The grey water footprint of milk due to nitrate leaching from dairy farms in Canterbury, New Zealand

The Canterbury Region of New Zealand has undergone rapid and significant land use intensification over the last three decades resulting in a substantial increase of nitrate-nitrogen leached to the environment. In this article, we determined the nitrate grey water footprint of milk, which is the amount of water needed to dilute nitrogen leached past the root zone to meet different receiving water nitrate standards per milk production unit. Our analysis revealed the nitrate grey water footprint for Canterbury ranged from 433 to 11,110 litres of water per litre of milk, depending on the water standards applied. This footprint is higher than many estimates for global milk production, and reveals that footprints are very dependent on inputs included in the analyses and on the water quality standards applied to the receiving water. The extensive dairy farming in Canterbury is leading to significant pollution of the region's groundwater, much of which is used for drinking water. Dairy farming at this intensity is unsustainable and if not reduced could pose a significant risk to human health and the market perception of the sustainability of the New Zealand dairy industry and its products.