226Ra and 228Ra in mineral waters of southeast Brazil

Abstract

This paper reports the experimental steps for measuring the natural radioactivity due to 226Ra and 228Ra in mineral waters occurring at Sao Paulo and Minas Gerais states, Brazil, that are extensively used for drinking in public places, bottling and bathing purposes, among other. The measurements of these alpha- and beta-emitting radionuclides were realized in 75 water sources located in 14 municipalities of those states. The 226Ra activity concentration was determined by alpha spectrometry from radon (222Rn) readings using the ionization chamber Alpha Guard PQ2000PRO equipped with an appropriate drive (Aquakit), following the protocol suggested by the manufacturer. The gamma spectrometry with an NaI(Tl) well-type detector was used for quantification of 228Ra due to its easy handling and fast response, where the data acquisition was performed taking into account the condition of secular equilibrium between 228Ra and its direct descendant, 228Ac. The 226Ra activity concentration ranged from 42 to 2,913 mBq/L, whereas the 228Ra activity concentration varied between <5.4 and 3,899 mBq/L. The data acquired have been utilized to perform dose calculations, whose values were compared with the World Health Organization (WHO) guidance level for the total effective dose (0.1 mSv/year). Adopting typical dose conversion factors for 226Ra and 228Ra, it has been verified that 48 % of the water sources (36 samples) exhibited values exceeding the WHO guideline reference value. This was mainly caused by the presence of dissolved 228Ra in water sources whose discharge occurs in areas characterized by the presence of enhanced levels of natural radioelements in rocks. The high 228Ra levels in some samples allowed to identify the presence of its short-lived daughters 212Pb and 208Tl in the liquid phase, whose implications in the dose calculations have been considered in this paper too.