Ways to replenish aquifer considered

The Cardrona River at the Larches, upstream of Wanaka. Photo by Mark Price.

The Otago Regional Council is looking at adding to the water stored in the underground Cardrona-Wanaka aquifer.

A report commissioned by the council suggests water from the Cardrona River could be collected in ''soakage basins'' upstream of Wanaka during the irrigation ''off season'' from May to September.

From there, it would filter through ''a significant thickness of unsaturated gravel'' to the aquifer, adding to the natural infiltration supplying the aquifer.

Then, in the dry months from December to April, the water would be available, via bores, for irrigation around Wanaka.

The report, a pre-feasibility assessment into the ''managed recharge'' of the aquifer, was completed by consultants Golder Associates in September last year.

It has become public as part of the agenda for today's monthly meeting of the council.

The object of the scheme would be to reduce the amount of surface water taken from the river for irrigation, so as to improve the flow of the river past Wanaka. The stretch of river dries up at times in the summer.

Golder Associates notes ''managed aquifer recharge'' in various forms is a ''proven technology used throughout the world''.

Its report said the quality of the water in the 40km Cardrona River was ''excellent'', with ''generally low levels of nutrients and sediment''.

E. coli concentrations and suspended solid concentrations had improved between 2006 and 2011, the report said. However, water quality deteriorated after significant rainfall, with higher concentrations of bacteria, phosphorus, nitrogen and sediment.

''These events occur naturally and presumably have the potential to influence the groundwater quality in the Wanaka-Cardrona aquifer.

''On that basis, the groundwater quality in the Wanaka-Cardrona aquifer already reflects the filtering capacity of the gravel aquifer and overlying unsaturated zone.''

River water soaks naturally into the aquifer along a 3km stretch between the Larches, at the entrance to the Cardrona Valley, and the Ballantyne Rd bridge.

A 2011 report to the council showed the amount was significantly influenced by the water take of the Mt Barker, Farrant and Wanaka water races, irrigating 825ha of farmland.

Director, engineering hazards and science, Gavin Palmer is recommending the council note the report and provide it to stakeholders.

An aquifer ''recharge'' scheme began diverting water from the Rangitata River into the Hinds-Hekeao aquifer in Mid Canterbury in June to improve the quality of water containing high levels of nitrogen.