One may ask why the natural concentration is used as a reference and
not the actual concentration in the receiving water body. The reason is that
the grey water footprint is an indicator of appropriated assimilation capacity.
The assimilation capacity of a receiving water body depends on the difference
between the maximum allowable and the natural concentration of a substance.
If one would compare the maximum allowable concentration with the actual
concentration of a substance, one would look at the remaining assimilation
capacity, which is obviously changing all the time, as a function of the actual
level of pollution at a certain time.