AWWA ACE56306

AWWA ACE56306 Decay Rates of Faecal Indicator Organisms and Pathogens: Use of Microcosm and In Situ Studies for the Estimation of Exposure Risk in Recreational Waters
Conference Proceeding by American Water Works Association, 06/16/2002

Craig, Duncan; Fallowfield, Howard; Cromar, Nancy

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This three year study compared the decay of the commonly used faecal indicatororganisms Escherichia coli, enterococci and coliphage in coastal water and sedimentusing laboratory based microcosms with that for the pathogens Salmonella typhimuriumand S. derby. Results from the laboratory study were validated with in-situ decay rates offaecal coliforms observed at a recreational coastal site following a significant stormwaterevent. Results demonstrated that for both indicators and pathogens, greater decay wasobserved in the overlying water compared with the surface sediment layer. In general, thedecay rates of Salmonella spp. were greater than either enterococci or coliphage inoverlying water and sediment. Decay rates of E. coli were similar to Salmonella spp. inoverlying water, although greater in sediment. Increased temperature resulted in anincreased decay rate for all organisms in the overlying water (and to a lesser extent in thesurface sediment layer). Results from a 12 month investigation into faecal coliformconcentrations at recreational coastal sites also demonstrated higher concentrations offaecal coliforms in sediment compared with overlying water. Sediments were thereforeillustrated to act as a reservoir for both faecal indicator and pathogenic microorganismsand may represent an increased exposure risk if these organisms are resuspended backinto the water column during recreational activities. Using measured decay rates andavailable dose-response data, a quantitative microbial risk assessment (QMRA) utilizingMonte Carlo simulation was undertaken to estimate the risk of infection to Salmonella spp.following exposure to recreational coastal water subject to a range of faecal contaminationlevels. For exposure to recreational water of extremely poor quality (106 CFU 100 mL-1) themaximum risk of infection (95% CI) on the day of the contamination event was above 2.0 x 10-1 and remained above 1 x 10-3 for three days following the initial high concentration. Includes 25 references, tables, figures.