Assessment of microbial infection risks posed by ingestion of water during domestic water use and full-contact recreation in a mid-southern African region

Abstract

A customised Water-related Quantitative Microbial Risk Assessment (WROMRA) process was used to determine risk of infection to water ingested by users in the south-eastern Free State, South Africa. The WRQMRA consisted of an observed-adverse-effect-level approach (OAELA) and a quantitative microbial risk assessment (QMRA). The OAELA was based on the occurrence of E coli in the study waters to determine the possible risk of infection and the QMRA probable risk of infection by salmonellae. The WRQMRA was applied to recreational surface resource waters as well as waters from an unprotected spring and waters from the treated municipal supply that were stored in containers for domestic purposes. E. coli numbers were measured against expected infection levels expressed in water quality guidelines, while Salmonella counts were calculated to give the probable infection risk (P-I). Ingestion was based on intake volumes compiled for the various water uses. E coli occurred in numbers <1 06 in the surface waters, while the untreated spring and treated supply water contained E Coliof < 10(2) and <10(1) respectively. Salmonella occurred in numbers of <10(3) in recreational waters, and <10(-1) in water used for domestic purposes. A single exposure to the mean (as well as 95th percentile) risk was calculated using a beta-Poisson dose-response model at ingestion volumes of 100 mL (for full-contact recreation) and 1,318 mL (for domestic water use). Both the OAELA and the QMRA approaches indicated a risk of infection to recreational and domestic water users, even for a single exposure event, with the OAELA either over- or under-estimating the risk of infection for singular exposure events. This indicated that this method, used on its own, could not reliably predict a realistic risk of infection. It is recommended that the full WRQMRA process be used, and further developed to address several uncertainties that became evident during this study.