Modeling fate and transport of Cryptosporidium parvum and rotavirus in overland flow

In the United States, there have been at least 1870 outbreaks associated with drinking water during the period of 1920 to 2002, causing 883,806 illnesses. Most of these outbreaks are resulted due to the presence of microbial pathogens in drinking water. Cryptosporidium parvum (C. parvum) has been recognized as one of the most frequently occurring microbial contaminants that can cause infection and diarrhea in many mammalian hosts, including humans. About 403,000 of an estimated 1.61 million residents in the Milwaukee area became ill with the stomach cramps, fever, diarrhea and dehydration caused by the outbreak of C. parvum in 1993. It was the largest waterborne disease outbreak in documented United States history. Similarly, rotavirus is the leading cause of death among children around the world. Each year more than two million children are hospitalized due to rotavirus infection and more than 500,000 children die from diarrheal disease caused by rotavirus. Past studies have demonstrated that environmental factors such as rainfall intensity and duration, slope, soil type and surface cover conditions significantly affect the transport of C. parvum oocyst and rotavirus in surface flow. Laboratory experiments conducted at the University of Illinois have demonstrated C. parvum oocysts and rotavirus transport is greatly influenced by climatic and soil-surface conditions like slope, soil types, soil texture, and ground cover. The objective of this study was to simulate the fate and transport of C. parvum and rotavirus in overland flow in different ground cover and slope conditions. Transport of pathogens in overland flow can be simulated mathematically by including terms for the concentration of the pathogens in the liquid phase (in suspension or free-floating) and the solid phase (adsorbed to the soil solid particles like clay, sand and silt). Advection, adsorption, and decay processes have been considered in the physically-based model. The mass balance equations have been solved using numerical technique to predict spatial and temporal changes in pathogen concentrations in two phases. In order to capture the dynamics of sediment-bound pathogen, the Water Erosion Prediction Project (WEPP) is coupled with the pathogen transport model. Outputs from WEPP simulations (flow velocity, depth, saturated conductivity and the soil particle fraction exiting in flow) are transferred as input to the pathogen transport model. Altogether four soil types (Alvin, Catlin, Darwin and Newberry), four slope conditions (1.5, 2.5, 3.0 and 4.5%), three rainfall intensities (2.54, 6.35 and 9.0 cm/hr), and three different surface cover conditions (bare, Brome grass and Fescue) have been used in the experimental investigations. Results of C. parvum and rotavirus transport from these conditions have been used in calibrating and validating the model simulation results. Model simulation results of C. parvum and rotavirus transport through soil surface with and without any ground cover (bare soil) have produced very good agreement between observed and predicted results in most cases. Experimental data on pathogen transport showed multiple peaks in few cases. It was noted that the model results could capture only first observed peak in pathogen break through curve but could not replicate multiple peaks in pathogen transport that were found in experimental results. Additionally, there were more parameters used in model calibration for vegetated surface compared to surface without ground cover. This study provides both success and challenges of the Cryptosporidium parvum and rotavirus modeling and list future activities so that a pathogen transport model can be reasonably used under different climatic and soil conditions