A Conceptual Model for Cryptosporidium Transport in Watersheds

This paper describes a conceptual model to estimate Cryptosporidium parvum oocyst transport from source to water treatment plant intake. The intent of the model is ultimately to be able to predict oocyst concentrations at an intake to an order-of-magnitude level. The transport and fate mechanisms included are: oocyst detachment from waste or soil, generation of runoff, overland transport, reservoir and in-stream transport, and oocyst die-off. The model is formulated in finite difference form, and deals with both non-point sources from manure-applied areas, and point sources from wastewater treatment plants. An impor-tant contribution of this work is the recognition that the settling rates of free and floc- or particle-associated oocysts can be considerably different. This has important implications for their transport. A finite difference scheme was developed for five sections of a hypothet-ical watershed: a point source, a lake or reservoir (which can be modelled as either a continuous stirred tank reactor or an ideal rectangular setting tank), the section of stream channel from the outlet of the lake or reservoir to the conflu