This regulation applies to a community water system (CWS). A CWS, both surface and ground water, designated by the Approving Authority as vulnerable shall sample for beta particle and photon radioactivity. Monitoring. Suppliers of water shall collect quarterly samples for beta emitters and annual samples for tritium and strontium-90 at each entry point to the distribution system, after this called a sampling point, beginning within the next calendar quarter after being notified by the Approving Authority. Suppliers of water at systems already designated as vulnerable by the Approving Authority shall continue to sample until the Approving Authority reviews and either reaffirms or removes the designation. If the gross beta particle activity minus the naturally occurring potassium-40 beta particle activity at a sampling point has a running annual average (computed quarterly) less than or equal to 50 picocuries per liter (pCi/l) (screening level), the Approving Authority may reduce the frequency of monitoring at that sampling point to once every 3 years. CWS in the Vicinity of a Nuclear Facility. The Approving Authority may allow the supplier of water to utilize environmental surveillance data collected by the nuclear facility instead of monitoring data collected at the system's entry points if the Approving Authority determines that the data is applicable to a particular water system. If there is a release from a nuclear facility, a supplier of water at a CWS that uses surveillance data shall begin monitoring at the system's entry points in accordance with §B(1) and (2) of this regulation. Suppliers of water designated by the Approving Authority as utilizing waters contaminated by effluents from nuclear facilities shall monitor as specified in 40 CFR §141.26(b)(2). Suppliers of water designated by the Approving Authority to monitor for beta particle and photon radioactivity may not apply for a waiver from the monitoring frequencies that are specified in §A or C of this regulation. Analysis for gross beta particle activity shall be conducted in accordance with 40 CFR §141.26(b)(4) and (5). Sampling When Sampling Points Exceed the Maximum Contaminant Level. Suppliers of water shall monitor monthly at the sampling points that exceed the maximum contaminant level in 40 CFR §141.66(d)(2) beginning the month after the exceedance occurs. Suppliers of water shall continue monthly monitoring until the CWS has established, by a rolling average of three monthly samples, that the MCL is being met. Water suppliers who establish that the MCL is being met shall return to quarterly monitoring as set forth in §B, C, or D of this regulation as determined by the Approving Authority.