Activity Concentration of Natural Radionuclides in Surface Sediments of Major River Watersheds in Korea and Assessment of Radiological Hazards

Abstract

The assessment of potential radiation hazards in accumulated sediments in aquatic ecosystems is vital for the management and disposal of sediments. Furthermore, preemptive management of radionuclides in terrestrial ecosystems is critical for marine ecosystem conservation. We analyzed the activity concentrations of natural radionuclides (²²⁶Ra,²³²Th, ²³⁸U, and ⁴⁰K) in the surface sediments of major river watersheds in Korea and evaluated the radiation hazards stemming from these activity concentrations. The mean activity concentrations of ²²⁶Ra and ²³⁸U were lower than the global average, whereas those of ²³²Th and ⁴⁰K were higher. The mean values of radium equivalent activity, external hazard index, and internal hazard index calculated from these activity concentrations did not exceed the recommended maximum values. The mean values of absorbed gamma dose rate in air and annual outdoor effective dose rate (AEDR_out) were higher than the global average by the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) but remarkably lower than the recommended and background values by the International Commission on Radiological Protection (ICRP) and the Korea Institute of Nuclear Safety (KINS). The contribution of ⁴⁰K and ²³²Th to the AEDR_out mean value was predominant. In conclusion, the surface sediments of major river watersheds in Korea are associated with negligible radiation hazards. These findings provide fundamental data for the management and treatment of sediments in terrestrial and marine ecosystems.

1. Introduction

Sources of radiation exposure in the environment that affect human health include natural and artificial radionuclides, which occur naturally from cosmic rays and the earth’s crust, and anthropogenic sources such as nuclear tests and nuclear power plant accidents [1]. In particular, external exposure to radiation from natural radionuclides is the primary contributor to the average annual radiation dose received by humans, of which terrestrial gamma radiation accounts for 85% of the total annual average ionizing radiation [2]. In addition, continuous exposure to even low levels of terrestrial gamma radiation can adversely affect human health [3]. The major natural sources of terrestrial gamma radiation include ⁴⁰K and radionuclides derived from the decay series of long-lived radionuclides, such as uranium (²³⁸U) and thorium (²³²Th) [1]. ²³⁸U and ²³²Th radionuclides undergo a very complex radioactive decay series to produce the inert gases radon (²²²Rn) and thoron (²²⁰Rn), which can negatively impact human health via the respiratory process and internal exposure [1,4,5].

Natural radionuclides are widely present in various environmental spheres such as the lithosphere, biosphere, and pedosphere [3,6,7,8,9] and are distributed at different concentration levels depending on various geologic and geographical factors [1,10,11]. Therefore, the activity concentrations and distribution of natural radionuclides in rocks or soils related to their crustal composition are globally inhomogeneous [12]. In addition, the activity concentrations of natural radionuclides in river, lake, and coastal sediments accumulated as a result of weathering and the erosion of these rocks or soils are known to vary [1,8,12,13,14]. Furthermore, certain building materials used in Egypt, such as bricks, gypsum, cement, and sand, contain high concentrations of natural radionuclides [15]. In particular, sediments not only provide useful information for assessing the sources and environmental behavior of radionuclides in aquatic environments, but also serve as a direct source of radiation exposure to benthic organisms [8,12,16,17]. Furthermore, river and lake sediments comprise sand and gravel of various grain sizes, which are often utilized as construction materials [3,15,17,18,19]. Therefore, an understanding of the natural radionuclides in aquatic ecosystems is crucial for determining the background radiation levels to assess the radiation exposure effects in humans [20,21]. Furthermore, sediments contaminated by various factors are dredged and disposed of on land where they are either left untreated or utilized for other purposes such as construction materials [22,23,24]. These dredged sediments produce ionizing radiation, rendering the assessment of their potential as a radiation hazard for proper management and disposal imperative [24,25]. Therefore, the assessment of radiation hazard from natural radionuclides in sediments present in various environments such as rivers, lakes, and oceans has been performed by numerous researchers worldwide [7,8,12,17,19,23,24,26,27,28,29,30,31,32].

A national sediment monitoring network, operational in Korea since 2012, evaluates sediment quality in the aquatic environment according to the water environment information system (http://water.nier.go.kr) [33]. However, the monitoring network has only focused on assessing the temporal changes and contamination levels of nutrients, organic matter, and heavy metals; information on natural radionuclides is not available. Furthermore, studies examining natural radionuclides in sediments in Korea have been conducted only in a limited number of rivers, and no integrated study of major river watersheds has been attempted. River sediments containing natural radionuclides can eventually flow into coastal environments, affecting not only the coastal ecosystems but also humans engaged in marine activities. Therefore, preemptive management is essential.

This study aimed to investigate the distribution of the activity concentration of natural radionuclides (²²⁶Ra, ²³²Th, ²³⁸U, and ⁴⁰K) in surface sediments of major river watersheds in Korea. The objective was to evaluate the radiation hazard based on the concentrations of these radionuclides to provide fundamental data for the proper management and disposal of sediments containing natural radionuclides.

2. Materials and Methods

2.1. Study Area

According to the water environment information system of the Ministry of Environment, Korea’s major watersheds are divided based on five major rivers: Han River (HR), Geum River (GR), Nakdong River (NR), Yeongsan River (YR), and Seomjin River (SR) (http://water.nier.go.kr) [33]. These rivers eventually flow to the western and southern coasts of Korea. The average annual precipitation of each watershed is 1360 mm, 1258 mm, 1337 mm, and 1392 mm for HR, GR, NR, and Y-SR, respectively, with the GR watershed receiving the least amount of precipitation [34]. The region is characterized by a monsoon climate with more than 70% of annual precipitation recorded in summer. The area of each watershed is HR 41,957 km², NR 23,690 km², GR 9912 km², SR 4914 km², and YR 3371 km². In this study, the survey sites were categorized into HR, GR, NR, YR, and SR, and a total of 58 sites including 15 sites in HR (H1–H15), 13 sites in GR (G1–G13), 16 sites in NR (N1–N16), 9 sites in YR (Y1–Y9), and 5 sites in SR (S1–S5) were selected from the national sediment monitoring network operating in these watersheds (Figure 1).

2.2. Sampling and Pretreatment

Surface sediment samples were mixed after consistently securing five or more surface layers using a Ponar grab and a scoop. The mixed samples were homogenized to be considered as representative samples of each site. The collected surface sediment samples were air-dried after removing impurities, passed through a 2 mm sieve, dried at 105 °C, and then ground into a very fine powder. The ground samples were placed into measuring sample containers (50 mm wide and 70 mm high) and sealed completely with a glue gun to prevent the loss of radon gas. The samples were then stored for approximately one month until secular equilibrium was reached between ²²⁶Ra and ²³²Th and their daughter radionuclides [35,36,37].

2.3. Measurement of Radionuclides

The analysis of natural radionuclides was performed using a gamma spectrometer consisting of a high-purity germanium detector (GEM-MX7080P4, Ortec, USA) and a multichannel analyzer (DSPEC-50, Ortec), which was used to analyze radiation equilibrium surface sediment samples and background activity for 80,000 s per sample. Background activity was examined via routine measurements once a week. The high-purity germanium detector of the gamma spectrometer had a relative efficiency of 66% at ⁶⁰Co (1332 keV), an energy resolution of 1.9 keV, and a peak/Compton ratio of 75:1. The measured spectra were analyzed with an analytical program (Gamma Vision, Ortec) for daughter radionuclides of the ²²⁶Ra (²¹⁴Pb, 352.0 keV; ²¹⁴Bi, 609.3 keV), ²³²Th (²¹²Pb, 238.6 keV; ²²⁸Ac, 911.2 keV), and ²³⁸U (²³⁴Th, 63.3 keV) families and a gamma ray energy of ⁴⁰K (1460.8 keV) [36,37]. The activity concentrations of ²²⁶Ra and ²³²Th were obtained by averaging the measured concentrations of each daughter radionuclide. All surface sediment samples were corrected to exclude background activity and account for radioactive decay depending on the date of collection and measurement time before calculating the activity concentrations. The minimum detectable activity (MDA) of the gamma spectrometer at the 95% confidence level for natural radionuclides was calculated as follows [36]:

$$\mathrm{MDA} (\mathrm{Bq} {\mathrm{kg}}^{-1})={\displaystyle \frac{4.66 \times \sqrt{B}}{T \times \gamma \times \epsilon \times \omega }}$$

(1)

where B is the background counts, T is the counting time (s), γ is the gamma emission probability, ε is the absolute efficiency of the detector, and ω is the sample weight (kg). The MDA values of the ²²⁶Ra, ²³²Th, ²³⁸U, and ⁴⁰K radionuclides were 0.465 Bq kg⁻¹, 0.435 Bq kg⁻¹, 2.34 Bq kg⁻¹, and 2.64 Bq kg⁻¹, respectively.

The standard samples used for the energy and efficiency calibration of the gamma spectrometer were obtained from the Korea Research Institute of Standards and Science (KRISS), which provided volumetric liners containing several gamma radionuclides (²⁴¹Am, ¹⁰⁹Cd, ⁵⁷Co, ¹³⁹Ce, ⁵¹Cr, ¹¹³Sn, ⁸⁵Sr, ¹³⁷Cs, ⁸⁸Y, and ⁶⁰Co). Standard volumetric sources were assessed for 30,000 s, and energy corrections and measurement efficiencies were calculated with an analytical program (Gamma Vision, Ortec). Quality control of the natural radionuclides analyzed by gamma spectrometry was performed using the National Institute of Standards and Technology (NIST) standard reference material (SRM 4353A) for Rocky Flats soil, which was assessed under the same conditions as those employed for the surface sediment samples to validate the reliability of the assessments. The activity concentrations of the natural radionuclides analyzed in the SRM soil were in good agreement with the NIST reference values, as follows: 40.2 ± 1.6 Bq kg⁻¹ at ²¹⁴Bi (NIST value 40.6 Bq kg⁻¹), 46.9 ± 1.6 Bq kg⁻¹ at ²¹⁴Pb (NIST value 43.2 Bq kg⁻¹), 46.3 ± 7.1 Bq kg⁻¹ at ²²⁶Ra (NIST value 42.4 Bq kg⁻¹), and 590 ± 13 Bq kg⁻¹ at ⁴⁰K (NIST value 589 Bq kg⁻¹).

2.4. Assessment of Radiological Hazards

The assessment of radiation hazard for humans from ²²⁶Ra, ²³²Th, and ⁴⁰K radionuclides in the sediments was estimated based on the five most widely used indices (Radium Equivalent Activity, Ra_eq; External Hazard Index, H_ex; Internal Hazard Index, H_in; Absorbed Gamma Dose Rate in Air, AGDR; and Annual Outdoor Effective Dose Rate, AEDR_out).

Ra_eq was defined as the radiation estimate that yields the same gamma ray dose rate as radium and is assumed to be the same if ²²⁶Ra,²³²Th, and ⁴⁰K activity concentrations are 370 Bq kg⁻¹, 259 Bq kg⁻¹, and 4810 Bq kg⁻¹, respectively. ²²⁶Ra, ²³²Th, and ⁴⁰K are conveniently used for the comparative assessment of external radiation exposure in sediments with inhomogeneously distributed activity concentration. The following equation [1,38] was used for the assessments:

Ra_eq (Bq kg⁻¹) = C_Ra + 1.43C_Th + 0.077C_K

(2)

where C_Ra, C_Th, and C_K represent the measured ²²⁶Ra, ²³²Th, and ⁴⁰K activity concentrations (Bq kg⁻¹), respectively, and the acceptable limit for the Ra_eq values in surface sediments containing them was 370 Bq kg⁻¹.

H_ex is used to assess the hazard of natural gamma radiation. Its use is proposed to restrict the allowable dose equivalent for the public to 1 mSv y⁻¹ [39]. A value of H_ex less than 1 is considered to reflect a minor radiation hazard. It was calculated via the following equation [1,38]:

H_ex = C_Ra/370 + C_Th/259 + C_k/4810

(3)

where C_Ra, C_Th, and C_K represent the measured ²²⁶Ra, ²³²Th, and ⁴⁰K activity concentrations (Bq kg⁻¹), respectively.

H_in refers to internal exposure to radon and its decay products and is used to assess the radiological hazard stemming from the inhalation of the short-lived radionuclides radon (²²²Rn) and thoron (²²⁰Rn). If the value of H_in is lower than 1, the radiation hazard is considered minor. It was calculated via the following equation [1,38]:

H_in = C_Ra/185 + C_Th/259 + C_k/4810

(4)

where C_Ra, C_Th, and C_K represent the measured ²²⁶Ra, ²³²Th, and ⁴⁰K activity concentrations (Bq kg⁻¹), respectively.

AGDR is characterized as the external exposure to gamma radiation from terrestrial radionuclides at a height of 1 m above the earth’s surface, with ²²⁶Ra, ²³²Th, and ⁴⁰K being the highest contributors due to their close relationship with activity concentrations [8,40]. AGDR was used to derive conversion factors from the ²²⁶Ra, ²³²Th, and ⁴⁰K activity concentrations (0.462 nGy h⁻¹ per Bq kg⁻¹ for ²²⁶Ra, 0.604 nGy h⁻¹ per Bq kg⁻¹ for ²³²Th, and 0.0417 nGy h⁻¹ per Bq kg⁻¹ for ⁴⁰K), assuming that the ²²⁶Ra, ²³²Th, and ⁴⁰K radionuclides were uniformly distributed, and the ²³⁸U and ²³²Th decay series were in secular equilibrium, as shown in the following equation [1]:

AGDR (nGy h⁻¹) = 0.462C_Ra + 0.604C_Th + 0.0417C_K

(5)

where C_Ra, C_Th, and C_K represent the measured activity concentrations (Bq kg⁻¹) of ²²⁶Ra, ²³²Th, and ⁴⁰K, respectively.

As per the recommendation of the ICRP, the AEDR_out should be used in the units of the dose equivalent (Sv) to optimize protection from radiation exposure [39]. Therefore, a conversion coefficient (0.7) and an outdoor occupancy factor (0.2) based on the absorbed gamma dose rate in the air should be applied. It was calculated using the following equation [1]:

AEDR_out (µSv y⁻¹) = AGDR (nGy h⁻¹) × 8760 (h y⁻¹) × 0.7 (Sv Gy⁻¹) × 0.2 × 10⁻³

(6)

3. Results and Discussion

3.1. Activity Concentration of ²²⁶Ra, ²³²Th, ²³⁸U, and ⁴⁰K

The activity concentrations of ²²⁶Ra, ²³²Th, ²³⁸U, and ⁴⁰K radionuclides in surface sediments collected from major river watersheds in South Korea are shown in

Table 1. Activity concentrations of ²²⁶Ra, ²³²Th, ²³⁸U, and ⁴⁰K in surface sediments collected from major river watersheds in Korea.

Watershed	226Ra (Bq kg−1)	232Th (Bq kg−1)	238U (Bq kg−1)	40K (Bq kg−1)
Han River (n = 15)	28.2 ± 15.2 (8.73–65.4) a	60.9 ± 24.7 (23.6–103)	22.4 ± 11.4 (9.25–46.6)	989 ± 118 (881–1319)
Geum River (n = 13)	33.8 ± 9.7 (23.6–58.0)	65.6 ± 20.2 (41.4–111)	30.4 ± 10.5 (19.8–59.1)	1016 ± 111 (870–1257)
Nakdong River (n = 16)	24.2 ± 11.2 (11.3–46.7)	45.3 ± 25.3 (16.0–112)	19.7 ± 12.0 (<2.64 b–46.2)	858 ± 62 (763–1004)
Yeongsan River (n = 9)	40.3 ± 15.3 (19.3–59.9)	62.1 ± 22.1 (36.4–93.1)	36.5 ± 9.9 (21.7–50.2)	763 ± 100 (581–915)
Seomjin River (n = 5)	37.1 ± 19.5 (16.9–64.9)	64.3 ± 33.0 (29.4–104)	35.2 ± 24.3 (15.9–76.4)	837 ± 134 (608–959)
Total (n = 58)	31.0 ± 14.2 (8.73–65.4)	58.1 ± 24.8 (16.0–112)	26.8 ± 13.8 (<2.64–76.4)	911 ± 136 (581–1319)

Notes: ^a Mean ± standard deviation (min–max), ^b less than minimum detectable activity.

.

The mean activity concentrations of ²²⁶Ra, ²³²Th, ²³⁸U, and ⁴⁰K radionuclides in the surface sediments of all watersheds were 31.0 Bq kg⁻¹, 58.1 Bq kg⁻¹, 26.8 Bq kg⁻¹, and 911 Bq kg⁻¹, respectively, in the order of ⁴⁰K > ²³²Th > ²²⁶Ra > ²³⁸U. The mean activity concentrations of these radionuclides across the watersheds were also consistent with the overall trend of ⁴⁰K > ²³²Th > ²²⁶Ra > ²³⁸U, with the ⁴⁰K radionuclide being the most dominant. These results are consistent with the natural radionuclide concentrations in sediments from the east coast of India and the Yangtze estuary in China (⁴⁰K > ²³²Th > ²³⁸U) [8,19]. In general, potassium is the most abundant element in the earth’s crust compared to thorium and uranium. Thorium is insoluble and accumulates in particles relatively better than uranium [8,41,42]. The mean activity concentrations of these radionuclides in the major river watersheds were as follows: 28.2 Bq kg⁻¹, 60.9 Bq kg⁻¹, 22.4 Bq kg⁻¹, and 989 Bq kg⁻¹ for HR; 33.8 Bq kg⁻¹, 65.6 Bq kg⁻¹, 30.4 Bq kg⁻¹, and 1016 Bq kg⁻¹ for GR; 24.2 Bq kg⁻¹, 45.3 Bq kg⁻¹, 19.7 Bq kg⁻¹, and 858 Bq kg⁻¹ for NR; 40.3 Bq kg⁻¹, 62.1 Bq kg⁻¹, 36.5 Bq kg⁻¹, and 763 Bq kg⁻¹ for YR; and 37.1 Bq kg⁻¹, 64.3 Bq kg⁻¹, 35.2 Bq kg⁻¹, and 837 Bq kg⁻¹ for SR. ²²⁶Ra, ²³²Th, ²³⁸U, and ⁴⁰K exhibited the mean activity concentrations in the following order: YR ≈ SR ≈ GR > HR > NR, GR ≈ SR ≈ YR ≈ HR > NR, YR ≈ SR > GR > HR > NR, and GR ≈ HR > NR ≈ SR ≈ YR, respectively. The mean activity concentrations of ²²⁶Ra, ²³²Th, and ²³⁸U tended to be lower in the HR and NR watersheds, whereas that of ⁴⁰K mean tended to be lower in the SR and YR watersheds.

A comparison with studies from other countries was conducted to validate the levels of ²²⁶Ra, ²³²Th, ²³⁸U, and ⁴⁰K radionuclide activity concentrations investigated in this study. The results are presented in

Table 2. Comparison of activity concentrations of ²²⁶Ra, ²³²Th, ²³⁸U, and ⁴⁰K in surface sediments collected from major river watersheds in Korea and other countries in the world.

Country	River/Lake/ Estuary	226Ra (Bq kg−1)	232Th (Bq kg−1)	238U (Bq kg−1)	40K (Bq kg−1)	Reference
Algeria	Beni Haroun	24.7 (9–66) a	26.0 (14–37)	- b	208 (177–288)	[26]
Bangladesh	Karnaphuli	35.9 (18.4–85.2)	65.5 (50.8–88.4)	37.2 (20.0–89.7)	272 (217–320)	[27]
	Shango	27.8 (24.0–31.9)	57.5 (52.4–61.7)	25.4 (21.6–28.3)	255 (212–292)	[27]
China	Wei	21.8 (10.4–39.9)	33.1 (15.3–54.8)	-	833 (515–1176)	[12]
	Yangtze	24.3 (13.7–52.3)	40.9 (26.1–71.9)	32.8 (14.1–62.3)	628 (392–898)	[19]
Egypt	Nile	52.0	76.2	-	352	[32]
India	Ponnaiyar	-	46.9 (<MDA c–106)	7.31 (<MDA–11.6)	384 (201–468)	[31]
	Bharathapuzha	41.9 (21.2–66.0)	54.9 (33.5–93.1)	-	478 (232–900)	[18]
Nigeria	Oguta	47.9 (7.23–135)	55.4 (<MDA–140)	-	1023 (189–2401)	[17]
	Ogun	12.7 (5.57–20.4)	11.8 (5.04–23.1)	-	499 (371–608)	[45]
Pakistan	Gilgit & Indus	50.7 (21.4–111)	70.2 (11.7–172)	-	532 (174–825)	[7]
Thailand	Chao Phraya	-	64.9 (60.7–69.1)	60.2 (55.3–65.2)	432 (393–478)	[30]
Turkey	Fırtına	49.9 (15–167)	38 (17–87)	39 (16–113)	573 (51–1605)	[28]
	Çoruh	-	18.3 (8.4–46.8)	11.4 (<3.5 c–29.9)	510 (129–969)	[13]
USA	Reedy	21.4 (11.4–41.2)	45.3 (13.0–186)	37.8 (11.1–74.2)	609 (386–1047)	[29]
Worldwide		40 (8–160)	35 (4–130)	35 (4–140)	370 (100–700)	[1]
Korea		31.0 (8.73–65.4)	58.1 (16.0–112)	26.8 (<2.64–76.4)	911 (581–1319)	This study

Notes: ^a Mean (min–max), ^b No data, ^c less than minimum detectable activity (MDA).

. The activity concentrations of ²²⁶Ra, ²³²Th, ²³⁸U, and ⁴⁰K radionuclides reported in different countries varied in the range of 12.7–52.0 Bq kg⁻¹, 11.8–76.2 Bq kg⁻¹, 7.31–60.2 Bq kg⁻¹, and 208–1023 Bq kg⁻¹, respectively. The results of this study are within the range of these activity concentrations observed in different countries. Furthermore, compared with the global average values, low levels of the ²²⁶Ra and ²³⁸U radionuclides and high levels of the ²³²Th and ⁴⁰K radionuclides were observed here. In particular, the ⁴⁰K radionuclide activity in this study was notably higher than the levels found in many countries (except for a few countries) and more than twice as high as the global average. These ⁴⁰K radionuclide activity concentrations are consistent with those found in Korean soils, which supports the high levels observed [35,43]. Furthermore, these levels are also closely related to the geological characteristics of Korea, which is dominated by granite distribution [44].

3.2. Spatial and Frequency Distribution of ²²⁶Ra, ²³²Th, ²³⁸U, and ⁴⁰K

The spatial and frequency distributions of ²²⁶Ra, ²³²Th, ²³⁸U, and ⁴⁰K radionuclides in the surface sediments collected from major river watersheds in Korea are shown in Figure 2 and Figure 3.

According to the spatial distribution of ²²⁶Ra, ²³²Th, ²³⁸U, and ⁴⁰K radionuclides (Figure 2), the highest activity concentrations of these radionuclides were as follows: 65.4 Bq kg⁻¹ at site H4 in the HR watershed, 112 Bq kg⁻¹ at site N5 in the NR watershed, 76.4 Bq kg⁻¹ at site S3 in the SR watershed, and 1319 Bq kg⁻¹ at site H9 in the HR watershed. In contrast, the lowest activity concentrations of these radionuclides were recorded as follows: 8.73 Bq kg⁻¹ at site H3 in the HR watershed, 16.0 Bq kg⁻¹ at site N10 in the NR watershed, below the minimum detectable activity at sites N14 and N16 in the NR watershed, and 581 Bq kg⁻¹ at site Y3 in the YR watershed. Based on the calculated skewness (²²⁶Ra, 0.72; ²³²Th, 0.44; ²³⁸U, 0.92; ⁴⁰K, 0.43) and kurtosis (²²⁶Ra, −0.14; ²³²Th, −0.44; ²³⁸U, 2.01; ⁴⁰K, 1.38) values, the activity concentrations of these radionuclides generally showed an asymmetric and peaked distribution, but with a high-frequency distribution centered around the mean value (Figure 3).

The ²²⁶Ra, ²³²Th, and ²³⁸U radionuclides all showed highly correlated spatial distributions, whereas the ⁴⁰K radionuclide showed no association with these radionuclides (Figure 2). The correlation between the radionuclides presented in Figure 4 also reflected a trend consistent with their spatial distribution. The correlation coefficients (r) were 0.887 (p < 0.01) for ²³²Th–²²⁶Ra, 0.838 (p < 0.01) for ²³⁸U–²³²Th, and 0.928 (p < 0.01) for ²³⁸U–²²⁶Ra. These relationships have been reported to be closely related to the geologic characteristics of the study area [8,46,47,48].

3.3. Assessment of Radiological Hazards

The radiation hazards according to five indices (Ra_eq, H_ex, H_in, AGDR, and AEDR_out) were calculated based on the activity concentrations of ²²⁶Ra, ²³²Th, and ⁴⁰K radionuclides in the surface sediments collected from major river watersheds in Korea. The results are presented in

Table 3. Radiological hazard indices calculated based on the activity concentrations of ²²⁶Ra, ²³²Th, and ⁴⁰K in surface sediments collected from major river watersheds in Korea.

Watershed	Raeq a (Bq kg−1)	Hex b	Hin c	AGDR d (nGy h−1)	AEDRout e (µSv y−1)
Han River (n = 15)	191 ± 48 (116–281) f	0.517 ± 0.130 (0.314–0.759)	0.593 ± 0.168 (0.337–0.936)	91.0 ± 20.9 (58.2–130)	112 ± 26 (71.4–159)
Geum River (n = 13)	206 ± 36 (164–284)	0.556 ± 0.097 (0.444–0.766)	0.647 ± 0.119 (0.507–0.923)	97.6 ± 15.5 (79.0–130)	120 ± 19 (96.9–160)
Nakdong River (n = 16)	155 ± 44 (100–266)	0.419 ± 0.119 (0.270–0.717)	0.484 ± 0.148 (0.304–0.844)	74.3 ± 18.7 (50.3–121)	91.2 ± 22.9 (61.7–148)
Yeongsan River (n = 9)	188 ± 50 (122–253)	0.507 ± 0.136 (0.329–0.683)	0.616 ± 0.176 (0.390–0.845)	87.9 ± 22.3 (57.7–118)	108 ± 27 (70.8–144)
Seomjin River (n = 5)	194 ± 62 (126–282)	0.523 ± 0.169 (0.340–0.762)	0.623 ± 0.221 (0.386–0.938)	90.9 ± 26.8 (61.8–130)	111 ± 33 (75.8–159)
Total (n = 58)	184 ± 48 (99.8–284)	0.498 ± 0.131 (0.270–0.766)	0.581 ± 0.166 (0.304–0.938)	87.4 ± 21.1 (50.3–130)	107 ± 26 (61.7–160)

Notes: ^a Radium equivalent activity, ^b external hazard index, ^c Internal hazard index, ^d absorbed gamma dose rate in air, ^e annual effective dose rate outdoors, ^f mean (min–max).

.

The mean concentrations of Ra_eq in the HR, GR, NR, YR, and SR watersheds were 191 Bq kg⁻¹, 206 Bq kg⁻¹, 155 Bq kg⁻¹, 188 Bq kg⁻¹, and 194 Bq kg⁻¹, respectively. The corresponding H_ex values were 0.517, 0.556, 0.419, 0.507, and 0.523, the H_in values were 0.593, 0.647, 0.484, 0.616, and 0.623, the AGDR values were 91.0 nGy h⁻¹, 97.6 nGy h⁻¹, 74.3 nGy h⁻¹, 87.9 nGy h⁻¹, and 90.9 nGy h⁻¹, and the AEDR_out values were 112 µSv y⁻¹, 120 µSv y⁻¹, 91.2 µSv y⁻¹, 108 µSv y⁻¹, and 111 µSv y⁻¹, respectively. The values of all five indices were highest in the GR watershed and lowest in the NR watershed. Their mean values for all sites in the five major river watersheds were evaluated as 184 Bq kg⁻¹, 0.498, 0.581, 87.4 nGy h⁻¹, and 107 µSv y⁻¹, respectively.

The mean value of Ra_eq obtained in this study is lower than the recommended maximum value (370 Bq kg⁻¹), and the mean values of H_ex and H_in also do not exceed the recommended value (1), indicating that the radiation hazard was negligible [1,38]. The AGDR mean value of 87.4 nGy h⁻¹ is approximately 1.5-fold higher than the global mean value presented by the UNSCEAR (59 nGy h⁻¹) [1]. However, these values are within the range of background values measured by a high-pressure ionization chamber on land in Korea (23–148 nGy h⁻¹) [44] and those investigated in river or lake sediments in several countries [7,12,18,19,26,28,30,31]. The mean value of AEDR_out calculated based on AGDR is 107 µSv y⁻¹, which is approximately 1.5-fold higher (70 µSv y⁻¹) than the global mean value reported by the UNSCEAR [1]; however, the value is within the range of values reported from different countries [12,18,19,26,28,30]. Nevertheless, the values reported in this study are remarkably lower than those recommended by the ICRP (1000 µSv y⁻¹) and the background values measured by a high-pressure ionization chamber at the Korea Institute of Nuclear Safety (KINS) (438–2628 µSv y⁻¹), indicating a negligible radiation hazard [39,49].

When the contribution of ²²⁶Ra, ²³²Th, and ⁴⁰K radionuclides to the mean value of AEDR_out was evaluated (Figure 5), we found that the contribution to the total watershed was dominated by ⁴⁰K radionuclides, in the following order ⁴⁰K (45%) > ²³²Th (38%) > ²²⁶Ra (17%). In general, the ²³⁸U and ²³²Th series and the ⁴⁰K radionuclides had a high contribution to the AEDR_out value [1], which is consistent with the results of many studies conducted on soils [36,50,51,52,53]. The contribution of these radionuclides by major river watershed was as follows: ⁴⁰K (48%) > ²³²Th (39%) > ²²⁶Ra (13%) for HR; ⁴⁰K (44%) > ²³²Th (40%) > ²²⁶Ra (16%) for GR; ⁴⁰K (56%) > ²³²Th (30%) > ²²⁶Ra (14%) for NR; ²³²Th (42%) > ⁴⁰K (38%) > ²²⁶Ra (20%) for YR; and ⁴⁰K (42%) > ²³²Th (40%) > ²²⁶Ra (18%) for SR. Although the overall radionuclide contributions were consistent among all watersheds, the contribution of ⁴⁰K radionuclides was higher in the NR than in the other watersheds, and the contribution of ²³²Th radionuclides was the highest in the YR watershed. Overall, the contribution to the AEDR_out value was dominated by the ⁴⁰K and ²³²Th radionuclides. These results tend to be consistent with those obtained in previous studies, which are as follows: China Wei River, ⁴⁰K (50%) > ²³²Th (33%) > ²²⁶Ra (17%); and India Bharathapuzha River, ²³²Th (49%) > ⁴⁰K (28%) > ²²⁶Ra (23%) [12,18]. Notably, the sediments from streams and rivers around Kanyakumari, India, where background radiation is high, indicated a very high contribution of ²³²Th radionuclides, up to approximately 87% [25]. The differences in the contribution of these radionuclides to the AEDR_out depend on the activity concentrations in the study area, and the activity concentrations of these radionuclides are closely related to geologic characteristics. Future studies should aim to understand the geology around the watersheds to clearly evaluate the differences in the contribution of these radionuclides.

4. Conclusions

In this study, we determined, for the first time, the activity concentrations of ²²⁶Ra,²³²Th, ²³⁸U, and ⁴⁰K radionuclides in the surface sediments of major river watersheds in Korea and evaluated the radiation hazards stemming from these radionuclide activity concentrations. The mean activity concentrations of ²²⁶Ra, ²³²Th, ²³⁸U, and ⁴⁰K radionuclides in surface sediments of all watersheds were 31.0 Bq kg⁻¹, 58.1 Bq kg⁻¹, 26.8 Bq kg⁻¹, and 911 Bq kg⁻¹, respectively. Spatially, the mean activity concentrations of ²²⁶Ra, ²³²Th, and ²³⁸U radionuclides tended to be higher in the HR and NR watersheds, and the mean activity concentration of ⁴⁰K radionuclides tended to be lower in the SR and YR watersheds. The activity concentration levels of these radionuclides showed low levels of ²²⁶Ra and ²³⁸U and high levels of ²³²Th and ⁴⁰K compared to the global average. In particular, the ⁴⁰K radionuclide activity concentration levels were not only higher than the levels presented in many countries but also more than twice as high as the global average. The ²²⁶Ra, ²³²Th, and ²³⁸U radionuclides showed a highly correlated spatial distribution, whereas the ⁴⁰K radionuclide did not show any association with these radionuclides. The activity concentrations of these radionuclides showed a high-frequency distribution centered around the mean value, although they generally showed an asymmetric and peaked distribution.

In the assessment of radiation hazards (Ra_eq, H_ex, H_in, AGDR, and AEDR_out) by major river watersheds, the mean values for Ra_eq, H_ex, and H_in did not exceed the recommended values presented by the UNSCEAR, indicating a negligible radiation hazard. The mean values of AGDR and AEDR_out were approximately 1.5-fold higher than the global mean values presented by the UNSCEAR. However, these values were remarkably lower than the ICRP-recommended values and the Korean background values, indicating negligible radiation hazards. In contrast, the contributions of the ²²⁶Ra,²³²Th, and ⁴⁰K radionuclides to the AEDR_out mean values were predominantly influenced by the ⁴⁰K radionuclide, in the order of ⁴⁰K (45%) > ²³²Th (38%) > ²²⁶Ra (17%), across all watersheds, and the contributions of these radionuclides by watersheds were also consistent with the results for the whole watersheds. However, the contribution of the ⁴⁰K radionuclide was relatively higher in the NR watershed than in other watersheds, and the contribution of the ²³²Th radionuclide was the highest in the YR watershed. Overall, the contribution to the AEDR_out value was dominated by the ⁴⁰K and ²³²Th radionuclides.

In conclusion, the ²²⁶Ra, ²³²Th, ²³⁸U, and ⁴⁰K radionuclides in surface sediments of major Korean river watersheds showed various concentration distributions, indicating that the radiation hazard from sediments is negligible. Therefore, the results obtained in this study can be used as an important resource for the proper management and treatment of accumulated sediments in various ecosystems, such as terrestrial and marine, in the future.