Frequent Questions

Is there a maximum contaminant level (MCL) for heterotrophic plate count (HPC) in drinking water?

EPA has not promulgated or proposed an MCL for HPC. The Safe Drinking Water Act requires EPA to promulgate an MCL as close to the maximum contaminant level goal (MCLG) as feasible when EPA develops an MCLG for a particular contaminant (SDWA 1412(b)(4)(B)). EPA cannot specify a scientifically rational MCLG for HPC (other than zero) at which no adverse health effects occur because HPC analysis measures both pathogenic and harmless (innocuous) bacteria. Drinking water with any level of HPC might contain numerous, few, or no pathogens. EPA considers the health benefits of complying with a bacteria concentration near zero versus some higher level (e.g., 500/mL) as unquantifiable and probably negligible. Additionally, excessive amounts of disinfectant would be needed to achieve such a level and could result in excessive levels of disinfection byproducts (which carry their own health risks) in finished drinking water. Based on these considerations, EPA did not propose an MCLG for HPC and therefore, did not propose an MCL (52 FR 42178, 42180; November 3, 1987).
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