Destructive and Nondestructive Determination of ²²⁶Ra and ²²⁸Ra in Drinking Water by Gamma Spectrometry

Abstract

The Safe Drinking Water Act (SDWA) mandates that the drinking water should be monitored for 226 Ra and 228 Ra isotopes and establishes the Maximum Contaminant Level of 185 mBq/L (5 pCi · L -1 ) for the sum. In addition, SDWA regulates the Detection Limit (DL) of 37.0 mBq/L (1 pCi/L) for each isotope. The purpose of this work is to develop a working method for the determination of radium isotopes in drinking water satisfying the regulatory requirements of U.S. Environmental Protection Agency by utilizing our extensive experience in low-background gamma spectrometry at this laboratory. Two versions of the method were studied: destructive and non-destructive. Destructive method used the BaSO4 coprecipitation as well as 133Ba tracer for chemical recovery. We have used three gamma spectrometers: low-background 102% and 134% efficient with top muon guards, as well as an ultralow-background 140% efficient with full muon guard. We obtained a range of DLs from 5.3 to 22.6 mBq/L for 226 Ra and from 7.4 to 30.4 mBq/L for 228 Ra using the destructive method. For non-destructive method, the DL range was 26.0 to 26.9 mBq/L for 226 Ra and 27.6 to 28.6 mBq/L for 228 Ra using the 140% detector. To verify the methods, 7 to 10 laboratory control samples were spiked with both 226 Ra and 228 Ra at two different activities of 37.0 and 185 mBq/L. The results were evaluated by performing a combined location/variance chi-square test at a right-tail significance of 0.01 (99% Confidence Level), as stipulated by EPA. The verification results passed the chi-square tests at both activity levels. The destructive method can be accomplished using low-background gamma spectrometry, whereas non-destructive method requires ultralow-background gamma spectrometry.