Mass Balance and the Control Volume

Three possible outcomes exist for a chemical present at a specific location in the environment at a particular time the chemical can remain in that location, can be carried elsewhere by a transport process, or can be eliminated through transformation into another chemical. This very simple observation is known as mass balance or mass conservation. Mass balance is a concept around which an analysis of the fate and transport of any environmental chemical can be 

Implicit in the application of the mass balance concept is the need to choose a control volume. A control volume is any closed volume, across whose boundaries we propose to account for all transport of a chemical, and within whose boundaries we propose to account for all the chemical initially present (stored), as well as all processes (sources or sinks) that produce or consume the chemical. The mass balance expression for any chemical in any control volume during any time interval can be written as 

Change in storage of mass = mass transported in — mass transported out 

The mass balance expression in a control volume can also be written in terms of rates, that is, mass per time [M/T]  

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