Does the Decay Rate Matter in the Spread of Constituents in Water Distribution Systems?

When a constituent such as chlorine, fluoride, bacteria, antibiotics, stimulants, depressants, or other pharmaceuticals enter a water distribution system, it spreads throughout the system in various ways. There are many constituents being discharged to water sources through the wastewater treatment plant that are not removed. These are termed "emerging contaminants." The decay rates of many of these emerging contaminants are not yet known. Their behavior is complex, since a water system is composed of potentially thousands of pipes in various loops at different elevations, pipe sizes, and roughness values being pumped by multiple pumps and stored in multiple tanks. In addition, the rate of user demands at each junction or neighborhood varies throughout the day and night depending on water use habits. These vary in different ways depending on whether they are domestic or industrial use. To understand this better, a modeling study was performed in which the decay rate for various systems was varied and the time for the contaminant to reach locations throughout the system was documented. It was found that the decay rate did play an important part in the time for the constituent to spread. For one system, in particular, it was found that the time to reach a location did not change for decay rates varying from -0.00001 to -0.005 units per day, but values higher than that caused the travel times to decrease quite significantly. When the contaminant reached a storage tank, it took a long time for it to fill the tank and then exit.