Contaminants to be considered for regulation are selected from the Contaminant Candidate List (CCL), which was authorized by the 1996 SDWA amendments. Microbial entries on the CCL are likely to be waterborne health risks that may not be controlled by treatment. The CCL was published in March 1998 and includes 10 microorganisms or groups of microorganisms. One of the microbial contaminants on the CCL is freshwater algae and its toxins. The SDWA also authorized drinking water monitoring for unregulated contaminants which lack sufficient occurrence data to make a regulatory determination. This provision was implemented as the Unregulated Contaminant Monitoring Rule (UCMR) in Sept 1999. Contaminants to be monitored by the UCMR will be selected from the occurrence priorities list of the CCL, which is a list of contaminants needing more occurrence information. Of the 10 CCL pathogens, 8, including algae and toxins, are listed as occurrence priorities. Analytical methods that are suitable for a national monitoring program are necessary before monitoring can be done. The UCMR was designed with different monitoring options for contaminants that have or do not have analytical methods. The first of three monitoring options is assessment monitoring. Assessment monitoring will be done for contaminants with methods that were well developed and validated when the UCMR was promulgated in Sept., 1999. It includes all water systems serving more than 10,000 people plus a representative sample of 800 small systems. Sampling will be 4 times a year for surface water systems and 2 times a year for groundwater systems. Samples are taken at the entry point to the distribution system or a compliance monitoring location. The next monitoring option is a screening survey. Screening surveys will be done for contaminants that did not have fully developed methods when the UCMR was promulgated, but will have methods ready by the time of the screening survey. They include a representative sample of 120 large systems and 180 small systems for a total of 300 systems. Screening surveys will be done for one year, with flexible sample times, locations within the distribution system and sampling frequency. The third monitoring option is the prescreen survey. The prescreen survey is for contaminants that don't have well developed methods, or methods that are expensive. Analytical methods may be evaluated with these surveys. Prescreen surveys include up to 200 systems, but may be much smaller if necessary. If possible, samples will be taken from water distribution systems which are vulnerable to contaminant occurrence, and at times and locations within the systems where contaminants may occur most frequently. The purpose of selecting vulnerable samples is to maximize the likelihood of detecting the contaminant. Includes reference.