Not all of the mass of a non-decaying species that enters the pathway necessarily has to exit the pathway. This is because when creating the fracture network, not all of the locations where flow is discharged from the system need to be identified as sinks. Mass that exits the system elsewhere is effectively "lost" and will never re-enter other GoldSim pathways. Note, however, that when GoldSim reports the total mass in the pathway, it includes this "lost" mass.
If you do not wish to "lose" any mass in a Network pathway, all discharge locations for the fracture network must be specifically mapped sinks.