Abstract: This work presents the chemical composition and concentration of radioactive and stable isotopes in water samples collected from nine well-known therapeutic centers located in north Vietnam. Excluding the Thanh Thuy – Phu Tho waters, which are hosted in Pleistocene sandstone with marble and gravel, the rest of studied waters are hosted in Paleozoic limestone formations. The thermal waters are extracted from 10 to 400 m below the surface, but the Keng Ga Ninh Binh intake is of an artesian nature. The temperature measured in the outlet waters ranges from 39 °C to 61 °C. The δ18O and δ2H (δD) vary from −9.13 to −5.09 ‰ and −63.9 to −34.1 ‰ respectively. All the thermal waters examined originate from infiltration water. This is also confirmed by the values of selected calculated hydrochemical indices. The hydrochemical types of the waters are HCO3-Ca-Mg, HCO3-Na, Cl-Na, SO4-HCO3-Ca-Mg, SO4-Ca-Mg and SO4-Ca. The mineralization of the concerned waters varies from 244 mgL−1 to more than 11,500 mgL−1. The concentrations of uranium and radium in the waters are in the broad interval from about 4 mBqL−1 to 680 mBqL−1 for 238U and from below 5 mBqL−1 (limit of detection - LOD) to above 3400 mBqL−1 for 226Ra. There is no 228Ra in most of the waters (below 10 mBqL−1- LOD), excluding two waters in the Thanh Thuy-Phu Tho district, where 228Ra is present with 18 and 27 mBqL−1. In all the waters studied, there is a very low concentration of polonium (210Po) which varies from 0.5 mBqL−1 to 8.2 mBqL−1. High mineralization and high radium concentration should be associated with a long residence time in the given geological formation. The linear relationship between the depth of the host water formations and the water temperature suggests that the study waters are mostly heated from the geogradient of heat energy. But the water from the My Lam district is probably heated by intrusive magmatic fluid. From a radiological point of view, the waters used for therapeutic and relaxation purposes are safe, however, there would be some problems if they were to be used for drinking purposes.

More information http://doi.org/https://doi.org/10.1016/j.ecolind.2022.108856. 

Free full text https://www.sciencedirect.com/science/article/pii/S1470160X22003272.