QMRA: Exposure model sensitivity to input parameters, critical review of dose-response models, and application in assessing risk of aging drinking water distribution systems
Van Abel, Nicole
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In the United States (US) and around the world, aging water infrastructure is a public health problem because drinking water infrastructure is coming to the end of its useful life with components in some areas being over 100 years old. Aging water infrastructure leads to an increase in the number of water main breaks and repairs in distribution systems, which has been identified as a trend relevant to the deterioration of water quality. The issue of aging pipes is of concern because of the potential relationship between waterborne disease outbreaks and main breaks. With distribution system failures as a result of aging drinking water infrastructure contributing to contamination of drinking water, Quantitative Microbial Risk Assessment (QMRA) can be used to estimate the human health risk from pathogen contamination during water main breaks and repairs in a water distribution system. Therefore, this research designed a comprehensive model coupling hydraulic analysis with risk analysis to estimate this human health risk. The first aim was to investigate the sensitivity of QMRA to assumptions about exposure to multiple consumption events per day because QMRAs of contaminated drinking water usually assume the daily intake volume is consumed once a day. However, individuals could consume water at multiple time points over one day. The second aim was to perform a comprehensive critical review of the numerous Norovirus dose-response models including describing all published dose-response data, describing the mathematics involved as well as all assumptions, and discussing the differences in the models such as the ID50 values. The third aim investigated the application of Norovirus dose-response models in QMRA by comparing the predicted risk estimates from waterborne exposures (recreational and drinking) and evaluating the ability of dose-response models to accurately estimate published attack rates from oyster outbreaks. The final aim was to estimate the risk of infection from ingestion of Cryptosporidium contaminated drinking water as a result of increased watermain breaks due to aging. The conclusions of this research are important for informing future QMRAs. The results indicated that the number of consumption events per day did have an impact on the probability of infection during short term contamination events because when the number of consumption events per day was greater than 1 then the microbial risk increased. The results demonstrated the complexity of Norovirus dose-response and discussed aggregation, exposed populations, low dose exposure, and differences in genogroups. Aggregation was determined to be an important consideration of Norovirus dose-response that yielded very different ID50 values. The results also demonstrated that the choice of Norovirus dose-response model is critical for QMRA because the numerous models led to very different risk estimates. Finally, this work demonstrated that aging will have an impact on distribution systems as a 4 to 8% increase in the amount of people impacted by main break failures was observed over a 50 year period. This has implications for water utilities as replacement rates of pipes may not aggressive enough to adequately combat the increase in break frequency observed with aging.
- Environmental health