When the load into a flowing water body reaches a certain ‘critical load’, the
grey water footprint will be equal to the run-off, which means that full runoff
is appropriated for waste assimilation. The critical load (Lcrit, in mass/time)
is the load of pollutants that will fully consume the assimilation capacity of
the receiving water body. It can be calculated by multiplying the run-off of
the water body (R, in volume/time) by the difference between the maximum
acceptable and natural concentration: