Decay Rates of Faecal Indicator Organisms and Pathogens: Use of Microcosm and In Situ Studies for the Estimation of Exposure Risk in Recreational Waters

This three year study compared the decay of the commonly used faecal indicator organisms Escherichia coli, enterococci and coliphage in coastal water and sediment using laboratory based microcosms with that for the pathogens Salmonella typhimurium and S. derby. Results from the laboratory study were validated with in-situ decay rates of faecal coliforms observed at a recreational coastal site following a significant stormwater event. Results demonstrated that for both indicators and pathogens, greater decay was observed in the overlying water compared with the surface sediment layer. In general, the decay rates of Salmonella spp. were greater than either enterococci or coliphage in overlying water and sediment. Decay rates of E. coli were similar to Salmonella spp. in overlying water, although greater in sediment. Increased temperature resulted in an increased decay rate for all organisms in the overlying water (and to a lesser extent in the surface sediment layer). Results from a 12 month investigation into faecal coliform concentrations at recreational coastal sites also demonstrated higher concentrations of faecal coliforms in sediment compared with overlying water. Sediments were therefore illustrated to act as a reservoir for both faecal indicator and pathogenic microorganisms and may represent an increased exposure risk if these organisms are resuspended back into the water column during recreational activities. Using measured decay rates and available dose-response data, a quantitative microbial risk assessment (QMRA) utilising Monte 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 contamination levels. For exposure to recreational water of extremely poor quality (10 CFU 100 mL) the maximum risk of infection (95% CI) on the day of the contamination event was above 2.0 × 10 and remained above 1 × 10 for three days following the initial high concentration.