Microbial risk assessment for urban pluvial flooding

Abstract

Public health risks of urban pluvial flooding have so far received little attention in technical discussions. In this paper, the results of pathogen measurements in the sewer system of Utrecht and an urban flooding experiment are presented and used in an application of Quantitative Microbial Risk Assessment, an existing risk analysis method for the quantification of infection probabilities. This method uses ingested doses of pathogenic organisms for the calculation of infection probabilities. Ingested dose estimations are based on pathogen measurements. These samples have been analysed for concentrations of Campylobacter, Cryptosporidium and Giardia. Dose-response relations from literature are used to calculate infection probabilities for flood events. The results show that mean probabilities of obtaining a Campylobacter or Giardia infection as a result of contact with urban flood water are 2.8% and 0.6% per event respectively for adults and at least 5.7% and 1.0% per event for children, respectively. Infection probabilities for Cryptosporidium are about 1000 times lower than for Giardia. The infection probabilities found indicate that the health risk of urban flooding is higher than that of swimming in recreational freshwater environments, based on a comparison to the values for ‘acceptable risk’ as defined by the WHO for bathing water.