Levels of natural and anthropogenic radionuclides in selected environmental radioactivity monitoring sites in Singapore

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Wong, H.J. Tan, J.A. Corcho Alvarado This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-4747517/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract This paper provides insights into the radioactivity levels in soil, sediment and surface seawater from selected monitoring sites in Singapore. The environmental samples were measured for naturally occurring and anthropogenic radionuclides. In soils, the decay series of 232 Th and 238 U are the highest contributors to the total radioactivity, while in sediments, 40 K is the main contributor to the total radioactivity. The levels of 90 Sr, 137 Cs and 239+240 Pu are also reported. The data demonstrates that the main source of anthropogenic radionuclides in soils is the global fallout from nuclear weapons testings; in sediments, contributions from the Pacific Proving Grounds have been identified. This study also highlights significant differences in radioactivity levels in soils between the northern and central regions of Singapore, which are attributed to the histories of the monitoring sites. Nuclear Chemistry Environmental Chemistry Analytical Chemistry Naturally occurring radionuclides anthropogenic radionuclides Singapore soil sediment Figures Figure 1 Figure 2 Figure 3 Figure 4 Introduction The Fukushima Daiichi Nuclear Power Plant accident in March 2011 released significant amounts of radioactive materials into the environment and highlighted the transboundary nature of a nuclear accident. Since 2018, Singapore routinely monitors radioactivity levels in environmental matrices to establish an ambient baseline while the South-east Asian region is still free of nuclear activities. Soil, sediment and surface seawater, as main receptors of radioactive contamination, are among the environmental matrices that are routinely monitored (Table 1 ). The monitoring programme focuses on gamma emitters and gross alpha and gross beta radioactivity. Tritium ( 3 H, half-life of 12.5 yrs) levels are also routinely monitored in water samples. Table 1 Overview of Singapore’s Environmental Baseline Radioactivity Monitoring Programme Sample Matrix Sample Details Sampling Locations Frequency Radiochemical Analyses Gamma Spectrometry Gross Alpha and Beta Analysis 3 H Analysis Soil Soil profiling (0-30cm depth) 12 Every 5-yearly ✓ ✓ Surface soil 12 Biannually ✓ ✓ Water Seawater 9 Biannually ✓ ✓ ✓ Reservoir water 9 Biannually ✓ ✓ Rainwater 5 Quarterly ✓ Sediment Surface and 50cm depth 1 Annually ✓ ✓ In addition to cosmic radiation, terrestrial radiation from naturally occurring radionuclides such as the uranium-238 ( 238 U, half-life of 4.47 x 10 9 yrs) and thorium-232 ( 232 Th, half-life of 14.02 x 10 9 yrs) decay chains and potassium-40 ( 40 K, half-life of 1.25 x 10 9 yrs) represent the main sources of natural radiation exposure for humans and biota in Singapore. The natural background radiation dose in Singapore is estimated to be about 0.1 µSv/h. Anthropogenic radionuclides such as strontium-90 ( 90 Sr, half-life of 28.80 yrs), cesium-137 ( 137 Cs, half-life of 30.02 yrs), plutonium-239 ( 239 Pu, half-life of 24100 yrs) and plutonium-240 ( 239 Pu, half-life of 6561 yrs), which have relatively long half-lives and were released during nuclear weapons testings (NWT) in the 1950s and 1960s or from nuclear accidents may also be present in the environment [ 1 , 2 ]. However, these radionuclides have not been investigated in Singapore. In this paper, selected sites under Singapore’s environmental radioactivity monitoring programme were investigated (Fig. 1 ). Environmental samples were collected by the National Radiochemistry Laboratory (NRL, Singapore). Radionuclide analyses were performed at NRL and Spiez Laboratory (LS, Switzerland). This paper contains evaluation of analysis results from LS and NRL, comparison to the levels reported in the region, as well as an interlaboratory comparison with the analysis results from NRL. This paper also provides insights on the sources of the anthropogenic radionuclides found in Singapore’s environment. It is shown that while soils are mainly contaminated with Pu from global fallout, sediments contain a fraction originating from the Pacific Proving Grounds (PPG) in the Marshall Islands. Sampling and analytical methods Sampling and sample preparation Singapore, with a total land area of approximately 750 km 2 , is located at the southern tip of the Malay Peninsula, about 137 km north of the Equator (Fig. 1 ). The main island of Singapore is separated from Peninsular Malaysia to the north by the Straits of Johor, a narrow channel that is less than 1 km long. The southern limits of Singapore to Indonesia run through the Straits of Singapore (Fig. 1 ). Singapore has several nature areas and nature reserves that are protected and conserved under legislations and land use development plans to safeguard Singapore’s key indigenous ecosystems. Beyond these nature areas, most parts of Singapore are urbanized or reclaimed. The selection of sites for soil sampling for the Singapore’s environmental baseline radioactivity monitoring programme was therefore challenging. The sites were selected based on the following criteria: i) open space, away from trees and constructions to avoid any interception of fallout and rainfall; and ii) minimal human activities and no plans for future development to avoid disturbance to soil stratification and composition and minimize interference to continual monitoring of radioactivity levels. In this study, soil samples from two of the monitoring sites, one in the central region and one in the northern regions, were investigated (Fig. 1 , Table 2 ). Both monitoring sites have different histories of land developments. While the central region was mostly conserved nature areas since the 1910s, the northern region was developed later in the 1980s. Table 2 Description of samples investigated in this study Region (Fig. 1 ) Sample code Sample matrix Depth (cm) Sampling date Surface area or sample mass North (N) N-1 Soil profile 0–5 18.05.2023 86.6 cm 2 , 0.8 kg N-2 5–10 18.05.2023 86.6 cm 2 , 0.8 kg N-3 10–15 18.05.2023 86.6 cm 2 , 0.8 kg N-4 15–30 18.05.2023 28.9 cm 2 , 0.8 kg Central (C) C-1 Soil surface 0–5 12.04.2018 86.6 cm 2 , 0.8 kg C-2 0–5 31.10.2018 86.6 cm 2 , 0.8 kg C-3 0–5 11.04.2019 86.6 cm 2 , 0.8 kg C-4 0–5 03.10.2019 86.6 cm 2 , 0.8 kg East (E) E-1 Sediment 5 19.01.2023 2 kg E-2 50 19.01.2023 2 kg South-west (SW) SW-1 Seawater 5 08.03.2023 36.1 kg Soil samples were collected using a 15 cm core sampler that is lined with a disposable plastic tube and attached onto a slide hammer (Fig. 2 , Table 2 ). A sampling area of approximately 60 cm by 60 cm was identified and geo-referenced by GPS coordinates and distance referencing. Within the sampling area, nine sampling points were randomly selected to collect soil using the core sampler. The soil was removed from the plastic liner and separated into 3 segments of 5 cm, making up the surface soil layer (0–5 cm) and core soil layers (5–10 cm and 10–15 cm). Three of the nine points were selected to further collect soil at 15–30 cm depth. Singapore is bound by the Straits of Johor and Straits of Singapore. It is a crucial gateway between Asia and Europe and is exposed to transboundary effects by ocean circulation from the Pacific Ocean. In this study, a surface seawater sample collected from the south-western Straits of Singapore as well as sediment samples collected from the eastern coast of Singapore were analyzed (Fig. 1 , Table 2 ). High-volume surface seawater sampling was performed manually using a 5 L bucket attached to a rope. The seawater was transferred into a 25 L carboy using a funnel, and not conditioned on-site. Sediment samples were collected by core sampling at 5 cm and 50 cm depths. The sediment was allowed to settle in double-layered resealable bags before decanting excess seawater. Soil samples were dried in an oven at 60°C, while sediment samples were dried in an oven at 40°C over 3 days followed by freeze-drying, until constant dry mass was achieved. The dry and wet mass values were recorded. After drying, soil and sediment samples were ground using a planetary ball miller. The ground samples were transferred into a pre-weighed container over a sieve. The samples were then homogenized using a shaker mixer. For radiochemical analysis, an aliquot of each sample was ashed at 520 o C for about 20 h. Radionuclide analysis For gamma-ray spectrometry analysis, approximately 70–150 g of ground soil and sediment samples were packed into appropriate counting geometries (cylindrical, h: 29.3 mm, diam. 67.8 mm, Vol. 79.1 cm 3 ), measured on high-purity germanium (HPGe) detectors (CANBERRA®, n-type, carbon window, 25–30% R.E., and/or Ortec®) and analyzed by the spectroscopy software APEX-Gamma (V1.4.1) using Genie 2000 (V3.4.1). The efficiency calibrations were calculated by the calibration software LabSOCS™ (Laboratory Sourceless Calibration Software, CANBERRA®) with the according approximated matrix and density. The counting times varied between 3 and 12 days to achieve desirable detection limits. The soil samples were used for an interlaboratory comparison between LS and NRL. The results of this exercise are presented in the Supplementary Information (Fig. 1 -S; Table 1 -S). Both laboratories displayed a good agreement with data within 25% difference (Fig. 2 -S). As the data sets were comparable, the one from LS was used in the results and discussion section. Cs radioisotopes in seawater were preconcentrated in two stacked 2 mL columns, each containing approximately 1 mL of potassium nickel ferrocyanate (KNiFC-PAN) resin as described elsewhere [ 3 ]. With the use of KNiFC-PAN resin, acidification of the seawater was optional and, in this case, not performed. The processed seawater was collected in a pre-weighed carboy and the mass of seawater was recorded. For the analysis of 137 Cs in the KNiFC-PAN resin, the 2 mL column geometry was modelled and the efficiency calibration calculated with the software LabSOCS™. The 2 mL resin columns were counted on CANBERRA® and/or Ortec® HPGe detectors for a minimum of 3 days. Radionuclides of Pu and U were determined in the ashed materials as described elsewhere [ 4 – 8 ]. Up to 5 g of ash were spiked with known amounts of 242 Pu (chemical recovery tracer) and 115 In (internal standard). The samples were digested by lithium borate fusion and then dissolved in 4.5 M HNO 3 . Aliquots were filtered and diluted for quantitative analysis of 238 U as reported elsewhere [ 6 , 8 ]. After filtration, ammonium iron (II) sulphate hexahydrate was added to the solutions and Pu was separated in a TEVA extraction chromatography resin as described elsewhere [ 6 ]. The breakthrough solutions of the TEVA columns containing U radioisotopes were collected for further analysis. The TEVA columns were rinsed with 6 M HCl to remove Th and then with 3 M HNO 3 . Pu radioisotopes were eluted with 0.2% HNO 3 / 0.2% HF. Pu radioisotopes were analyzed with a double-focusing magnetic sector field inductively coupled plasma mass spectrometer (SF-ICP-MS) Element 2 (Thermo Fisher Scientific). An Apex nebulizing system connected to an ACM desolvator and a self-aspirating PFA-ST-nebuliser (all from Elemental Scientific Incorporation, USA) were used for the introduction of samples into the system. Pu isotope concentrations were calculated from the signals of the 242 Pu tracer. The contributions of the Pu isotopes from the tracer and tailing from U and Th were corrected mathematically based on the isotopic ratios from the certificates and abundance sensitivity measurements of U and Th standards. In the separation of Pu using TEVA resin, it was found that the existing method was insufficient to wash out the high levels of U found in Singapore’s soil. With the existing method [ 6 , 7 ], U separation factors varying between 3000 and 8500 were obtained. A significant contribution of the 238 U tailing on 239 Pu was observed. This, combined with the low content of Pu in most of the soils, impeded a quantification of the Pu isotopes. Hence, an additional Pu purification through a TEVA column was necessary. After the second TEVA separation, higher U separation factors varying between 1 x 10 6 and 13 x 10 6 were obtained and, the low levels of Pu in the soil could be quantified. The UTEVA extraction chromatography resin was used to separate the U radioisotopes in the breakthrough solutions from the TEVA resins. The breakthrough solutions were loaded onto pre-conditioned 2 mL UTEVA columns as described elsewhere [ 8 ]. The UTEVA columns were rinsed with 6 M HCl and then with 3 M HNO 3 . U radioisotopes were eluted with 0.2% HNO 3 / 0.002% HF. A known amount of 115 In standard solution was added to the U fractions. The analysis of the U radioisotopes was carried out using a multi-collector inductively coupled plasma mass spectrometer (MC-ICP-MS) Neptune (Thermo Fisher Scientific) as described elsewhere [ 7 ]. For the determination of the Sr radioisotopes, about 20 g of ashed material was analyzed as described elsewhere [ 5 ]. Sample aliquots were digested, first in 8 M HNO 3 and then in 2 M HNO 3 . A known amount of stable Sr was then added as carrier and chemical recovery tracer. The Ca and Sr oxalates were precipitated by adding C 2 H 2 O 4 and adjusting to pH 3. The precipitates were centrifuged, ashed at 500°C and then dissolved in 3 M HNO 3 . The Sr resin columns were conditioned in 3 M HNO 3 . The sample solutions were loaded onto the columns, and then washed with 3 M HNO 3 to remove most of the alkaline-earth metal interferences. Sr was finally stripped with 0.005 M HNO 3 . Sr was precipitated as SrCO 3 and filtered through pre-weighed glass microfiber filters. The filters were ashed at 500°C for 1 h and then counted in a low-level gas proportional counter (LLC) LB-770 system (Berthold Technologies, GmbH, Germany). The chemical recovery for Sr was determined gravimetrically or by ICP-OES. The certified reference materials IAEA-375 (Soil, Russia), IAEA-384 (Sediment, Fangataufa Lagoon) and IAEA-414 (Sediment, Pacific Ocean) from the International Atomic Energy Agency (IAEA, Vienna, Austria) were used as quality control over the analytical methods. With each series of six to eight samples, certified reference materials of relevant matrices and blanks were analyzed. Results and discussion Levels of naturally occurring radionuclides The activity concentrations of 238 U, 235 U, 234 U, 232 Th and 40 K in soils ranged from 46–138 Bq/kg, 2.1–6.4 Bq/kg, 42–134 Bq/kg, 68–252 Bq/kg, and 17–60 Bq/kg, respectively (Table 3 , Fig. 3 a). Soil samples from the northern region of Singapore contained about 2 to 3 times more U and Th than in the central region. For a better understanding of these differences, the levels in soil should be linked up with the underlying geological composition. Furthermore, anthropogenic activities and development carried out in the northern part in the late 1980s may most likely be behind these differences. Table 3 Levels of naturally occurring radionuclides in soil and sediment (LS Data). Expanded uncertainties with k = 2 are reported. Sample code Activity concentration, in Bq/kg 234 U 235 U 238 U 232 Th 40 K 210 Pb 214 Bi 214 Pb 234 Th N-1 72 ± 2 3.6 ± 0.1 77 ± 2 200 ± 5 25 ± 3 106 ± 30 65 ± 4 67 ± 6 85 ± 16 N-2 85 ± 2 4.1 ± 0.1 90 ± 2 228 ± 6 27 ± 4 81 ± 24 81 ± 4 90 ± 8 113 ± 18 N-3 96 ± 3 4.6 ± 0.1 101 ± 3 252 ± 7 27 ± 3 103 ± 30 89 ± 4 92 ± 8 107 ± 18 N-4 134 ± 4 6.4 ± 0.2 138 ± 4 242 ± 7 60 ± 5 131 ± 38 109 ± 6 111 ± 10 138 ± 22 C-1 46 ± 1 2.3 ± 0.1 49 ± 1 85 ± 2 29 ± 6 77 ± 22 40 ± 2 41 ± 4 59 ± 10 C-2 42 ± 1 2.1 ± 0.1 46 ± 1 68 ± 2 26 ± 3 81 ± 24 38 ± 2 39 ± 4 54 ± 8 C-3 48 ± 1 2.4 ± 0.1 52 ± 1 71 ± 2 24 ± 4 101 ± 28 43 ± 2 43 ± 4 55 ± 10 C-4 55 ± 2 2.7 ± 0.1 58 ± 2 84 ± 2 17 ± 5 92 ± 26 49 ± 2 50 ± 4 67 ± 12 E-1 51 ± 1 2.4 ± 0.1 51 ± 1 87 ± 2 491 ± 41 92 ± 26 31 ± 2 33 ± 2 58 ± 10 E-2 51 ± 1 2.4 ± 0.1 51 ± 1 84 ± 2 518 ± 44 86 ± 24 31 ± 2 32 ± 2 59 ± 12 IAEA-384 41 ± 1 1.6 ± 0.1 36 ± 1 0.015 ± 0.006 n.a. n.a. n.a. n.a. n.a. IAEA-412 30 ± 1 1.5 ± 0.1 32 ± 1 35.9 ± 1.0 n.a. n.a. n.a. n.a. n.a. n.a.: not analyzed. According to UNSCEAR (2008) [ 3 ], the worldwide average levels for 238 U and 232 Th in soils are about 33 Bq/kg and 45 Bq/kg respectively. The levels found in Singapore were on average 2 to 3 times these levels. Notwithstanding, it was noted that the worldwide average levels were low with large variations, and up to 1000 Bq/kg 238 U and 360 Bq/kg 232 Th were being reported [ 3 ]. Comparing to the region, the levels of U and Th in the soils of Singapore were comparable to the levels reported in soils of Malaysia ( 238 U: 30–234 Bq/kg, 232 Th: 63–332 Bq/kg) and Thailand ( 238 U: 3–370 Bq/kg, 232 Th: 7–120 Bq/kg) [ 3 , 4 ]. Sediment samples contained 51 Bq/kg, 2.4 Bq/kg, 51 Bq/kg, 84–87 Bq/kg, 491–518 Bq/kg of 238 U, 235 U, 234 U, 232 Th and 40 K respectively, with no significant differences at 5 cm and 50 cm depths (Table 3 , Fig. 3 a). The levels of natural radionuclides were in the same range as those reported in sediments of Malaysia (32 Bq/kg 238 U) and Vietnam (36 Bq/kg 232 Th) [ 5 ]. The uranium isotopic composition in soils was comparable to that of natural uranium, with 48.6% 238 U, 49.2% 234 U and 2.2% 235 U by activity [ 6 ]. 236 U activity was below the detection limit of the method (< 50 µBq/kg). The activity concentrations of 234 Th in soils and sediments were comparable to those of its mother 238 U (Fig. 3 a). The activity concentrations of 214 Pb and 214 Bi (post-radon daughter radionuclides) were comparable, with 214 Bi/ 214 Pb ratios of approximately 1. In most samples, the activities of 214 Pb and 214 Bi were slightly lower than those of 238 U (Fig. 3 a). This was expected as the samples were not vacuum-sealed to prevent escape of gaseous radon ( 222 Rn), hence the decay chain was likely not in secular equilibrium. Moreover, because soil and sediment are “open systems” affected by many hydrogeochemical processes and the elements of the decay series have different chemical behaviors, a radioactive disequilibrium is not unexpected in those compartments [ 7 – 9 ]. In most surface soils, the activity concentration of 210 Pb was significantly higher than that of its 214 Pb and 214 Bi parents. This trend can be explained by an enrichment of 210 Pb in surface soils after the decay of 222 Rn that diffused from the deeper grounds. Notwithstanding, 210 Pb activity concentrations would be useful for subsequent 210 Po studies and dose assessment studies. The levels of 40 K in soils were about 17 times lower than in sediments (Table 3 , Fig. 3 a), and 15 times lower than the worldwide average of 420 Bq/kg [ 3 ]. 40 K levels in the soils of Singapore were nonetheless comparable to the levels reported in soils of Thailand (7–712 Bq/kg) [ 3 ]. The low 40 K levels in well-developed soils from tropical regions has been attributed to the leaching of this element by the high rainfall rates [ 10 ]. In sediments, the levels of 40 K were comparable to those reported in the sediments of Malaysia (median value of 250 Bq/kg) and the Philippines (median value of 809 Bq/kg) [ 5 ]. Levels of anthropogenic radionuclides The activity concentrations of 239+240 Pu, 137 Cs and 90 Sr in the soil and sediment samples from Singapore are reported in Table 4 and displayed in Fig. 3 b. The activity concentrations of these radionuclides were significantly lower than those of the naturally occurring radionuclides (Fig. 3 ). Moreover, soil samples from the northern region contained significantly lower levels of anthropogenic radionuclides than those in the central region (Fig. 3 b). This is likely related to the fact that soils in the northern part were disturbed by anthropogenic activities, as exposing of deeper layers of soil without fallout, carried out in the late 1980s. Table 4 Levels of anthropogenic radionuclides in soil and sediment (LS data). 90 Sr was analyzed by low-level gas proportional counting, 137 Cs by gamma-ray spectrometry and 239+240 Pu by ICP-MS. Expanded uncertainties with k = 2 are reported. Sample code Activity concentration, in Bq kg − 1 240 Pu/ 239 Pu atom ratio 239+240 Pu/ 137 Cs 137 Cs/ 90 Sr 90 Sr 137 Cs 239+240 Pu N-1 n.a. < 0.30 0.007 ± 0.002 0.10 ± 0.07 - - N-2 n.a. < 0.30 0.005 ± 0.001 0.14 ± 0.06 - - N-3 n.a. < 0.30 0.002 ± 0.001 0.10 ± 0.17 - - N-4 n.a. < 0.30 0.008 ± 0.001 0.08 ± 0.02 - - C-1 0.34 ± 0.07 0.65 ± 0.14 0.057 ± 0.004 0.17 ± 0.03 0.09 ± 0.02 1.7 ± 0.3 C-2 0.28 ± 0.07 0.52 ± 0.16 0.038 ± 0.004 0.19 ± 0.03 0.07 ± 0.02 1.6 ± 0.4 C-3 0.24 ± 0.06 0.48 ± 0.11 0.031 ± 0.002 0.18 ± 0.02 0.06 ± 0.02 1.8 ± 0.3 C-4 0.29 ± 0.06 0.41 ± 0.10 0.050 ± 0.003 0.17 ± 0.02 0.12 ± 0.03 1.3 ± 0.3 E-1 0.17 ± 0.04 0.46 ± 0.11 0.294 ± 0.009 0.24 ± 0.01 0.64 ± 0.15 2.6 ± 0.3 E-2 0.15 ± 0.05 0.54 ± 0.16 0.239 ± 0.006 0.26 ± 0.01 0.44 ± 0.13 3.5 ± 0.4 SW-1 - 0.0011 ± 0.0002 - - - - IAEA-384 - - 116 ± 3 0.050 ± 0.001 - - IAEA-412 - - 0.67 ± 0.02 0.17 ± 0.01 - - IAEA-375 98 ± 10 - - - - - n.a.: not analyzed. The activity concentrations of 239+240 Pu in soils vary between 0.002 Bq/kg and 0.057 Bq/kg (Table 4 , Fig. 3 b), a range comparable to those reported in surface soils of south China (0.003–0.469 Bq/kg) [ 11 ] and Vietnam (0.008 to 0.365 Bq/kg) [ 12 ]. In sediments, 239+240 Pu activity concentrations were 0.2–0.3 Bq/kg (Table 4 , Fig. 3 b). This range of activities is comparable to the range of 0.21–0.45 Bq/kg reported in surface sediments from the east coast of Peninsular Malaysia [ 13 ], and slightly below the range reported in surface sediments from the Indonesian throughflow [ 14 ]. 137 Cs activity concentrations in soils of Singapore were in the range of 0.24 Bq/kg to 0.34 Bq/kg (Table 4 ). In the northern region, the 137 Cs activity concentrations were below the detection limit of the method (0.3 Bq/kg). The 137 Cs levels in soils were similar to those reported in surface soils of Malaysia and Timor Leste (about 1 Bq/kg, decay corrected to 2024) [ 15 – 17 ], but slightly lower than those reported for West Java (about 1 Bq/kg, decay corrected to 2024) [ 18 ] and for Vietnam (0.19–9.2 Bq/kg, decay corrected to 2024) [ 12 ]. The levels of 137 Cs in sediments of the eastern coast of Singapore were of 0.46–0.54 Bq/kg (Table 4 ). These levels are comparable to those reported in coastal sediments from Philippines and Indonesia (0.52 Bq/kg and 0.47 Bq/kg respectively, decay corrected to 2024) [ 15 ] and the east coast of Peninsular Malaysia (< 1.0 Bq/kg to 2.0 Bq/kg, decay corrected to 2024) [ 13 ]. The level of 137 Cs in seawater from the south-western coast of Singapore was 0.0011 ± 0.0002 Bq/kg (Table 4 ), comparable to the levels reported in seawater from the east coast of Peninsular Malaysia [ 13 ], and in other sites in Asia at similar latitudes [ 19 ]. 90 Sr activity concentrations in soils from the central region of Singapore were in the range of 0.24 to 0.34 Bq/kg (Table 4 ). In the coastal sediments, the 90 Sr activity concentrations were 0.15 Bq/kg and 0.17 Bq/kg (Table 4 ). These values are comparable to the levels reported in sediments from Vietnam (about 0.24 Bq/kg, decay corrected to 2024) [ 5 ]. Activity and isotope ratios Pu isotope ratios vary significantly in the environment depending on source, making them well-suited as fingerprints for determining the origin of a contamination. 240 Pu/ 239 Pu atom ratios in soil from Singapore varied between 0.10 ± 0.07 and 0.19 ± 0.03 (Table 4 , Fig. 4 a). The soils in the central region have 240 Pu/ 239 Pu ratios consistent with the ratio 0.18 ± 0.01 [ 20 ] reported in global fallout from the NWT in the 1950s and 1960s (Fig. 4 a). In the northern region, ratios tend to be below the ratio in global fallout. However, these ratios contain relatively large uncertainties and the data therefore cannot be used to precisely identify the origin of the Pu contamination. In sediments, 240 Pu/ 239 Pu atom ratios in the range 0.24–0.26 were detected, consistently higher than the global fallout ratio (Table 4 , Fig. 4 a). The high 240 Pu/ 239 Pu ratios could be related to the exposure of the Straits of Singapore to transboundary effects of ocean circulation from the PPG, which has a fingerprint ratio of about 0.33 [ 14 , 21 , 22 ]. In the Indo-Pacific Intersection (Indonesian Seas and South China Sea), 240 Pu/ 239 Pu atom ratios ranging from 0.194 to 0.258 (average of 0.223 ± 0.021) were reported in surface seawater [ 23 ]. These elevated ratios were attributed to the contribution of Pu transported from the PPG [ 14 , 23 ]. The 239,240 Pu/ 137 Cs activity ratios also provide valuable information for a better understanding of the sources of a radioactive contamination. For example, 239,240 Pu/ 137 Cs activity ratios of 0.04–0.08 (decay correction to 2024) are characteristics of soils contaminated mainly by global fallout [ 24 , 25 ]. In soils heavily contaminated by the Chernobyl accident, much lower ratios of 0.014–0.021 (decay correction to 2024) were reported [ 26 ]. In Singapore soils, the 239,240 Pu/ 137 Cs activity ratios varied between 0.06 and 0.12, further confirming that the origin of the radionuclides is global fallout from the NWTs. In the coastal sediments of Singapore, the 239,240 Pu/ 137 Cs activity ratios varied between 0.44 and 0.56 (Fig. 4 b), a range above the range expected from global fallout (0.04–0.08). High 239,240 Pu/ 137 Cs activity ratios have been reported in sediments from other coastal sites in Asia, such as in Malaysia (0.1–1.2, decay corrected to 2024) [ 13 ], in the Sagami Bay (1.13 ± 0.3, decay corrected to 2024) [ 27 ] and the Tokyo Bay (0.87 ± 0.24, decay corrected to 2024) [ 28 ]. The high 239,240 Pu/ 137 Cs activity ratios in the sediments is likely explained by a preferential scavenging of Pu over Cs by suspended particulates [ 13 , 29 ]. In seawater, the two chemical elements show different affinities to suspended particulate, but also solubilities and biological uptake. 137 Cs and 90 Sr are produced by fission of U or Pu with relatively high yields. They also decay with similar half-lives of 30.06 y and 28.8 y, respectively, and have remained in the environment after their releases by the NWTs and/or nuclear accidents. Activity ratios of 137 Cs to 90 Sr are also a useful tool to identify sources of radioactive releases and gain better understanding of environmental behaviors of these radionuclides. The ratio of 137 Cs/ 90 Sr in the soils of Singapore was on average 1.6 ± 0.3 (Table 4 ), which is comparable to the ratio of 1.5 reported in global fallout [ 3 , 30 ]. In the sediment samples, the 137 Cs/ 90 Sr ratios were slightly higher than the ratio reported in global fallout (Table 4 ), likely as a result of fractionation processes affecting these radionuclides in the ocean. Conclusions The levels of natural and anthropogenic radionuclides in four monitoring sites of Singapore were investigated. This study highlighted significant differences in radioactivity levels in soils from the northern and central regions of Singapore. These differences might be attributed to the histories of the monitoring sites. While the central region was mostly conserved nature areas since the 1910s, the northern region was developed later in the 1980s. The levels of anthropogenic radionuclides in soils were relatively low and displayed a typical signature from the global fallout of the nuclear weapons testings in the 1950s and 1960s. 240 Pu/ 239 Pu atom ratios and 137 Cs/ 90 Sr activity ratios of 0.18 ± 0.01 and 1.6 ± 0.3 respectively were observed in the soil samples. The levels of anthropogenic radionuclides in sediments were slightly higher than in soils, and their fingerprints indicated a contribution of radionuclides released from the Pacific Proving Grounds. This was evidenced by the elevated 240 Pu/ 239 Pu atom ratios of about 0.25 ± 0.01. Further studies would be performed on the rest of the monitoring sites under the environmental baseline radioactivity monitoring programme to illustrate the distribution of radioactivity levels across Singapore. Declarations Acknowledgements This study was conducted in the frame of a technical cooperation project of the IAEA in Singapore (SIN7001: Enhancing Capabilities in Environmental Radioactivity Monitoring). We would like to acknowledge the IAEA for their generous financial support of two fellowships: FS-SIN7001-2200807, and FS-SIN7001-2305395. We are also grateful to Spiez Laboratory (an IAEA Collaborating Centre) staff who in one way or another were related to the completion of the two fellowships. Conflicts of interest statement The authors have no competing interests to declare that are relevant to the content of this article. Data availability statement The authors confirm that the data supporting the findings of this study are available within the article and its supplementary materials. References Duran EB et al (2004) 137Cs and 239 + 240Pu levels in the Asia-Pacific regional seas. J Environ Radioact 76(1):139–160 UNSCEAR, Sources and effects of ionizing radiation. UNSCEAR 2000 report to the General Assembly. Volume I: Sources UNSCEAR, Editor. (2000) UN: New York. p. 657 UNSCEAR, Sources and effects of ionizing radiation.UNSCEAR 2008 report to the General Assembly with Scientific Annexes . (2011) UN: New York Apriantoro N, Ramli A, Sutisna S (2013) Activity Concentration of 238 U, 232 Th and 40 K Based on Soil Types in Perak State, Malaysia IAEA MARIS, 2023. IAEA Marine Radioactivity Information System. In: Division of IAEA Environment Laboratories [online]. Monaco. [17.08.2023] https://maris.iaea.org.2023 U.S.NRC. Natural uranium (2023) ; https://www.nrc.gov/reading-rm/basic-ref/glossary/natural-uranium.html Dickson BL (1995) Uranium-series disequilibrium in Australian soils and its effect on aerial gamma-ray surveys. J Geochem Explor 54(3):177–186 Sam AK (2003) The State of Disequilibrium between U and Th Series Isotopes in Marine Sediments from the Sudanese Red Sea Coast. Radiochemistry 45(1):90–95 Yusoff A, Mohamed CAR (2016) Mini Review Uranium-Thorium Decay Series in the Marine Environment of the Southern South China Sea. Journal of Geology & Geophysics, p 05 Luiz doC, Leal A et al (2020) Spatial distributions of natural radionuclides in soils of the state of Pernambuco, Brazil: Influence of bedrocks, soils types and climates. J Environ Radioact 211:106046 Feng D et al (2023) An overview of plutonium isotopes in soils, China: Distribution, spatial patterns, and sources. Environ Res 216:114677 Quang NH et al (2004) 239 + 240Pu, 90Sr and 137Cs inventories in surface soils of Vietnam. J Environ Radioact 75(3):329–337 Yii M-W, Wan ZUy, Mahmood (2011) Radioactivity of plutonium isotopes, 137Cs and their ratio in sediment, seawater and biota from the east coast of Peninsular Malaysia. J Radioanal Nucl Chem 289(3):819–833 Pittauer D et al (2017) Continuous transport of Pacific-derived anthropogenic radionuclides towards the Indian Ocean. Sci Rep 7(1):44679 Furuichi T, Wasson RJ (2013) Caesium-137 in Southeast Asia: Is there enough left for soil erosion and sediment redistribution studies? J Asian Earth Sci 77:108–116 de Neergaard A, Magid J, Mertz O (2008) Soil erosion from shifting cultivation and other smallholder land use in Sarawak, Malaysia, vol 125. Agriculture, Ecosystems & Environment, pp 182–190. 1 Gharibreza M et al (2013) Land Use Changes and Soil Redistribution Estimation using 137Cs in the Tropical Bera Lake Catchment, Malaysia. Soil and Tillage Research, p 131 Suhartini N (2016) The Influence of Cultivation System on Distribution Profile Of 137cs and Erosion / Deposition Rate. Forum Geografi 21:33–42 Brochot S et al (2004) Modelling of the bioleaching of sulphide ores: application for the simulation of the bioleaching/gravity section of the Kasese Cobalt Company Ltd process plant. Miner Eng 17(2):253–260 Krey P et al (1976) Mass isotopic composition of global fallout plutonium in soil. , in Transuranium nuclides in the environment . IAEA: Vienna, Austria Buesseler KO (1997) The isotopic signature of fallout plutonium in the North Pacific. J Environ Radioact 36(1):69–83 Wu J et al (2018) Sources and accumulation of plutonium in a large Western Pacific marginal sea: The South China Sea. Sci Total Environ, 610–611: p. 200–211 Xie T et al (2021) Sources and transport of plutonium in the Indo-Pacific Intersection: Implications for South China Sea freshwater transport into Indonesian Seas. Chem Geol 580:120367 Hodge V, Smith C, Whiting J (1996) Radiocesium and plutonium: still together in background soils after more than thirty years. Chemosphere 32(10):2067–2075 Bunzl K, Kracke W (1988) Cumulative deposition of 137Cs, 238Pu, 239 + 240Pu and 241Am from global fallout in soils from forest, grassland and arable land in Bavaria (FRG). J Environ Radioact 8(1):1–14 Muramatsu Y et al (2000) Concentrations of 239 Pu and 240 Pu and Their Isotopic Ratios Determined by ICP-MS in Soils Collected from the Chernobyl 30-km Zone , vol 34. Environmental Science & Technology, pp 2913–2917. 14 Yamada M, Nagaya Y (2000) Vertical Profiles, Inventories, and Activity Ratios of 239 + 240Pu and 137Cs in Sediments from Sagami Bay, Western Northwest Pacific Margin. J Radioanal Nucl Chem 246(2):369–378 Yamada M, Nagaya Y (2000) 239 + 240Pu and 137Cs in Sediments from Tokyo Bay: Distribution and Inventory. J Radioanal Nucl Chem 245(2):273–279 Yamada M, Oikawa S (2022) 239Pu, 240Pu, 241Pu, 241Am, 137Cs, and 210Pb in seafloor sediments in the western North Pacific Ocean and the Sea of Japan: distributions, sources and budgets. Journal of Radioanalytical and Nuclear Chemistry, 331(6): pp. 2689–2703 Bossew P et al (2007) Activity ratios of 137Cs, 90Sr and 239 + 240Pu in environmental samples. J Environ Radioact 97(1):5–19 Bouisset P et al (2021) Contribution of close-in fallout from the French atmospheric tests in inventories of 137Cs, 241Am and plutonium (238, 239, 240) in Gambier Islands (French Polynesia) – Signatures of stratospheric fallout in the Southern Hemisphere. J Environ Radioact, 235–236: p. 106624 Lee SH et al (2001) Analysis of plutonium isotopes in marine samples by radiometric, ICP-MS and AMS techniques. J Radioanal Nucl Chem 248(3):757–764 Diamond H et al (1960) Heavy Isotope Abundances in Mike Thermonuclear Device. Phys Rev 119(6):2000–2004 Muramatsu Y et al (2001) Measurement of 240 Pu/ 239 Pu isotopic ratios in soils from the Marshall Islands using ICP-MS . Sci Total Environ 278(1–3):151–159 Yamamoto M et al (1996) 237 Np in hemp-palm leaves of Bontenchiku for fishing gear used by the Fifth Fukuryu-Maru: 40 years after Bravo . Health Phys 70(5):744–748 Johansen MP et al (2019) Plutonium and other radionuclides persist across marine-to-terrestrial ecotopes in the Montebello Islands sixty years after nuclear tests. Sci Total Environ 691:572–583 Kelley JM, Bond LA, Beasley TM (1999) Global distribution of Pu isotopes and 237 Np . Sci Total Environ, 237–238: p. 483–500 Supplementary Informations Supplementary Information is not available with this version. Additional Declarations The authors declare no competing interests. Cite Share Download PDF Status: Posted Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-4747517","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":327511910,"identity":"a688f222-d859-43e3-b242-658dda2846d6","order_by":0,"name":"E. Wong","email":"","orcid":"","institution":"National Environment Agency","correspondingAuthor":false,"prefix":"","firstName":"E.","middleName":"","lastName":"Wong","suffix":""},{"id":327512270,"identity":"d680e246-f2a6-4bfa-9a40-b816e7adfa55","order_by":1,"name":"H.J. Tan","email":"","orcid":"","institution":"National Environment Agency","correspondingAuthor":false,"prefix":"","firstName":"H.J.","middleName":"","lastName":"Tan","suffix":""},{"id":327512271,"identity":"ca102d95-b263-46ca-ac4b-4cf85b35ca40","order_by":2,"name":"J.A. Corcho Alvarado","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAABA0lEQVRIiWNgGAWjYBACxgYYS4KB8TEDwwEQ04AhAUwS1sJsTJQWBJBgYJOGa0GQmIC5vf3hA4Yam3z+2c3Hqgtq7tgzsB/euuHhDgZ5c1wO6zljbMBwLM1yxp1jabdnHHuW2MCTVnYj8QyD4c4GHFpm5LBJMDYcNjCQyDG7zcN2GOiLHLMbiW0MCQYHcGlJf/4DpqWY599hewb+N4S0JJgxwLQw87YdZmyQIGQL0C8SCcfSDCRupCVLz+x7ltgm8QzolzYJww04tBgCQ+zDhxobA/4ZyQc/F3y7Y8/Pn7zt5s82G3lcthg2AIkEZBE2CCWBXT0QyOOUGQWjYBSMglEAAwAUhV0brZampgAAAABJRU5ErkJggg==","orcid":"","institution":"Spiez Laboratory","correspondingAuthor":true,"prefix":"","firstName":"J.A.","middleName":"Corcho","lastName":"Alvarado","suffix":""}],"badges":[],"createdAt":"2024-07-16 06:35:20","currentVersionCode":1,"declarations":{"humanSubjects":false,"vertebrateSubjects":false,"conflictsOfInterestStatement":false,"humanSubjectEthicalGuidelines":false,"humanSubjectConsent":false,"humanSubjectClinicalTrial":false,"humanSubjectCaseReport":false,"vertebrateSubjectEthicalGuidelines":false},"doi":"10.21203/rs.3.rs-4747517/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-4747517/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":60515012,"identity":"124be933-15f1-4df2-9d65-cb0c0ec2c6b0","added_by":"auto","created_at":"2024-07-17 15:18:53","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":1495104,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cem\u003eLocation of Singapore in the South-east Asian region, map of Singapore and study sites. Source: Google Earth (2024).\u003c/em\u003e\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-4747517/v1/8a9a1baedec561fa491f85ab.png"},{"id":60514161,"identity":"ac08c9db-2573-4759-9ef0-8bb4279620a2","added_by":"auto","created_at":"2024-07-17 15:10:53","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":1104908,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cem\u003eCore sampling set consisting of a slide hammer, a core sampler and a plastic tube, and illustration of collecting 15 cm depth of soil. Source: National Radiochemistry Laboratory, Singapore (2019)\u003c/em\u003e\u003c/p\u003e","description":"","filename":"Figure2Soilsampling.png","url":"https://assets-eu.researchsquare.com/files/rs-4747517/v1/947744e6506f83c5f4e0708e.png"},{"id":60513676,"identity":"f69e4086-2cc2-4ca4-b184-c7e8c2d8efb7","added_by":"auto","created_at":"2024-07-17 15:02:53","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":192126,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cem\u003eComparison of activity concentrations of: a) naturally occuring radionuclides, and b) anthropogenic radionuclides, measured in soil and sediment samples from Singapore.\u003c/em\u003e\u003c/p\u003e","description":"","filename":"Figure3Allradionuclides.png","url":"https://assets-eu.researchsquare.com/files/rs-4747517/v1/425cc25f3fe652ae4a4697f4.png"},{"id":60513673,"identity":"5b5207c7-34ca-4669-9771-9fc167f60fbd","added_by":"auto","created_at":"2024-07-17 15:02:53","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":111897,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cem\u003ea) \u003c/em\u003e\u003csup\u003e\u003cem\u003e240\u003c/em\u003e\u003c/sup\u003e\u003cem\u003ePu/\u003c/em\u003e\u003csup\u003e\u003cem\u003e239\u003c/em\u003e\u003c/sup\u003e\u003cem\u003ePu isotope ratios and b) \u003c/em\u003e\u003csup\u003e\u003cem\u003e239+240\u003c/em\u003e\u003c/sup\u003e\u003cem\u003ePu/\u003c/em\u003e\u003csup\u003e\u003cem\u003e137\u003c/em\u003e\u003c/sup\u003e\u003cem\u003eCs activity ratios measured in environmental samples from Singapore, compared to ratios reported for known sources of these radionuclides in the environment and some characteristic ratios. Known Pu isotope ratios are: fallout from nuclear weapon testing sites of French Polynesia [\u003c/em\u003e\u003ca href=\"#_ENREF_31\" title=\"Bouisset, 2021 #1571\"\u003e\u003cem\u003e31\u003c/em\u003e\u003c/a\u003e\u003cem\u003e, \u003c/em\u003e\u003ca href=\"#_ENREF_32\" title=\"Lee, 2001 #1210\"\u003e\u003cem\u003e32\u003c/em\u003e\u003c/a\u003e\u003cem\u003e], Castle Bravo [\u003c/em\u003e\u003ca href=\"#_ENREF_33\" title=\"Diamond, 1960 #1263\"\u003e\u003cem\u003e33-35\u003c/em\u003e\u003c/a\u003e\u003cem\u003e] and Montebello Islands [\u003c/em\u003e\u003ca href=\"#_ENREF_36\" title=\"Johansen, 2019 #1577\"\u003e\u003cem\u003e36\u003c/em\u003e\u003c/a\u003e\u003cem\u003e]; and global fallout [\u003c/em\u003e\u003ca href=\"#_ENREF_20\" title=\"Krey, 1976 #1465\"\u003e\u003cem\u003e20\u003c/em\u003e\u003c/a\u003e\u003cem\u003e, \u003c/em\u003e\u003ca href=\"#_ENREF_37\" title=\"Kelley, 1999 #1242\"\u003e\u003cem\u003e37\u003c/em\u003e\u003c/a\u003e\u003cem\u003e]. Known \u003c/em\u003e\u003csup\u003e\u003cem\u003e239+240\u003c/em\u003e\u003c/sup\u003e\u003cem\u003ePu/\u003c/em\u003e\u003csup\u003e\u003cem\u003e137\u003c/em\u003e\u003c/sup\u003e\u003cem\u003eCs activity ratios are: global fallout [\u003c/em\u003e\u003ca href=\"#_ENREF_24\" title=\"Hodge, 1996 #2151\"\u003e\u003cem\u003e24\u003c/em\u003e\u003c/a\u003e\u003cem\u003e], fallout from the Chernobyl accident [\u003c/em\u003e\u003ca href=\"#_ENREF_26\" title=\"Muramatsu, 2000 #1248\"\u003e\u003cem\u003e26\u003c/em\u003e\u003c/a\u003e\u003cem\u003e] and in coastal sediments of southasia[\u003c/em\u003e\u003ca href=\"#_ENREF_13\" title=\"Yii, 2011 #2110\"\u003e\u003cem\u003e13\u003c/em\u003e\u003c/a\u003e\u003cem\u003e, \u003c/em\u003e\u003ca href=\"#_ENREF_27\" title=\"Yamada, 2000 #2155\"\u003e\u003cem\u003e27\u003c/em\u003e\u003c/a\u003e\u003cem\u003e, \u003c/em\u003e\u003ca href=\"#_ENREF_28\" title=\"Yamada, 2000 #2156\"\u003e\u003cem\u003e28\u003c/em\u003e\u003c/a\u003e\u003cem\u003e]. Reference date for decay correction is 01.06.2024.\u003c/em\u003e\u003c/p\u003e","description":"","filename":"Figure4Ratios.png","url":"https://assets-eu.researchsquare.com/files/rs-4747517/v1/08fe36d44dd60fea39285fed.png"},{"id":60515914,"identity":"7b9c19fd-9182-422d-8784-2c95c80faa76","added_by":"auto","created_at":"2024-07-17 15:26:55","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":4151801,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-4747517/v1/4c755e93-61fd-4e9a-bedc-6dbd1fa6440a.pdf"}],"financialInterests":"The authors declare no competing interests.","formattedTitle":"\u003cp\u003eLevels of natural and anthropogenic radionuclides in selected environmental radioactivity monitoring sites in Singapore\u003c/p\u003e","fulltext":[{"header":"Introduction","content":"\u003cp\u003eThe Fukushima Daiichi Nuclear Power Plant accident in March 2011 released significant amounts of radioactive materials into the environment and highlighted the transboundary nature of a nuclear accident. Since 2018, Singapore routinely monitors radioactivity levels in environmental matrices to establish an ambient baseline while the South-east Asian region is still free of nuclear activities. Soil, sediment and surface seawater, as main receptors of radioactive contamination, are among the environmental matrices that are routinely monitored (Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). The monitoring programme focuses on gamma emitters and gross alpha and gross beta radioactivity. Tritium (\u003csup\u003e3\u003c/sup\u003eH, half-life of 12.5 yrs) levels are also routinely monitored in water samples.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eOverview of Singapore\u0026rsquo;s Environmental Baseline Radioactivity Monitoring Programme\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"7\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eSample Matrix\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eSample Details\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eSampling Locations\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eFrequency\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colspan=\"3\" nameend=\"c7\" namest=\"c5\"\u003e \u003cp\u003eRadiochemical Analyses\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eGamma Spectrometry\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003eGross Alpha and Beta Analysis\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c7\"\u003e \u003cp\u003e\u003csup\u003e3\u003c/sup\u003eH Analysis\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eSoil\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSoil profiling \u003c/p\u003e \u003cp\u003e(0-30cm depth)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e12\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eEvery \u003c/p\u003e \u003cp\u003e5-yearly\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e✓\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e✓\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSurface soil\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e12\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eBiannually\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e✓\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e✓\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"2\" rowspan=\"3\"\u003e \u003cp\u003eWater\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSeawater\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e9\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eBiannually\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e✓\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e✓\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e✓\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eReservoir water\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e9\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eBiannually\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e✓\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e✓\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eRainwater\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eQuarterly\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e✓\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSediment\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSurface and \u003c/p\u003e \u003cp\u003e50cm depth\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eAnnually\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e✓\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e✓\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eIn addition to cosmic radiation, terrestrial radiation from naturally occurring radionuclides such as the uranium-238 (\u003csup\u003e238\u003c/sup\u003eU, half-life of 4.47 x 10\u003csup\u003e9\u003c/sup\u003e yrs) and thorium-232 (\u003csup\u003e232\u003c/sup\u003eTh, half-life of 14.02 x 10\u003csup\u003e9\u003c/sup\u003e yrs) decay chains and potassium-40 (\u003csup\u003e40\u003c/sup\u003eK, half-life of 1.25 x 10\u003csup\u003e9\u003c/sup\u003e yrs) represent the main sources of natural radiation exposure for humans and biota in Singapore. The natural background radiation dose in Singapore is estimated to be about 0.1 \u0026micro;Sv/h. Anthropogenic radionuclides such as strontium-90 (\u003csup\u003e90\u003c/sup\u003eSr, half-life of 28.80 yrs), cesium-137 (\u003csup\u003e137\u003c/sup\u003eCs, half-life of 30.02 yrs), plutonium-239 (\u003csup\u003e239\u003c/sup\u003ePu, half-life of 24100 yrs) and plutonium-240 (\u003csup\u003e239\u003c/sup\u003ePu, half-life of 6561 yrs), which have relatively long half-lives and were released during nuclear weapons testings (NWT) in the 1950s and 1960s or from nuclear accidents may also be present in the environment [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e, \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]. However, these radionuclides have not been investigated in Singapore.\u003c/p\u003e \u003cp\u003eIn this paper, selected sites under Singapore\u0026rsquo;s environmental radioactivity monitoring programme were investigated (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). Environmental samples were collected by the National Radiochemistry Laboratory (NRL, Singapore). Radionuclide analyses were performed at NRL and Spiez Laboratory (LS, Switzerland). This paper contains evaluation of analysis results from LS and NRL, comparison to the levels reported in the region, as well as an interlaboratory comparison with the analysis results from NRL. This paper also provides insights on the sources of the anthropogenic radionuclides found in Singapore\u0026rsquo;s environment. It is shown that while soils are mainly contaminated with Pu from global fallout, sediments contain a fraction originating from the Pacific Proving Grounds (PPG) in the Marshall Islands.\u003c/p\u003e"},{"header":"Sampling and analytical methods","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003eSampling and sample preparation\u003c/h2\u003e \u003cp\u003eSingapore, with a total land area of approximately 750 km\u003csup\u003e2\u003c/sup\u003e, is located at the southern tip of the Malay Peninsula, about 137 km north of the Equator (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). The main island of Singapore is separated from Peninsular Malaysia to the north by the Straits of Johor, a narrow channel that is less than 1 km long. The southern limits of Singapore to Indonesia run through the Straits of Singapore (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eSingapore has several nature areas and nature reserves that are protected and conserved under legislations and land use development plans to safeguard Singapore\u0026rsquo;s key indigenous ecosystems. Beyond these nature areas, most parts of Singapore are urbanized or reclaimed. The selection of sites for soil sampling for the Singapore\u0026rsquo;s environmental baseline radioactivity monitoring programme was therefore challenging. The sites were selected based on the following criteria: i) open space, away from trees and constructions to avoid any interception of fallout and rainfall; and ii) minimal human activities and no plans for future development to avoid disturbance to soil stratification and composition and minimize interference to continual monitoring of radioactivity levels.\u003c/p\u003e \u003cp\u003eIn this study, soil samples from two of the monitoring sites, one in the central region and one in the northern regions, were investigated (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e, Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). Both monitoring sites have different histories of land developments. While the central region was mostly conserved nature areas since the 1910s, the northern region was developed later in the 1980s.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eDescription of samples investigated in this study\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"6\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eRegion\u003c/p\u003e \u003cp\u003e(Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSample code\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eSample matrix\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eDepth\u003c/p\u003e \u003cp\u003e(cm)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eSampling\u003c/p\u003e \u003cp\u003edate\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003eSurface area or sample mass\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"3\" rowspan=\"4\"\u003e \u003cp\u003eNorth (N)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eN-1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\" morerows=\"3\" rowspan=\"4\"\u003e \u003cp\u003eSoil profile\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0\u0026ndash;5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e18.05.2023\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e86.6 cm\u003csup\u003e2\u003c/sup\u003e, 0.8 kg\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eN-2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e5\u0026ndash;10\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e18.05.2023\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e86.6 cm\u003csup\u003e2\u003c/sup\u003e, 0.8 kg\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eN-3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e10\u0026ndash;15\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e18.05.2023\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e86.6 cm\u003csup\u003e2\u003c/sup\u003e, 0.8 kg\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eN-4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e15\u0026ndash;30\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e18.05.2023\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e28.9 cm\u003csup\u003e2\u003c/sup\u003e, 0.8 kg\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"3\" rowspan=\"4\"\u003e \u003cp\u003eCentral (C)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eC-1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\" morerows=\"3\" rowspan=\"4\"\u003e \u003cp\u003eSoil surface\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0\u0026ndash;5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e12.04.2018\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e86.6 cm\u003csup\u003e2\u003c/sup\u003e, 0.8 kg\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eC-2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0\u0026ndash;5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e31.10.2018\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e86.6 cm\u003csup\u003e2\u003c/sup\u003e, 0.8 kg\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eC-3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0\u0026ndash;5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e11.04.2019\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e86.6 cm\u003csup\u003e2\u003c/sup\u003e, 0.8 kg\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eC-4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0\u0026ndash;5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e03.10.2019\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e86.6 cm\u003csup\u003e2\u003c/sup\u003e, 0.8 kg\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eEast (E)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eE-1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eSediment\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e19.01.2023\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e2 kg\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eE-2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e50\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e19.01.2023\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e2 kg\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSouth-west (SW)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSW-1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eSeawater\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e08.03.2023\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e36.1 kg\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eSoil samples were collected using a 15 cm core sampler that is lined with a disposable plastic tube and attached onto a slide hammer (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e, Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). A sampling area of approximately 60 cm by 60 cm was identified and geo-referenced by GPS coordinates and distance referencing. Within the sampling area, nine sampling points were randomly selected to collect soil using the core sampler. The soil was removed from the plastic liner and separated into 3 segments of 5 cm, making up the surface soil layer (0\u0026ndash;5 cm) and core soil layers (5\u0026ndash;10 cm and 10\u0026ndash;15 cm). Three of the nine points were selected to further collect soil at 15\u0026ndash;30 cm depth.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eSingapore is bound by the Straits of Johor and Straits of Singapore. It is a crucial gateway between Asia and Europe and is exposed to transboundary effects by ocean circulation from the Pacific Ocean. In this study, a surface seawater sample collected from the south-western Straits of Singapore as well as sediment samples collected from the eastern coast of Singapore were analyzed (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e, Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). High-volume surface seawater sampling was performed manually using a 5 L bucket attached to a rope. The seawater was transferred into a 25 L carboy using a funnel, and not conditioned on-site. Sediment samples were collected by core sampling at 5 cm and 50 cm depths. The sediment was allowed to settle in double-layered resealable bags before decanting excess seawater.\u003c/p\u003e \u003cp\u003eSoil samples were dried in an oven at 60\u0026deg;C, while sediment samples were dried in an oven at 40\u0026deg;C over 3 days followed by freeze-drying, until constant dry mass was achieved. The dry and wet mass values were recorded. After drying, soil and sediment samples were ground using a planetary ball miller. The ground samples were transferred into a pre-weighed container over a sieve. The samples were then homogenized using a shaker mixer. For radiochemical analysis, an aliquot of each sample was ashed at 520 \u003csup\u003eo\u003c/sup\u003eC for about 20 h.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003eRadionuclide analysis\u003c/h2\u003e \u003cp\u003eFor gamma-ray spectrometry analysis, approximately 70\u0026ndash;150 g of ground soil and sediment samples were packed into appropriate counting geometries (cylindrical, h: 29.3 mm, diam. 67.8 mm, Vol. 79.1 cm\u003csup\u003e3\u003c/sup\u003e), measured on high-purity germanium (HPGe) detectors (CANBERRA\u0026reg;, n-type, carbon window, 25\u0026ndash;30% R.E., and/or Ortec\u0026reg;) and analyzed by the spectroscopy software APEX-Gamma (V1.4.1) using Genie 2000 (V3.4.1). The efficiency calibrations were calculated by the calibration software LabSOCS\u0026trade; (Laboratory Sourceless Calibration Software, CANBERRA\u0026reg;) with the according approximated matrix and density. The counting times varied between 3 and 12 days to achieve desirable detection limits.\u003c/p\u003e \u003cp\u003eThe soil samples were used for an interlaboratory comparison between LS and NRL. The results of this exercise are presented in the Supplementary Information (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e-S; Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e-S). Both laboratories displayed a good agreement with data within 25% difference (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e-S). As the data sets were comparable, the one from LS was used in the \u003cspan refid=\"Sec5\" class=\"InternalRef\"\u003eresults and discussion\u003c/span\u003e section.\u003c/p\u003e \u003cp\u003eCs radioisotopes in seawater were preconcentrated in two stacked 2 mL columns, each containing approximately 1 mL of potassium nickel ferrocyanate (KNiFC-PAN) resin as described elsewhere [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e]. With the use of KNiFC-PAN resin, acidification of the seawater was optional and, in this case, not performed. The processed seawater was collected in a pre-weighed carboy and the mass of seawater was recorded. For the analysis of \u003csup\u003e137\u003c/sup\u003eCs in the KNiFC-PAN resin, the 2 mL column geometry was modelled and the efficiency calibration calculated with the software LabSOCS\u0026trade;. The 2 mL resin columns were counted on CANBERRA\u0026reg; and/or Ortec\u0026reg; HPGe detectors for a minimum of 3 days.\u003c/p\u003e \u003cp\u003eRadionuclides of Pu and U were determined in the ashed materials as described elsewhere [\u003cspan additionalcitationids=\"CR5 CR6 CR7\" citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]. Up to 5 g of ash were spiked with known amounts of \u003csup\u003e242\u003c/sup\u003ePu (chemical recovery tracer) and \u003csup\u003e115\u003c/sup\u003eIn (internal standard). The samples were digested by lithium borate fusion and then dissolved in 4.5 M HNO\u003csub\u003e3\u003c/sub\u003e. Aliquots were filtered and diluted for quantitative analysis of \u003csup\u003e238\u003c/sup\u003eU as reported elsewhere [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e, \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]. After filtration, ammonium iron (II) sulphate hexahydrate was added to the solutions and Pu was separated in a TEVA extraction chromatography resin as described elsewhere [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e]. The breakthrough solutions of the TEVA columns containing U radioisotopes were collected for further analysis. The TEVA columns were rinsed with 6 M HCl to remove Th and then with 3 M HNO\u003csub\u003e3\u003c/sub\u003e. Pu radioisotopes were eluted with 0.2% HNO\u003csub\u003e3\u003c/sub\u003e / 0.2% HF.\u003c/p\u003e \u003cp\u003ePu radioisotopes were analyzed with a double-focusing magnetic sector field inductively coupled plasma mass spectrometer (SF-ICP-MS) Element 2 (Thermo Fisher Scientific). An Apex nebulizing system connected to an ACM desolvator and a self-aspirating PFA-ST-nebuliser (all from Elemental Scientific Incorporation, USA) were used for the introduction of samples into the system. Pu isotope concentrations were calculated from the signals of the \u003csup\u003e242\u003c/sup\u003ePu tracer. The contributions of the Pu isotopes from the tracer and tailing from U and Th were corrected mathematically based on the isotopic ratios from the certificates and abundance sensitivity measurements of U and Th standards.\u003c/p\u003e \u003cp\u003eIn the separation of Pu using TEVA resin, it was found that the existing method was insufficient to wash out the high levels of U found in Singapore\u0026rsquo;s soil. With the existing method [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e, \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e], U separation factors varying between 3000 and 8500 were obtained. A significant contribution of the \u003csup\u003e238\u003c/sup\u003eU tailing on \u003csup\u003e239\u003c/sup\u003ePu was observed. This, combined with the low content of Pu in most of the soils, impeded a quantification of the Pu isotopes. Hence, an additional Pu purification through a TEVA column was necessary. After the second TEVA separation, higher U separation factors varying between 1 x 10\u003csup\u003e6\u003c/sup\u003e and 13 x 10\u003csup\u003e6\u003c/sup\u003e were obtained and, the low levels of Pu in the soil could be quantified.\u003c/p\u003e \u003cp\u003eThe UTEVA extraction chromatography resin was used to separate the U radioisotopes in the breakthrough solutions from the TEVA resins. The breakthrough solutions were loaded onto pre-conditioned 2 mL UTEVA columns as described elsewhere [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]. The UTEVA columns were rinsed with 6 M HCl and then with 3 M HNO\u003csub\u003e3\u003c/sub\u003e. U radioisotopes were eluted with 0.2% HNO\u003csub\u003e3\u003c/sub\u003e / 0.002% HF. A known amount of \u003csup\u003e115\u003c/sup\u003eIn standard solution was added to the U fractions. The analysis of the U radioisotopes was carried out using a multi-collector inductively coupled plasma mass spectrometer (MC-ICP-MS) Neptune (Thermo Fisher Scientific) as described elsewhere [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eFor the determination of the Sr radioisotopes, about 20 g of ashed material was analyzed as described elsewhere [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e]. Sample aliquots were digested, first in 8 M HNO\u003csub\u003e3\u003c/sub\u003e and then in 2 M HNO\u003csub\u003e3\u003c/sub\u003e. A known amount of stable Sr was then added as carrier and chemical recovery tracer. The Ca and Sr oxalates were precipitated by adding C\u003csub\u003e2\u003c/sub\u003eH\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e and adjusting to pH 3. The precipitates were centrifuged, ashed at 500\u0026deg;C and then dissolved in 3 M HNO\u003csub\u003e3\u003c/sub\u003e. The Sr resin columns were conditioned in 3 M HNO\u003csub\u003e3\u003c/sub\u003e. The sample solutions were loaded onto the columns, and then washed with 3 M HNO\u003csub\u003e3\u003c/sub\u003e to remove most of the alkaline-earth metal interferences. Sr was finally stripped with 0.005 M HNO\u003csub\u003e3\u003c/sub\u003e. Sr was precipitated as SrCO\u003csub\u003e3\u003c/sub\u003e and filtered through pre-weighed glass microfiber filters. The filters were ashed at 500\u0026deg;C for 1 h and then counted in a low-level gas proportional counter (LLC) LB-770 system (Berthold Technologies, GmbH, Germany). The chemical recovery for Sr was determined gravimetrically or by ICP-OES.\u003c/p\u003e \u003cp\u003eThe certified reference materials IAEA-375 (Soil, Russia), IAEA-384 (Sediment, Fangataufa Lagoon) and IAEA-414 (Sediment, Pacific Ocean) from the International Atomic Energy Agency (IAEA, Vienna, Austria) were used as quality control over the analytical methods. With each series of six to eight samples, certified reference materials of relevant matrices and blanks were analyzed.\u003c/p\u003e \u003c/div\u003e"},{"header":"Results and discussion","content":"\u003cdiv id=\"Sec6\" class=\"Section2\"\u003e \u003ch2\u003eLevels of naturally occurring radionuclides\u003c/h2\u003e \u003cp\u003eThe activity concentrations of \u003csup\u003e238\u003c/sup\u003eU, \u003csup\u003e235\u003c/sup\u003eU, \u003csup\u003e234\u003c/sup\u003eU, \u003csup\u003e232\u003c/sup\u003eTh and \u003csup\u003e40\u003c/sup\u003eK in soils ranged from 46\u0026ndash;138 Bq/kg, 2.1\u0026ndash;6.4 Bq/kg, 42\u0026ndash;134 Bq/kg, 68\u0026ndash;252 Bq/kg, and 17\u0026ndash;60 Bq/kg, respectively (Table\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e, Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea). Soil samples from the northern region of Singapore contained about 2 to 3 times more U and Th than in the central region. For a better understanding of these differences, the levels in soil should be linked up with the underlying geological composition. Furthermore, anthropogenic activities and development carried out in the northern part in the late 1980s may most likely be behind these differences.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab3\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 3\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eLevels of naturally occurring radionuclides in soil and sediment (LS Data). Expanded uncertainties with k\u0026thinsp;=\u0026thinsp;2 are reported.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"10\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\"\u0026plusmn;\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\"\u0026plusmn;\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\"\u0026plusmn;\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\"\u0026plusmn;\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c8\" colnum=\"8\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c9\" colnum=\"9\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c10\" colnum=\"10\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eSample code\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colspan=\"9\" nameend=\"c10\" namest=\"c2\"\u003e \u003cp\u003eActivity concentration, in Bq/kg\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u003csup\u003e234\u003c/sup\u003eU\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003e\u003csup\u003e235\u003c/sup\u003eU\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u003csup\u003e238\u003c/sup\u003eU\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e 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colname=\"c6\"\u003e \u003cp\u003e27\u0026thinsp;\u0026plusmn;\u0026thinsp;4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e81\u0026thinsp;\u0026plusmn;\u0026thinsp;24\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e81\u0026thinsp;\u0026plusmn;\u0026thinsp;4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e90\u0026thinsp;\u0026plusmn;\u0026thinsp;8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c10\"\u003e \u003cp\u003e113\u0026thinsp;\u0026plusmn;\u0026thinsp;18\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eN-3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c2\"\u003e \u003cp\u003e96\u0026thinsp;\u0026plusmn;\u0026thinsp;3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c3\"\u003e 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\u003cp\u003e107\u0026thinsp;\u0026plusmn;\u0026thinsp;18\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eN-4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c2\"\u003e \u003cp\u003e134\u0026thinsp;\u0026plusmn;\u0026thinsp;4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c3\"\u003e \u003cp\u003e6.4\u0026thinsp;\u0026plusmn;\u0026thinsp;0.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c4\"\u003e \u003cp\u003e138\u0026thinsp;\u0026plusmn;\u0026thinsp;4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c5\"\u003e \u003cp\u003e242\u0026thinsp;\u0026plusmn;\u0026thinsp;7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e60\u0026thinsp;\u0026plusmn;\u0026thinsp;5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e131\u0026thinsp;\u0026plusmn;\u0026thinsp;38\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e109\u0026thinsp;\u0026plusmn;\u0026thinsp;6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e111\u0026thinsp;\u0026plusmn;\u0026thinsp;10\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c10\"\u003e \u003cp\u003e138\u0026thinsp;\u0026plusmn;\u0026thinsp;22\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eC-1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c2\"\u003e \u003cp\u003e46\u0026thinsp;\u0026plusmn;\u0026thinsp;1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c3\"\u003e \u003cp\u003e2.3\u0026thinsp;\u0026plusmn;\u0026thinsp;0.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c4\"\u003e \u003cp\u003e49\u0026thinsp;\u0026plusmn;\u0026thinsp;1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c5\"\u003e \u003cp\u003e85\u0026thinsp;\u0026plusmn;\u0026thinsp;2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e29\u0026thinsp;\u0026plusmn;\u0026thinsp;6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e77\u0026thinsp;\u0026plusmn;\u0026thinsp;22\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e40\u0026thinsp;\u0026plusmn;\u0026thinsp;2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e41\u0026thinsp;\u0026plusmn;\u0026thinsp;4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c10\"\u003e \u003cp\u003e59\u0026thinsp;\u0026plusmn;\u0026thinsp;10\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eC-2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c2\"\u003e \u003cp\u003e42\u0026thinsp;\u0026plusmn;\u0026thinsp;1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c3\"\u003e \u003cp\u003e2.1\u0026thinsp;\u0026plusmn;\u0026thinsp;0.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c4\"\u003e \u003cp\u003e46\u0026thinsp;\u0026plusmn;\u0026thinsp;1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c5\"\u003e \u003cp\u003e68\u0026thinsp;\u0026plusmn;\u0026thinsp;2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e26\u0026thinsp;\u0026plusmn;\u0026thinsp;3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e81\u0026thinsp;\u0026plusmn;\u0026thinsp;24\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e38\u0026thinsp;\u0026plusmn;\u0026thinsp;2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e39\u0026thinsp;\u0026plusmn;\u0026thinsp;4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c10\"\u003e \u003cp\u003e54\u0026thinsp;\u0026plusmn;\u0026thinsp;8\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eC-3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c2\"\u003e \u003cp\u003e48\u0026thinsp;\u0026plusmn;\u0026thinsp;1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c3\"\u003e \u003cp\u003e2.4\u0026thinsp;\u0026plusmn;\u0026thinsp;0.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c4\"\u003e \u003cp\u003e52\u0026thinsp;\u0026plusmn;\u0026thinsp;1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c5\"\u003e \u003cp\u003e71\u0026thinsp;\u0026plusmn;\u0026thinsp;2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e24\u0026thinsp;\u0026plusmn;\u0026thinsp;4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e101\u0026thinsp;\u0026plusmn;\u0026thinsp;28\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e43\u0026thinsp;\u0026plusmn;\u0026thinsp;2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e43\u0026thinsp;\u0026plusmn;\u0026thinsp;4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c10\"\u003e \u003cp\u003e55\u0026thinsp;\u0026plusmn;\u0026thinsp;10\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eC-4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c2\"\u003e \u003cp\u003e55\u0026thinsp;\u0026plusmn;\u0026thinsp;2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c3\"\u003e \u003cp\u003e2.7\u0026thinsp;\u0026plusmn;\u0026thinsp;0.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c4\"\u003e \u003cp\u003e58\u0026thinsp;\u0026plusmn;\u0026thinsp;2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c5\"\u003e \u003cp\u003e84\u0026thinsp;\u0026plusmn;\u0026thinsp;2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e17\u0026thinsp;\u0026plusmn;\u0026thinsp;5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e92\u0026thinsp;\u0026plusmn;\u0026thinsp;26\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e49\u0026thinsp;\u0026plusmn;\u0026thinsp;2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e50\u0026thinsp;\u0026plusmn;\u0026thinsp;4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c10\"\u003e \u003cp\u003e67\u0026thinsp;\u0026plusmn;\u0026thinsp;12\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eE-1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c2\"\u003e \u003cp\u003e51\u0026thinsp;\u0026plusmn;\u0026thinsp;1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c3\"\u003e \u003cp\u003e2.4\u0026thinsp;\u0026plusmn;\u0026thinsp;0.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c4\"\u003e \u003cp\u003e51\u0026thinsp;\u0026plusmn;\u0026thinsp;1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c5\"\u003e \u003cp\u003e87\u0026thinsp;\u0026plusmn;\u0026thinsp;2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e491\u0026thinsp;\u0026plusmn;\u0026thinsp;41\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e92\u0026thinsp;\u0026plusmn;\u0026thinsp;26\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e31\u0026thinsp;\u0026plusmn;\u0026thinsp;2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e33\u0026thinsp;\u0026plusmn;\u0026thinsp;2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c10\"\u003e \u003cp\u003e58\u0026thinsp;\u0026plusmn;\u0026thinsp;10\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eE-2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c2\"\u003e \u003cp\u003e51\u0026thinsp;\u0026plusmn;\u0026thinsp;1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c3\"\u003e \u003cp\u003e2.4\u0026thinsp;\u0026plusmn;\u0026thinsp;0.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c4\"\u003e \u003cp\u003e51\u0026thinsp;\u0026plusmn;\u0026thinsp;1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c5\"\u003e \u003cp\u003e84\u0026thinsp;\u0026plusmn;\u0026thinsp;2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e518\u0026thinsp;\u0026plusmn;\u0026thinsp;44\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e86\u0026thinsp;\u0026plusmn;\u0026thinsp;24\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e31\u0026thinsp;\u0026plusmn;\u0026thinsp;2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e32\u0026thinsp;\u0026plusmn;\u0026thinsp;2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c10\"\u003e \u003cp\u003e59\u0026thinsp;\u0026plusmn;\u0026thinsp;12\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eIAEA-384\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c2\"\u003e \u003cp\u003e41\u0026thinsp;\u0026plusmn;\u0026thinsp;1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c3\"\u003e \u003cp\u003e1.6\u0026thinsp;\u0026plusmn;\u0026thinsp;0.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c4\"\u003e \u003cp\u003e36\u0026thinsp;\u0026plusmn;\u0026thinsp;1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c5\"\u003e \u003cp\u003e0.015\u0026thinsp;\u0026plusmn;\u0026thinsp;0.006\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003en.a.\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003en.a.\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003en.a.\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003en.a.\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c10\"\u003e \u003cp\u003en.a.\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eIAEA-412\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c2\"\u003e \u003cp\u003e30\u0026thinsp;\u0026plusmn;\u0026thinsp;1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c3\"\u003e \u003cp\u003e1.5\u0026thinsp;\u0026plusmn;\u0026thinsp;0.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c4\"\u003e \u003cp\u003e32\u0026thinsp;\u0026plusmn;\u0026thinsp;1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c5\"\u003e \u003cp\u003e35.9\u0026thinsp;\u0026plusmn;\u0026thinsp;1.0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003en.a.\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003en.a.\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003en.a.\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003en.a.\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c10\"\u003e \u003cp\u003en.a.\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003ctfoot\u003e \u003ctr\u003e\u003ctd colspan=\"10\"\u003e\u003cem\u003en.a.: not analyzed.\u003c/em\u003e\u003c/td\u003e\u003c/tr\u003e \u003c/tfoot\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eAccording to UNSCEAR (2008) [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e], the worldwide average levels for \u003csup\u003e238\u003c/sup\u003eU and \u003csup\u003e232\u003c/sup\u003eTh in soils are about 33 Bq/kg and 45 Bq/kg respectively. The levels found in Singapore were on average 2 to 3 times these levels. Notwithstanding, it was noted that the worldwide average levels were low with large variations, and up to 1000 Bq/kg \u003csup\u003e238\u003c/sup\u003eU and 360 Bq/kg \u003csup\u003e232\u003c/sup\u003eTh were being reported [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e]. Comparing to the region, the levels of U and Th in the soils of Singapore were comparable to the levels reported in soils of Malaysia (\u003csup\u003e238\u003c/sup\u003eU: 30\u0026ndash;234 Bq/kg, \u003csup\u003e232\u003c/sup\u003eTh: 63\u0026ndash;332 Bq/kg) and Thailand (\u003csup\u003e238\u003c/sup\u003eU: 3\u0026ndash;370 Bq/kg, \u003csup\u003e232\u003c/sup\u003eTh: 7\u0026ndash;120 Bq/kg) [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e, \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eSediment samples contained 51 Bq/kg, 2.4 Bq/kg, 51 Bq/kg, 84\u0026ndash;87 Bq/kg, 491\u0026ndash;518 Bq/kg of \u003csup\u003e238\u003c/sup\u003eU, \u003csup\u003e235\u003c/sup\u003eU, \u003csup\u003e234\u003c/sup\u003eU, \u003csup\u003e232\u003c/sup\u003eTh and \u003csup\u003e40\u003c/sup\u003eK respectively, with no significant differences at 5 cm and 50 cm depths (Table\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e, Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea). The levels of natural radionuclides were in the same range as those reported in sediments of Malaysia (32 Bq/kg \u003csup\u003e238\u003c/sup\u003eU) and Vietnam (36 Bq/kg \u003csup\u003e232\u003c/sup\u003eTh) [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eThe uranium isotopic composition in soils was comparable to that of natural uranium, with 48.6% \u003csup\u003e238\u003c/sup\u003eU, 49.2% \u003csup\u003e234\u003c/sup\u003eU and 2.2% \u003csup\u003e235\u003c/sup\u003eU by activity [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e]. \u003csup\u003e236\u003c/sup\u003eU activity was below the detection limit of the method (\u0026lt;\u0026thinsp;50 \u0026micro;Bq/kg). The activity concentrations of \u003csup\u003e234\u003c/sup\u003eTh in soils and sediments were comparable to those of its mother \u003csup\u003e238\u003c/sup\u003eU (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea). The activity concentrations of \u003csup\u003e214\u003c/sup\u003ePb and \u003csup\u003e214\u003c/sup\u003eBi (post-radon daughter radionuclides) were comparable, with \u003csup\u003e214\u003c/sup\u003eBi/\u003csup\u003e214\u003c/sup\u003ePb ratios of approximately 1. In most samples, the activities of \u003csup\u003e214\u003c/sup\u003ePb and \u003csup\u003e214\u003c/sup\u003eBi were slightly lower than those of \u003csup\u003e238\u003c/sup\u003eU (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea). This was expected as the samples were not vacuum-sealed to prevent escape of gaseous radon (\u003csup\u003e222\u003c/sup\u003eRn), hence the decay chain was likely not in secular equilibrium. Moreover, because soil and sediment are \u0026ldquo;open systems\u0026rdquo; affected by many hydrogeochemical processes and the elements of the decay series have different chemical behaviors, a radioactive disequilibrium is not unexpected in those compartments [\u003cspan additionalcitationids=\"CR8\" citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e]. In most surface soils, the activity concentration of \u003csup\u003e210\u003c/sup\u003ePb was significantly higher than that of its \u003csup\u003e214\u003c/sup\u003ePb and \u003csup\u003e214\u003c/sup\u003eBi parents. This trend can be explained by an enrichment of \u003csup\u003e210\u003c/sup\u003ePb in surface soils after the decay of \u003csup\u003e222\u003c/sup\u003eRn that diffused from the deeper grounds. Notwithstanding, \u003csup\u003e210\u003c/sup\u003ePb activity concentrations would be useful for subsequent \u003csup\u003e210\u003c/sup\u003ePo studies and dose assessment studies.\u003c/p\u003e \u003cp\u003eThe levels of \u003csup\u003e40\u003c/sup\u003eK in soils were about 17 times lower than in sediments (Table\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e, Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea), and 15 times lower than the worldwide average of 420 Bq/kg [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e]. \u003csup\u003e40\u003c/sup\u003eK levels in the soils of Singapore were nonetheless comparable to the levels reported in soils of Thailand (7\u0026ndash;712 Bq/kg) [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e]. The low \u003csup\u003e40\u003c/sup\u003eK levels in well-developed soils from tropical regions has been attributed to the leaching of this element by the high rainfall rates [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e]. In sediments, the levels of \u003csup\u003e40\u003c/sup\u003eK were comparable to those reported in the sediments of Malaysia (median value of 250 Bq/kg) and the Philippines (median value of 809 Bq/kg) [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e].\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec7\" class=\"Section2\"\u003e \u003ch2\u003eLevels of anthropogenic radionuclides\u003c/h2\u003e \u003cp\u003eThe activity concentrations of \u003csup\u003e239+240\u003c/sup\u003ePu, \u003csup\u003e137\u003c/sup\u003eCs and \u003csup\u003e90\u003c/sup\u003eSr in the soil and sediment samples from Singapore are reported in Table\u0026nbsp;\u003cspan refid=\"Tab4\" class=\"InternalRef\"\u003e4\u003c/span\u003e and displayed in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb. The activity concentrations of these radionuclides were significantly lower than those of the naturally occurring radionuclides (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e). Moreover, soil samples from the northern region contained significantly lower levels of anthropogenic radionuclides than those in the central region (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb). This is likely related to the fact that soils in the northern part were disturbed by anthropogenic activities, as exposing of deeper layers of soil without fallout, carried out in the late 1980s.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab4\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 4\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eLevels of anthropogenic radionuclides in soil and sediment (LS data). \u003csup\u003e90\u003c/sup\u003eSr was analyzed by low-level gas proportional counting, \u003csup\u003e137\u003c/sup\u003eCs by gamma-ray spectrometry and \u003csup\u003e239+240\u003c/sup\u003ePu by ICP-MS. Expanded uncertainties with k\u0026thinsp;=\u0026thinsp;2 are reported.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"7\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eSample code\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colspan=\"3\" nameend=\"c4\" namest=\"c2\"\u003e \u003cp\u003eActivity concentration, in Bq kg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003e\u003csup\u003e240\u003c/sup\u003ePu/\u003csup\u003e239\u003c/sup\u003ePu\u003c/p\u003e \u003cp\u003eatom ratio\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003e\u003csup\u003e239+240\u003c/sup\u003ePu/\u003csup\u003e137\u003c/sup\u003eCs\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c7\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003e\u003csup\u003e137\u003c/sup\u003eCs/\u003csup\u003e90\u003c/sup\u003eSr\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u003csup\u003e90\u003c/sup\u003eSr\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003e\u003csup\u003e137\u003c/sup\u003eCs\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u003csup\u003e239+240\u003c/sup\u003ePu\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eN-1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003en.a.\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.30\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.007\u0026thinsp;\u0026plusmn;\u0026thinsp;0.002\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.10\u0026thinsp;\u0026plusmn;\u0026thinsp;0.07\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eN-2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003en.a.\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.30\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.005\u0026thinsp;\u0026plusmn;\u0026thinsp;0.001\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.14\u0026thinsp;\u0026plusmn;\u0026thinsp;0.06\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eN-3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003en.a.\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.30\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.002\u0026thinsp;\u0026plusmn;\u0026thinsp;0.001\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.10\u0026thinsp;\u0026plusmn;\u0026thinsp;0.17\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eN-4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003en.a.\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.30\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.008\u0026thinsp;\u0026plusmn;\u0026thinsp;0.001\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.08\u0026thinsp;\u0026plusmn;\u0026thinsp;0.02\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eC-1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.34\u0026thinsp;\u0026plusmn;\u0026thinsp;0.07\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.65\u0026thinsp;\u0026plusmn;\u0026thinsp;0.14\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.057\u0026thinsp;\u0026plusmn;\u0026thinsp;0.004\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.17\u0026thinsp;\u0026plusmn;\u0026thinsp;0.03\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e0.09\u0026thinsp;\u0026plusmn;\u0026thinsp;0.02\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e1.7\u0026thinsp;\u0026plusmn;\u0026thinsp;0.3\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eC-2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.28\u0026thinsp;\u0026plusmn;\u0026thinsp;0.07\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.52\u0026thinsp;\u0026plusmn;\u0026thinsp;0.16\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.038\u0026thinsp;\u0026plusmn;\u0026thinsp;0.004\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.19\u0026thinsp;\u0026plusmn;\u0026thinsp;0.03\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e0.07\u0026thinsp;\u0026plusmn;\u0026thinsp;0.02\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e1.6\u0026thinsp;\u0026plusmn;\u0026thinsp;0.4\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eC-3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.24\u0026thinsp;\u0026plusmn;\u0026thinsp;0.06\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.48\u0026thinsp;\u0026plusmn;\u0026thinsp;0.11\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.031\u0026thinsp;\u0026plusmn;\u0026thinsp;0.002\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.18\u0026thinsp;\u0026plusmn;\u0026thinsp;0.02\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e0.06\u0026thinsp;\u0026plusmn;\u0026thinsp;0.02\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e1.8\u0026thinsp;\u0026plusmn;\u0026thinsp;0.3\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eC-4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.29\u0026thinsp;\u0026plusmn;\u0026thinsp;0.06\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.41\u0026thinsp;\u0026plusmn;\u0026thinsp;0.10\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.050\u0026thinsp;\u0026plusmn;\u0026thinsp;0.003\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.17\u0026thinsp;\u0026plusmn;\u0026thinsp;0.02\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e0.12\u0026thinsp;\u0026plusmn;\u0026thinsp;0.03\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e1.3\u0026thinsp;\u0026plusmn;\u0026thinsp;0.3\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eE-1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.17\u0026thinsp;\u0026plusmn;\u0026thinsp;0.04\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.46\u0026thinsp;\u0026plusmn;\u0026thinsp;0.11\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.294\u0026thinsp;\u0026plusmn;\u0026thinsp;0.009\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.24\u0026thinsp;\u0026plusmn;\u0026thinsp;0.01\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e0.64\u0026thinsp;\u0026plusmn;\u0026thinsp;0.15\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e2.6\u0026thinsp;\u0026plusmn;\u0026thinsp;0.3\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eE-2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.15\u0026thinsp;\u0026plusmn;\u0026thinsp;0.05\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.54\u0026thinsp;\u0026plusmn;\u0026thinsp;0.16\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.239\u0026thinsp;\u0026plusmn;\u0026thinsp;0.006\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.26\u0026thinsp;\u0026plusmn;\u0026thinsp;0.01\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e0.44\u0026thinsp;\u0026plusmn;\u0026thinsp;0.13\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e3.5\u0026thinsp;\u0026plusmn;\u0026thinsp;0.4\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSW-1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.0011\u0026thinsp;\u0026plusmn;\u0026thinsp;0.0002\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eIAEA-384\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e116\u0026thinsp;\u0026plusmn;\u0026thinsp;3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.050\u0026thinsp;\u0026plusmn;\u0026thinsp;0.001\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eIAEA-412\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.67\u0026thinsp;\u0026plusmn;\u0026thinsp;0.02\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.17\u0026thinsp;\u0026plusmn;\u0026thinsp;0.01\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eIAEA-375\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e98\u0026thinsp;\u0026plusmn;\u0026thinsp;10\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003ctfoot\u003e \u003ctr\u003e\u003ctd colspan=\"7\"\u003e\u003cem\u003en.a.: not analyzed.\u003c/em\u003e\u003c/td\u003e\u003c/tr\u003e \u003c/tfoot\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eThe activity concentrations of \u003csup\u003e239+240\u003c/sup\u003ePu in soils vary between 0.002 Bq/kg and 0.057 Bq/kg (Table\u0026nbsp;\u003cspan refid=\"Tab4\" class=\"InternalRef\"\u003e4\u003c/span\u003e, Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb), a range comparable to those reported in surface soils of south China (0.003\u0026ndash;0.469 Bq/kg) [\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e] and Vietnam (0.008 to 0.365 Bq/kg) [\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e]. In sediments, \u003csup\u003e239+240\u003c/sup\u003ePu activity concentrations were 0.2\u0026ndash;0.3 Bq/kg (Table\u0026nbsp;\u003cspan refid=\"Tab4\" class=\"InternalRef\"\u003e4\u003c/span\u003e, Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb). This range of activities is comparable to the range of 0.21\u0026ndash;0.45 Bq/kg reported in surface sediments from the east coast of Peninsular Malaysia [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e], and slightly below the range reported in surface sediments from the Indonesian throughflow [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e].\u003c/p\u003e \u003cp\u003e \u003csup\u003e137\u003c/sup\u003eCs activity concentrations in soils of Singapore were in the range of 0.24 Bq/kg to 0.34 Bq/kg (Table\u0026nbsp;\u003cspan refid=\"Tab4\" class=\"InternalRef\"\u003e4\u003c/span\u003e). In the northern region, the \u003csup\u003e137\u003c/sup\u003eCs activity concentrations were below the detection limit of the method (0.3 Bq/kg). The \u003csup\u003e137\u003c/sup\u003eCs levels in soils were similar to those reported in surface soils of Malaysia and Timor Leste (about 1 Bq/kg, decay corrected to 2024) [\u003cspan additionalcitationids=\"CR16\" citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e], but slightly lower than those reported for West Java (about 1 Bq/kg, decay corrected to 2024) [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e] and for Vietnam (0.19\u0026ndash;9.2 Bq/kg, decay corrected to 2024) [\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eThe levels of \u003csup\u003e137\u003c/sup\u003eCs in sediments of the eastern coast of Singapore were of 0.46\u0026ndash;0.54 Bq/kg (Table\u0026nbsp;\u003cspan refid=\"Tab4\" class=\"InternalRef\"\u003e4\u003c/span\u003e). These levels are comparable to those reported in coastal sediments from Philippines and Indonesia (0.52 Bq/kg and 0.47 Bq/kg respectively, decay corrected to 2024) [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e] and the east coast of Peninsular Malaysia (\u0026lt;\u0026thinsp;1.0 Bq/kg to 2.0 Bq/kg, decay corrected to 2024) [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eThe level of \u003csup\u003e137\u003c/sup\u003eCs in seawater from the south-western coast of Singapore was 0.0011\u0026thinsp;\u0026plusmn;\u0026thinsp;0.0002 Bq/kg (Table\u0026nbsp;\u003cspan refid=\"Tab4\" class=\"InternalRef\"\u003e4\u003c/span\u003e), comparable to the levels reported in seawater from the east coast of Peninsular Malaysia [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e], and in other sites in Asia at similar latitudes [\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e].\u003c/p\u003e \u003cp\u003e \u003csup\u003e90\u003c/sup\u003eSr activity concentrations in soils from the central region of Singapore were in the range of 0.24 to 0.34 Bq/kg (Table\u0026nbsp;\u003cspan refid=\"Tab4\" class=\"InternalRef\"\u003e4\u003c/span\u003e). In the coastal sediments, the \u003csup\u003e90\u003c/sup\u003eSr activity concentrations were 0.15 Bq/kg and 0.17 Bq/kg (Table\u0026nbsp;\u003cspan refid=\"Tab4\" class=\"InternalRef\"\u003e4\u003c/span\u003e). These values are comparable to the levels reported in sediments from Vietnam (about 0.24 Bq/kg, decay corrected to 2024) [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e].\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003eActivity and isotope ratios\u003c/h2\u003e \u003cp\u003ePu isotope ratios vary significantly in the environment depending on source, making them well-suited as fingerprints for determining the origin of a contamination. \u003csup\u003e240\u003c/sup\u003ePu/\u003csup\u003e239\u003c/sup\u003ePu atom ratios in soil from Singapore varied between 0.10\u0026thinsp;\u0026plusmn;\u0026thinsp;0.07 and 0.19\u0026thinsp;\u0026plusmn;\u0026thinsp;0.03 (Table\u0026nbsp;\u003cspan refid=\"Tab4\" class=\"InternalRef\"\u003e4\u003c/span\u003e, Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea). The soils in the central region have \u003csup\u003e240\u003c/sup\u003ePu/\u003csup\u003e239\u003c/sup\u003ePu ratios consistent with the ratio 0.18\u0026thinsp;\u0026plusmn;\u0026thinsp;0.01 [\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e] reported in global fallout from the NWT in the 1950s and 1960s (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea). In the northern region, ratios tend to be below the ratio in global fallout. However, these ratios contain relatively large uncertainties and the data therefore cannot be used to precisely identify the origin of the Pu contamination.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eIn sediments, \u003csup\u003e240\u003c/sup\u003ePu/\u003csup\u003e239\u003c/sup\u003ePu atom ratios in the range 0.24\u0026ndash;0.26 were detected, consistently higher than the global fallout ratio (Table\u0026nbsp;\u003cspan refid=\"Tab4\" class=\"InternalRef\"\u003e4\u003c/span\u003e, Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea). The high \u003csup\u003e240\u003c/sup\u003ePu/\u003csup\u003e239\u003c/sup\u003ePu ratios could be related to the exposure of the Straits of Singapore to transboundary effects of ocean circulation from the PPG, which has a fingerprint ratio of about 0.33 [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e, \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e, \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e]. In the Indo-Pacific Intersection (Indonesian Seas and South China Sea), \u003csup\u003e240\u003c/sup\u003ePu/\u003csup\u003e239\u003c/sup\u003ePu atom ratios ranging from 0.194 to 0.258 (average of 0.223\u0026thinsp;\u0026plusmn;\u0026thinsp;0.021) were reported in surface seawater [\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e]. These elevated ratios were attributed to the contribution of Pu transported from the PPG [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e, \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eThe \u003csup\u003e239,240\u003c/sup\u003ePu/\u003csup\u003e137\u003c/sup\u003eCs activity ratios also provide valuable information for a better understanding of the sources of a radioactive contamination. For example, \u003csup\u003e239,240\u003c/sup\u003ePu/\u003csup\u003e137\u003c/sup\u003eCs activity ratios of 0.04\u0026ndash;0.08 (decay correction to 2024) are characteristics of soils contaminated mainly by global fallout [\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e, \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e]. In soils heavily contaminated by the Chernobyl accident, much lower ratios of 0.014\u0026ndash;0.021 (decay correction to 2024) were reported [\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e]. In Singapore soils, the \u003csup\u003e239,240\u003c/sup\u003ePu/\u003csup\u003e137\u003c/sup\u003eCs activity ratios varied between 0.06 and 0.12, further confirming that the origin of the radionuclides is global fallout from the NWTs.\u003c/p\u003e \u003cp\u003eIn the coastal sediments of Singapore, the \u003csup\u003e239,240\u003c/sup\u003ePu/\u003csup\u003e137\u003c/sup\u003eCs activity ratios varied between 0.44 and 0.56 (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eb), a range above the range expected from global fallout (0.04\u0026ndash;0.08). High \u003csup\u003e239,240\u003c/sup\u003ePu/\u003csup\u003e137\u003c/sup\u003eCs activity ratios have been reported in sediments from other coastal sites in Asia, such as in Malaysia (0.1\u0026ndash;1.2, decay corrected to 2024) [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e], in the Sagami Bay (1.13\u0026thinsp;\u0026plusmn;\u0026thinsp;0.3, decay corrected to 2024) [\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e] and the Tokyo Bay (0.87\u0026thinsp;\u0026plusmn;\u0026thinsp;0.24, decay corrected to 2024) [\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e]. The high \u003csup\u003e239,240\u003c/sup\u003ePu/\u003csup\u003e137\u003c/sup\u003eCs activity ratios in the sediments is likely explained by a preferential scavenging of Pu over Cs by suspended particulates [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e, \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e]. In seawater, the two chemical elements show different affinities to suspended particulate, but also solubilities and biological uptake.\u003c/p\u003e \u003cp\u003e \u003csup\u003e137\u003c/sup\u003eCs and \u003csup\u003e90\u003c/sup\u003eSr are produced by fission of U or Pu with relatively high yields. They also decay with similar half-lives of 30.06 y and 28.8 y, respectively, and have remained in the environment after their releases by the NWTs and/or nuclear accidents. Activity ratios of \u003csup\u003e137\u003c/sup\u003eCs to \u003csup\u003e90\u003c/sup\u003eSr are also a useful tool to identify sources of radioactive releases and gain better understanding of environmental behaviors of these radionuclides. The ratio of \u003csup\u003e137\u003c/sup\u003eCs/\u003csup\u003e90\u003c/sup\u003eSr in the soils of Singapore was on average 1.6\u0026thinsp;\u0026plusmn;\u0026thinsp;0.3 (Table\u0026nbsp;\u003cspan refid=\"Tab4\" class=\"InternalRef\"\u003e4\u003c/span\u003e), which is comparable to the ratio of 1.5 reported in global fallout [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e, \u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e]. In the sediment samples, the \u003csup\u003e137\u003c/sup\u003eCs/\u003csup\u003e90\u003c/sup\u003eSr ratios were slightly higher than the ratio reported in global fallout (Table\u0026nbsp;\u003cspan refid=\"Tab4\" class=\"InternalRef\"\u003e4\u003c/span\u003e), likely as a result of fractionation processes affecting these radionuclides in the ocean.\u003c/p\u003e \u003c/div\u003e"},{"header":"Conclusions","content":"\u003cp\u003eThe levels of natural and anthropogenic radionuclides in four monitoring sites of Singapore were investigated. This study highlighted significant differences in radioactivity levels in soils from the northern and central regions of Singapore. These differences might be attributed to the histories of the monitoring sites. While the central region was mostly conserved nature areas since the 1910s, the northern region was developed later in the 1980s.\u003c/p\u003e \u003cp\u003eThe levels of anthropogenic radionuclides in soils were relatively low and displayed a typical signature from the global fallout of the nuclear weapons testings in the 1950s and 1960s. \u003csup\u003e240\u003c/sup\u003ePu/\u003csup\u003e239\u003c/sup\u003ePu atom ratios and \u003csup\u003e137\u003c/sup\u003eCs/\u003csup\u003e90\u003c/sup\u003eSr activity ratios of 0.18\u0026thinsp;\u0026plusmn;\u0026thinsp;0.01 and 1.6\u0026thinsp;\u0026plusmn;\u0026thinsp;0.3 respectively were observed in the soil samples. The levels of anthropogenic radionuclides in sediments were slightly higher than in soils, and their fingerprints indicated a contribution of radionuclides released from the Pacific Proving Grounds. This was evidenced by the elevated \u003csup\u003e240\u003c/sup\u003ePu/\u003csup\u003e239\u003c/sup\u003ePu atom ratios of about 0.25\u0026thinsp;\u0026plusmn;\u0026thinsp;0.01.\u003c/p\u003e \u003cp\u003eFurther studies would be performed on the rest of the monitoring sites under the environmental baseline radioactivity monitoring programme to illustrate the distribution of radioactivity levels across Singapore.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAcknowledgements\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis study was conducted in the frame of a technical cooperation project of the IAEA in Singapore (SIN7001: Enhancing Capabilities in Environmental Radioactivity Monitoring). We would like to acknowledge the IAEA for their generous financial support of two fellowships: FS-SIN7001-2200807, and FS-SIN7001-2305395. We are also grateful to Spiez Laboratory (an IAEA Collaborating Centre) staff who in one way or another were related \u0026nbsp;to the completion of the two fellowships.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConflicts of interest statement\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors have no competing interests to declare that are relevant to the content of this article.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eData availability statement\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors confirm that the data supporting the findings of this study are available within the article and its supplementary materials.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eDuran EB et al (2004) 137Cs and 239\u0026thinsp;+\u0026thinsp;240Pu levels in the Asia-Pacific regional seas. 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Health Phys 70(5):744\u0026ndash;748\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eJohansen MP et al (2019) Plutonium and other radionuclides persist across marine-to-terrestrial ecotopes in the Montebello Islands sixty years after nuclear tests. Sci Total Environ 691:572\u0026ndash;583\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eKelley JM, Bond LA, Beasley TM (1999) \u003cem\u003eGlobal distribution of Pu isotopes and\u003c/em\u003e \u003csup\u003e\u003cem\u003e237\u003c/em\u003e\u003c/sup\u003e\u003cem\u003eNp\u003c/em\u003e. Sci Total Environ, 237\u0026ndash;238: p. 483\u0026ndash;500\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"},{"header":"Supplementary Informations","content":"\u003cp\u003eSupplementary Information is not available with this version.\u003c/p\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":true,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"Naturally occurring radionuclides, anthropogenic radionuclides, Singapore, soil, sediment","lastPublishedDoi":"10.21203/rs.3.rs-4747517/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-4747517/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThis paper provides insights into the radioactivity levels in soil, sediment and surface seawater from selected monitoring sites in Singapore. The environmental samples were measured for naturally occurring and anthropogenic radionuclides. In soils, the decay series of \u003csup\u003e232\u003c/sup\u003eTh and \u003csup\u003e238\u003c/sup\u003eU are the highest contributors to the total radioactivity, while in sediments, \u003csup\u003e40\u003c/sup\u003eK is the main contributor to the total radioactivity. The levels of \u003csup\u003e90\u003c/sup\u003eSr, \u003csup\u003e137\u003c/sup\u003eCs and \u003csup\u003e239+240\u003c/sup\u003ePu are also reported. The data demonstrates that the main source of anthropogenic radionuclides in soils is the global fallout from nuclear weapons testings; in sediments, contributions from the Pacific Proving Grounds have been identified. This study also highlights significant differences in radioactivity levels in soils between the northern and central regions of Singapore, which are attributed to the histories of the monitoring sites.\u003c/p\u003e","manuscriptTitle":"Levels of natural and anthropogenic radionuclides in selected environmental radioactivity monitoring sites in Singapore","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-07-17 15:02:48","doi":"10.21203/rs.3.rs-4747517/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"eab8d471-904b-4dba-8e48-49127ec51aff","owner":[],"postedDate":"July 17th, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[{"id":34659725,"name":"Nuclear Chemistry"},{"id":34659726,"name":"Environmental Chemistry"},{"id":34659727,"name":"Analytical Chemistry"}],"tags":[],"updatedAt":"2024-07-17T15:02:49+00:00","versionOfRecord":[],"versionCreatedAt":"2024-07-17 15:02:48","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-4747517","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-4747517","identity":"rs-4747517","version":["v1"]},"buildId":"qtupq5eGEP_6zYnWcrvyt","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}