The radioactivity concentration in sediments
The radioactivity concentration of 40K ranged from 506 to 914 Bq/kg, with an average of 656 Bq/kg. The average concentration decreased to 752 Bq/kg, 733 Bq/kg, 710 Bq/kg, 697 Bq/kg, 687 Bq/kg, 661 Bq/kg, 624 Bq/kg, 613 Bq/kg, 587 Bq/kg, 581 Bq/kg, and 572 Bq/kg at positions HPCII, HPK4, BLCI, HPCI, TB1, BLCII, CĐ4, KSCI, KSCII, BL1, and KSCIII. The values at positions were above the global average of 400 Bq/kg (Table 2, Fig. 2a).
Table 2
Radioactivity concentration of 40K, 232Th, 226Ra and their radiological hazard indices (min-max; average ± standard deviation)
Core | 40K (Bq/kg) | 232Th (Bq/kg) | 226Ra (Bq/kg) | Raeq (Bq/kg) | ADR (nGy/h) | AEDE (mSv/y) | Iγr | AUI | Hex | AGDE (µSv/y) |
KSCI (n = 17) | 576–676; 613 ± 32 | 36.7–44.3; 41.2 ± 2.1 | 28.4–33.8; 31.3 ± 1.9 | 131.0-145.3; 137.5 ± 5.1 | 61.6–68.5; 64.8 ± 2.4 | 0.08–0.08; 0.08 ± 0.00 | 0.98–1.09; 1.03 ± 0.04 | 0.76–0.89; 0.84 ± 0.04 | 0.35–0.39; 0.37 ± 0.01 | 439–488; 462 ± 17 |
KSCII (n = 13) | 531–654; 587 ± 32 | 37.2–58.1; 49.4 ± 4.8 | 19.0-34.8; 28.0 ± 5.3 | 127.7-159.3; 143.8 ± 9.7 | 59.5–73.8; 67.1 ± 4.4 | 0.07–0.09; 0.08 ± 0.01 | 0.96–1.18; 1.07 ± 0.07 | 0.79–1.03; 0.90 ± 0.08 | 0.34–0.43; 0.39 ± 0.03 | 424–524; 477 ± 31 |
KSCIII (n = 21) | 506–700; 572 ± 48 | 39.9–50.8; 44.9 ± 3.2 | 24.5–30.6; 27.6 ± 1.7 | 125.2-147.4; 135.9 ± 6.2 | 58.9–69.2; 63.7 ± 3.0 | 0.07–0.09; 0.08 ± 0.00 | 0.94–1.10; 1.01 ± 0.05 | 0.77–0.92; 0.84 ± 0.04 | 0.34–0.40; 0.37 ± 0.02 | 419–491; 453 ± 22 |
CĐ4 (n = 24) | 581–736; 624 ± 34 | 40.7–60.6; 46.6 ± 4.9 | 35.2–48.4; 43.5 ± 4.0 | 138.4-189.6; 158.2 ± 11.8 | 65.2–88.7; 74.3 ± 5.4 | 0.08–0.11; 0.09 ± 0.01 | 1.03–1.41; 1.17 ± 0.09 | 0.86–1.22; 1.02 ± 0.09 | 0.37–0.51; 0.43 ± 0.03 | 463–627; 525 ± 38 |
BLCI (n = 11) | 687–735; 710 ± 18 | 43.9–49.9; 47.0 ± 1.8 | 35.9–48.6; 42.7 ± 5.4 | 151.8-172.2; 164.6 ± 6.8 | 71.9–81.0; 77.7 ± 3.1 | 0.09–0.10; 0.10 ± 0.00 | 1.14–1.28; 1.23 ± 0.05 | 0.92–1.10; 1.02 ± 0.06 | 0.41–0.46; 0.44 ± 0.02 | 511–573; 551 ± 21 |
BLCII (n = 16) | 530–764; 661 ± 78 | 30.8–58.4; 49.0 ± 7.9 | 24.4–42.0; 34.7 ± 5.8 | 114.0-177.4; 154.6 ± 23.3 | 54.3–83.2; 72.5 ± 10.7 | 0.07–0.10; 0.09 ± 0.01 | 0.86–1.33; 1.15 ± 0.17 | 0.66–1.10; 0.96 ± 0.16 | 0.31–0.48; 0.42 ± 0.06 | 388–593; 516 ± 76 |
BL1 (n = 25) | 520–643; 581 ± 33 | 39.4–48.1; 45.4 ± 2.0 | 41.6–48.7; 44.3 ± 2.0 | 142.0-161.8; 153.9 ± 5.3 | 66.8–76.0; 72.1 ± 2.5 | 0.08–0.09; 0.09 ± 0.00 | 1.05–1.20; 1.14 ± 0.04 | 0.92–1.06; 1.00 ± 0.04 | 0.38–0.44; 0.42 ± 0.01 | 471–538; 509 ± 18 |
TB1 (n = 23) | 641–914; 687 ± 56 | 44.1–64.2; 47.3 ± 4.2 | 35.5–48.7; 41.0 ± 4.3 | 150.1-209.7; 161.6 ± 13.1 | 70.8–98.8; 76.2 ± 6.2 | 0.09–0.12; 0.09 ± 0.01 | 1.12–1.57; 1.20 ± 0.10 | 0.93–1.29; 1.01 ± 0.08 | 0.41–0.57; 0.44 ± 0.04 | 502–702; 540 ± 44 |
HPCI (n = 17) | 593–756; 697 ± 41 | 47.7–64.2; 58.9 ± 4.3 | 30.0-46.8; 35.6 ± 5.6 | 152.4-190.3; 173.4 ± 10.9 | 71.3–88.7; 81.1 ± 5.0 | 0.09–0.11; 0.10 ± 0.01 | 1.13–1.41; 1.29 ± 0.08 | 0.98–1.24; 1.10 ± 0.08 | 0.41–0.51; 0.47 ± 0.03 | 505–627; 575 ± 35 |
HPCII (n = 23) | 706–813; 752 ± 32 | 49.4–69.4; 60.6 ± 4.6 | 25.9–46.9; 34.6 ± 5.7 | 157.3-200.8; 179.2 ± 11.3 | 74.5–93.8; 84.0 ± 5.1 | 0.09–0.12; 0.10 ± 0.01 | 1.19–1.50; 1.34 ± 0.08 | 0.92–1.27; 1.11 ± 0.09 | 0.42–0.54; 0.48 ± 0.03 | 532–666; 596 ± 35 |
HPK4 (n = 23) | 684–781; 733 ± 37 | 44.5–62.7; 55.6 ± 5.1 | 34.3–42.3; 37.5 ± 2.6 | 153.1-191.2; 173.4 ± 11.5 | 72.5–89.6; 81.5 ± 5.3 | 0.09–0.11; 0.10 ± 0.01 | 1.15–1.42; 1.29 ± 0.08 | 0.93–1.21; 1.08 ± 0.08 | 0.41–0.52; 0.47 ± 0.03 | 515–635; 578 ± 37 |
Average | 656 | 49.8 | 37.0 | 158.7 | 74.5 | 0.09 | 1.18 | 1.00 | 0.40 | 496.4 |
UNSCEAR 2000 | 400 | 30 | 35 | 370 | 84 | 0.46 | 0.5 | 1 | 1 | 300 |
n = sample numbers; min-max; average ± stand deviation |
The radioactivity concentration of 232Th ranged from 30.8 to 69.4 Bq/kg, with an average of 49.8 Bq/kg. The average concentration decreased to 60.6 Bq/kg, 58.9 Bq/kg, 55.6 Bq/kg, 49.4 Bq/kg, 49.0 Bq/kg, 47.3 Bq/kg, 47.0 Bq/kg, 46.6 Bq/kg, 45.4 Bq/kg, 44.9 Bq/kg, and 41.2 Bq/kg at positions HPCII, HPCI, HPK4, KSCII, BLCII, TB1, BLCI, CĐ4, BL1, KSCIII, and KSCI, respectively. At the positions, the radioactivity concentration of 232Th was above the global average (30 Bq/kg) (Table 2, Fig. 2b).
The 226Ra radioactivity concentration ranged from 19.0 to 48.7 Bq/kg, with an average of 37.0 Bq/kg. The average concentration of 226Ra at positions BL1, CĐ4, BLCI, TB1, HPK4, HPCI, BLCII, HPCII, KSCI, KSCII, and KSCIII decreased to 44.3 Bq/kg, 43.5 Bq/kg, 42.7 Bq/kg, 41.0 Bq/kg, 37.5 Bq/kg, 35.6 Bq/kg, 34.7 Bq/kg, 34.6 Bq/kg, 31.3 Bq/kg, 28.0 Bq/kg, 27.6 Bq/kg (Table 2). The radioactivity concentration of 226Ra was above 35 Bq/kg at 6 positions (BL1, CĐ4, BLCI, TB1, HPK4, HPCI) while it was below 35 Bq/kg at 5 positions (BLCII, HPCII, KSCI, KSCII, and KSCIII) (Fig. 2c).
The radiological hazard indices
The Raeq ranged from 114.0 to 209.7 Bq/kg, with an average of 158.7 Bq/kg. The average Raeq decreased to 179.2 Bq/kg, 173.4 Bq/kg, 173.4 Bq/kg, 164.6 Bq/kg, 161.6 Bq/kg, 158.2 Bq/kg, 154.6 Bq/kg, 153.9 Bq/kg, 143.8 Bq/kg, 137.4 Bq/kg, and 135.9 Bq/kg at positions HPCII, HPCI, HPK4, BLCI, TB1, CĐ4, BLCII, BL1, KSCII, KSCI, and KSCIII, respectively. The Raeq was below the UNSCEAR’s recommended level at all positions (370 Bq/kg) (Table 2, Fig. 3a).
The ADR ranged from 54.3 to 98.8 nGy/h, with an average of 74.5 nGy/h. The avreage ADR decreased to 84.0 nGy/h, 81.5 nGy/h, 81.1 nGy/h, 77.7 nGy/h, 76.2 nGy/h, 74.3 nGy/h, 72.5 nGy/h, 72.1 nGy/h, 67.1 nGy/h, 64.8 nGy/h, and 63.7 nGy/h at stites HPCII, HPK4, HPCI, BLCI, TB1, CĐ4, BLCII, BL1, KSCII, KSCI, and KSCIII. The ADR at positions was lower than or equal the UNSCEAR’s recommended level (84 nGy/h) (Table 2, Fig. 3b).
The AEDE was between 0.07 and 0.12 mSv/y, with an average of 0.09 mSv/y. The average AEDE at positions HPCII, HPK4, HPCI, BLCI, TB1, CĐ4, BLCII, BL1, KSCII, KSCI, and KSCIII were 0.10 mSv/y, 0.10 mSv/y, 0.10 mSv/y, 0.10 mSv/y, 0.09 mSv/y, 0.09 mSv/y, 0.09 mSv/y, 0.09 mSv/y, 0.08 mSv/y, 0,08 mSv/y, and 0.08 mSv/y. The AEDE was several times below the UNSCEAR’s recommended level (0.46 mSV/y) (Table 2, Fig. 3c).
The Iγr ranged from 0.86 to 1.57 and an average 1.18. The Iγr was above the UNSCEAR’s recommended level (0.50). The average Iγr at positions reduced by 1.34, 1.29, 1.29, 1.23, 1.20, 1.17, 1.15, 1.14, 1.07, 1.03, and 1.01 for HPCII, HPK4, HPCI, BLCI, TB1, CĐ4, BLCII, BL1, KSCII, KSCI, and KSCIII (Table 2, Fig. 3d).
The AUI ranged from 0.66 to 1.29, with an average of 1.00. AUI decreased to 1.11, 1.10, 1.08, 1.02, 1.02, 1.01, 1.00, 0.96, 0.90, 0.84, and 0.84 at positions HPCII, HPCI, HPK4, CĐ4, BLCI, TB1, BL1, BLCII, KSCII, KSCIII, and KSCI. The AUI was above 1 at 7 positions (HPK4, HPCII, HPCI, TB1, BLCI, CĐ4, BL1) and below 1 at 4 positions (KSCII, KSCI, KSCII, BLCII) (Table 2, Fig. 3e).
The Hex ranged from 0.31 to 0.57, with an average of 0.40 below 1 across all positions. Average Hex decreased to 0.48, 0.47, 0.47, 0.44, 0.43, 0.42, 0.42, 0.39, 0.37, and 0.37 at positions HPCII, HPCI, HPK4, BLCI, TB1, CĐ4, BLCII, BL1, KSCII, KSCI, and KSCIII (Table 2, Fig. 3f).
The AGDE ranged from 388 to 702 µSv/y, with an average of 496.4 µSv/y. The average AGDE decreased to 596 µSv/y, 578 µSv/y, 575 µSv/y, 551 µSv/y, 540 µSv/y, 525 µSv/y, 516 µSv/y, 509 µSv/y, 477 µSv/y, 462 µSv/y, 453 µSv/y at positions HPCII, HPK4, HPCI, BLCI, TB1, CĐ4, BLCII, BL1, KSCII, KSCI, KSCIII (Table 2, Fig. 3g). The AGDE at all positions above UNSCEAR’s recommended level (300 µSv/y).
The correlation between radionuclides and radiological hazard indices
The positive correlation between radionuclides and the radiological hazard indices was mostly moderate to strong, but in some cases not significant. The moderately positive correlation was between 40K and 232Th. There was a strong positive correlation between 40K, 232Th and the radiological hazard indices. There was not significant correlation between 226Ra and 40K, 232Th, the radiological hazard indices (Fig. 4).
The comparison of radionuclides in different parts of the world
The radioactivity concentrationof 40K, 226Ra, 232Th in the Red River Delta (in this study) was lower than in the surface sediment of the Red River (Duong et al., 2023). The radioactivity concentration of 40K, 226Ra in Ha Long Bay was lower than in the Red River Delta while 232Th was higher (Carvalho et al., 2021) (Table 3).
Table 3
Comparison of radioactivity concentration in sediments with some parts of the world
TT | Areas | 40K (Bq/kg) | 232Th (Bq/kg) | 226Ra (Bq/kg) | Refference |
1 | Red River Delta, VietNam | 656.0 | 49.8 | 37.0 | This study |
2 | Along Red River, Vietnam | 685.0 | 83.7 | 59.6 | Duong et al. 2023 |
3 | Ha Long Bay, Vietnam | 424.0 | 52.0 | 25.0 | Carvalho et al. 2021 |
4 | Gulf of Tokin, China | 39.6 | 9.6 | 6.9 | Liu and Lin 2018 |
5 | Gulf of Tokin, China | 263.0 | - | 23.0 | Lin et al. 2020 |
6 | Yangtze Estuary, China | 628.0 | 40.9 | 24.3 | Wang et al. 2017 |
7 | Daya Bay, China | 432.0 | - | 20.9 | Zhou et al. 2015 |
8 | Penang Island, Malaysia | 79–416 | 11-2086 | 13-1023 | Shuaibu et al. 2017 |
9 | Saudi Arabian, Arabia | 153.8 | 6.7 | 11.3 | Al-Ghamdi et al. 2016 |
10 | Tamilnadu, India | 360.2 | 14.3 | - | Ravisankar et al. 2015 |
11 | Tamil Nadu, India | 328.0 | 40.5 | - | Sivakumar et al. 2018 |
12 | Port of Tema, Ghana | 320.0 | 30.0 | 14.0 | Botwe et al. 2017 |
13 | Intertidal flat Oman | 44.8 | 2.3 | 20.5 | Al Shaaibi et al. 2021 |
14 | Martin Island, Bangladesh | 388.0 | 36.8 | 30.7 | Islam et al. 2019 |
15 | Algiers Bay, Algeria | - | - | 20.5 | Taieb Errahmani et al. 2020 |
16 | Marmara Sea, Turkey | 558.6 | 23.2 | 23.1 | Otansev et al. 2016 |
17 | Siberia, Russusia | 418.0 | 40.7 | 30.8 | Charkin et al. 2022 |
| Global average | 400.0 | 30.0 | 35.0 | UNSCEAR 2000 |
Compared to the China Sea such as the Gulf of Tonkin, Daya Bay, and the Yangtze River Estuary. The radioactivity concentration of 226Ra and 232Th in the Red River Delta was higher than that of the Gulf of Tonkin and the Yangtze River Estuary (Table 4). Only the 40K radioactivity concentration in the Yangtze River Estuary was higher than that in the Red River Delta (Table 3) (Liu and Lin, 2018; Lin et al., 2020; Wang et al., 2017; Zhou et al., 2015).
Table 4
Radioactivity and radiological hazard indices in groups
Group | Cores | 40K (Bq/kg) | 232Th (Bq/kg) | 226Ra (Bq/kg) | Raeq (Bq/kg) | ADR (nGy/h) | AEDE (mSv/y) | Iγr | AUI | Hex | AGDE (µSv/y) |
1 | 6 | 606.2 | 46.1 | 34.9 | 147.3 | 69.1 | 0.08 | 1.1 | 0.9 | 0.4 | 490.2 |
2 | 5 | 715.9 | 53.9 | 38.3 | 170.5 | 80.1 | 0.10 | 1.3 | 1.1 | 0.5 | 568.3 |
UNSCEAR 2000 | 400 | 30 | 35 | 370 | 84 | 0.46 | 0.5 | 1 | 1 | 300 |
Compared to the sand beach of Penang Island in Malaysia, only 40K radioactivity concentration in Red River Delta was higher than Penang Island, while 226Ra and 232Th were lower (Table 3) (Shuaibu et al., 2017).
Compared to other seas such as Saudi Arabian (Arabia) (Al-Ghamdi et al., 2016), Tamil Nadu (India) (Ravisankar et al., 2015; Sivakumar et al., 2018), Port of Tema (Ghana) (Botwe et al., 2017), intertidal flat Oman (Oman) (Al Shaaibi et al., 2021), Martin Island (Bangladesh) (Islam et al., 2019), Algiers Bay (Algeria) (Taieb Errahmani et al., 2020), Marmara Sea (Turkey) (Otansev et al., 2016), Siberia (Russia) (Charkin et al., 2022), radioactivity concentration of 40K, 226Ra, 232Th were lower than in the Red River Delta (Table 3).
Sediment groups and their influences
Based on clustering of average radioactivity conentration of 40K, 232Th, 226Ra, and radiological hazard indices, the sediment (Fig. 5a) and the sediment parameters (Fig. 5b) were divided into two groups.
Sediment group 1 consisted of six positions (KSCI, KSCII, KSCIII, BL1, CĐ4, BLCII) (Table 4, Fig. 5a), distributed southward. The radioactivity concentration of 40K, 232Th exceeded global average, and AGDE and Iγr were higher than UNSCEAR’s recommended levels. Sediment group 2 included of 5 positions (BLCI, TB1, HPCI, HPCII, and HPK4), distributed north. The radioactivity concentration of 40K, 232Th, 226Ra were higher than group 1 and higher than the global average, and the AUI, Iγr, and AGDE indices were higher than UNSCEAR’s recommended levels (Table 4, Fig. 5a). The group 1 sediment parameters consisted of 40K, 232Th with radiological hazard indices. The group 2 sediment parameters were 226Ra (Fig. 5b).
The result of the factor analysis (F) showed that 40K and 232Th affected the radiological hazard indices by 86.79% (F1), while 226Ra affected 10.68% (F2) (Table 5).
Table 5
Result of factor analysis of radioactivity and radiological hazard indices
Parameters | F1 | F2 | F3 |
40K | 0.90 | -0.15 | -0.40 |
232Th | 0.83 | -0.51 | 0.23 |
226Ra | 0.48 | 0.88 | 0.04 |
Raeq | 1.00 | 0.00 | 0.02 |
ADR | 1.00 | 0.02 | -0.01 |
AEDE | 1.00 | 0.02 | -0.01 |
Iγr | 1.00 | -0.02 | -0.01 |
AUI | 0.98 | 0.11 | 0.18 |
Hex | 1.00 | 0.00 | 0.02 |
AGDE | 1.00 | 0.00 | -0.03 |
Variance | 8.68 | 1.07 | 0.25 |
Percent of variance (%) | 86.79 | 10.68 | 2.53 |
Different natural conditions, such as tides and sediments affected radioactivity concentrations and radiological hazard indices in sediment groups. According to Thuy (1984), the time of spring tide equaled the time of neap tide in the northern area, while spring tide is shorter in the southern area. Radionuclides from rivers are transported to the sea through water and TSS. Under tide action, in the north, the TSS was brought back to the coast, while in the south, the action of the tide was weaker. In the north, fine sediments predominated, whereas in the south, coarser sediments predominated. Quartz increased from the north to the south while clay minerals (kaolinite, illite, chlorite, and motmoriolite), feldspar, and gothite decreased from the north to the middle to the south (Nhon et al., 2019). Radionuclides were absorbed in fine sediments, clay minerals explained for high radioactivity concentration in the north area and decreased in the south area, the positive correlation between 40K, 226Ra, and 232Th with fine sediments and clay mineral were studied (Yang et al., 2013; Du et al., 2021).