**1. 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 [2–7]. 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 [7–15].

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. 228Ra originates from the 232Th series, and in contrast to the typically not very soluble of Th element, 228Ra can be partially mobilized in natural waters, giving information on geochemical conditions and enabling contributions to the potential public exposure. 226Ra is a long-lived daughter of the 238U decay series, and it is also found in the water in trace quantities. The concentrations of 238U, 228Ra and 226Ra 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 226Ra, 228Ra and 238U concentrations in drinking water allows understanding their distribution and evaluating their impact on human health.

**Citation:** Duong, V.-H.; Nguyen, T.-D.; Hegedus, M.; Kocsis, E.; Kovacs, T. Study of Well Waters from High-Level Natural Radiation Areas in Northern Vietnam. *IJERPH* **2021**, *18*, 469.

https://doi.org/10.3390/ijerph18020469

Received: 7 December 2020 Accepted: 5 January 2021 Published: 8 January 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

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 ( 226Ra, 228Ra 238U) in the area surrounding these mines are investigated. Based on the activity concentrations, the radiological health hazards are also evaluated.

#### **2. Materials and Methods**

#### *2.1. 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.

**Figure 1.** Location of the study areas (map was modified from Hung et al., 2016) [18].

#### *2.2. 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)SO4 for radium isotopes, then the uranium isotopes were subsequently precipitated as (NH4)2U2O7 together with MnO2 [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 226Ra after sealing, and approximately 100 days for 238U after precipitation).

#### *2.3. Methods*
