Federated Farmers: Project part of solution for nutrient enrichment

The Central Arizona project delivers nearly two trillion cubic metres of Colorado River water annually to central and southern Arizona.

A proposal to recharge the shallow aquifer in the lower Hinds catchment of mid-Canterbury is designed to dilute nitrate in ground and surface water.

A managed aquifer recharge (MAR) trial will pour up to 500 litres per second of water from the Rangitata River into an infiltration basin to percolate into the aquifer.

Ironically, the need for MAR is partly because irrigation has become more efficient in the catchment, with the shift from border dyke to spray irrigation and the piping of irrigation delivery systems. This results in less drainage from irrigated areas and delivery infrastructure and, therefore, less aquifer recharge.

MAR will top up the aquifer and provide more water for shallow wells, spring-fed streams and drains.

However, questions remain about the effectiveness of this approach to dilute nitrate-N and there are concerns about adding more water to an area which is low-lying and has a high water table.

Augmenting particular streams through existing irrigation infrastructure might be a more effective approach.

The history of water management in the Hinds catchment highlights the need to manage systems as a whole, rather than focusing on isolated parts without regard for unintended effects.

MAR is often used to recharge depleted aquifers. A pilot project near Gisborne aims to replenish the Makauri Aquifer to sustain ground water yields beneath the Poverty Bay Flats.

The Makauri aquifer is more confined than those on the Canterbury Plains. It is more like a bucket or tank and is therefore more suited to water storage.

Managed aquifer recharge is practiced around the world to store or 'bank' water.

One example is the Meyers Water Bank and Wildlife Project in California's Central Valley. Up to 9.8 million cubic metres of water from the Sacramento-San Joaquin River Delta is delivered into 37 hectares of recharge ponds.

The surfacewater infiltrates the shallow aquifer for underground storage. Only 95 per cent of the banked water is later allowed to be extracted, so the aquifer is left in a better net state as a result of the project. A wetland wildlife area is another environmental bonus.

Underground storage is an attractive option here because it avoids evaporative loss, which can be up to 30 peer cent in mid-summer.

The much larger Central Arizona Project diverts up to 480 million cubic metres of Colorado River water annually into underground storage.

The Arizona groundwater recharge also diminishes the risk of land subsidence and improves water quality by natural filtration.