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Implications of coliform variability in the assessment of the sanitary quality of recreational waters

SUMMARY The most widely used indicator of the sanitary quality of recreational waters is the coliform group of bacteria. Present techniques of coliform enumeration are imprecise, and this fact is too often overlooked in routine water quality surveys as well as in research efforts seeking quantitative relationships between coliform density and the health effects of recreational waters. To illustrate this point, three years of data gathered by the New York City Department of Health as part of their routine beach water sampling programme were re-analysed, taking the limited precision of each coliform estimate into account. Re-analysis showed 56·6% of the data were not significantly different (P > 0·05) from the standard being used. This large percentage of the data was of little value in determining the acceptability of the waters being sampled relative to the standard being used and thus represented a substantial waste of time and expense. Of the remaining data, half indicated acceptable water quality and half indicated unacceptable water quality relative to the standard. These three years of data, therefore, gave little information on the acceptability of the water quality at this location with respect to the standard being used. The data further suggest significant differences in coliform density within sample dates. It is recommended that in future water quality surveys, or in studies of the health effects of recreational waters as related to coliform density, emphasis should shift from maximizing the number of sample dates to maximizing the number of replicate determinations made per sample date.