The natural concentration in a receiving water body is the concentration in
the water body that would occur if there were no human disturbances in the
catchment. For human-made substances that naturally do not occur in water,
cnat = 0. When natural concentrations are not known precisely but are estimated
to be low, for simplicity one may assume cnat = 0. This will, however, result in an
underestimated grey water footprint when cnat is actually not equal to zero.