The Safe Drinking Water Act (SDWA) mandates that the drinking water should be monitored for 226Ra and 228Ra 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 226Ra and from 7.4 to 30.4 mBq/L for 228Ra using the destructive method. For non-destructive method, the DL range was 26.0 to 26.9 mBq/L for 226Ra and 27.6 to 28.6 mBq/L for 228Ra using the 140% detector. To verify the methods, 7 to 10 laboratory control samples were spiked with both 226Ra and 228Ra 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.
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