Why are microorganisms such as Giardia, Legionella, and viruses regulated through a treatment technique rather than a maximum contaminant level?
EPA believes it is not economically or technologically feasible to measure (i.e., monitor) for Giardia, Legionella, and viruses in drinking water; therefore, EPA regulates these microorganisms using a treatment technique. EPA believes it is not feasible because water system personnel generally do not have levels of expertise required by available analytical methods; analysis by independent laboratories is generally very expensive; sample validation procedures have not yet been established; systems would have to take inordinately large numbers and frequent samples of water to ensure no significant health risk (e.g., failure to detect Giardia in one or a few samples does not provide assurance that Giardia does not occur at significant levels in the water supply); and it is not possible to assure that a monitoring program will detect the microorganisms before they actually cause or contribute to an increased risk to health (52 FR 42178, 42180; November 3, 1987).
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