The most intuitive use of a chemical fate and transport model is the prediction of a concentration arising from certain emissions to the environment. For instance, let us consider a simple case of three point sources (P1, P2, P3) producing a continuous emission of a water contaminant to the stream network. Let us also assume that the three point sources are the only sources of that contaminant present in a catchment. We assume emission rates , , , respectively, for the three points. Moreover, we assume the emitted chemical has a removal half-life in water days. Under the plug-flow steady-state assumptions of Equation (2–45) in Chapter 2, we may compute concentration at any point of the stream network downstream. The steady-state plug-flow model enables measuring water trajectories in terms of *travel time*, that is, the length of the trajectory divided by crossing velocity, which can be computed as a weighted downstream flow length as we have shown in Chapter ...

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