Author: Thomas Harter, thharter@ucdavis.edu, Cooperative Extension Specialist, Groundwater Hydrology
Once in a stream, pathogens may travel several tens of miles within the first 24 hours. Pathogens can survive days to several weeks in the stream environment, particularly in warmer streams, so there is no limit to how far a pathogen can travel. (Note that with almost 4,000 miles, the Mississippi-Missouri River system is the third-longest on earth. The water travel time along the 2,300-mile Mississippi River from Lake Itasca, Minnesota, to the Gulf of Mexico near New Orleans is approximately three months). However, many pathogens die off relatively quickly (see FAQ 27152 "What is the longevity of pathogens once they enter a surface water?"). Also, pathogens are removed from stream flow by being captured in the streambed. This natural in-stream filtration occurs as the result of two processes. Some pathogens (e.g., Cryptosporidium parvum) associate with suspended sediments in the stream water. The sediments are relatively heavy and gradually sink to the stream bottom. Natural filtration of stream water also occurs as river water is constantly exchanged with the underlying stream bed sediments, especially in smaller streams. Thus, a stream is able to reduce its pathogen load naturally. Scientists have estimated that the natural in-stream attenuation of Cryptosporidium parvum, for example, is one order of magnitude for every 0.2 miles (best case: small stream, heavy sediment load) to 5 miles (worst case: large stream, negligible sediment load) traveled.
Pathogen removal in streams also depends on weather conditions. During a storm, when pathogens enter streams from overland flow (e.g., from a manure field or from a cow pasture), stream velocity is much faster than during dry weather, there is more turbulence, and pathogens travel longer distances despite the presence of much suspended sediments in the stream flow.
