Harvesting precipitation from rooftops is part of many green infrastructure projects to manage storm water and supplement water supplies. One of the barriers for implementing rooftop rainwater harvesting in the U.S. is limited risk-based treatment guidance for the use of roof-harvested rainwater. Guidance is limited due to data gaps (e.g., microbial pathogen characterization in rainwater), regional differences, varying roof-harvested rainwater infrastructure, materials, and maintenance, as well as the variability of pathogen characteristics (i.e., infection rate, prevalence, intensity, and virulence) in roof-harvested rainwater. To work toward the creation of guidance in the face of these issues a causal model was created to quantify the pathogenic risk of non-potable rainwater use. The model assumes the main source of pathogens in rainwater is from the feces of rooftop animals; therefore, there is a direct relationship between the pathogens in rainwater and animal feces. The prevalence of pathogens in animal feces was characterized using log normal distribution parameters, that were determined from a systematic review and meta-analysis. Using the assumptions, the prevalence of animal fecal pathogens was used to model pathogen concentration in rainwater based on the relative loadings of fecal indicator bacterial in rainwater collections and the animal feces. Modeled pathogen distributions in rainwater were then used in the hazard identification step of quantitative microbial risk assessment (QMRA) to support the development of risk-based guidance when using rainwater for non-potable use such as landscape irrigation, garden watering, and vehicle washing.
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