COLIFORMS IN THE DRINKING-WATER SUPPLY - WHAT INFORMATION DO THE 0 100-ML SAMPLES PROVIDE/

The majority of 100-mL samples taken from drinking-water supplies in t he UK register zero coliforms. Those samples may present a false sense of security because water company operators do not know how close eac h zero sample was to registering a coliform or how many were close to registering coliforms. Furthermore, large deteriorations in water qual ity with respect to coliforms could occur without detection at the 100 -mL level. The variability in total heterotrophic bacteria densities a cross UK drinking-water supply zones during a year is shown to be acco mmodated by the log-normal distribution. Some of that variability is s easonal. In this paper, coliform densities in the drinking-water suppl y are also modelled as log-normal distributions. This approach suggest s that coliform densities may vary by as much as 10(8)-fold across the supply and that large volumes could be void of coliforms. Monitoring coliforms in 100-mL volumes is effectively an indicator of microbiolog ical clustering within the drinking-water supply and does not alert op erators to dilute homogeneously dispersed coliforms. For this reason, the coliform standard may be more effective at protecting public healt h against less infectious pathogens(e.g. Vibrio cholerae) than highly infectious pathogens (e.g. rotavirus). Analysing coliform densities in larger volumes of water would provide better information for monitori ng and controlling quality.