As a final note, it is observed here that grey water footprints are measured
based on the (human-induced) loads that enter into freshwater bodies, not on the
basis of the loads that can finally be measured in the river or groundwater flow at
some downstream point. Since water quality evolves over time and in the course of
the water flow as a result of natural processes, the load of a certain chemical at a downstream point can be distinctly different from the sum of the loads that once
entered the stream (upstream). The choice to measure the grey water footprint
at the point where pollutants enter the groundwater or surface water system
has the advantage that it is relatively simple – because one does not need to
model the processes that change water quality along the river – and safe – because
water quality may improve along the flow of a river by decay processes – but it is
unclear why one should take improved water quality downstream as an indicator
instead of measuring the immediate impact of a load at the point where it enters
the system. While the grey water footprint indicator thus does not account for
natural processes that may improve water quality along the water flow, it does
also not account for processes that consider the combined effect of pollutants,
which may sometimes be greater than what one may expect on the basis of the
concentrations of chemicals when considered separately. In the end, the grey
water footprint strongly depends on ambient water quality standards (maximum
acceptable concentrations), which is reasonable given the fact that such standards
are set based on the best available knowledge about the possible harmful effects of
chemicals including their possible interaction with other chemicals.