The approach taken in grey water footprint accounting is the same as the
so-called critical-load approach (Box 3.5). In both cases, the basic recognition
is that the room for waste uptake of a water body is limited by the difference
between the maximum and the natural concentration. The critical load refers
to the situation where the room for waste uptake has been fully consumed. At
the critical load, the grey water footprint will be equal to the available water
flow, which is then required in full to dilute the chemicals down to acceptable
concentrations