a set of equations representing the indestructibility of material as it flows through a system and which is given by the condition that all the mass (fluid, pollutant, etc.) entering a system during a given time period must leave the system, be stored within the system, or be converted into energy, the relationship between input and output of a specified substance in a defined system, for example in a lake, river or sewage treatment works, taking into account the formation or decomposition of that substance in the system