There are no different standards although the Agency is working on a specific guidance along these lines having to do with radiological emergencies specifically. However, the guidance is not finalized yet.
EPA and States apply enforcement discretion in dealing with emergencies. In some cases, SDWA Section 1431 authority will be used, which allows EPA to issue orders to take action regardless of existing standards, to formalize emergency actions and the standards to be used during that time period. The usage of Section 1431 is rare.