Determination of Critical Rainfall Events for Quantitative Microbial Risk Assessment of Land-Applied Soil Amendments

Abstract

The occurrence of rainfall events has been significantly linked to waterborne-disease outbreaks in the United States and other parts of the world, indicating that wet-weather events may have significant impacts on microbial risk. An important step in developing a quantitative microbial risk assessment for exposure to pathogens originating from land application of soil amendments is to characterize the risk of storm-induced infiltration and runoff, the hydrologic processes most likely to introduce soil amendment-associated pathogens to source water. This paper develops an approach to estimate the risks of producing sufficient infiltration to saturate the soil to the water table and runoff to surface water on the basis of widely available intensity-duration-frequency curves for rainfall events. An infiltration model is developed to determine the runoff volume and infiltration depth associated with specific storm events. For a given return period the maximum infiltration and maximum runoff (which will typically be produced by different storm events) are then associated with that return period. For a specified return period 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 longer duration events become less intense. The inverse of the return period of the critical rainfall events provide the probabilities of infiltration and runoff that may introduce fertilizer-associated pathogens to source waters. This approach provides a sound method for determining the storm events with the greatest potential for mobilization of pathogens in the environment and the greatest risk to human health.