Quantification of the dependence of the emanation of radon from water on the pH and temperature of water.

Uranium- and thorium-series isotopes are able to dissolve in hot spring waters and decay into radon, which could potentially cause health risk of the public. Besides, several environmental investigations can be carried out by measuring radon concentrations in hot spring waters. Accordingly, the measurement of radon in hot spring water is a significant issue. A practical method for measuring radon in hot spring water was developed by using a silicon semiconductor detector, RAD-7. The overall effect of detection efficiency and radon leakage of the measuring system were examined to calibrate the measured concentration of radon in hot spring waters. In addition, the effect of water temperature and pH on emanation of radon from standard 226Ra solutions as well as that of the measuring time on precision of the measurement were investigated. The correlation among temperature, pH and emanation of radon was then obtained. Further, the established method was applied to in-situ measurement to investigate the concentration of radon in hot spring waters from different geological areas in Taiwan. According to the percent error of the measurement, the measuring time for environmental samples was set as 50 min. The overall effect of radon leakage and detection efficiency of the measuring system was estimated to be 99.19 ± 7.47% with a standard rock source of 222Rn from NIST. Standard sources of 222Rn were prepared from 226Ra standard solutions with different pH values. The 226Ra standard solutions were sealed and stand for 38 days in a 250 mL vial specific for the RAD-7. After measured by the WAT250 mode of the RAD-7, the data were measured as (52.5 ± 8.9)%∼(83.6 ± 11.0)% of the prepared concentration of 222Rn (from 226Ra). The emanation rate of 222Rn from standard solution under pH 1.44–9.07 was (53.2 ± 6.9)% ∼(84.3 ± 11.1)% and decreased with pH to a minimum value at 7.00. Moreover, the effect of temperature increases with pH. High correlated trend curves (r2 = 0.9829–0.9999) for 222Rn emanation rate versus pH under different temperature were obtained. A smooth curved surface fitted by the MATLAB for emanation rate of 222Rn was accordingly derived. The 222Rn concentrations in waters of the Beitou Hot Spring (acid-digestion sulfate spring on sedimentary rock strata), Wulai Hot Spring (sodium bicarbonate spring on metamorphic rock strata), Choshuitan (sodium bicarbonate spring on mudstone strata) and Zhaori Hot Spring (acid-digestion sulfate spring on andesite strata) were measured with RAD-7 by WAT250 for 50 min and then calibrated by 222Rn emanation rate estimated above, which were 215 ± 37 (Bq/m3), 6797 ± 1073 (Bq/m3), 2124 ± 577 (Bq/m3) and 26644 ± 2768 (Bq/m3). • A practical method for measuring Rn in hot spring water was developed. • Correlation among water temperature, pH and emanation of Rn was obtained. • A smooth curved surface fitted by the MATLAB for Rn emanation was derived. • This method was applied to in-situ measurement of Rn in hot spring waters in Taiwan. [ABSTRACT FROM AUTHOR]