Both ambient water quality standards and natural background concentrations
vary for surface and groundwater bodies. Thresholds in groundwater are
often based on requirements for drinking water, while maximum acceptable
concentrations in surface waters are typically determined by ecological considerations.
One could therefore propose to calculate the grey water footprint separately
for surface and groundwater systems. The problem with doing so, however, is
that groundwater generally ends up as surface water, so that for a pollutant
load to groundwater one can better take the difference between water quality
standard and natural background concentration for the most critical water body
(either the groundwater system or the surface water system). For loads to the
surface water system one can take the relevant data as for the surface water
system. When it is precisely known which loads arrive (first) in the groundwater
system and which loads in the surface water system, it makes sense to show two
components of the grey water footprint: the grey groundwater footprint and the
grey surface-water footprint