Quantitative Microbial Risk Assessments (QMRA) have focused on drinking water system components upstream of distribution to customers, for nominal and event conditions. Yet some 15—33% of waterborne outbreaks are reported to be caused by contamination events in distribution systems. In the majority of these cases and probably in all non-outbreak contamination events, no pathogen concentration data was available. Faecal contamination events are usually detected or confirmed by the presence of E.coli or other faecal indicators, although the absence of this indicator is no guarantee of the absence of faecal pathogens. In this paper, the incidence and concentrations of various coliforms and sources of faecal contamination were used to estimate the possible concentrations of faecal pathogens and consequently the infection risks to consumers in event-affected areas. The results indicate that the infection risks may be very high, especially from Campylobacter and enteroviruses, but also that the uncertainties are very high. The high variability of pathogen to thermotolerant coliform ratios estimated in environmental samples severely limits the applicability of the approach described. Importantly, the highest ratios of enteroviruses to thermotolerant coliform were suggested from soil and shallow groundwaters, the most likely sources of faecal contamination that are detected in distribution systems. Epidemiological evaluations of non-outbreak faecal contamination of drinking water distribution systems and thorough tracking and characterisation of the contamination sources are necessary to assess the actual risks of these events.

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