Study of Well Waters from High-Level Natural Radiation Areas in Northern Vietnam

The determination of natural radionuclide concentrations plays an important role for assuring public health and in the estimation of the radiological hazards. This is especially true for high level radiation areas. In this study, 226Ra, 228Ra and 238U concentrations were measured in well waters surrounding eight of the high-level natural radiation areas in northern Vietnam. The 226Ra, 228Ra and 238U activity concentrations vary from <1.2 × 10−3–2.7 (0.46), <2.6 × 10−3–0.43 (0.07) and <38 × 10−3–5.32 Bq/L (0.50 of median), respectively. 226Ra and 238U isotopes in most areas are in equilibrium, except for the DT-Thai Nguyen area. The calculated radiological hazard indices are generally higher than WHO (World Health Organization) recommendations. Average annual effective dose and excess lifetime cancer risk values due to drinking well water range from to 130 to 540 μSv/year and 7.4 × 10−6 to 3.1 × 10−5, respectively.


Introduction
Human beings are always exposed to a wide range of natural radionuclides [1]. Natural radionuclides can be present in the whole environment, including soil, water, air, food and even our bodies. Radionuclides in soil, air and water come from different sources, such as the weathering of the earth's crust, mining activities or fertilizer materials [2][3][4][5][6][7]. The radionuclides in water can enter the food chain, if the water is used for drinking or irrigation purposes. Determination of natural radionuclide concentrations in all the environments plays an important role for public health, because it can be used to assess the population's exposure to radiation and estimate the radiological hazard.
Investigations on natural radiation have received particular attention throughout the world in the last decade, which led to extensive studies in many countries, especially in or surrounding the high-level natural radiation areas. Studies regarding the natural radioactivity in water from different sources were widely conducted [7][8][9][10][11][12][13][14][15].
Among natural radionuclides, uranium leaches out from the bedrock and is present in water (surface and underground water) in various dissolved and suspended particulate forms. Other sources can be from the dry or wet deposition of aerosol from air. 228 Ra originates from the 232 Th series, and in contrast to the typically not very soluble of Th element, 228 Ra can be partially mobilized in natural waters, giving information on geochemical conditions and enabling contributions to the potential public exposure. 226 Ra is a long-lived daughter of the 238 U decay series, and it is also found in the water in trace quantities. The concentrations of 238 U, 228 Ra and 226 Ra in the water depend on the lithology, geomorphology and other geological conditions [16]. Thus, the concentration of these radionuclides varies from one site to another. The study concerning 226 Ra, 228 Ra and 238 U concentrations in drinking water allows understanding their distribution and evaluating their impact on human health. In Northern Vietnam, there are several mines, which contain higher than average concentrations of radioactive elements such as the rare earth mines in NX (Lai Chau), DP (Lai Chau), MH (Lao Cai) and YP (Yen Bai); there is also a polymetallic mine (also containing high uranium concentration) in DT (Thai Nguyen); finally, there is uranium ore in BY (Son La), TS (Phu Tho) and NB (Cao Bang). These mines were recently reported to have a high radioactive background by unpublished data from the Geological Division for Radioactive and Rare Minerals, Hanoi, Vietnam. This presents a possible public health concern. Therefore, in this study, the natural radionuclide concentrations in well water ( 226 Ra, 228 Ra 238 U) in the area surrounding these mines are investigated. Based on the activity concentrations, the radiological health hazards are also evaluated.

Study Areas
The eight areas in Northern Vietnam, including NX-Lai Chau, DP-Lai Chau, MH-Lao Cai, BY-Son La, TS-Phu Tho, YP-Yen Bai, DT-Thai Nguyen and NB-Cao Bang were selected for this study. The location of these areas is presented in Figure 1. The NX mine is one of the largest rare earth element (REE) mines in Vietnam, with probable reserves of about 7.7 million tons. DP mine ranks the second, with probable reserves of about 3.7 million tons and is followed by MH with approximately 400,000 tons and YP with about 5000 tons [17]. BY (Son La), TS (Phu Tho) and NB (Cao Bang) have uranium ore deposits, while DT (Thai Nguyen) is the largest polymetallic mine in Vietnam. geomorphology and other geological conditions [16]. Thus, the concentration of these radionuclides varies from one site to another. The study concerning 226 Ra, 228 Ra and 238 U concentrations in drinking water allows understanding their distribution and evaluating their impact on human health. In Northern Vietnam, there are several mines, which contain higher than average concentrations of radioactive elements such as the rare earth mines in NX (Lai Chau), DP (Lai Chau), MH (Lao Cai) and YP (Yen Bai); there is also a polymetallic mine (also containing high uranium concentration) in DT (Thai Nguyen); finally, there is uranium ore in BY (Son La), TS (Phu Tho) and NB (Cao Bang). These mines were recently reported to have a high radioactive background by unpublished data from the Geological Division for Radioactive and Rare Minerals, Hanoi, Vietnam. This presents a possible public health concern. Therefore, in this study, the natural radionuclide concentrations in well water ( 226 Ra, 228 Ra 238 U) in the area surrounding these mines are investigated. Based on the activity concentrations, the radiological health hazards are also evaluated.

Study Areas
The eight areas in Northern Vietnam, including NX-Lai Chau, DP-Lai Chau, MH-Lao Cai, BY-Son La, TS-Phu Tho, YP-Yen Bai, DT-Thai Nguyen and NB-Cao Bang were selected for this study. The location of these areas is presented in Figure 1. The NX mine is one of the largest rare earth element (REE) mines in Vietnam, with probable reserves of about 7.7 million tons. DP mine ranks the second, with probable reserves of about 3.7 million tons and is followed by MH with approximately 400,000 tons and YP with about 5000 tons [17]. BY (Son La), TS (Phu Tho) and NB (Cao Bang) have uranium ore deposits, while DT (Thai Nguyen) is the largest polymetallic mine in Vietnam.

Sample Collection and Preparation
In each study location, 20 water samples were collected from local wells during 2018-2019. These wells were dug manually in the soil to the depth of about 5 to 10 m, and these wells provide drinking water for the local population. A total of 160 water samples with 50 L for each sample were collected for this study. Each water sample was stored in a big, 50 L plastic container. Each water sample was co-precipitated as Ba(Ra)SO 4 for radium isotopes, then the uranium isotopes were subsequently precipitated as (NH 4 ) 2 U 2 O 7 together with MnO 2 [15,19,20]. The solid precipitate was then filtered. Together with study samples, a blank sample was prepared using distilled water in order to determine the background. The obtained precipitated sample was dried and milled to powder, then they were pressed into cylindrical plastic containers, weighted and finally hermetically sealed. The samples were stored for 4 half-lives in order to reach the secular equilibrium (16 days for 226 Ra after sealing, and approximately 100 days for 238 U after precipitation).

Measurements of Activity Concentration of 238 U, 228 Ra and 226 Ra in Water
After the samples reached equilibrium, activity concentration measurements were performed using a high-resolution detector HPGe with a low background made by Ortec™. The analysis was performed using Gamma Vision software. The detector's energy resolution was 1.9 keV at the 1.33 MeV 60 Co gamma-ray peak. To reduce the effects of background radiation at the laboratory, the detector was shielded by a 10-cm thick old-lead cylinder with a 1 mm cadmium and 1 mm copper inner lining. The samples were counted for two days to minimize the statistical counting error and activity calculation and calibration were carried out based on standard reference materials (IAEA-375). The level of background radiation present in the laboratory and introduced by the chemical process was determined using the blank sample.
The activity concentration of each sample was determined based on its respective gamma lines. The gamma lines of 609.3 keV, 1120.3 keV and 1764.5 keV of 214 Bi were used to determine the activity concentration of 226 Ra, the 911.1 keV line of 228 Ac was used for 228 Ra while the 1001 keV line of 234m Pa was used for 238 U (which was verified by 235 U measurement using the 186 keV line). The lowest limit detection were 0.0012, 0.0026 and 0.038 Bq/L for 226 Ra, 228 Ra and 238 U, respectively (the values were used for a studied sample volume of 50 L).
The activity concentrations of 226 Ra, 228 Ra and 238 U are calculated based on the following Equation (1) [7]: where: A sp and A st is activity concentration of studied and standard samples; N sp , M sp and N st, M st are the net measured intensity and mass of the sample and standard sample, respectively; C i is the correction factor for the differences between the densities of the samples and the standard sample for the i isotope; and C di is the correction fraction for the precipitation efficiency for the i isotope.

Evaluation of Radiological Hazard Indices
The annual effective dose (AED) due to the ingestion of the drinking well water was estimated to assess the radiological hazards for the local population by using Equation (2) [21]: where AED is the annual effective dose due to ingestion of radionuclides; A is the activity concentration of radionuclides; Cw is the annual water consumption for a person (730 L/year for adults) [22]. DCF is the ingestion dose conversion factor for the corresponding radionuclides (0.28, 0.69 and 0.045 µSv/Bq for 226 Ra, 228 Ra and 238 U, respectively) [21,23]. We all know that there are some other isotopes, like 210 Po, which can contribute to a higher annual effective dose caused by drinking well waters, but in this study we only used the 226 Ra, 228 Ra and 238 U values to calculate the AED.

Activity Concentration
The range and average values of activity concentration of 226 Ra, 228 Ra and 238 U measured in the well water samples are given in Table 1 [25]. The concentrations of 226 Ra, 228 Ra and 238 U are less than 1 Bq/L in most areas, except for DT-Thai Nguyen (Table 1). In the case of DT-Thai Nguyen, the concentrations of 226 Ra, 228 Ra and 238 U are comparatively high and are in the ranges of 0.36-2.70, 0.05-0.43 and 0.33-5.32 Bq/L, respectively. There, the 226 Ra concentration can reach levels multiple times higher than the WHO guideline (1 Bq/L) [26]. The high concentrations of 226 Ra and 238 U in DT-Thai Nguyen can be attributed to the polymetallic mine (which contains high uranium concentration) in this area. There are some activities, such as exploitation and the process of ore sorting going on, which can influence activity concentrations. It should be noted that the water samples in this study were taken from wells with depth of less than 10 m. These type of wells depend on rainfall and surface water as their source of water. Accordingly, they are easily contaminated by surface water and various human activities. Thus, the human activities in the polymetallic mine can lead to a relatively high concentration of 226 Ra, 228 Ra and 238 U in well water.   228 Ra and 238 U in well water in the areas observed in this study are significantly higher than those in Hoa Binh, Vietnam. In addition, the observed concentrations are higher than those in reported for many other countries [8][9][10][11][12]16], whereas they are lower than some values reported for tube wells in India. The concentrations observed in well water significantly depend on the type of aquifer rock as well as the chemical and physical characteristics of water [27], thus such differences can be expected. The concentration of studied radionuclides observed in well water in this study is within the worldwide range [28]. Regarding the concentration ratio of 226 Ra/ 238 U in well water samples, as shown in Table 1, the average value ranges from 0.57 (DT-Thai Nguyen) to 1.09 (BY-Son La). The data presented in Table 1 also shows that on average there is near equilibrium between 226 Ra and 238 U, except for DT-Thai Nguyen. Kumar et al. (2016) reported that the concentration of 226 Ra/ 238 U in groundwater in southwestern Punjab in India was varied from 0.08 to 0.22 [29]. In groundwater in Finland, Asikainen (1981) also showed that the ratio of 226 Ra/ 238 U ranged from 0.05 to 1. By contrast, other previous studies reported the enrichment of 226 Ra in groundwater [30]. For examples, Gascoyne (1989) indicated that the 226 Ra/ 238 U ratios in Canadian groundwater varied from 0.026 to 5300; this ratio in Konnngara Australian groundwater was from 0.02 to 89 [31]. Recently, the research results of Almasoud et al. (2020) indicated that the ratios of 226 Ra/ 238 U in groundwater samples in Saudi Arabia ranged from 1.25 to 20.4 [32]. The issue is further complicated by the effects of the recoil from the emission of an alpha particle, which can increase the mobility of the daughter nuclide due to the Szilárd-Chalmer effect. On the other hand, the 234 Th or 234 U can be fixed to more weathering resistant mineral phases, resulting in relatively more 238 U dissolving into groundwater [31]. The depletion of 234 U in groundwater can also be observed based on the relative abundances of U under various geochemical conditions [30] The relationship between activity concentrations of 238 U and 226 Ra in well water samples in this study is shown in Figure 2. A significant positive correlation was found between the two radionuclides with a Pearson correlation coefficient, 0.9402 and a p value < 0.00001 for the overall dataset, due to the influence of the higher values observed at DT-Thai Nguyen. The high value of correlation between 238 U and 226 Ra shows that these radionuclides have leached from the similar host rock [16]. Excluding DT-Thai Nguyen, there is moderate positive correlation with a Pearson correlation coefficient of 0.6326, and a p value < 0.00001. Similarly, a strong positive correlation was observed both between 238 U and 228 Ra (Pearson correlation coefficient: 0.8411, with a p value < 0.00001) and 226 Ra and 228 Ra (Pearson correlation coefficient: 0.7834, with a p value < 0.00001) for the overall dataset, however the effect of the higher values at DT-Thai Nguyen improving the correlation are observable here as well.

Radiological Hazards
The calculated radiation hazard indices based on the average activity concentrations for some drinking well water in northern Vietnam are listed in Table 3. As shown in this table, the annual effective dose (AED) for 226 Ra is significantly higher than that for 238 U, while 228 Ra is in the middle despite having a higher dose conversion coefficient due to the comparatively low activity concentrations. The average total annual effective dose for adults due to the consumption of water ranges from 130 to 540 µSv/year with the mean value of 240 µSv/year. The average excess life cancer risk (ELCR) due to drinking the investigated well water is from 7.4 × 10 −6 to 3.1 × 10 −5 (7 to 31 cases per 1 million people) with the average of 1.4 × 10 −5 (14 cases per 1 million people). Specific wells can have higher values; the overall maximum activity concentrations were observed in a well in YP-Yen Bai translating to a total annual effective dose of 540 µSv/y for adults and an ELCR of 7.0 × 10 −5 (70 cases per 1 million people). As reported by the WHO (2017), the reference values for AED and ELCR due to drinking water are 100 µSv/year and 1.0 × 10 −5 , respectively. It can be seen that the results of AED and ELCR due to consumption of well water in this study are higher on average for each area from the observed radionuclides alone than the values suggested by the WHO (2017), with the exception of ELCR for YP-Yen Bai. This indicates that there is a need for defining local policy regarding the wells in high-level natural radiation areas, northern Vietnam (in the observed areas), especially DT-Thai Nguyen.

Conclusions
The concentrations of 226 Ra, 228 Ra and 238 U in well waters in different locations surrounding the high-level radiation areas in northern Vietnam were extensively measured and evaluated. The research results show that the concentrations of 226 Ra, 228 Ra and 238 U in well water samples in the observed mining areas of northern Vietnam are comparatively higher than those reported for other areas of Vietnam and other countries. The highest concentrations of 226 Ra, 228 Ra and 238 U are observed in DT-Thai Nguyen. The research also shows that the concentration of 226 Ra and 238 U for most locations on average are around equilibrium, except for DT-Thai Nguyen. Regarding the radiological hazards assessment, the calculated results of AED and ELCR due to the consumption of well water are often higher, and for DT-Thai Nguyen multiple times higher, than the WHO reference values. The results generated from this study provide important baseline data for the impact assessment of the mining activities in the region in the future.