The importance of uranium isotope composition for gross alpha activity regulation

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 ²³⁴U equals ²³⁸U. Here, we present two decades of uranium isotope data measured by alpha spectrometry that shows ²³⁴U activity typically exceeds ²³⁸U. 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.