Radioactivity concentration and risk indices in intertidal sediments of the Red River Delta, Vietnam

The radioactivity concentration in eleven intertidal sediment cores in the Red River Delta was determined 40 K, 232 Th, and 226 Ra as well as radiological hazard indices for environmental risk assessment. The radioactivity concentrations of 40 K, 232 Th, and 226 Ra were 656 Bq/kg, 49.8 Bq/kg, and 37.0 Bq/kg, respectively. The radium equivalent activity (Raeq) was 158.7 Bq/kg; the absorbed dose rate (ADR) was 74.5 nGy/h; the annual effective dose equivalent (AEDE) was 0.09 mSv/y; the representative gamma index (I γ r) was 1.18; the activity utilization index (AUI) was 1.0; the external hazard index (Hex) was 0.40; the annual gonadal dose equivalent (AGDE) was 496.4 µSv/y. Two sediment groups were split. Group 1 with lower the radioactivity concentrations than Group 2, 40 K and 232 Th were above the global average while 226 Ra was below, I γ r and AGDE were above UNSCEAR’s recommended levels. Group 2 radioactivity concentrations of 40 K, 226 Ra, and 232 Th were higher than global average; radiological hazard indices were higher than Group 1; the AUI, I γ r, and AGDE indices were above UNSCEAR’s recommended level. The radioactivity concentrations of 40 K, 232 Th, and 226 Ra were above global average. The AUI, I γ r and AGDE indices were above UNSCEAR’s recommended levels. It is recommended that coastal sediments should not be used for construction purposes as this can affect organisms in intertidal areas.


Introduction
The Red River and Thai Binh River systems today form the Red River Delta, which is of high value to the coastal provinces from Hai Phong to Ninh Binh, due to the sediment ows from the mainlands to the coastal areas, creating new lands.However, sediment ows from the continents can also transport pollutants of natural and anthropogenic origins, comprising radionuclides that can affect aquatic life and humans.
In nature, radionuclides mainly come from the chains of uranium (U), thorium (Th) and potassium ( 40 K) that commonly found in minerals in rocks, clastic sediments.In addition, nuclear tests and nuclear accidents such as Chernobyl (1986) and Fukushima Daiichi (2011) also released amounts of radioactive isotopes into the atmosphere and then into topsoil and sediments.For Southeast Asia, this contribution was only local and insigni cant compared to the amount of radioactivity from previous nuclear weapons tests (UNSCEAR 1988(UNSCEAR , 1993(UNSCEAR , 2013)).The worldwide average annual effective dose per capita from natural radiation was approximately 2.4 mSv, of which the external dose was 0.9 mSv (37.5%) and the internal dose was 1.5 mSv (62.5%) (UNSCEAR 2000).Both natural and arti cial radioisotopes produced ionizing radiation that could cause damage to the cells and genetic materials, resulting in the death of the cell or potentially harmful changes in the DNA (Little 2003;Ravisankar et al. 2014).
With respect to radiation safety, radioactivity levels and hazard indices have been evaluated for beach materials and nearshore marine sediments in many regions of the world such as Orissa Beach in India (Sulekha Rao et al. 2009), Cumuruxatiba Beach in Brazil (Vasconcelos et  (2023) determined the radioactivity of 226 Ra, 232 Th and 238 U in sediment, seawater and seafood samples colleted at some places in northen Vietnam (Hai Phong, Quang Ninh and Ha Tinh Provinces) and assessed the radiological risks associated with natural radionuclides using the Erica software.Data on the concentration of naturally occurring radioisotopes in intertidal sediments in the Red River Delta, and the assessment of radiological hazard from them are scarce.Studies conducted in recent years showed that natural radioactivities at some sites in the upstream basin of the Red River Delta have been quite high (Phan et al. 2019;Duong et al. 2021Duong et al. , 2023)).In this study we determined the radioactivity of 226 Ra, 232 Th and 40 K in intertidal sediments at 11 coastal regions where sediments came from the Red River and Thai Binh River catchments, and assessed the radiological risks associated with natural radionuclides.

Regional studies
The coastal area of the Red River System spans 4 provinces and cities.The total number of people is 6854100 while Hai Phong is 2.088000, Thai Binh is 1878500, Nam Dinh is 1876900, and Ninh Binh is 1010700.The population density is 1368, 1185, 1125, and 716 inhabitants/km 2 in Hai Phong, Thai Binh, Nam Dinh, and Ninh Binh, respectively.In the coastal area, industrial activities, tourism and leisure activities, aquaculture and nature conservation contribute signi cantly to the economic development of the coastal provinces (GSO 2022).
In the coastal area, there are 12 estuaries that ow into the coastal region.The total volumes of water and total suspended solids (TSS) to the shore were changed before and after nishing the construction of the Hoa Binh Dam.
Before the construction of the Hoa Binh Dam (1960Dam ( -1979)), the water volumes and TSS in the coastal area were 130.5 x 10 9 m 3 /year and 77.3 x 10 6 tons/year, respectively.After nishing the Hoa Binh Dam (1989-2010), the total volumes of water and TSS decreased signi cantly by 120.3 x 10 9 m 3 /year and 35.1 x 10 6 tons/year (Vu et al. 2014).
The studied area is in uenced by the monsoons with two seasons, the southwest monsoon (rainy season, from May to September) and the northeast moonsoon (dry season, from October to April next year).The average rainfall was 1800-2542mm/year in the period of 2018-2022, while the monthly average rainfall in the rainy season is 172.2-653.0mm and in the dry season is 10.7-102.2mm.The air temperature ranges from 26.4 to 30.4°C in the rainy season and from 15.1 to 24.9°C in the dry season (GSO 2022).
The geological environment consists mainly of Quaternary sedimentary formations of the Thai Binh and Hai Hung Formations with typical compositions of sand, silt, clay.They are marine, aluvial-marine origin and are distributed from Hai Phong to Ninh Binh Provinces.Along the coast of Hai Phong, there are geological formations: The Do Son Formation (D 2 đs), composed of conglomerate, sandstones, and violet-red argillite; the Ban Pap Formation (D 2 bp), composed of gray, dark limestone, interbedded with chert and cherty limestone; the Pho Han Formation (D 3 -C 1 ph) and the Bac Son Formation (C-P bs), composed of limestone, cherty limestone, and claystone.The area has sites where illmenite-zircon minerals occur in Hai Phong and Nam Dinh Provinces (Ky et al. 1999).

Materials and methods
The sampling took place from 2009 to 2012 (Fig. 1).In 2009, two sediment cores were collected (HPCI and HPCII); in 2011 ve sediment cores were collected (KSCI, KSCII, KSCIII, BLCI and BLCII), and in 2012 four sediment cores were collected (CĐ, BL1, TB1, HPK4) (Table 1).A manual piston core was used to collect samples.The tubes for the cores were made of Plexiglas and had an inner diameter of 60 mm.The length of the sediment cores varied from 20cm to 90 cm, and were cut into 1 cm thick from 0-1 cm, 2 cm thick from 1-21 cm, 3 cm thick from 21-51 cm, and 4 cm thick from 51 cm to the end of the core.Samples were stored in polypropylene tubes in ice boxes at 4°C until the laboratory, at the laboratory they were dried in air-conditioned at 16°C.Based on the activity of radioisotopes 226 Ra, 232 Th, and 40 K in the samples, Raeq was calculated by using formula [1] ( Beretka and Mathew, 1985).
The AEDE rate was calculated by using formula [3].
AGDE (µSv/y) = 3.09A Ra + 4.18A Th + 0.314A K [7] Where A Ra , A Th and A K in the formulas [1] to [7] were the activities of 226 Ra, 232 Th and 40 K in Bq/kg in samples.
Data processing was statistically analyzed by using Origin Pro 2021: maximum, minimum, mean, standard deviation, Pearson's correlation coe cient, factor analysis (F), and cluster analysis.The relationship between radionuclides and risk indices was determined by using Pearson's correlation analysis, factor analysis to identify the variables that in uenced each other, and cluster analysis of stations with similar characteristics.
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 were lower than in the Red River Delta (Table 3).

Sediment groups and their in uences
Based on clustering of average radioactivity conentration of 40 K, 232 Th, 226 Ra, and radiological hazard indices, the sediment (Fig. 5a) and the sediment parameters (Fig. 5b) were divided into two groups.
The group 1 sediment parameters consisted of 40 K, 232 Th with radiological hazard indices.The group 2 sediment parameters were 226 Ra (Fig. 5b).
The result of the factor analysis (F) showed that 40 K and 232 Th affected the radiological hazard indices by 86.79% (F1), while 226 Ra affected 10.68% (F2) (Table 5).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, ne 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 ne sediments, clay minerals explained for high radioactivity concentration in the north area and decreased in the south area, the positive correlation between 40  The radiological hazard indices correlated positively with radionuclides ranging from moderate to strong.The radiological hazard indices were strongly affected ed by 40 K and 232 Th while 226 Ra was weak.In the northern area, the radioactivity concentration and the radiological hazard indices were higher than in the southern area.
Coastal sediments that were considered unsafe to use as building material or land ll material and could affect intertidal animals were recommended.Group of sediments and sediment parameters al. 2011), Srilanka Beach (Withanage and Mahawatte 2012), Sithonian Beach in Greece (Papadopoulos et al. 2014), Calabria Beach in Italy (Caridi et al. 2016), Namibia Beach (Onjefu et al. 2017), the coastline of Guangxi Province in China (Liu and Lin 2018), Beibu Gulf in South China Sea (Lin et al., 2020), East Coast of Tamilnadu in India (Devanesan et al. 2020), Vefsnfjord in Norway (Heldal et al. 2021), Egyptian Red Sea coast (Zakaly et al. 2021), Jeddah Coast in Saudi Arabia (Bandar et al. 2022).In the coastal area of Vietnam, the assessment of radiological hazard from natural radionuclides to humans and aquatic creatures was still very limitted.Nguyen et al. (2020) determined radioactive concentrations of 90 Sr and 137 Cs in seawater and sediment at several sites in the Gulf of Tonkin.Sidorov et al. (2022) determined the activity of 137 Cs, 40 K and 210 Po in water, bottom sediments and suspended matter at some places in the Can Gio Biosphere Reserve.Vo et al.

Figure 1 Sample
Figure 1

Table 1
(Murray et al., 1987)centrations of the226Ra,232Th,40K isotopes were determined by gamma spectrometry using GMXtype high purity germanium (HPGe) detectors with a relative e ciency of 30-50%(Murray et al., 1987).The 226 Ra activity was determined through its progeny, namely 214 Pb (measured at the 295 keV and 352 keV gamma peaks) and214Bi (measured at the 609 keV peak) after equilibrating222Rn in the sample.The 232 Th activity was determined by its daughter228Ra assuming radioactive equilibrium reached, where228Ra was determined by its short-lived228Ac daughter (measured at the 338 keV and 911 keV lines).The 40 K radioactivity was measured at the 1462 keV peak.Thi analysis was performed at the VILAS 525 laboratory of Dalat Nuclear Research Institute, which has been recognized by the Vietnam Laboratory Accreditation Scheme (VILAS) complying with the ISO/IEC 17025:2017.The radioactivity of isotopes is used to calculate radiological hazard indices in sediments. The are considered safe if the following indices are met: the radium equivalent radioactivity (Raeq) < 370 Bq/kg; absorbed dose rate (ADR) < 84 nGy/h; the annual effective dose equivalent (AEDE) < 0.46 mSv/y; representative gamma index (Iγr) < 0.5; activity utilization index (AUI) < 1; external hazard index (Hex) < 1; annual gonadal dose equivalent (AGDE) < 300 µSv/y (NEA-OECD, 1979; UNSCEAR, 2000).