Use of microcosm and in-situ studies for the estimation of exposure risk from recreational coastal waters and sediments

Author: Duncan L Craig

Craig, Duncan L, 2005 Use of microcosm and in-situ studies for the estimation of exposure risk from recreational coastal waters and sediments, Flinders University, School of Medicine

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The interaction of microorganisms with sediments can enhance their survival by reducing exposure to various stressors and thus marine sediments may act as reservoirs for pathogenic microorganisms. In coastal waters there can be an increased risk of infection to humans due to the possible re-suspension of these microorganisms during recreational activities. This research attempts to more accurately identify environmental exposure in the first stage of a health risk assessment in recreational coastal waters. Techniques were developed to successfully separate microorganisms from sediment particles. Of the methods investigated, subjecting diluted sediment samples to a sonication bath for 10 minutes was found to be the most efficient separation technique over a range of sediment types. This method was therefore used in the subsequent studies to enumerate organisms from the surface sediment layer, as distinct from the water column. Faecal coliforms were enumerated by membrane filtration in both water and sediment from three Adelaide metropolitan recreational coastal sites, chosen to represent different physical sediment characteristics, over a 12-month period. All sites investigated met current National Health and Medical Research Council Guidelines for primary contact recreation. Faecal coliform concentrations were generally greater in sediment compared with overlying water for all samples. This was most evident in sediment consisting of greater silt/clay and organic carbon content (with up to 1000 times higher concentrations in the surface sediment layer compared with overlying water). For coastal recreational sites impacted by stormwater or river discharges, high faecal coliform concentrations were found to be associated with rainfall. A laboratory-based microcosm study utilising intact sediment cores was undertaken to determine the decay rates of faecal indicator organisms (E. coli, enterococci and somatic coliphage) and pathogens (Salmonella derby and S. typhimurium) in both overlying water and in various sediment types. For all organisms tested, temperature had an inverse relationship with survival. Greater decay was observed in the overlying water compared to the surface sediment layer. Small particle size and high organic carbon content was found to be more conducive to microbial survival. In general, decay rates of E. coli were significantly greater than enterococci and coliphage. Although no significant correlations were observed between decay rates of the pathogens and indicator organisms, decay of Salmonella spp. in overlying water more closely resembled that of E. coli than that of other indicators. Using decay rates measured in the microcosm study 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. and rotavirus following exposure to recreational coastal water subject to a range of faecal contamination levels. For modelling purposes, the assumption was made that rotavirus decay was equivalent to coliphage decay. The probability of infection from rotavirus due to exposure to contaminated recreational coastal water was greater than that for Salmonella spp. under all scenarios. This increased probability of infection is linked to the high infectivity of rotavirus compared to Salmonella spp. Results of this research highlight the limited effectiveness of using prescribed faecal coliform concentrations in the water column alone to estimate the risk of exposure to pathogenic microorganisms during recreational activity at coastal areas. It demonstrated that coastal sediments act as a reservoir for both indicator and pathogenic organisms released into the coastal environment. This suggests an increased exposure risk if these organisms are resuspended back into the water column during recreational activity. A combined risk-based monitoring program would provide a more robust and reliable estimate of health risk associated with coastal recreational areas.

Keywords: microcosm,recreational waters,coastal waters,sediment

Subject: Environmental Health thesis

Thesis type: Doctor of Philosophy
Completed: 2005
School: School of Medicine
Supervisor: Nancy Cromar