AWWA PATH50779

A conceptual model using a finite difference method was developed to estimate Cryptosporidium parvum oocyst transport from source to water treatment plant intake. Subsurface oocyst transport is neglected because the size of oocysts would minimize their transport through soil. Based on the literature and theoretical calculations, oocysts are divided in two types: free and floc-associated oocysts. This is important because of the difference in settling rates based on the physical properties of the two types of oocysts. For oocyst viability, die-off is implemented as a simple first order decay function. A detachment function, adapted from microorganisms detachment, is used to estimate oocyst transfer from a field to runoff water. The advection-dispersion equation is used to estimate in-stream transport for point and nonpoint oocyst sources, with an assumption that the flow in the stream does not change. Within a lake, two different sub-models explain settling and die-off of oocysts. A normalized sensitivity analysis showed the significance of parameters for oocyst transport (in decreasing order) as follows: for a point source upstream of a lake, the point source concentration is the most significant followed by the lake residence time and the die-off rate. For nonpoint sources, the amount of rainfall is most important. This is followed by the number of oocysts on a field, the detachment ratio, and river velocity (all equal), then the land slope, and finally the distance from the manure-applied area to the watercourse. Includes 26 references, tables, figures.