Frequent Questions

The maximum contaminant level (MCL) for total coliform is based on its presence or absence in a water sample (40 CFR 141.63(a)). Why is the total coliform MCL based on presence or absence rather than an estimation of coliform density?

The total coliform presence-absence based MCL is used because there is no relationship between coliform densities and either pathogen density or the potential for a waterborne disease outbreak (52 FR 42224, 42227; November 3, 1987). Additionally, coliform presence or absence determination is easier to make then to determine coliform density, is less influenced by sample transit time than a density determination, and is not subject to the calculation difficulties implicit in the statistical methodology of coliform density calculations (54 FR 27544, 27548; June 29, 1989).
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