growing populations. It is also friendlier to the environment for not contributing to adverse effects as burning of fossil fuels. However, there are potential risks of planned and unplanned releases of radionuclide into the marine environment which need to be addressed. The Chernobyl incident proves that any accident occurring in a nuclear power plant introduces artificial radioisotopes such as 134Cs, 137Cs, 239,240Pu, 238Pu and
n
Corresponding author. Tel.: + 6 03 8925 0510; fax: + 6 03 8928 2977. E-mail addresses: [email protected], [email protected] (Z. Ahmad). 0969-8043/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.apradiso.2010.04.012
241
Am into the environment. However, higher demand of energy for development purposes in some countries has forced them to use nuclear power as an alternative energy resource due to its sustainability. The presence of the above artificial radionuclides for the past 60 years, particularly in the marine environment, originated from nuclear weapon tests, releases from nuclear facilities, radioactive waste dumping, the Chernobyl accident and nuclear submarine and aircraft accidents. It is believed that around 380 and 40 nuclear weapon tests were carried out in the Northern and Southern Hemisphere, respectively (Godoy et al., 1998). Although most of the radionuclide were released and deposited in the Northern Hemisphere, they could also be transported, partially through stratospheric circulation, into the Southern Hemisphere. Amongst these radioisotopes, anthropogenic radionuclides 137Cs and 239 + 240 Pu with half-lives of 30.2 years and 2.4 104 years, respectively, are important indicators for radioactive contamination in marine environment, and are of primary interest because of existing inventories and possible health effects (Duran et al., 2004). Cs-137 can also serve as a conservative tracer of the transport and accumulation patterns of contaminants in seawater. Aarkrog (2003) has estimated that the total input of 137Cs to the World Ocean was 659 PBq (604 PBq from fallout, 40 PBq from nuclear reprocessing and 16 PBq from the Chernobyl accident). It has also been estimated that 239 + 240Pu inventories by the year 2000 in the Pacific and Indian Oceans were 4.3 PBq from global stratospheric fallout and 2.0 PBq from close-in fallout (Aarkrog, 2003).
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Monitoring of radioactivity content in Malaysian marine environment is of interest, not only for additional information to represent baseline data for the nation but also for worldwide database. This is due to the fact that there is still inadequate data on 137Cs and 239 + 240Pu inventory within the Malaysian marine environment, even though some work had been carried out by other researchers in the nearby region as reported in ASPAMARD (Duran et al., 2004). Furthermore, there is a great concern on the potential risks of releases of radioactive materials from nuclear facilities in nearby countries within the region to the nation and its residents. The aim of this study is to investigate the present level of 137Cs and 239 + 240Pu activity concentrations in the Exclusive Economic Zone (EEZ) of east coast Peninsular Malaysia, whereby the study area was extended from an approximately 28,052 km2 (from previous study by Yii and Zaharudin, 2004) in the coastal area to cover an area of 140,972 km2 of the zone.
2. Experimental 2.1. Study area and sample collection The Exclusive Economic Zone of east coast Peninsular Malaysia covers an area between 11 14.040 to 71 48.920 N latitude, and 1021 5.030 to 1051 48.770 E longitude, with approximately 1150 km length and has a maximum width of 417 km. It is relatively shallow, with an average depth of 60–70 m. The seawater is generally well mixed throughout the water column and the prevailing surface currents are closely associated with the monsoon seasons (Mohsin and Mohamed, 1988). Seawater samples were collected during sampling expeditions on board the K.L. PAUS (Malaysian Fisheries Institute) from June
11 to 30, 2008. A total of 30 locations were selected and samples were subsequently collected and analyzed. Duplicate surface water samples (up to a depth of 5 m) were collected and labeled according to its sampling location as in Fig. 1. Due to an expected low concentration of 137Cs and 239 + 240Pu in the marine environment, large volumes of seawater (approximately 200 liters) were collected. Different co-precipitation techniques were employed to concentrate cesium and plutonium, respectively. Physical parameters, particularly temperature, salinity, pH and turbidity of the water column were also measured to serve as supporting parameters (Table 1).
2.2. Determination of
137
Cs in seawater samples
Approximately 4–8 Bq of 134Cs in 1% saline solution (Isotope Products Laboratories, 989-72) was used as tracer for yield determination. All chemicals used are of AnalaR grade, including potassium hexacyanoferrate (II) trihydrate [K4Fe(CN)6 3H2O] and copper (II) nitrate 2.5 hydrate [Cu(NO3)2 2.5 H2O]. Detailed procedure for precipitating and collecting of 137 Cs from seawater was described in the previous publication (Yii and Zaharudin, 2007). The precipitate was oven dried prior to 137Cs activity concentration determination. The dried precipitate was finely ground and sealed in 6 ml polyethylene containers as of the calibration standard. Counting was carried out with a p-type well germanium detector for 24 h. The activity of 137Cs was determined by measuring the peak area under the photo-peak of the 661.62 keV gamma-ray and correcting it for the detection efficiency, gamma-ray abundance, recovery of 134Cs tracer and averaged over the volume of seawater used.
Fig. 1. Map showing sampling site locations.
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Table 1 Physical parameters and activity concentration of Station
SF01 SF02 SF03 SF04 SF05 SF06 SF07 SF08 SF09 SF10 SF11 SF12 SF13 SF14 SF15 SF16 SF17 SF18 SF19 SF20 SF21 SF22 SF23 SF24 SF25 SF26 SF27 SF28 SF29 SF30
Temp. (1C)
137
Salinity PSU
30.91 30.46 30.29 30.04 30.03 30.11 30.25 30.41 28.66 30.26 30.03 29.59 29.85 29.63 29.61 29.68 29.94 29.76 29.50 29.72 29.64 29.63 29.89 29.75 29.03 29.21 29.25 29.74 29.48 30.91
32.66 33.04 33.17 32.98 33.53 33.57 33.27 33.17 33.98 33.75 33.55 33.69 33.58 33.57 33.29 33.38 33.97 33.65 34.01 33.91 33.84 33.83 33.48 33.73 33.98 34.03 33.93 33.99 33.78 32.66
Cs and
239 + 240
pH
8.18 8.23 8.22 8.21 8.18 8.21 8.19 8.20 8.16 8.23 8.19 8.20 8.21 8.20 8.22 8.20 8.15 8.16 8.15 8.22 8.24 8.20 8.22 8.23 8.17 8.24 8.18 8.23 8.16
Pu in surface seawater. Turbidity
2.80 2.60 2.70 2.70 2.63 2.70 2.60 2.67 2.65 2.55 2.67 2.80 2.65 2.55 2.50 2.60 2.70 2.58 2.80 2.75 2.65 2.70 2.70 2.65 2.67 2.85 2.70 2.75 2.80 5.97
2.3. Gamma spectrometry counting system The p-type gamma spectrometry counting system (cryostat well 40 mm depth 16 mm diameter, GCW 2523) was calibrated using a customized gamma multinuclide standard source solution (comprising 210Pb, 241Am, 109Cd, 57Co, 123Te, 51Cr, 113Sn, 85Sr, 137 Cs, 88Y and 60Co) with known activities, prepared by Isotope Products Laboratories, USA (source no. 1290-84). The energy and the efficiency calibration of the gamma-spectrometer system were then confirmed by using the Certified Reference Material (Soil-6 and IAEA-326) on the same counting geometry (hence efficiency). The peak resolution, FWHM, at the 122–1332 keV was 1.16 and 1.90 keV, respectively. The performance of the instrument was monitored regularly to ensure fitness for the purpose (Zaharudin et al., 2003).
2.4. Determination of
1841
239 + 240
Pu in seawater samples
Radionuclide 242Pu (National Physical Laboratory, R 1944) was used as tracer for chemical yield determination. All chemical used are of AnalaR grade, including potassium permanganate [KMnO4], sodium hydroxide [NaOH] and manganese chloride tetrahydrate [MnCl2 4 H2O]. Collected seawater samples were immediately acidified to pHr2.0 with the addition of concentrated HCl. Approximately 10 mBq of 242Pu was added into the sample as yield determinant and 25 ml of saturated KMnO4 subsequently. The sample was then stirred thoroughly and left for an hour. Approximately 10 M of NaOH was added to adjust the pH to 7–8. A 1 M MnCl2 solution was then added slowly (a few ml at a time) to form precipitate (re-adjust pH with 10 M NaOH) until further addition of MnCl2 did not change the pH of the sample. Precipitates formed were left
239 + 240
Pu/137 Cs activity ratio ( 10 3)
Activity Cs-137 (Bq/m3)
Pu-239, 240 (mBq/m3)
5.29 70.31 5.55 70.32 5.27 70.30 5.23 70.30 4.93 70.29 4.62 70.27 4.44 70.26 5.85 70.33 4.96 70.28 5.74 70.33 4.65 70.27 4.39 70.26 5.85 70.34 4.40 70.26 4.07 70.24 3.40 70.20 5.10 70.29 5.89 70.34 5.11 70.30 5.21 70.30 5.03 70.29 4.83 70.28 4.38 70.26 5.49 70.32 4.38 70.25 5.81 70.34 5.67 70.33 5.25 70.30 4.82 70.28 5.33 70.30
3.38 7 0.29 2.86 7 0.24 2.33 7 0.20 2.90 7 0.25 2.63 7 0.23 3.39 7 0.30 4.23 7 0.37 2.36 7 0.20 4.32 7 0.37 5.24 7 0.45 4.65 7 0.40 4.52 7 0.39 4.56 7 0.39 4.63 7 0.40 2.74 7 0.24 4.61 7 0.39 3.43 7 0.29 3.61 7 0.31 3.76 7 0.32 3.21 7 0.27 3.77 7 0.33 4.92 7 0.42 3.41 7 0.29 2.66 7 0.23 4.36 7 0.37 3.78 7 0.33 3.40 7 0.30 3.48 7 0.30 4.45 7 0.38 7.95 7 0.68
0.64 0.52 0.44 0.55 0.53 0.73 0.95 0.40 0.87 0.91 1.00 1.03 0.78 1.05 0.67 1.36 0.67 0.61 0.73 0.62 0.75 1.02 0.78 0.49 1.00 0.65 0.60 0.66 0.92 1.49
to settle overnight before decanting the clear water. High quality HDPE container was used for storing the precipitates obtained. The container was sealed, labeled and transferred to the laboratory for analysis. Rinsing of precipitates with distilled water was done twice prior to oven drying process. Chemical leaching processes for the determination of 239 + 240Pu activity were then done by dissolving the dried manganese dioxide precipitates with 8 M HNO3, sodium nitrite and hydrogen peroxide, subsequently. These processes were done while heating for ease of dissolution and decomposition of excess H2O2 and NaNO2. Solution obtained was then loaded onto conditioned anion exchange column (BIO-RAD, Ag 1-X2, 50–100 mesh, Cl form). Americium, uranium, polonium, thorium and iron were desorbed from the column by passing through 8 M HNO3 (60 ml) and 8 M HCl (30 ml). Plutonium was eluted using 0.7 M HNO3 (40 ml) and 0.7 M HNO3 (50 ml) with the presence of 0.075 g hydroquinone [C6H4(OH)2]. Hydroquinone was eliminated by heating and addition of concentrated HNO3 into the sample. The sample was then evaporated to dryness. A solution of 8 M HNO3 was then added into the dried sample and heated. Dissolved sample was then loaded onto conditioned anion exchange column (BIO-RAD, AG1-X8, 100–200 mesh, Cl form). Impurities were removed from the column by passing through 8 M HNO3 (3 5 ml) and 10 M HCl (3 5 ml). Purified plutonium was eluted from the column using 20–25 ml of freshly prepared NH4I-HCl (5.1 g of NH4I dissolved in 100 ml of water mixed with concentrated HCl with the ratio of 29:71 (v/v)). Solution obtained was then heated so as to eliminate iodine content prior to addition of concentrated HNO3. The solution was then evaporated to dryness. Sample was then ready for electro-deposition. Electro-deposition was done by adding 5 ml of 10% (v/v) H2SO4 into the sample and neutralized by adding NH4OH 25% (drop-
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wise). Neutralized solution was then transferred into an electrodeposition cell and the beaker was rinsed with 2 ml of H2SO4 1% and 3 ml of water. Plutonium was then electrodeposited onto polished stainless steel discs (pH 2.0 to 2.2, constant current of 1.0 A for 1 h) and assayed by alpha spectrometry for 3 days. 2.5. Alpha spectrometry counting system The alpha activities were measured with 450 mm2 Passivated ion Implanted Silicon, PIPS detectors placed in multichamber analyzer (Canberra). The vacuum in the chambers was kept under 10 Torr and the source to detector distance was less than 1 cm, with the registration effectiveness of 25%. The background count for each spectroscopy channel was less than 24 counts per day. Data treatment was by comparison of the peak area to the amount of yield monitor, 242Pu, added.
3. Results and discussion The activity concentration of 137Cs and 239 + 240Pu, and the Pu/137Cs activity ratios in the Exclusive Economic Zone of east coast Peninsular Malaysia seawater obtained in this study is summarized as in Table 1. Activities are reported in Bq/m3 and mBq/m3 for 137Cs and 239 + 240Pu, respectively. Cs-137 activity concentrations at the sampling period ranged from 3.40 to 5.89 Bq/m3 with mean value of 5.03 Bq/m3. On the other hand, 239 + 240 Pu activity concentrations were found to be in the range of 2.33–7.95 mBq/m3 with mean value of 3.85 mBq/m3. Approximately 90% and 50–80% of 134Cs and 242Pu tracers, respectively were recovered successfully throughout this study. Errors quoted for 137Cs and 239 + 240Pu activities were 1s values derived from counting statistics. The ranges of all values were large, mainly due to the large area of the Exclusive Economic Zone. Median values for both 137Cs and 239 + 240Pu activity concentrations are calculated as 5.11 and 3.68 mBq/m3, respectively, which are quite close to their respective mean values. Student-t test indicated that the difference between median and mean values are insignificant due to the calculated t value for 137Cs (t¼ 0.680) and 239 + 240Pu (t¼ 0.835) are less than t-critical, 2.045 (Miller and Miller, 1993), suggesting that the activity concentrations of both radionuclides are uniformly distributed. Study for ASPAMARD by Duran et al. (2004) found that the median value for 137 Cs activity concentration in seawater was 2.8 Bq/m3, whilst the median value for 239 + 240Pu was 5.9 mBq/m3. Wide study area and larger range of activity concentrations in ASPAMARD might be the reason for the difference of median values for both this and ASPAMARD studies. Comparison with previous study performed in the East Coast of peninsular Malaysia (Yii and Zaharudin, 2004) showed that there are elevations in 137Cs activity concentrations within the studied area. Cs-137 activity concentrations in the previous study were found to be in the range of 2.46–5.28 Bq/m3 with mean value of 3.79 Bq/m3, approximately a fold lower than the activity concentrations of the same radionuclide found in this study (4.02–6.96 Bq/m3 with mean value of 5.95 Bq/m3). Both studies have median values of 3.89 and 6.04 Bq/m3, respectively. Values from both studies were corrected to March 2001. Distributions of elements, suspended materials and organic matters in seawater and seawater surface are highly influenced by current, circulation, wind flows and seafloor depth. Results from this study showed high concentration of 137Cs activity along the coastal area of the studied zone, and decreased with distance into the open sea. The high 137Cs activity along the coastal shelf can be explained by its shallower water depth and higher particulate materials content compared to the open sea (Nagaya and 239 + 240
Nakamura, 1992). Studies showed that the concentration of 137 Cs activity decreases with increasing water depth (Hirose et al., 1999). Therefore, the shallower depth along the coastal shelf limits the distribution of the radionuclide, thus results the high activity concentration. Moreover, although 137Cs is known to be easily soluble in seawater, it also tend to be adsorbed on colloids surfaces, suspended particles and organic matters before deposited to form sediment layers (Lusa et al., 2009; Park et al., 2004). Study by Park et al. (2004) concluded that adsorption of 137Cs onto suspended materials and organic matters in seawater are dependable on the particle size. The study showed that the concentration of 137Cs activity increases with decreasing of particle size. Fine particles, especially organic matters content are higher in the coastal area rather than pelagic area (Park et al., 2004), thus explains the high concentration of 137Cs activity in seawater of coastal area found in this study. Seawater flow and eddy also plays an important factor in the distribution pattern of the radionuclide in the Exclusive Economic Zone of Peninsular Malaysia. A study on South China Sea current by Chu et al. (1999) showed that the seawater current, especially on the coastal or continental shelf is dependable on the monsoon. The study discovered that the seawater current in May–August is quite slow with a velocity of 0.5 ms-1. The seawater flows from the southeast towards the north, towards east coast Peninsular Malaysia and the Gulf of Thailand and bends northwest into the open sea, thus explains the higher content of 137Cs activity concentration in the coastal shelf of Exclusive Economic Zone of Peninsular Malaysia. The activity of 239 + 240Pu, however, was found to be higher towards the south of the studied zone. Pu-239 +240 are known to be easily adsorbed onto particulate matters in comparison with 137 Cs which is more soluble in seawater. However, it regenerates into soluble form as it moves vertically in deeper waters as a result of microbial decomposition of the particles (Hirose et al., 2006). Moreover, Pu(III) state, one of the main Pu oxidation states, is stable in aqueous solution, but is easily oxidized to Pu(IV) by a-oxidation. The Pu(IV) state exists as Pu(OH)4, which has a very low solubility in seawater. Generally, seawater is a weak base with a pH of about 8.0 and the oxidation state of Pu is predominantly Pu(VI) in solution, which exists in the form of PuO2CO3OH– and falls to the bottom due to heavy molecular mass (Lin et al., 2006). The high 239 + 240Pu activity concentration in the southern area of the EEZ might be due to ‘obstructions’ created by Anambas and Natuna Isles located at the southern end of South China Sea, which limits the seawater circulation in the area. Seawater flow and eddy may also play an important factor in the distribution pattern. More studies need to be done to clarify this phenomenon. On the other hand, the northern area of the EEZ is free from any ‘obstruction’, thus, 239 + 240Pu activities are lower and quite evenly distributed. Furthermore, turbidity tests of seawater collected showed high turbidity in the southern part of the EEZ as compared to the northern part of the zone. Using the SPSS Pearson correlation analysis, it is shown that there is no significant correlation between 137Cs and 239 + 240Pu activity concentrations with temperature, salinity, turbidity and pH of the samples. Therefore, the activity concentrations are independent of these parameters. Mapping of radio-contaminant activity concentrations is the best option to better understand the distribution pattern of 137Cs and 239 + 240Pu within the area of interest. This is generated by using Kriging prediction from ArcGIS 9.3.1 The software predicts the respective radionuclide distribution pattern based on the results from the 30 sampling points. The
1
ESRI ArcGIS 9.3.
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distribution map for 137Cs and 239 + 240Pu activity concentrations within east coast of Peninsular Malaysia EEZ are shown as in Figs. 2 and 3. The legend for the figures was selected to align with the activity concentration as reported by ASPAMARD.
Fig. 2. Distributions of
Fig. 3. Distributions of
The distribution trend for 137Cs activity concentration as predicted by ArcGIS 9.3 software shows increment of the radiocontaminant towards the shore, as shown in Fig. 2. Although there is no clear correlation between turbidity with 137Cs activity
137
Cs activity concentration (Bq/m3) in seawater.
239 + 240
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Pu activity concentration (mBq/m3) in seawater.
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Table 2 Comparison of
137
Cs activity concentrations in Asia Pacific regional seawater (decay corrected to March 2001).
Region
137
239 + 240
This study East coast Peninsular Malaysia Straits of Malacca Straits of Malacca 0 001 00 –051N Vietnam (1999–2002) Philippines (Sulu Sea) Philippines (South China Sea) Western North Pacific North Pacific Ocean South Pacific Ocean Korea (Yangnam) Japan (Jan 1986–Aug 1987) ASPAMARD Arctic Oceana Black Seaa
4.02–6.96 2.46–5.28 1.88–5.09 3.60–5.90 1.70–4.30 0.68–3.67 0.15–2.69 0.12–2.62 2.37–2.99 (1997) – 1.37–1.74 (2003) 2.40–4.50 3.40–7.30 0.26–11.47 9.4–13.6 18.79
2.3–7.9 – – – – – 3.01–3.67 2.38–2.75 1.52–1.85 1.5–9.2 0.5–4.1 – – 0.8–84.3 14.0–23.0 –
a
Cs activity (Bq/m3)
Pu activity (mBq/m3)
References
Yii and Zaharudin (2004) Yii and Zaharudin (2004) Zaharudin et al. (2003) Duran et al. (2004) Nguyen and Nguyen (2003) Yamada et al. (2006) Yamada et al. (2006) Yamada et al. (2006) Hirose et al. (2007a) Hirose et al. (2007b) Park et al. (2004) Aoyama and Hirose (1995) Duran et al. (2004) Mitchell et al. (1999) Gulin and Stokozov (2005)
Areas influenced directly by inputs from discharge of nuclear reprocessing plant, nuclear activities or Chernobyl accident.
concentrations, distribution pattern analysis using ArcGIS application suggested higher activity concentration of 137Cs in high turbidity locations, which is higher at the coastal and towards the south of Peninsular Malaysia. This may be due to the high affinity of the element towards suspended particulates. Alternatively, the distribution pattern for 239 + 240Pu as generated by the software shows that the activity concentrations of the respective radiocontaminant are tend to accumulate offshore and towards the south of Peninsular Malaysia. However, reasons for this phenomenon is not yet well understood. The 239 + 240Pu/137Cs activity ratios in surface waters reflect biogeochemical processes in the surface layer (Hirose et al., 1992; Yamada et al., 1996). Cesium behaves rather conservatively in most water bodies as compared to plutonium, which is particle reactive and redox sensitive, indicating that geochemical process of plutonium can take place. Unlike cesium, plutonium does not accumulate and enter the food chains (Holm et al., 1996). A short residence time and higher affinity for removal of plutonium on sinking particles is supported by the general findings that particulate 239 + 240Pu concentrations range up to 20% of the total activity concentrations in surface waters compared with less than 1% for 137Cs. (Fowler et al., 1983; Hirose et al., 1992). The concentrations of the radionuclides found in this study are very low with typical values due to fallout deposition. Table 2 summarizes the comparison of 137Cs and 239 + 240Pu activity concentrations found in this study to some other areas. The 239 + 240Pu/137Cs ratios in this study (0.00044–0.00149) are lower in comparison to areas influenced by direct inputs from discharge of nuclear reprocessing plant, nuclear activities or the Chernobyl accident. For instance, a study by Hirose et al. (2007a) found that the 239 + 240Pu/137Cs ratios in the western North Pacific subtropical region are higher (0.038–0.13), probably from the input of plutonium due to close-in fallout of the US nuclear explosion in the Marshall Islands. The ratio for global fallout observed in Japan was estimated at 0.015 in 2002 (Hirose et al., 2007a). The differences of suspended particulate affinity and biological uptake of 239 + 240Pu and 137Cs in the Exclusive Economic Zone marine ecosystem might be the factor which resulted in the lower activity ratios of 239 + 240Pu/137Cs found in this study, as compared to global fallout. In addition, higher activity ratio was also observed in the western South Pacific (0.00063–0.00193) and South Pacific subtropical gyre (0.00063–0.00252) as compared to this study. However, 239 + 240Pu/137Cs activity ratios found in eastern South Pacific were lower as compared to this study (Hirose et al., 2007b). The activity ratios from this study were also
Table 3 Comparison of
239 + 240
Pu/137Cs activity ratios with other studies.
Region
239 + 240
Pu/137Cs activity ratio ( 10 3)
This study 0.44–1.49 Western north Pacific 38–130 Global fallout 15 Western south Pacific 0.63–1.93 South Pacific subtropical gyre 0.63–2.52 Eastern south Pacific 0.68–21 Mediterranean Sea 6.3 7 0.7 Sulu and Indonesian Sea 1.32 7 0.39 South China Sea 0.93 7 0.04 ASPAMARD 1.9
References
Hirose et al. (2007a) Hirose et al. 2007a Hirose et al. (2007b) Hirose et al. (2007b) Hirose et al. (2007b) Sang-Han Lee et al. (2003) Yamada et al. (2006) Yamada et al. (2006) Duran et al. (2004)
found to be lower than that reported in ASPAMARD as tabulated in Table 3.
4. Conclusions Generally, findings in this study showed that 137Cs in the Exclusive Economic Zone of peninsular Malaysia are higher in the coastal zone compared to the open sea. Whilst, at the same sampling depth, the concentration of 239 + 240Pu are found to be lower along the coastal zone but higher towards the south. The activity concentrations of 137Cs and 239 + 240Pu were in the range of 3.40–5.89 Bq/m3 and 2.3–7.9 mBq/m3, respectively. These data represent reference values for Malaysia and will be used to estimate radionuclide inventory in Malaysian marine environment, particularly at the Exclusive Economic Zone of East Coast peninsular Malaysia. These estimations will also serve as a baseline for detection of any future nuclear activities. The 239 + 240 Pu/137Cs activity ratios indicate that there are no new inputs of nuclear radionuclides in the study area.
Acknowledgments Funding from MOSTI, under ScienceFund 04-03-01-SF0020, is gratefully acknowledged. We would like to thank the captain and crews of KL PAUS for their assistance during sampling expedition and staff members of the Radiochemistry and Environment Group for their support and able technical assistance throughout this
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study. We are grateful to Dr. Khairuddin Abdul Rahim for his support and assistance. References Aarkrog, A., 2003. Input of anthropogenic radionuclides into the World Ocean. Deep-Sea Res. Part 2 Top. Stud. Oceanogr. 50, 2597–2606. Aoyama, M., Hirose, K., 1995. The temporal and spatial variation of 137Cs concentration in the western North Pacific and its marginal seas during the period from 1979 to 1988. J. Environ. Radioact. 29, 57–74. Chu, P.C., Edmons, N.L., Fan, C., 1999. Dynamical mechanisms for the South China Sea seasonal circulation and thermohaline variabilities. J. Phys. Oceanogr. 29 (11), 2971–2989. Duran, E.B., Povinec, P.P., Fowler, S.W., Airey, P.L., Hong, G.H., 2004. 137Cs and 239 + 240Pu levels in the Asia-Pacific regional seas. J. Environ. Radioact. 76, 139–160. Fowler, S.W., Ballestra, S., La-Rosa, J., Fukai, R., 1983. Vertical transport of particulate-associated plutonium and americium in the Northwest Pacific. Deep-Sea Res. 7, 35. Godoy, J.M, Schuch, L.A., Nordemann, D.J.R., Reis, V.R.G., Ramalho Recio, M., Brio, J.C., Olech, R.R.A.M.A., 1998. 137Cs, 226, 228Ra, 210Pb and 40 K concentrations in Antarctic soil, sediment and selected moss and lichen samples. J. Environ. Radioact. 41, 33–45. Gulin, S.B., Stokozov, N.A., 2005. 137Cs concentrations in Atlantic and western Antarctic surface waters: results of the 7th Ukrainian Antarctic Expedition, 2002. J. Environ. Radioact. 83, 1–7. Holm, E., Roos, P., Josefsson, D., Persson, B., 1996. Radioactivity from the North Pole to the Antartic, in: P. Guegueniat, P. Germain, H. Metivier (Eds), Radionuclides in the Oceans: Inputs and Inventories, Les Editions de Physiques, 59. Hirose, K., Amano, H., Baxter, M.S., Chaykovskaya, E., Chumichev, V.B., Hong, G.H., Isogai, K., Kim, C.K., Kim, S.H., Miyao, T., Morimoto, T., Nikitin, A., Oda, K., Pettersson, H.B.L., Povinec, P.P., Seto, Y., Tkalin, A., Togawa, O., Veletova, N.K., 1999. Anthropogenic radionuclides in seawater in the East Sea/Japan Sea: results of the first-stage Japanese–Korean–Russian expedition. J. Environ. Radioact. 43, 1–13. Hirose, K., Aoyama, M., Kim, C.S., Kim, C.K., Povinec, P.P., 2006. Plutonium isotopes in seawater of the north pacific: effects of close-in fallout. In: Povinec, P., Sanchez-Cabeza, J.A. (Eds.), Radioactivity in the Environment, vol. 8, pp. 67–82. Hirose, K., Aoyama, M., Kim, C.S., 2007a. Plutonium in seawater of the Pacific Ocean. J. Radioanal. Nucl. Chem. 274, 635–638. Hirose, K., Aoyama, M., Fukasawa, M., Kim, C.S., Komura, K., Povinec, P., SanchezCabeza, J.A., 2007b. Plutonium and 137Cs in surface water of the South Pacific Ocean. Sci. Total Environ. 381, 243–255. Hirose, K., Sugimura, Y., Aoyama, M., 1992. Plutonium and 137Cs in the western North Pacific: estimation of residence time of plutonium in surface waters. In: Mann, W.B., McLaughlin, W.L. (Eds.), Low-Level-Radioactivity. Measuring Techniques and Alpha-Particle Spectrometry, Int. J. Radiat. Appl. Instrum. Part A, 43; 1992, pp. 349–359.
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International Atomic Energy Agency (IAEA), Energy, electricity and nuclear power estimates for the period up to 2020, 2001. IAEA-RDS-1/21, p. 13; Website IAEA. http://www.iaea.org/worldatom/Programmes/energy/pess/rds-1/RDS1-21scr. pdf. Lin, X-J., Park, G., Kwak, J-H., Kim, W., Kang, H-D., Lee, H-L., Kim, Y-G., Doh, S-H., Kim, D-S., Kim, C-K., 2006. 239 + 240Pu concentrations of sediments and seawater in the coastal sea of Korea. J. Radioanal. Nucl. Chem. 268 (1), 103–108. Lusa, M., Lehto, J., Leskinen, A., Jaakkola, T., 2009. 137Cs, 239, 240Pu and 241Am in ¨ anne, ¨ bottom sediments and surface water of Lake Paij Finland. J. Environ. Radioact. 100, 468–476. Mohsin, K.M., Mohamed, M.I. (Eds.), 1988. Ekspedisi Matahari ’87: A study on the offshore waters of the Malaysian EEZ. Universiti Pertanian Malaysia. Occasional Publication No. 8, p. 168. Miller, J.C., Miller, J.N., 1993. Statistics for Analytical Chemistry 3rd ed. Ellis Harwood Ptr Prentice Hall, New York. Mitchell, P.I., Holm, E., Dahlgaard, H., Nielsen, S.P., Bopust, D., Leonard, K.S., Papucci, C., Salbu, B., Strand, P., Sanchez-Cabeza, J.A., Rissanen, K., Pollard, D., Gasco, C., Christensen, G.C., Leon-Vintro, L., McMaho, C.A., Herrmann, J., Nies, H., 1999. Results and analysis of the ARMARA sea water intercomparison for 137 Cs, 238Pu and 239,240Pu. In: Proceedings of On the Marine Pollution, 5–9 October, 1998, Monaco. IAEA, Vienna (IAEA-TECDOC-1094). Nagaya, Y., Nakamura, K., 1992. 239,240Pu and 137Cs in the East China and the Yellow Seas. J. Oceanogr. 48, 23–35. Nguyen T.N., Nguyen T.B., 2003. Project formulation meeting on enhancing the marine coastal environment. In: Project Progress Meeting, 3–7 March 2003, Kuala Lumpur. Park, G., Lin, X.J., Kim, W., Kang, H.D., Lee, H.L., Kim, Y., Doh, S.H., Kim, D.S., Yun, S.G., Kim, C.K., 2004. Properties of 137Cs in marine sediments off Yangnam, Korea. J. Environ. Radioact. 77, 285–299. Sang-Han Lee, S-H., La Rosa, J.J, Levy-Palomo, I., Beniamino Oregioni, B., Pham, M.K., Povinec, P.P., Wyse, E., 2003. Recent inputs and budgets of 90Sr, 137Cs, 239, 240Pu and 241Am in the northwest Mediterranean Sea. Deep-Sea Research II 50, 2817–2834. World Nuclear Association, World Nuclear Power Reactors 2007–09 and Uranium Requirements, 2008. Website of the World Nuclear Association. /http://www. world-nuclear.org/info/reactors.htmlS [cited 30 September 2008]. Yamada, M., Aono, T, Hirano, S., 1996. 239 + 240Pu and 137Cs distributions in seawater from the Yamato Basin and the Tsushima Basin in the Japan Sea. J Radioanal. Nucl. Chem. 210, 129–136. Yamada, M., Wang, Z-L., Zheng, J., 2006. The extremely high 137Cs inventory in the Sulu Sea: a possible mechanism. J. Environ. Radioact. 90, 163–171. Yamada, M., Zheng, J., Wang, Z-L., 2006. 137Cs, 239 + 240Pu and 240Pu/239Pu atom ratios in the surface waters of the western North Pacific Ocean, eastern Indian Ocean and their adjacent seas. Sci. Total Environ. 366, 242–252. Yii, M.W., Zaharudin, A., 2004. Determination of 137Cs in seawater surrounding peninsular Malaysia—A case study. J. Nucl. Relat. Technol. 1 (2), 17. Yii, M.W., Zaharudin, A., 2007. Concentration of 137Cs in seawater surrounding East Malaysia. J. Radioanal. Nucl. Chem. 274, 323–329. Zaharudin, A., Yii, M.W., Kamarozaman, I., Wood, A.K., 2003. Analytical Chemistry: Application and Current Issues. Lee Ming Press, Kuching 131.
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Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
Spatial distributions of 137Cs and 239 + 240Pu in surface seawater within the Exclusive Economic Zone of East Coast Peninsular Malaysia Zaharudin Ahmad n, Yii Mei-Wo, Ahmad Sanadi Abu Bakar, Hidayah Shahar Radiochemistry and Environmental Group, Malaysian Nuclear Agency (Nuclear Malaysia), Bangi, 43000 Kajang, Malaysia
a r t i c l e in fo
abstract
Article history: Received 5 June 2009 Received in revised form 4 January 2010 Accepted 1 April 2010
The studies of 137Cs and 239 + 240Pu distributions in surface seawater at South China Sea within the Exclusive Economic Zone (EEZ) of Peninsular Malaysia were carried out in June 2008. The analysis results will serve as additional information to the expanded baseline data for Malaysia’s marine environment. Thirty locations from extended study area were identified in the EEZ from which large volumes of surface seawater samples were collected. Different co-precipitation techniques were employed to concentrate cesium and plutonium separately. A known amount of 134Cs and 242Pu tracers were used as yield determinant. The precipitate slurry was collected and oven dried at 60 oC for 1–2 days. Cesium precipitate was fine-ground and counted using gamma-ray spectrometry system at 661.62 keV, while plutonium was separated from other radionuclides using anion exchange, electrodeposited and counted using alpha spectrometry. The activity concentrations of 137Cs and 239 + 240 Pu were in the range of 3.40–5.89 Bq/m3 and 2.3–7.9 mBq/m3, respectively. The 239 + 240Pu/137Cs ratios indicate that there are no new inputs of these radionuclides into the area. & 2010 Elsevier Ltd. All rights reserved.
Keywords: 137 Cs 239 + 240 Pu Seawater East coast Peninsular Malaysia Baseline data
1. Introduction Rapid development of nuclear power industry in the recent years, especially in the Asia Pacific region, has raised some concern to the countries within the area. As of October 1, 2008, there were 439 nuclear power plants in operation, 36 nuclear power plants under construction, 99 were in planning and 232 were proposed to build (World Nuclear Association, 2008). In East and South Asia alone, there are over 109 nuclear power reactors in operation, 18 under construction and a further 110 are planned to build. China and India have established 9 new nuclear power plants in the last 4 years and have 10 more under construction. Japan has the highest number of nuclear power plants in Asia, with 53 in operation (International Atomic Energy Agency, 2001). Nuclear power generation has tremendous benefits in meeting the electricity needs of growing populations. It is also friendlier to the environment for not contributing to adverse effects as burning of fossil fuels. However, there are potential risks of planned and unplanned releases of radionuclide into the marine environment which need to be addressed. The Chernobyl incident proves that any accident occurring in a nuclear power plant introduces artificial radioisotopes such as 134Cs, 137Cs, 239,240Pu, 238Pu and
n
Corresponding author. Tel.: + 6 03 8925 0510; fax: + 6 03 8928 2977. E-mail addresses: [email protected], [email protected] (Z. Ahmad). 0969-8043/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.apradiso.2010.04.012
241
Am into the environment. However, higher demand of energy for development purposes in some countries has forced them to use nuclear power as an alternative energy resource due to its sustainability. The presence of the above artificial radionuclides for the past 60 years, particularly in the marine environment, originated from nuclear weapon tests, releases from nuclear facilities, radioactive waste dumping, the Chernobyl accident and nuclear submarine and aircraft accidents. It is believed that around 380 and 40 nuclear weapon tests were carried out in the Northern and Southern Hemisphere, respectively (Godoy et al., 1998). Although most of the radionuclide were released and deposited in the Northern Hemisphere, they could also be transported, partially through stratospheric circulation, into the Southern Hemisphere. Amongst these radioisotopes, anthropogenic radionuclides 137Cs and 239 + 240 Pu with half-lives of 30.2 years and 2.4 104 years, respectively, are important indicators for radioactive contamination in marine environment, and are of primary interest because of existing inventories and possible health effects (Duran et al., 2004). Cs-137 can also serve as a conservative tracer of the transport and accumulation patterns of contaminants in seawater. Aarkrog (2003) has estimated that the total input of 137Cs to the World Ocean was 659 PBq (604 PBq from fallout, 40 PBq from nuclear reprocessing and 16 PBq from the Chernobyl accident). It has also been estimated that 239 + 240Pu inventories by the year 2000 in the Pacific and Indian Oceans were 4.3 PBq from global stratospheric fallout and 2.0 PBq from close-in fallout (Aarkrog, 2003).
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Monitoring of radioactivity content in Malaysian marine environment is of interest, not only for additional information to represent baseline data for the nation but also for worldwide database. This is due to the fact that there is still inadequate data on 137Cs and 239 + 240Pu inventory within the Malaysian marine environment, even though some work had been carried out by other researchers in the nearby region as reported in ASPAMARD (Duran et al., 2004). Furthermore, there is a great concern on the potential risks of releases of radioactive materials from nuclear facilities in nearby countries within the region to the nation and its residents. The aim of this study is to investigate the present level of 137Cs and 239 + 240Pu activity concentrations in the Exclusive Economic Zone (EEZ) of east coast Peninsular Malaysia, whereby the study area was extended from an approximately 28,052 km2 (from previous study by Yii and Zaharudin, 2004) in the coastal area to cover an area of 140,972 km2 of the zone.
2. Experimental 2.1. Study area and sample collection The Exclusive Economic Zone of east coast Peninsular Malaysia covers an area between 11 14.040 to 71 48.920 N latitude, and 1021 5.030 to 1051 48.770 E longitude, with approximately 1150 km length and has a maximum width of 417 km. It is relatively shallow, with an average depth of 60–70 m. The seawater is generally well mixed throughout the water column and the prevailing surface currents are closely associated with the monsoon seasons (Mohsin and Mohamed, 1988). Seawater samples were collected during sampling expeditions on board the K.L. PAUS (Malaysian Fisheries Institute) from June
11 to 30, 2008. A total of 30 locations were selected and samples were subsequently collected and analyzed. Duplicate surface water samples (up to a depth of 5 m) were collected and labeled according to its sampling location as in Fig. 1. Due to an expected low concentration of 137Cs and 239 + 240Pu in the marine environment, large volumes of seawater (approximately 200 liters) were collected. Different co-precipitation techniques were employed to concentrate cesium and plutonium, respectively. Physical parameters, particularly temperature, salinity, pH and turbidity of the water column were also measured to serve as supporting parameters (Table 1).
2.2. Determination of
137
Cs in seawater samples
Approximately 4–8 Bq of 134Cs in 1% saline solution (Isotope Products Laboratories, 989-72) was used as tracer for yield determination. All chemicals used are of AnalaR grade, including potassium hexacyanoferrate (II) trihydrate [K4Fe(CN)6 3H2O] and copper (II) nitrate 2.5 hydrate [Cu(NO3)2 2.5 H2O]. Detailed procedure for precipitating and collecting of 137 Cs from seawater was described in the previous publication (Yii and Zaharudin, 2007). The precipitate was oven dried prior to 137Cs activity concentration determination. The dried precipitate was finely ground and sealed in 6 ml polyethylene containers as of the calibration standard. Counting was carried out with a p-type well germanium detector for 24 h. The activity of 137Cs was determined by measuring the peak area under the photo-peak of the 661.62 keV gamma-ray and correcting it for the detection efficiency, gamma-ray abundance, recovery of 134Cs tracer and averaged over the volume of seawater used.
Fig. 1. Map showing sampling site locations.
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Table 1 Physical parameters and activity concentration of Station
SF01 SF02 SF03 SF04 SF05 SF06 SF07 SF08 SF09 SF10 SF11 SF12 SF13 SF14 SF15 SF16 SF17 SF18 SF19 SF20 SF21 SF22 SF23 SF24 SF25 SF26 SF27 SF28 SF29 SF30
Temp. (1C)
137
Salinity PSU
30.91 30.46 30.29 30.04 30.03 30.11 30.25 30.41 28.66 30.26 30.03 29.59 29.85 29.63 29.61 29.68 29.94 29.76 29.50 29.72 29.64 29.63 29.89 29.75 29.03 29.21 29.25 29.74 29.48 30.91
32.66 33.04 33.17 32.98 33.53 33.57 33.27 33.17 33.98 33.75 33.55 33.69 33.58 33.57 33.29 33.38 33.97 33.65 34.01 33.91 33.84 33.83 33.48 33.73 33.98 34.03 33.93 33.99 33.78 32.66
Cs and
239 + 240
pH
8.18 8.23 8.22 8.21 8.18 8.21 8.19 8.20 8.16 8.23 8.19 8.20 8.21 8.20 8.22 8.20 8.15 8.16 8.15 8.22 8.24 8.20 8.22 8.23 8.17 8.24 8.18 8.23 8.16
Pu in surface seawater. Turbidity
2.80 2.60 2.70 2.70 2.63 2.70 2.60 2.67 2.65 2.55 2.67 2.80 2.65 2.55 2.50 2.60 2.70 2.58 2.80 2.75 2.65 2.70 2.70 2.65 2.67 2.85 2.70 2.75 2.80 5.97
2.3. Gamma spectrometry counting system The p-type gamma spectrometry counting system (cryostat well 40 mm depth 16 mm diameter, GCW 2523) was calibrated using a customized gamma multinuclide standard source solution (comprising 210Pb, 241Am, 109Cd, 57Co, 123Te, 51Cr, 113Sn, 85Sr, 137 Cs, 88Y and 60Co) with known activities, prepared by Isotope Products Laboratories, USA (source no. 1290-84). The energy and the efficiency calibration of the gamma-spectrometer system were then confirmed by using the Certified Reference Material (Soil-6 and IAEA-326) on the same counting geometry (hence efficiency). The peak resolution, FWHM, at the 122–1332 keV was 1.16 and 1.90 keV, respectively. The performance of the instrument was monitored regularly to ensure fitness for the purpose (Zaharudin et al., 2003).
2.4. Determination of
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239 + 240
Pu in seawater samples
Radionuclide 242Pu (National Physical Laboratory, R 1944) was used as tracer for chemical yield determination. All chemical used are of AnalaR grade, including potassium permanganate [KMnO4], sodium hydroxide [NaOH] and manganese chloride tetrahydrate [MnCl2 4 H2O]. Collected seawater samples were immediately acidified to pHr2.0 with the addition of concentrated HCl. Approximately 10 mBq of 242Pu was added into the sample as yield determinant and 25 ml of saturated KMnO4 subsequently. The sample was then stirred thoroughly and left for an hour. Approximately 10 M of NaOH was added to adjust the pH to 7–8. A 1 M MnCl2 solution was then added slowly (a few ml at a time) to form precipitate (re-adjust pH with 10 M NaOH) until further addition of MnCl2 did not change the pH of the sample. Precipitates formed were left
239 + 240
Pu/137 Cs activity ratio ( 10 3)
Activity Cs-137 (Bq/m3)
Pu-239, 240 (mBq/m3)
5.29 70.31 5.55 70.32 5.27 70.30 5.23 70.30 4.93 70.29 4.62 70.27 4.44 70.26 5.85 70.33 4.96 70.28 5.74 70.33 4.65 70.27 4.39 70.26 5.85 70.34 4.40 70.26 4.07 70.24 3.40 70.20 5.10 70.29 5.89 70.34 5.11 70.30 5.21 70.30 5.03 70.29 4.83 70.28 4.38 70.26 5.49 70.32 4.38 70.25 5.81 70.34 5.67 70.33 5.25 70.30 4.82 70.28 5.33 70.30
3.38 7 0.29 2.86 7 0.24 2.33 7 0.20 2.90 7 0.25 2.63 7 0.23 3.39 7 0.30 4.23 7 0.37 2.36 7 0.20 4.32 7 0.37 5.24 7 0.45 4.65 7 0.40 4.52 7 0.39 4.56 7 0.39 4.63 7 0.40 2.74 7 0.24 4.61 7 0.39 3.43 7 0.29 3.61 7 0.31 3.76 7 0.32 3.21 7 0.27 3.77 7 0.33 4.92 7 0.42 3.41 7 0.29 2.66 7 0.23 4.36 7 0.37 3.78 7 0.33 3.40 7 0.30 3.48 7 0.30 4.45 7 0.38 7.95 7 0.68
0.64 0.52 0.44 0.55 0.53 0.73 0.95 0.40 0.87 0.91 1.00 1.03 0.78 1.05 0.67 1.36 0.67 0.61 0.73 0.62 0.75 1.02 0.78 0.49 1.00 0.65 0.60 0.66 0.92 1.49
to settle overnight before decanting the clear water. High quality HDPE container was used for storing the precipitates obtained. The container was sealed, labeled and transferred to the laboratory for analysis. Rinsing of precipitates with distilled water was done twice prior to oven drying process. Chemical leaching processes for the determination of 239 + 240Pu activity were then done by dissolving the dried manganese dioxide precipitates with 8 M HNO3, sodium nitrite and hydrogen peroxide, subsequently. These processes were done while heating for ease of dissolution and decomposition of excess H2O2 and NaNO2. Solution obtained was then loaded onto conditioned anion exchange column (BIO-RAD, Ag 1-X2, 50–100 mesh, Cl form). Americium, uranium, polonium, thorium and iron were desorbed from the column by passing through 8 M HNO3 (60 ml) and 8 M HCl (30 ml). Plutonium was eluted using 0.7 M HNO3 (40 ml) and 0.7 M HNO3 (50 ml) with the presence of 0.075 g hydroquinone [C6H4(OH)2]. Hydroquinone was eliminated by heating and addition of concentrated HNO3 into the sample. The sample was then evaporated to dryness. A solution of 8 M HNO3 was then added into the dried sample and heated. Dissolved sample was then loaded onto conditioned anion exchange column (BIO-RAD, AG1-X8, 100–200 mesh, Cl form). Impurities were removed from the column by passing through 8 M HNO3 (3 5 ml) and 10 M HCl (3 5 ml). Purified plutonium was eluted from the column using 20–25 ml of freshly prepared NH4I-HCl (5.1 g of NH4I dissolved in 100 ml of water mixed with concentrated HCl with the ratio of 29:71 (v/v)). Solution obtained was then heated so as to eliminate iodine content prior to addition of concentrated HNO3. The solution was then evaporated to dryness. Sample was then ready for electro-deposition. Electro-deposition was done by adding 5 ml of 10% (v/v) H2SO4 into the sample and neutralized by adding NH4OH 25% (drop-
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wise). Neutralized solution was then transferred into an electrodeposition cell and the beaker was rinsed with 2 ml of H2SO4 1% and 3 ml of water. Plutonium was then electrodeposited onto polished stainless steel discs (pH 2.0 to 2.2, constant current of 1.0 A for 1 h) and assayed by alpha spectrometry for 3 days. 2.5. Alpha spectrometry counting system The alpha activities were measured with 450 mm2 Passivated ion Implanted Silicon, PIPS detectors placed in multichamber analyzer (Canberra). The vacuum in the chambers was kept under 10 Torr and the source to detector distance was less than 1 cm, with the registration effectiveness of 25%. The background count for each spectroscopy channel was less than 24 counts per day. Data treatment was by comparison of the peak area to the amount of yield monitor, 242Pu, added.
3. Results and discussion The activity concentration of 137Cs and 239 + 240Pu, and the Pu/137Cs activity ratios in the Exclusive Economic Zone of east coast Peninsular Malaysia seawater obtained in this study is summarized as in Table 1. Activities are reported in Bq/m3 and mBq/m3 for 137Cs and 239 + 240Pu, respectively. Cs-137 activity concentrations at the sampling period ranged from 3.40 to 5.89 Bq/m3 with mean value of 5.03 Bq/m3. On the other hand, 239 + 240 Pu activity concentrations were found to be in the range of 2.33–7.95 mBq/m3 with mean value of 3.85 mBq/m3. Approximately 90% and 50–80% of 134Cs and 242Pu tracers, respectively were recovered successfully throughout this study. Errors quoted for 137Cs and 239 + 240Pu activities were 1s values derived from counting statistics. The ranges of all values were large, mainly due to the large area of the Exclusive Economic Zone. Median values for both 137Cs and 239 + 240Pu activity concentrations are calculated as 5.11 and 3.68 mBq/m3, respectively, which are quite close to their respective mean values. Student-t test indicated that the difference between median and mean values are insignificant due to the calculated t value for 137Cs (t¼ 0.680) and 239 + 240Pu (t¼ 0.835) are less than t-critical, 2.045 (Miller and Miller, 1993), suggesting that the activity concentrations of both radionuclides are uniformly distributed. Study for ASPAMARD by Duran et al. (2004) found that the median value for 137 Cs activity concentration in seawater was 2.8 Bq/m3, whilst the median value for 239 + 240Pu was 5.9 mBq/m3. Wide study area and larger range of activity concentrations in ASPAMARD might be the reason for the difference of median values for both this and ASPAMARD studies. Comparison with previous study performed in the East Coast of peninsular Malaysia (Yii and Zaharudin, 2004) showed that there are elevations in 137Cs activity concentrations within the studied area. Cs-137 activity concentrations in the previous study were found to be in the range of 2.46–5.28 Bq/m3 with mean value of 3.79 Bq/m3, approximately a fold lower than the activity concentrations of the same radionuclide found in this study (4.02–6.96 Bq/m3 with mean value of 5.95 Bq/m3). Both studies have median values of 3.89 and 6.04 Bq/m3, respectively. Values from both studies were corrected to March 2001. Distributions of elements, suspended materials and organic matters in seawater and seawater surface are highly influenced by current, circulation, wind flows and seafloor depth. Results from this study showed high concentration of 137Cs activity along the coastal area of the studied zone, and decreased with distance into the open sea. The high 137Cs activity along the coastal shelf can be explained by its shallower water depth and higher particulate materials content compared to the open sea (Nagaya and 239 + 240
Nakamura, 1992). Studies showed that the concentration of 137 Cs activity decreases with increasing water depth (Hirose et al., 1999). Therefore, the shallower depth along the coastal shelf limits the distribution of the radionuclide, thus results the high activity concentration. Moreover, although 137Cs is known to be easily soluble in seawater, it also tend to be adsorbed on colloids surfaces, suspended particles and organic matters before deposited to form sediment layers (Lusa et al., 2009; Park et al., 2004). Study by Park et al. (2004) concluded that adsorption of 137Cs onto suspended materials and organic matters in seawater are dependable on the particle size. The study showed that the concentration of 137Cs activity increases with decreasing of particle size. Fine particles, especially organic matters content are higher in the coastal area rather than pelagic area (Park et al., 2004), thus explains the high concentration of 137Cs activity in seawater of coastal area found in this study. Seawater flow and eddy also plays an important factor in the distribution pattern of the radionuclide in the Exclusive Economic Zone of Peninsular Malaysia. A study on South China Sea current by Chu et al. (1999) showed that the seawater current, especially on the coastal or continental shelf is dependable on the monsoon. The study discovered that the seawater current in May–August is quite slow with a velocity of 0.5 ms-1. The seawater flows from the southeast towards the north, towards east coast Peninsular Malaysia and the Gulf of Thailand and bends northwest into the open sea, thus explains the higher content of 137Cs activity concentration in the coastal shelf of Exclusive Economic Zone of Peninsular Malaysia. The activity of 239 + 240Pu, however, was found to be higher towards the south of the studied zone. Pu-239 +240 are known to be easily adsorbed onto particulate matters in comparison with 137 Cs which is more soluble in seawater. However, it regenerates into soluble form as it moves vertically in deeper waters as a result of microbial decomposition of the particles (Hirose et al., 2006). Moreover, Pu(III) state, one of the main Pu oxidation states, is stable in aqueous solution, but is easily oxidized to Pu(IV) by a-oxidation. The Pu(IV) state exists as Pu(OH)4, which has a very low solubility in seawater. Generally, seawater is a weak base with a pH of about 8.0 and the oxidation state of Pu is predominantly Pu(VI) in solution, which exists in the form of PuO2CO3OH– and falls to the bottom due to heavy molecular mass (Lin et al., 2006). The high 239 + 240Pu activity concentration in the southern area of the EEZ might be due to ‘obstructions’ created by Anambas and Natuna Isles located at the southern end of South China Sea, which limits the seawater circulation in the area. Seawater flow and eddy may also play an important factor in the distribution pattern. More studies need to be done to clarify this phenomenon. On the other hand, the northern area of the EEZ is free from any ‘obstruction’, thus, 239 + 240Pu activities are lower and quite evenly distributed. Furthermore, turbidity tests of seawater collected showed high turbidity in the southern part of the EEZ as compared to the northern part of the zone. Using the SPSS Pearson correlation analysis, it is shown that there is no significant correlation between 137Cs and 239 + 240Pu activity concentrations with temperature, salinity, turbidity and pH of the samples. Therefore, the activity concentrations are independent of these parameters. Mapping of radio-contaminant activity concentrations is the best option to better understand the distribution pattern of 137Cs and 239 + 240Pu within the area of interest. This is generated by using Kriging prediction from ArcGIS 9.3.1 The software predicts the respective radionuclide distribution pattern based on the results from the 30 sampling points. The
1
ESRI ArcGIS 9.3.
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distribution map for 137Cs and 239 + 240Pu activity concentrations within east coast of Peninsular Malaysia EEZ are shown as in Figs. 2 and 3. The legend for the figures was selected to align with the activity concentration as reported by ASPAMARD.
Fig. 2. Distributions of
Fig. 3. Distributions of
The distribution trend for 137Cs activity concentration as predicted by ArcGIS 9.3 software shows increment of the radiocontaminant towards the shore, as shown in Fig. 2. Although there is no clear correlation between turbidity with 137Cs activity
137
Cs activity concentration (Bq/m3) in seawater.
239 + 240
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Pu activity concentration (mBq/m3) in seawater.
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Table 2 Comparison of
137
Cs activity concentrations in Asia Pacific regional seawater (decay corrected to March 2001).
Region
137
239 + 240
This study East coast Peninsular Malaysia Straits of Malacca Straits of Malacca 0 001 00 –051N Vietnam (1999–2002) Philippines (Sulu Sea) Philippines (South China Sea) Western North Pacific North Pacific Ocean South Pacific Ocean Korea (Yangnam) Japan (Jan 1986–Aug 1987) ASPAMARD Arctic Oceana Black Seaa
4.02–6.96 2.46–5.28 1.88–5.09 3.60–5.90 1.70–4.30 0.68–3.67 0.15–2.69 0.12–2.62 2.37–2.99 (1997) – 1.37–1.74 (2003) 2.40–4.50 3.40–7.30 0.26–11.47 9.4–13.6 18.79
2.3–7.9 – – – – – 3.01–3.67 2.38–2.75 1.52–1.85 1.5–9.2 0.5–4.1 – – 0.8–84.3 14.0–23.0 –
a
Cs activity (Bq/m3)
Pu activity (mBq/m3)
References
Yii and Zaharudin (2004) Yii and Zaharudin (2004) Zaharudin et al. (2003) Duran et al. (2004) Nguyen and Nguyen (2003) Yamada et al. (2006) Yamada et al. (2006) Yamada et al. (2006) Hirose et al. (2007a) Hirose et al. (2007b) Park et al. (2004) Aoyama and Hirose (1995) Duran et al. (2004) Mitchell et al. (1999) Gulin and Stokozov (2005)
Areas influenced directly by inputs from discharge of nuclear reprocessing plant, nuclear activities or Chernobyl accident.
concentrations, distribution pattern analysis using ArcGIS application suggested higher activity concentration of 137Cs in high turbidity locations, which is higher at the coastal and towards the south of Peninsular Malaysia. This may be due to the high affinity of the element towards suspended particulates. Alternatively, the distribution pattern for 239 + 240Pu as generated by the software shows that the activity concentrations of the respective radiocontaminant are tend to accumulate offshore and towards the south of Peninsular Malaysia. However, reasons for this phenomenon is not yet well understood. The 239 + 240Pu/137Cs activity ratios in surface waters reflect biogeochemical processes in the surface layer (Hirose et al., 1992; Yamada et al., 1996). Cesium behaves rather conservatively in most water bodies as compared to plutonium, which is particle reactive and redox sensitive, indicating that geochemical process of plutonium can take place. Unlike cesium, plutonium does not accumulate and enter the food chains (Holm et al., 1996). A short residence time and higher affinity for removal of plutonium on sinking particles is supported by the general findings that particulate 239 + 240Pu concentrations range up to 20% of the total activity concentrations in surface waters compared with less than 1% for 137Cs. (Fowler et al., 1983; Hirose et al., 1992). The concentrations of the radionuclides found in this study are very low with typical values due to fallout deposition. Table 2 summarizes the comparison of 137Cs and 239 + 240Pu activity concentrations found in this study to some other areas. The 239 + 240Pu/137Cs ratios in this study (0.00044–0.00149) are lower in comparison to areas influenced by direct inputs from discharge of nuclear reprocessing plant, nuclear activities or the Chernobyl accident. For instance, a study by Hirose et al. (2007a) found that the 239 + 240Pu/137Cs ratios in the western North Pacific subtropical region are higher (0.038–0.13), probably from the input of plutonium due to close-in fallout of the US nuclear explosion in the Marshall Islands. The ratio for global fallout observed in Japan was estimated at 0.015 in 2002 (Hirose et al., 2007a). The differences of suspended particulate affinity and biological uptake of 239 + 240Pu and 137Cs in the Exclusive Economic Zone marine ecosystem might be the factor which resulted in the lower activity ratios of 239 + 240Pu/137Cs found in this study, as compared to global fallout. In addition, higher activity ratio was also observed in the western South Pacific (0.00063–0.00193) and South Pacific subtropical gyre (0.00063–0.00252) as compared to this study. However, 239 + 240Pu/137Cs activity ratios found in eastern South Pacific were lower as compared to this study (Hirose et al., 2007b). The activity ratios from this study were also
Table 3 Comparison of
239 + 240
Pu/137Cs activity ratios with other studies.
Region
239 + 240
Pu/137Cs activity ratio ( 10 3)
This study 0.44–1.49 Western north Pacific 38–130 Global fallout 15 Western south Pacific 0.63–1.93 South Pacific subtropical gyre 0.63–2.52 Eastern south Pacific 0.68–21 Mediterranean Sea 6.3 7 0.7 Sulu and Indonesian Sea 1.32 7 0.39 South China Sea 0.93 7 0.04 ASPAMARD 1.9
References
Hirose et al. (2007a) Hirose et al. 2007a Hirose et al. (2007b) Hirose et al. (2007b) Hirose et al. (2007b) Sang-Han Lee et al. (2003) Yamada et al. (2006) Yamada et al. (2006) Duran et al. (2004)
found to be lower than that reported in ASPAMARD as tabulated in Table 3.
4. Conclusions Generally, findings in this study showed that 137Cs in the Exclusive Economic Zone of peninsular Malaysia are higher in the coastal zone compared to the open sea. Whilst, at the same sampling depth, the concentration of 239 + 240Pu are found to be lower along the coastal zone but higher towards the south. The activity concentrations of 137Cs and 239 + 240Pu were in the range of 3.40–5.89 Bq/m3 and 2.3–7.9 mBq/m3, respectively. These data represent reference values for Malaysia and will be used to estimate radionuclide inventory in Malaysian marine environment, particularly at the Exclusive Economic Zone of East Coast peninsular Malaysia. These estimations will also serve as a baseline for detection of any future nuclear activities. The 239 + 240 Pu/137Cs activity ratios indicate that there are no new inputs of nuclear radionuclides in the study area.
Acknowledgments Funding from MOSTI, under ScienceFund 04-03-01-SF0020, is gratefully acknowledged. We would like to thank the captain and crews of KL PAUS for their assistance during sampling expedition and staff members of the Radiochemistry and Environment Group for their support and able technical assistance throughout this
ARTICLE IN PRESS Z. Ahmad et al. / Applied Radiation and Isotopes 68 (2010) 1839–1845
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