Water quality routing in conduit connections assumes that the conduit behaves like a continuously stirred tank reactor (CSTR). While a plug flow reactor assumption may be more realistic, the differences will be small if the travel time through the conduit is in the same order as the routing time step. The concentration of a constituent leaving the conduit at the end of a time step is found by integrating the mass conservation equation, using average values for quantities that may change over time, such as flow rate and conduit volume.
Modeling water quality at storage unit nodes follows the same approach used for conduits. For other types of nodes that do not have volume, the quality of the water leaving the node is simply the concentration of the mixture of all the water entering the node.
The pollutant concentration in a conduit and storage node will be reduced by a first-order decay reaction if the pollutant's first-order decay coefficient is not zero.