2.3.1. Measurements of Activity Concentration of 238U, 228Ra and 226Ra 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 60Co 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 214Bi were used to determine the activity concentration of 226Ra, the 911.1 keV line of 228Ac was used for 228Ra while the 1001 keV line of 234mPa was used for 238U (which was verified by 235U measurement using the 186 keV line). The lowest limit detection were 0.0012, 0.0026 and 0.038 Bq/L for 226Ra, 228Ra and 238U, respectively (the values were used for a studied sample volume of 50 L).

The activity concentrations of 226Ra, 228Ra and 238U are calculated based on the following Equation (1) [7]:

$$\mathbf{A\_{sp}} = \frac{\mathbf{N\_{sp}M\_{st}A\_{st}C\_iC\_{di}}}{\mathbf{N\_{st}M\_{sp}}} \tag{1}$$

where: Asp and Ast is activity concentration of studied and standard samples; Nsp, Msp and Nst, Mst are the net measured intensity and mass of the sample and standard sample, respectively; Ci is the correction factor for the differences between the densities of the samples and the standard sample for the i isotope; and Cdi is the correction fraction for the precipitation efficiency for the i isotope.

2.3.2. Evaluation of Radiological Hazard Indices

• Annual effective dose (AED)

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]:

$$\text{AED (\mu Sv/year)} = \text{A (Bq/L)} \times \text{Cw (L/year)} \times \text{DCF (\mu Sv/Bq)} \tag{2}$$

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 226Ra, 228Ra and 238U, respectively) [21,23]. We all know that there are some other isotopes, like 210Po, which can contribute to a higher annual effective dose caused by drinking well waters, but in this study we only used the 226Ra, 228Ra and 238U values to calculate the AED.

• Excess lifetime cancer risk (ELCR)

Based on the values of AED, excess lifetime cancer risks (ELCR) were calculated using the following Equation (3) [24]:

$$\text{ELCR} = \text{AED} \times \text{Life Expectancy (LE)} \times \text{Risk factor (RF)} \tag{3}$$

where LE is life expectancy of Vietnamese people in North Vietnam and mountainous areas (71 years) (https://www.gso.gov.vn/default\_en.aspx?tabid=774); RF the risk factor associated with radiation, which is equal to 0.057 Sv−<sup>1</sup> [24].

#### **3. Results and Discussion**

#### *3.1. Activity Concentration*

The range and average values of activity concentration of 226Ra, 228Ra and 238U measured in the well water samples are given in Table 1. It can be seen that the activity concentration of 226Ra, 228Ra and 238U ranges from <0.0012–2.7, <0.0026–0.43 and <0.038– 5.32 Bq/L, respectively. The highest concentrations of all three isotopes are found in DT-Thai Nguyen. This table shows only a slight difference in concentration between 226Ra, 228Ra and 238U in most cases, except for the DT-Thai Nguyen sampling site. 226Ra, 228Ra and 238U ratios near unity indicate recent contact with uranium bearing not yet weathered minerals [25]. The concentrations of 226Ra, 228Ra and 238U 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 226Ra, 228Ra and 238U 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 226Ra concentration can reach levels multiple times higher than the WHO guideline (1 Bq/L) [26]. The high concentrations of 226Ra and 238U 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 226Ra, 228Ra and 238U in well water.


**Table 1.** Concentration of natural radionuclides in well water samples in North, Vietnam.


**Table 1.** *Cont*.

\* during averaging values under the detection limit were taken as the detection limit to give a conservative estimate. \*\* uncalculable values were left out of the ratio calculation.

> Table 2 compares the 226Ra, 228Ra and 238U concentrations in the well water samples in this study with that of different water sources in different countries. The concentrations of 226Ra, 228Ra and 238U 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 [8–12,16], 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].

**Table 2.** 226Ra and 238U concentrations in water samples in different areas.


\* Equilibrium was assumed by the original authors.

Regarding the concentration ratio of 226Ra/238U 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 226Ra and 238U, except for DT-Thai Nguyen. Kumar et al. (2016) reported that the concentration of 226Ra/238U 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 226Ra/238U ranged from 0.05 to 1. By contrast, other previous studies reported the enrichment of 226Ra in groundwater [30]. For examples, Gascoyne (1989) indicated that the 226Ra/238U 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 226Ra/238U 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 234Th or 234U can be fixed to more weathering resistant mineral phases, resulting in relatively more 238U dissolving into groundwater [31]. The depletion of 234U in groundwater can also be observed based on the relative abundances of U under various geochemical conditions [30]

The relationship between activity concentrations of 238U and 226Ra 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 238U and 226Ra 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 238U and 228Ra (Pearson correlation coefficient: 0.8411, with a *p* value < 0.00001) and 226Ra and 228Ra (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.

**Figure 2.** Relationship between 238U and 226Ra concentrations.

#### *3.2. 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 226Ra is significantly higher than that for 238U, while 228Ra 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 × <sup>10</sup>−<sup>6</sup> to 3.1 × <sup>10</sup>−<sup>5</sup> (7 to 31 cases per 1 million people) with the average of 1.4 × <sup>10</sup>−<sup>5</sup> (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 × <sup>10</sup>−<sup>5</sup> (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 <sup>μ</sup>Sv/year and 1.0 × <sup>10</sup><sup>−</sup>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.


**Table 3.** Radiation hazard indices for well water samples in northern Vietnam.

## **4. Conclusions**

The concentrations of 226Ra, 228Ra and 238U 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 226Ra, 228Ra and 238U 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 226Ra, 228Ra and 238U are observed in DT-Thai Nguyen. The research also shows that the concentration of 226Ra and 238U 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.

**Author Contributions:** Conceptualization, V.-H.D., T.-D.N., and T.K.; methodology, M.H., V.-H.D., and T.K.; formal analysis, M.H., T.-D.N.; investigation, V.-H.D., T.-D.N., E.K., and T.K.; resources, V.-H.D., and T.K.; data curation, T.K. and M.H.; writing—original V.-H.D., and M.H. and T.K. draft preparation, V.-H.D.; writing—review and editing, M.H., E.K., V.-H.D., and T.K.; visualization, V.- H.D. and E.K.; supervision, T.K.; project administration, T.K.; funding acquisition, V.-H.D. and T.K. All authors have read and agreed to the published version of the manuscript.

**Funding:** The research work is supported by Grant of The National Foundation for Science and Technology Development (NAFOSTED), Vietnam, no 105.05-2019.10. and the TKP2020-IKA-07 project financed under the 2020-4.1.1-TKP2020 Thematic Excellence Programme by the National Research, Development and Innovation Fund of Hungary.

**Institutional Review Board Statement:** Not applicable for studies not involving humans or animals.

**Informed Consent Statement:** Not applicable for studies not involving humans.

**Data Availability Statement:** The data presented in this study are available on request from the corresponding author.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


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