Modeling Transport of Cryptosporidium Parvum Oocysts in Overland Flow

Cryptosporidium, a manure-borne protozoan parasite that is common in the environment, has recently been recognized as an important microbial contaminant of water. Cryptosporidium parvum (C. parvum) can cause infection and diarrhea in many mammalian hosts, including humans. Because cattle, particularly calves, are considered a major source of C. parvum , it is important to understand the movement of these pathogens from feedlots to water supplies. Vegetative filter strips (VFSs) are an established best management practice for sediment and nutrient removal from feedlots, but their influence on movement of microbial pathogens is not well understood. The potential of C. parvum oocysts from feedlots entering surface water through overland flow is perceived as one of the major threats to drinking water quality. VFSs are often positioned between the feedlot and surface water to remove a portion of the oocysts exiting the feedlot. Transport and partitioning of C. parvum in soil and water systems have been evaluated by microplot studies and saturated column experiments. In field-scale systems, the dynamics are often quite complex. The simplest case of overland-flow transport of oocysts is one in which a VFS infiltrates all runoff inflow. In this case, retention of all water also means retention of all oocysts. When runoff occurs from a VFS, it has been found that the number of oocysts exiting during a runoff event depends on several factors: the extent of entrainment of oocysts from manure, transport characteristics of oocysts (e.g., advection, dispersion, diffusion, and settling in the runoff water), adsorption and straining of oocysts induced by interactions with plants substrates, and death and predation of oocysts. This paper will describe the development and implementation of a model for simulating removal of C. parvum oocysts from overland flow. Transport of oocysts through a VFS is simulated mathematically by including terms for the concentration of the oocysts in the liquid phase (in suspension or free-floating) and the solid phase (adsorbed to the solid particles). Advection, adsorption, and decay processes are modeled. The model also accounts for the potential ranges of all hydraulic, transport, and die-off kinetic parameters. The hydrology and sediment deposition is determined using an existing VFS model (VFSMOD) that simulates the hydrology and sediment filtration in buffer strips, and generates output files containing values for water outflow and sediment trapping on the VFS. This paper describes the numerical solution of spatial and temporal changes in oocyst concentrations in two phases, and the development and implementation of the interface between the developed model and VFSMOD. Future work will compare the model results with field data.