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Article

Assessment of Soil Radioactivity Associated with Risk and Correlation with Soil Properties near Maanshan Nuclear Power Plant, Taiwan

by
Ting-Chien Chen
1,
Fan-Cheng Zeng
2,
Chih-Chung Lin
1,2,
Yi-Lung Yeh
1,3 and
Wei-Hsiang Huang
1,*
1
Disaster Prevention and Mitigation Technology Research Center, General Research Service Center, National Pingtung University of Science and Technology, Pingtung 91201, Taiwan
2
Department of Environmental Science and Engineering, National Pingtung University of Science and Technology, Pingtung 91201, Taiwan
3
Department of Civil Engineering, National Pingtung University of Science and Technology, Pingtung 91201, Taiwan
*
Author to whom correspondence should be addressed.
Appl. Sci. 2024, 14(20), 9239; https://doi.org/10.3390/app14209239
Submission received: 23 September 2024 / Revised: 7 October 2024 / Accepted: 8 October 2024 / Published: 11 October 2024
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Abstract

:
This study analyzes the concentration of radioactive material in the soil near the Maanshan nuclear power plant (NPP). Out of the thirty samples, only one sample was found to have 137Cs radioactivity measuring 2.58 Bq/kg. The activity concentrations were 77.2–517.7 Bq/kg, 3.9–31.6 Bq/Kg, and 5.3–39.1 Bq/kg, respectively, with mean values of 344.4 Bq/kg, 18.6 Bq/kg, and 26.5 Bq/kg for 40K, 226Ra, and 232Th, respectively. These levels are lower than the global average of soil activity concentrations. The activity concentrations varied, with the highest levels being 7–8 times greater than the lowest levels. Clay content had a positive correlation and sand content had a negative correlation with 40K, 226Ra, and 232Th activity concentrations. The activity concentrations followed a normal distribution for 40K, 226Ra, and 232Th. The activity ratios for 232Th/226Ra, 40K/232Th, and 40K/226Ra were 1.43 ± 0.22, 13.1 ± 1.9, and 18.8 ± 4.1, respectively, and ratios show light minerals in the soils. The average values for external hazard indices (Hex) and radium equivalent activity (Raeq) were 0.22 Bq/kg and 83.0 Bq/kg, respectively, both of which are below the recommended limit values of 1.0 Bq/kg and 370 Bq/kg, respectively. The outdoor absorbed dose rate (DRex) and annual effective dose equivalent (AEDex) were 39.0 nGy/h and 47.8 μSv/y, respectively, both of which are lower than the global soil average of 59 nGy/h and 70 μSv/y, respectively. These results indicate that local residents and tourists are not at significant risk of radiological hazards from the soil. The soil activity concentrations can serve as a baseline for continuous monitoring, even after the Maanshan NPP is decommissioned in 2025.

1. Introduction

It is important to understand that ionizing radiation is a widespread occurrence in our environment [1]. Exposure to this type of radiation from natural sources can have negative effects on both humans and other living organisms [2]. Natural sources account for 80% of human radiation exposure, with soil being the primary source [1]. This is because the soil is made up of minerals from weathered and disintegrated rocks that may contain naturally occurring radioactive nuclides, such as uranium (238U) and thorium (232Th) decay series, as well as potassium (40K) [3,4]. The amount of radiation released by these nuclides is significantly impacted by the geological and geographical characteristics of the soil and by many environmental processes [4,5,6].
It is worth noting that the level of radioactivity in the soil can differ significantly based on the type of rocks that make up the soil [7]. Research has revealed that igneous rocks typically have more radium, such as monazite, whereas sedimentary rocks contain less radium such as limestone [8].
Furthermore, analyzing variations in activity ratios, such as radioactivity ratio of 232Th/226Ra, of natural radionuclides can provide valuable information about geochemical processes in the environment and serve as a gauge for the occurrence of these radionuclides [9].
The Maanshan nuclear power plant (NPP) is situated in the Hengchun Peninsula, within the Kenting National Park (KNP), Taiwan. The Peninsula is surrounded by the Taiwan Strait, Bashi Channel, and the Pacific Ocean. This park is a popular tourist destination, attracting over a million visitors annually [10]. To ensure the safety of residents and visitors, it is crucial to assess the potential radiological hazards that may arise from naturally occurring and man-made radioactive substances within the soil [1]. Studies conducted by Tsai et al. [11] and Huang et al. [7] show that there are varying levels of natural radioactivity in the soil of the Hengchun Peninsula. Some areas have high activity concentrations of 268–390 Bq/kg, 24.0–25.8 Bq/kg, and 20.0–21.0 Bq/kg for 40K, 232Th, and 238U, respectively, while others have low activity concentrations of 17–74 Bq/kg, 2.1–4.0 Bq/kg, and 2.8–3.6 Bq/kg for 40K, 232Th, and 238U, respectively. This variation is due to the different soil compositions found in the region [7]. Additionally, the Maanshan NPP is set to be decommissioned in 2025, and, after decommissioning, there will be ongoing monitoring of environmental media such as soils to ensure that radioactivity levels remain safe [12].
The purpose of this research is to analyze the levels of gamma-ray-emitted natural nuclides, including 40K, 226Ra, and 232Th, as well as the artificial nuclide 137Cs, in soil samples surrounding the Maanshan NPP. To understand the dominate factors of radioactivity, the relationship between radioactivity and soil properties is examined. Through the use of radioactivity ratios, the mineral composition of the soils is investigated. To ensure the safety of both residents and visitors, the radiological hazard indices, such as radium activity equivalent (Raeq), external hazard index (Hex), external absorbed dose rate (DRex), and external annual effective dose equivalent (AEDex) are calculated [13,14]. The results of this study on soil radioactivity can serve as a baseline for future monitoring of soil radioactivity after the decommissioning of the Maanshan NPP.

2. Research Method and Material

2.1. Sampling Collection

The soil samples were collected from the area surrounding the Maanshan NPP. Figure 1 shows the geological map of the sampling locations and schematic sample sites and codes. In total, 30 soil samples were collected from the study area. The soil samples were collected from the area surrounding the Maanshan NPP on 11 July 2022. All the collected samples were stored in polyethylene bags and then transported to the laboratory for further analysis.
The soil sampling for this study was planned based on the geographical location and environment surrounding the Maanshan NPP. Owing to its coastal position and location within the Kenting National Park, the selection of sampling points was significantly influenced by these geographic features. The plant is surrounded by the Bashi Channel on one side, with roads and residential areas on the other three sides. Furthermore, the well-known irrigation wetland, Longluan Lake Wetland, is located north of the plant. We collected samples from areas where the soil was undisturbed by human activity, such as along unpaved roadsides and agricultural lands at a suitable distance from residential areas. This approach resulted in an irregular pattern of sampling locations.

2.2. Sample Preprocessing

After collecting the soil samples, they were taken to the laboratory where each sample was placed in a plastic tray and any impurities such as weeds, twigs, and other materials were removed. The soil samples were air dried at room temperature over a week. The dried samples were then pulverized into fine powder and sieved with a 10-mesh sieve (2.0 mm). The sieved dry samples were assessed with respect to soil properties including pH, electrical conductivity (EC), and organic matter (OM). The measuring method followed standard methods [15]. Soil texture was determined using the sieve method. A 10 g soil sample was sieved through a 2 mm sieve. The material retained on a 325-mesh sieve (0.47 mm) was classified as sand (0.47–2 mm). The material that passed through the sieve was washed into a sedimentation cylinder and water was added to make the volume 1000 mL. The settling time required by particles to fall 10 cm was calculated using Stokes’ law. A 25 mL pipette was used to extract 25 mL of clay suspension (<2 μm) at the 10 cm mark. This sample was dried and weighed to determine the clay content. The silt content was calculated by subtracting the sand and clay content from the total weight.
The soil properties are listed in Table 1. The average pH, EC, and OM were 7.20 ± 0.62, 165 ± 83 μS/cm, and 7.67 ± 3.52%, respectively. The soil textures exhibited a wide variation. The percentage ranges were 4.7–83.8%, 5.0–59.1%, and 11.2–66.4% for sand, silt, and clay, respectively. The average percentage order was: sand (47.4 ± 18.7%) > clay (35.0 ± 14.5%) > silt (17.6 ± 10.2%). The soil samples were categorized as clay, sandy clay, sandy clay loam, loamy sand, and sandy loam according to USDA classification.

2.3. Radionuclide Activity Measurement

The sieved soil (about 150 g) was transferred into an airtight plastic container. The samples were kept one month before the analysis under airtight conditions to allow secular equilibrium between thorium and radium and their decay products. All the samples were measured using a high purity germanium (HPGe) gamma spectrometry (ORTEC, GEM40P4-79-SMP, Oak Ridge, TN, USA) with a relative efficiency of 40% and a gamma vision analysis software. The activities of 40K and 137Cs are quantified by photopeaks of 1460.8 keV (40K) and 661.66 keV (137Cs), respectively. The activities of 226Ra and 232Th are calculated from the γ-rays of their progenies at 609.3 keV–45.4%, 1120.3 keV–14.9%, and 1764.5 keV–15.3% (214Bi/226Ra) and 583.2 keV–85.0% (208Tl/232Th) on the basis of the secular equilibrium of radium and its progenies. The uncertainty originated from counting statistics and was represented as one standard deviation (1δ). The MDAs for each radionuclide are as follows: 16.3 ± 1.8 Bq/kg for 40K, 3.1 ± 0.5 Bq/kg for 232Th, 2.2 ± 0.3 Bq/kg for 226Ra, 3.6 ± 0.5 Bq/kg for 228Ra, and 1.1 Bq/kg for 137Cs.

2.4. Radiological Hazard Indices

To ensure the safety of both residents and visitors, radiological hazard indices, such as the external hazard index (Hex, Equation (1)), radium equivalent activity (Raeq, Equation (2)), external absorbed gamma dose rate (DRex, Equation (3)), and external annual effective dose equivalent (AEDex, Equation (4)), were calculated for the studied soil samples [11,13,14,16,17,18,19]. Along with the indices, their recommended limit values and global soil average values are listed in Table 2.
Where CK, CRa, and CTh are the activity concentrations (Bq/kg) of 40K, 226Ra, and 232Th, respectively. For the annual effective dose equivalent, the conversion coefficient from absorbed dose in air to effective dose received by adults 0.2 for the outdoor occupancy factor.

2.5. Statistical Analysis

The statistical analyses were conducted using S-Plus (V 6.1, TIBCO Software Inc., Palo Alto, CA, USA) and Microsoft Excel (2010). A significance level of 0.05 was set. Descriptive statistics, such as mean, standard deviation, skewness, kurtosis, and median values, were calculated using both Excel (2010) and S-Plus (6.2). The results from both software packages were consistent across all descriptive statistical measures. Linear regression analyses, normality tests using the Kolmogorov–Smirnov (K–S) method [20,21], t-tests, and ANOVA analyses were performed using S-Plus.

3. Results

3.1. Activity Concentrations and Correlation of Radionuclides

Out of all the sites, only one sample from S8 displayed a 137Cs activity concentration of 2.58 Bq/kg. This level was considerably lower than the soil radioactivity in the region affected by radiation fall-out from the nuclear weapon testing and lower than that caused by the Chernobyl NPP accident [22,23]. The 137Cs concentrations affected by radiation fall-out are normally in the range of 15–30 Bq/kg [13]. For example, the reported mean 137Cs concentrations of soil range from 7.1 Bq/kg to 30.5 Bq/kg [2,4,23,24,25,26]. In Taiwan, the average detected soil 137Cs concentrations were less than 10 Bq/kg in the north east area [16,27], with the monitoring area located in the south tip of Taiwan which is less affected by radiation fall-out.
The activity concentrations of 40K, 226Ra, 228Ra, and 232Th of the soil samples are listed in Table 3. In all soil samples tested, there were detectable concentrations of 40K, 226Ra, 228Ra, and 232Th. The activity concentrations of 228Ra and 232Th showed a strong positive correlation (r = 0.97, p < 0.001, as seen in Figure 2a). The average activity ratio of 228Ra/232Th was 0.96 ± 0.10, with the ratio close to 1.0 indicating that the thorium series was in a state of secular equilibrium [28,29]. Since 228Ra is the progeny of the thorium decay series, the 232Th activity concentrations were used to represent the thorium series in this study. Additionally, as shown in Figure 2a, there was a strong positive linear correlation between 232Th activity concentrations and 226Ra (r = 0.79, p < 0.001).
Additionally, Figure 2b shows that there was a strong positive correlation (r = 0.89, p < 0.001) between the activity concentrations of 232Th and 40K, as well as a positive correlation (r = 0.66, p < 0.001) between 226Ra and 40K. This correlation suggests that these radionuclides may have come from the same original sources and that they coexist at the sampling sites [6,23,30]. It is also possible that these nuclides respond similarly to the soil’s geochemical behavior and to other environmental processes, as they are commonly distributed in the environment [10,14,30]. Some studies indicate positive correlations among the nuclides 226Ra, 232Th, and 40K at each concentration [10,14,23,30].
The activity concentrations ranged from 77.2 Bq/kg to 517.7 Bq/kg for 40K, 3.9 Bq/kg to 31.6 Bq/kg for 226Ra, and 5.3 Bq/kg to 39.1 Bq/kg for 232Th. The mean and standard deviation values were 344.4 ± 116.1 Bq/kg, 18.6 ± 5.5 Bq/kg, and 26.5 ± 8.4 Bq/kg for 40K, 226Ra, and 232Th, respectively. The activity concentrations of the radionuclides in the present study were lower than the world soil average activity concentrations of 420 Bq/kg, 32 Bq/kg, and 45 Bq/kg for 40K, 226Ra, and 232Th, respectively [14]. In these soil samples, 40K had significantly higher activity concentrations than 232Th and 226Ra, while 232Th had significantly higher activity concentrations than 226Ra (p < 0.001). The abundance of potassium in the earth’s crust [7,31] means that 40K was responsible for the majority (82.2–90.4%) of the activity concentrations found in the soil samples [1,11,18,29,32]. The levels of 232Th and 226Ra in the soil are impacted by the minerals present [1,3,7,32]. Some soil samples may have higher concentrations of 232Th, while others may have higher concentrations of 226Ra [4,18,33,34].
The levels of activity in the soil samples S14 and S15 were notably lower than those in the other samples. These two soil samples were collected near the coastline of the Bashi Channel, and their composition may be similar to that of the beach sand surrounding the Maanshan NPP, as shown in Figure 1. The beach sand is made up of debris from shells and coral reefs, resulting in significantly lower activity levels than soil. The white beach sands contain predominantly a mix of coral reef, shell, and light minerals, hence natural nuclides have low activity concentrations. Activity concentrations ranging from 10.7 Bq/kg to 58.3 Bq/kg, from 2.7 Bq/kg to 7.1 Bq/kg, and from 2.8 Bq/kg to 11.0 Bq/kg for 40K, 226Ra, and 232Th, respectively, have been found in beach sands [35,36,37]. When mixed with soil, the sand causes a considerable decrease in activity levels.
Soil activity concentrations vary across the world and are affected by factors such as mineral composition, particle size, geological conditions, and environmental processes specific to each region [5,29,38,39,40]. The size of soil particles in the samples varied extensively, as presented in Table 1. The average percentage of clay was 35.0 ± 14.5%, with a coefficient of variation (CV) of 41%. On the other hand, the mean percentage of sand was 47.4 ± 18.7%, with a CV of 39%. The sampled area was situated in the Maanshan Formation, which comprises sedimentary layers ranging from the Pliocene to the Pleistocene. These layers consist of limestone clasts, blue–gray shale, and gray sandstone, all of which contain diverse fossils such as shellfish and foraminifera [10]. The sedimentary soil found in this area has low concentrations of natural radionuclides [7,11].
Maanshan NPP nears the coastline of the Bashi Channel, and the sampling area is relatively small, covering less than 10 km2. Based on the variance in radioactivity, the sample seemed to contain a mix of soil, debris from shellfish, and foraminifera. Tsai et al. [11] measured the radioactivity of soils near nuclear power plants and storage facilities in Taiwan. Specifically, two soil samples were obtained from the vicinity of the Maanshan NPP. The radioactivity of one sample was 21.1 Bq/kg, 25.8 Bq/kg, and 268.3 Bq/kg and the radioactivity of the other sample was 3.6 Bq/kg, 4.0 Bq/kg, and 73.6 Bq/kg for 238U, 232Th, and 40K, respectively. The high radioactivity of the soil sample was similar to the average radioactivity in the current study and the low radioactivity sample was similar to the radioactivity measured in samples S14 and S15 in the current study. Hence, the activity concentrations were lower than in those in the soil from other areas in Taiwan [1,4,11,27,33,41,42,43].

3.2. Descriptive Statistics and Frequency Distribution of Activity Concentrations

The descriptive statistics were used to infer the types of frequency distributions of the radioactivity [6,18,43,44]. The statistical parameters included the mean, standard deviation (SD), minimum, maximum, median, coefficients of variation (CV), skewness, kurtosis, and p-values from a normality test. The computing methods for skewness (G1) and kurtosis (G2) followed Equations (5) and (6) [45]:
G 1 = { n n − 1 } n − 2 g 1
G 2 = n − 1 ( n − 2 ) ( n − 3 ) { n + 1 g 2 + 6 }
where g1 and g2 are traditional measures of skewness and kurtosis, respectively, and n is the sample size.
Table 4 lists the descriptive statistics of the activity concentrations for 40K, 226Ra, and 232Th. The mean and SD ranges are discussed in Section 3.1. To determine the spread of a probability distribution, the CV percentage was obtained by dividing the standard deviation by the mean value [6,46]. A smaller coefficient of variation implies less deviation in the data, translating into a lower risk with respect to the data. In this study, the CV percentages of the samples ranged from 29.5% to 33.7%, indicating moderate deviation in the activity concentrations [6]. This moderate deviation in radioactivity was due to certain samples, such as S14, S15, and S29, having significantly lower activity levels compared to the mean values.
To assess the degree of peakedness in activity concentrations compared to a normal distribution, the kurtosis parameter can be utilized [4,25,47]. In this study, the kurtosis values of three types of radioactivity were analyzed, and they were all positive, indicating a higher peak than the normal curve. The kurtosis values for 40K and 232Th were close to 0, while the kurtosis value for 226Ra was 2.79 due to a concentration of activity levels within a relatively small range. Specifically, 80% (24/30) of the 226Ra activity concentrations was found to fall between 15.0 Bq/kg and 25.0 Bq/kg.
The skewness parameter determines whether the activity concentration is symmetrically spread around the mean value, with positive and negative values indicating right and left tails of activity concentrations, respectively [4,25,47]. In the present study, the skewness values for the activity concentrations of three nuclides were negative (−0.80 to −1.04), indicating that the radioactivity was slightly skewed to the left of the mean.
Figure 3a–c shows the frequency distribution and the cumulative percentage curves of 40K, 232Th, and 226Ra activity concentrations. To test the normality of the activity concentration, the K–S method was employed. The results indicate that the activity concentrations were distributed normally, with significant values of normality test exceeding 0.05, as shown in Table 4. The activity concentration for the three radionuclides was mainly concentrated in the middle data. Around 80% of the 226Ra samples had activity concentrations in the range of 15–25 Bq/kg, 83% of the 232Th samples had activity concentrations in the range of 20–36 Bq/kg, and 80% of 40K samples had activity concentrations in the range of 250–500 Bq/kg. The normality of the distribution of 40K, 232Th, and 226Ra was reflected by the small values of kurtosis and skewness (except for 226Ra, which had a relatively high kurtosis value, Table 4) [4,25,34].

3.3. The Activity Concentrations Compared to Other Studies

Table 5 lists the radioactivity of natural nuclides from the present study and from previous studies in Taiwan, as well as from selected studies conducted in other countries. By comparing the soil activity concentrations across Taiwan, our results were found to be comparable to those obtained from soil surrounding a coal-fired power plant [48], soil in the whole island [7], and soil from a grass field [49]. However, the radioactivity levels were lower than those reported in most previous studies in Taiwan [1,7,11,16,41,48].
In this study, the levels of radioactivity for 40K, 226Ra, and 232Th were found to be similar to the levels found in the soil from several other countries [2,3,5,25,32,44,50], but lower than the levels found in some other areas [4,33,42,51]. Overall, the natural radioactivity levels observed were lower than the world average of 420 Bq/kg, 32 Bq/kg, and 45 Bq/kg for 40K, 226Ra, and 232Th, respectively [14]. This lower radioactivity can be attributed to the composition of the soil in the study area. The soil’s complex composition resulted in the high variability in radioactivity levels and in relatively lower levels compared to the world average.

3.4. Correlation of Activity Concentrations with Soil Properties

Table 6 lists the correlation coefficients between activity concentrations and soil properties. The results show that there was no significant correlation between activity concentrations and pH, EC, OM, and silt content (p > 0.05), respectively, except for 226Ra which had a positive correlation with EC (p = 0.03). The amount of soluble salts in the soil can be determined by measuring the EC content of the soil solution. The presence of 226Ra in the soil suggests that it may have been dissolved under highly soluble salt conditions [52]. The activity concentrations had a significantly positive correlation with clay content and a negative correlation with sand content. This is because the fine particles of clay contain mineral lattices with many nuclides, and the high specific surface area of the fine clay particles provides a stronger adsorption capacity for nuclides [22,26,39,40,53]. Some studies have found a strong correlation between clay content and activity concentrations [38]. While soil organic matter can affect radiation activity [3,17,30,39], this study did not find a significant correlation between OM and soil radioactivity, as it was the case for references [5,38]. This could be due to the complexity of the source of organic matter in the sampling area [5,7,11,30].

3.5. Activity Ratios of Activity Concentration

The activity ratio of 232Th to 226Ra can indicate the origin and stability of the nuclides within the 232Th and 238U decay series [9,30,42]. Furthermore, the activity ratios of 40K/232Th and 40K/226Ra can be used to evaluate the presence of heavy or light minerals in the sampling sites [9,54].
Figure 4a–c displays the activity ratios of 232Th/226Ra, 40K/232Th, and 40K/226Ra, respectively. The activity ratios of 232Th/226Ra ranged from 0.82 to 1.86, with an average value of 1.43 ± 0.22, as shown in Figure 4a. The majority of the ratios (90%) were greater than 1.20, with the exception of the S10 sample which had a ratio of less than 1.0. The activity ratios had a CV value of 15.6%, which indicates a weak variation of ratios. The 232Th/226Ra ratio was greater than 1.0, signifying that the thorium and uranium decay series were at disequilibrium and that Th-bearing minerals were higher than Ra-bearing minerals [9,54,55]. The average activity ratio was close to the ratio of the world soil average of 1.41, with mean concentrations of 45 Bq/kg and 32 Bq/kg for 232Th and 226Ra, respectively [14]. An average mean activity ratio close to 1.41 has been observed in China [56] and Egypt [30,57]. The ratio depends on the area’s local geological and geochemical settings [9]. Several studies have observed 232Th/226Ra activity ratios higher than those found in the present study [47,52,54,55,58]. In contrast, some studies have reported lower activity ratios than those found in this study [9,32].
The ratios of 40K/232Th (Figure 3b) varied from 8.47 to 15.67, with an average ratio of 13.1 ± 1.9. The CV value of 14.4% indicates low variation. The ratios of 40K/226Ra (Figure 3c) varied between 10.2 and 26.8, with an average ratio of 18.8 ± 4.1. The CV value of 22.1% indicates moderate variation. Additionally, the 40K/232Th and 40K/226Ra ratios in the soil samples were higher than the world average values of 9.3 and 13.1, respectively [14]. The higher ratios suggest that the mineral composition of the soil samples contained light minerals such as sandstone and limestone [9,55]. This confirmed the relatively low activity concentrations of natural radionuclides in comparison to world average concentrations in the soil.

3.6. Radiological Hazard Indices

Figure 5a–d shows the radiological hazard indices, which include the external hazard index (Hex), radium equivalent activity (Raeq), absorbed dose rate (DRex), and annual effective dose equivalent (AEDex). These indices were calculated using the equations listed in Table 2.
The external hazard index is an assessment of the hazard of the natural gamma radiation and is used to detect the radiological suitability of a material [14,19]. The Hex values ranged from 0.05 to 0.31, with an average of 0.22 (Figure 5a), all of which are less than the permissible value of unity.
The radium equivalent activity is widely used as a hazard index, with a formula for comparing the specific activity of materials containing different amounts of 226Ra, 232Th, and 40K [14,19]. It is defined as a single quantity that represents the combined specific activities of 226Ra, 232Th, and 40K, and provides a numerical indicator of an external dose to the public [19]. The Raeq values varied from 17.5 Bq/kg to 116.3 Bq/kg, with a mean of 83.0 Bq/kg (Figure 5b). These values are lower than the recommended limit of 370 Bq/kg [19].
The outdoor absorbed dose rate ranged from 8.3 nGy/h to 54.4 nGy/h, with an average value of 39.0 nGy/h. The calculated annual effective dose equivalent due to gamma radiation ranged from 10.1 μSv/y to 66.7 μSv/y, with a mean of 47.8 μSv/y. All DRex and AEDex values are also lower than the global population weight average of 59 nGy/h and 70 μSv/y, respectively [14]. Samples S14, S15, and S29 had much lower radiological hazard indices due to their low radioactivity.
The radiological risk values in the present study compared to soils around NPPs and nuclear facilities in other countries are listed in Table 7. The values from the present study are lower than those reported by previous studies across Taiwan [11,16], Thailand [17], and India [18] and are similar to those reported from Korea [13]. The radiological hazard indices Hex, Raeq, DRex, and AEDex values indicate that the area does not pose a significant radiological health risk. Therefore, the naturally occurring radionuclides in the soil around the Maanshan NPP do not pose a significant radiological risk to residents and tourists. The data obtained in this study provide useful information on the background radioactivity of the study area.

4. Conclusions

This study examines the levels of natural and artificial radiation in the soil near the Maanshan NPP. The results show that manmade radionuclides have not contaminated the soil. The average activity concentrations of natural nuclides 40K, 226Ra, and 232Th were below the global average activity concentrations in the soil. The activity ratios of 40K/226Ra and 40K/232Th were higher than global activity ratios, suggesting that the soil primarily comprises light minerals. The activity concentrations varied considerably, indicating that the soil composition is complex. However, the activity concentrations of natural nuclides were positively correlated with each other and clay content while they were negatively correlated with sand content. The difference in activity concentrations can be attributed to particle size and soil mineral composition. Some soil samples comprised light minerals, while a few contained shells and coral reef debris. The radiological hazard indices Raeq and Hex were lower than the safe recommended limits of 370 Bq/kg and 1.0, respectively. Additionally, all samples’ DRex and AEDex values were lower than the global average values. These findings suggest that the soil surrounding the Maanshan NPP does not pose a significant radiological risk to residents and tourists. These results provide a foundation for the future monitoring of soil activity concentrations around the NPP, particularly during its decommissioning in 2025.

Author Contributions

Conceptualization, T.-C.C., F.-C.Z., and W.-H.H.; methodology, C.-C.L. and Y.-L.Y.; formal analysis, W.-H.H. and F.-C.Z.; investigation, W.-H.H. and F.-C.Z.; resources, C.-C.L. and Y.-L.Y.; writing—original draft preparation, W.-H.H., F.-C.Z., and T.-C.C.; writing—review and editing, W.-H.H. and T.-C.C. All authors have read and agreed to the published version of the manuscript.

Funding

This research was partially funded by the nuclear safety commission, Taiwan, grant number 111-2001-02-28-05.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The raw data supporting the conclusions of this article will be made available by the authors on request.

Acknowledgments

The authors appreciate the technical assistance from the Radiation Monitoring Center, Nuclear Safety Commission.

Conflicts of Interest

The authors declare no conflicts of interest.

Abbreviations

The full name of abbreviation in the text:
AbbreviationFull Name
RaeqRadium activity equivalent
HexExternal hazard index
DRexExternal absorbed dose rate
AEDexExternal annual effective dose equivalent
PSPresent study
ECElectrical conductivity
OMOrganic matter
USDAUnited States Department of Agriculture
NPPNuclear power plant
KNPKenting National Park
HPGeHigh purity germanium
K-S methodKolmogorov–Smirnov method
CVCoefficient of variation
SDStandard deviation

References

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Applsci 14 09239 g001
Figure 1. The schematic location map of the sampling sites and the corresponding sample codes.
Figure 1. The schematic location map of the sampling sites and the corresponding sample codes.
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Applsci 14 09239 g002
Figure 2. Linear correlation of the activity concentrations of 232Th with 226Ra and 228Ra (a) and of 40K with 232Th and 226Ra (b) in the soil samples.
Figure 2. Linear correlation of the activity concentrations of 232Th with 226Ra and 228Ra (a) and of 40K with 232Th and 226Ra (b) in the soil samples.
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Figure 3. Histogram frequency distribution and cumulative percentage curve of 40K (a), 232Th (b), and 226Ra (c).
Figure 3. Histogram frequency distribution and cumulative percentage curve of 40K (a), 232Th (b), and 226Ra (c).
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Figure 4. The activity ratios of 232Th/226Ra (a), 40K/232Th (b), and 40K/226Ra (c), respectively, in the soil samples.
Figure 4. The activity ratios of 232Th/226Ra (a), 40K/232Th (b), and 40K/226Ra (c), respectively, in the soil samples.
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Figure 5. Radiological hazard indices of soil samples Hex (a), Raeq (b), DRex (c), and AEDex (d), with ‘PS’ indicating the present study.
Figure 5. Radiological hazard indices of soil samples Hex (a), Raeq (b), DRex (c), and AEDex (d), with ‘PS’ indicating the present study.
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Table 1. Soil properties of the samples from the tested sites.
Table 1. Soil properties of the samples from the tested sites.
Soil PropertiespHEC (μS/cm)OM (%)Clay (%)Silt (%)Sand (%)
Mean7.20164.57.6734.9517.6347.42
SD0.6282.73.5214.4810.1518.73
Min5.2057.73.2311.205.034.74
Max7.84433.019.3866.3759.0583.77
Table 2. Equations for the calculation of radiological hazard indices and their corresponding permissible and global average values.
Table 2. Equations for the calculation of radiological hazard indices and their corresponding permissible and global average values.
Radiological Hazard IndicesLimit and Average Values
(1) Hex = CK/4810 + CRa/370 + CTh/2591.0
(2) Raeq (Bq/kg) = 0.077CK + CRa + 1.43CTh370 Bq/kg
(3) DRex (nGy/h) = 0.0417CK + 0.462CRa + 0.604CTh59 nGy/h
(4) AEDex (μSv/y) = DRex × 8760 (h/y) × 0.2 × 0.7(Sv/Gy) × 10−370 μSv/y
Table 3. Activity concentrations of 40K, 226Ra, 228Ra, and 232Th in the soil samples.
Table 3. Activity concentrations of 40K, 226Ra, 228Ra, and 232Th in the soil samples.
Sample NOX CoordinateY Coordinate40K 226Ra232Th228Ra
Bq/kg
S121°58′30.39″ N120°45′02.80″ E485.8 ± 14.321.0 ± 1.334.9 ± 1.739.5 ± 1.7
S221°58′27.76″ N120°45′15.44″ E459.3 ± 13.720.0 ± 1.333.6 ± 2.137.6 ± 2.2
S321°58′23.37″ N120°45′15.44″ E517.7 ± 13.119.3 ± 1.135.8 ± 1.636.1 ± 1.4
S421°58′11.67″ N120°45′15.44″ E353.0 ± 12.318.5 ± 1.422.5 ± 1.924.9 ± 1.9
S521°58′12.71″ N120°45′18.21″ E455.8 ± 12.821.7 ± 1.330.5 ± 1.734.7 ± 1.7
S621°58′09.37″ N120°44′58.23″ E399.5 ± 13.417.6 ± 1.328.8 ± 2.228.7 ± 1.9
S721°57′56.73″ N120°44′36.92″ E200.9 ± 9.519.7 ± 1.323.7 ± 1.426.2 ± 1.4
S821°57′55.65″ N120°44′35.88″ E366.5 ± 12.722.7 ± 1.635.0 ± 2.035.8 ± 1.9
S921°57′23.97″ N120°44′23.99″ E317.3 ± 11.322.5 ± 1.524.3 ± 1.523.8 ± 1.7
S1021°57′13.60″ N120°44′24.43″ E321.8 ± 9.931.6 ± 2.225.9 ± 1.524.5 ± 1.8
S1121°57′05.57″ N120°44′25.40″ E254.5 ± 9.421.8 ± 1.423.0 ± 1.722.2 ± 1.8
S1221°56′53.08″ N120°44′34.47″ E231.3 ± 8.814.8 ± 0.917.0 ± 1.120.2 ± 1.0
S1321°56′39.94″ N120°44′36.16″ E292.3 ± 10.713.8 ± 1.120.3 ± 1.520.6 ± 1.5
S1421°56′49.66″ N120°44′37.32″ E79.8 ± 6.84.2 ± 0.75.8 ± 0.96.6 ± 0.8
S1521°56′45.60″ N120°44′36.16″ E77.2 ± 6.73.9 ± 0.85.3 ± 1.04.0 ± 0.1
S1621°57′56.98″ N120°45′28.87″ E437.7 ± 12.721.9 ± 1.532.2 ± 1.834.1 ± 2.2
S1721°57′39.20″ N120°45′47.01″ E322.5 ± 9.816.0 ± 1.121.5 ± 1.226.3 ± 1.1
S1821°57′37.83″ N120°45′44.06″ E334.1 ± 10.717.3 ± 1.223.6 ± 1.424.5 ± 1.2
S1921°57′35.06″ N120°45′43.27″ E359.0 ± 12.519.0 ± 1.527.2 ± 1.830.2 ± 2.2
S2021°58′05.37″ N120°45′30.45″ E390.6 ± 12.319.3 ± 1.226.3 ± 1.627.7 ± 1.6
S2121°58′06.23″ N120°45′30.34″ E443.3 ± 13.318.8 ± 1.129.2 ± 1.930.7 ± 1.8
S2221°58′06.09″ N120°45′31.60″ E375.0 ± 11.117.7 ± 1.126.5 ± 1.529.9 ± 1.2
S2321°57′57.09″ N120°44′25.65″ E327.5 ± 11.721.3 ± 1.532.6 ± 2.232.5 ± 1.7
S2421°57′57.92″ N120°44′28.78″ E433.7 ± 12.319.7 ± 1.334.3 ± 1.529.9 ± 1.9
S2521°57′28.93″ N120°44′28.03″ E472.1 ± 14.724.0 ± 1.839.1 ± 2.038.6 ± 2.3
S2621°58′00.37″ N120°44′27.85″ E261.6 ± 10.520.0 ± 1.329.4 ± 1.731.7 ± 1.7
S2721°58′03.82″ N120°44′34.97″ E353.5 ± 11.417.5 ± 1.226.5 ± 1.927.7 ± 1.6
S2821°58′01.99″ N120°44′36.70″ E499.0 ± 10.020.9 ± 1.634.0 ± 1.537.0 ± 1.8
S2921°58′01.84″ N120°44′23.56″ E129.7 ± 6.88.5 ± 1.011.1 ± 1.313.0 ± 1.4
S3021°57′54.82″ N120°44′24.10″ E379.6 ± 13.122.1 ± 1.635.6 ± 2.336.4 ± 2.1
Table 4. Descriptive statistics of activity concentrations for 40K, 226Ra, and 232Th.
Table 4. Descriptive statistics of activity concentrations for 40K, 226Ra, and 232Th.
Parameters40K232Th226Ra
Mean, Bq/kg344.426.518.6
SD *, Bq/kg116.18.45.5
CV (%)33.731.829.5
Minimum, Bq/kg77.25.33.9
Maximum, Bq/kg517.739.131.6
Median, Bq/kg356.226.919.5
Kurtosis0.290.982.79
Skewness−0.80−1.04−1.04
p-value **0.540.750.13
DistributionNormalNormalNormal
World average, Bq/kg4204532
* Standard deviation; ** normality test through the Kolmogorov–Smirnov method.
Table 5. Radioactivity in the sands studied in the present study compared with those from other countries.
Table 5. Radioactivity in the sands studied in the present study compared with those from other countries.
Location/CountryNumber40K226Ra232ThReferences
Bq/kg
Taiwan
Kenting Peninsula3034418.626.5PS *
Soil around coal-fired power plant1235918.726.8[48]
Soil in whole island503701727[7]
Soil in grass field33335 [49]
Soil in tobacco field970236.355.1[41]
Soil around NPPs1640722.233.4[11]
Soil around NPP IV1343624.232.1[16]
Soil in whole island735392341[1]
Soil in paddy field6659130.945.4[27]
Worldwide
Ho Chi Minh City/Vietnam3921524.033.0[32]
Karnataka/India2523828.121.6[44]
Nile Delta (Shoubra)/Egypt4027720.019.4[3]
Rawalpindi/Pakistan1430419.030.5[50]
Jeju Island/Korea1631432.435.6[5]
Anatolia/Turkey5843819.121.9[25]
Mtskheta–Mtianeti/Georgia1746424.026.9[2]
Iringa/Tanzania12583163.0143.0[51]
Cyprus3759483.753.6[4]
Kelantan/Malaysia3664382.0123.0[33]
Shangrao/China1674263.059.0[42]
World average 4203245[14]
* PS: present study.
Table 6. Linear correlation coefficient of 40K, 232Th, and 226Ra activity concentrations with soil properties.
Table 6. Linear correlation coefficient of 40K, 232Th, and 226Ra activity concentrations with soil properties.
NuclidepHEC (µs/cm)OM (%)Clay (%)Silt (%)Sand (%)
40K0.030.000.020.59 ***0.11−0.52 **
232Th−0.180.120.100.50 **0.19−0.48 **
226Ra−0.080.40 *0.050.44 *0.09−0.39 *
* p < 0.05, ** p < 0.01, *** p < 0.001.
Table 7. Radiological hazard indices in the soil from the present study and from soils around NPPs in other countries.
Table 7. Radiological hazard indices in the soil from the present study and from soils around NPPs in other countries.
LocationRaeqHexDRexAEDexReference
NPP/Taiwan98.2 a (55.2–145.3) b0.27 (0.15–0.39)46.6 (22.7–66.2)57.2 (27.9–81.7)[11]
NPP IV/Taiwan106.6 (14.92–171.1)0.27 (0.04–0.46)49.3 (7.33–82.8)60.5 (9.0–101.5)[16]
NPP/Thailand126 (14–446)0.34 (0.04–1.20)58 (6–203)71 (8–249)[17]
DIRAMS */Korea88.8 (71.5–94.7)0.24 (0.19–0.26)-- (37.6–45.3)50 (50–60)[13]
MAPS **/India189 (96.3–331.4)0.51 (0.26–0.89)74.6 (41.2–131.9)91.5 (50.5–161.7)[18]
NPP III/Taiwan83.0 (17.5–116.3)0.22 (0.05–0.31)39.0 (8.3–54.4)47.8 (10.1–66.7)Present Study
a mean value; b range minimum–maximum; * DIRAMS: Dongnam Institute of Radiological and Medical Science, Korea; ** MAPS: Madras Atomic Power Station, Kalpakkam, South India; -- not available.
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Chen, T.-C.; Zeng, F.-C.; Lin, C.-C.; Yeh, Y.-L.; Huang, W.-H. Assessment of Soil Radioactivity Associated with Risk and Correlation with Soil Properties near Maanshan Nuclear Power Plant, Taiwan. Appl. Sci. 2024, 14, 9239. https://doi.org/10.3390/app14209239

AMA Style

Chen T-C, Zeng F-C, Lin C-C, Yeh Y-L, Huang W-H. Assessment of Soil Radioactivity Associated with Risk and Correlation with Soil Properties near Maanshan Nuclear Power Plant, Taiwan. Applied Sciences. 2024; 14(20):9239. https://doi.org/10.3390/app14209239

Chicago/Turabian Style

Chen, Ting-Chien, Fan-Cheng Zeng, Chih-Chung Lin, Yi-Lung Yeh, and Wei-Hsiang Huang. 2024. "Assessment of Soil Radioactivity Associated with Risk and Correlation with Soil Properties near Maanshan Nuclear Power Plant, Taiwan" Applied Sciences 14, no. 20: 9239. https://doi.org/10.3390/app14209239

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