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Determination of Critical Rainfall Events for Quantitative Microbial Risk Assessment of Biosolids-Associated Pathogens

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A significant association has been found between rainfall events and waterborne-disease outbreaks in the United States and other parts of the world, indicating that wet-weather events can have significant impacts on microbial risk. In order to develop a quantitative microbial risk assessment model for exposure to biosolids-associated pathogens from ingestion of contaminated groundwater and surface water, the first step is to consider the effects of wet-weather events on infiltration and runoff values. This paper examines how runoff and infiltration vary as a function of soil hydraulic properties and precipitation intensity and duration. The results show that the selection of parameter values describing soil hydraulic properties is very important and the soil texture class needs to be identified clearly.

The second step in developing a risk assessment for biosolids-associated pathogens is to estimate the probabilities of critical wet weather events, such as those that create infiltration to groundwater or runoff to surface water. The generalized precipitation-frequency maps provided by the U.S. Weather Bureau provide return periods for rainfall intensities associated with different durations. However, the probabilities of different amounts of runoff and infiltration are not directly available. In this study these probabilities are derived by identifying the maximum runoff and the maximum infiltration associated with any of the duration storm events for a given return period. In general the maximum runoff is produced by intermediate duration events as very short storms do not saturate the soil and very long duration storms are of lower intensity and generally do not exceed the infiltration capacity of the soil. Infiltration amounts tend initially to increase with duration and then plateau as the longer duration events become less intense. The critical rainfall events (i.e., intensities and durations), identified in this manner, can be used to calculate infiltration depth and runoff volumes for use in exposure and infection risk models. The inverse of the return period of the critical rainfall event provides the probabilities of infiltration and runoff produced by that rainfall. The calculated exposure risk of infection is conditional on the occurrence of this amount of infiltration and runoff. The overall risk of infection during wet-weather events is determined by both of the infection risk conditional on the rainfall event and the probability of the rainfall event.
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Keywords: Biosolids; Infiltration and Runoff Modeling; Microbial Risk Assessment; Pathogens; Precipitation Frequency Analysis; Wet-Weather Events

Document Type: Research Article

Publication date: 2010-01-01

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