Grey water footprint

The grey water footprint refers to the volume of surface and groundwater polluted during the production process. This allows comparison between the volume of water consumed and the volume of water polluted. The grey water footprint is calculated as the volume of water that is required to assimilate pollutants in order to meet local water quality standards.
Since the grey water footprint is a measure of the polluted water volume, it needs to be considered in relation to the available runoff in a catchment. As long as the grey water footprint is smaller than the river runoff, the assimilative capacity of the river is not yet fully used. When the grey WF equals the runoff, the full assimilative capacity of the river is used. Pollution exceeds the assimilative capacity as soon as the grey WF is larger than the runoff.
A critical load is the load of pollutants that will fully consume the assimilation capacity of the receiving water body. When the load into a flowing water body reaches the critical load, the grey WF will be equal to the runoff, which means that full funoff is appropriated for waste assimilation. The critical load for a certain river can be calculated as the river runoff multiplied by the difference between the maximum allowable and the natural concentration of pollutant in a river. When the waste flow includes more than one form of pollution, we can first estimate the grey water footprint for different pollutants. Then, the overall grey water footprint is determined by the pollutant that is most critical. If there is enough water to assimilate this pollutant, all other pollutants are assumed to have been assimilated as well.

Example: Let's examine the grey WF for atrazine, a pesticide used in corn production in the US. The parameters are as follows. With an application rate of 1 kg per hectare of corn, astrazine is applied to 30 million hectares of corn annualy. Roughly 5% leaks into the groundwater or runs into surface water streams, so the load is 1.5 million kg per year. We calculate the grey WF by dividing the annual load by the difference between the maximum allowable and natural concentration. We then find a grey WF of 500 billion cubic metre per year, which is the volume of freshwater polluted annualy. This is equivalent to 16% of the total renewable water resources in the US.
We can also calculate the grey WF per unit of product, in this case per ton of maize. We then find a grey WF of 1850 litre of water per kg of maize.

Digging deeper: Types of water footprints

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