Uranium is a regulated contaminant (maximum contaminant level [MCL] 30 μg/L) that contributes to the gross alpha activity of groundwater. The adjusted gross alpha activity (MCL 15 pCi/L) is determined by subtracting the total uranium activity from the measured gross alpha activity. U.S. water utilities can use mass-based and radiochemical analysis methods for compliance monitoring of uranium. Mass-based measurements use a conversion factor of 0.67 pCi/μg of uranium to calculate the adjusted gross alpha activity. This conversion factor assumes that the activity of 234 U equals 238 U. Here, we present two decades of uranium isotope data measured by alpha spectrometry that shows 234 U activity typically exceeds 238 U. Using mass-based measurements, the total uranium activity is biased low causing artificial exceedances of the adjusted gross alpha activity. Therefore, water utilities with gross alpha activities >15 pCi/L should utilize radiochemical analyses for uranium for the most accurate calculation of adjusted gross alpha activity.alpha spectrometry, gross alpha activity, groundwater, uranium isotope ratios | INTRODUCTIONUranium is regulated by the United States Environmental Protection Agency (EPA) due to its toxicological properties (Dinocourt et al., 2015;Selden et al., 2009;Zamora et al., 1998). The radionuclides rule was updated in 2000 to include a uranium maximum contaminant level (MCL) of 30 μg/L (US EPA, 2015). The radionuclides rule also addresses gross alpha activity. When the gross alpha activity is measured above 15 pCi/L, analysis of uranium is required to determine the adjusted gross alpha activity (gross alpha minus the uranium and radon activities). Establishing compliance with the MCL requires four consecutive quarterly samples, and initial compliance is based on the running average of these four initial samples. Any exceedance of the MCL during this period or in subsequent sampling efforts triggers additional quarterly monitoring. Ultimately, any entry point in a drinking water distribution system must demonstrate four consecutive quarterly samples below the MCL to comply with the radionuclides rule for adjusted gross alpha activity. Because total uranium activity is subtracted from the gross alpha activity for compliance monitoring, quantification of the total uranium activity is a critical factor for adhering to this regulation.Both radiochemical and mass-based measurements are approved for compliance monitoring of uranium under federal rule when this analysis is triggered by a

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