40 CFR 141.26.  Radionuclides

The system shall collect one set of samples per entry point to the distribution system for four consecutive quarters by Dec. 31, 2008, or at initiation of water system operation. For Radium-226 and Radium-228, the combined value shall be used to determine compliance and future monitoring. If the Gross Alpha result does not exceed 5 pCi/L, the result may be substituted for Radium-226. If the Gross Alpha result does not exceed 15 pCi/L, the result may be substituted for Uranium.  If the Gross Alpha result is below the detection limit, then one-half the detection limit shall be used for substitution. If the quarterly annual average of any contaminant exceeds the respective maximum contaminant level(s) (MCL), the system shall sample the contaminant(s) quarterly. A system shall sample for a minimum of four consecutive quarters with results less than the MCL; then, with Drinking Water Branch approval, future monitoring shall be determined. If Gross Alpha exceeds the MCL, a Uranium sample shall also be collected quarterly and shall be (1) subtracted from the Gross Alpha result to determine compliance for Gross Alpha and (2) used to determine future monitoring for Uranium. Only systems designated as  “vulnerable” or “contaminated” shall monitor for beta particle and photon radioactivity.

Facts from EPA on radionuclides.