Increasing the chemical concentration in a water body by ‘taking water
out through evaporation while leaving the chemicals in’ is effectively the same
as adding a certain additional load. If one takes out X m3 of pure water, the
‘equivalent load’ is X m3 times the natural concentration in the water body (cnat
in mass per m3). The ‘equivalent load’ of X × cnat (expressed as a mass) is natural,
but no longer embedded in natural water, because the water was taken away
(it evaporated). This ‘equivalent load’ has to be assimilated by other natural
water. The grey water footprint related to this ‘equivalent load’ can be calculated
with the standard equation, whereby the grey water footprint is equal to the
‘equivalent load’ divided by the difference between the maximum and natural
concentration (Equation 3). This grey water footprint will come on top of grey
water footprints in the catchment related to real loads (in other words, chemical
loads added by human activities).