Despite figures showing Cromwell's terrace aquifer is technically one of Otago's over-allocated bodies of water, an investigation has shown there is actually "loads of water", Otago Regional Council groundwater scientist Jenns Rekker says.
The council recently assessed the groundwater resource and allocation of the 22.2sq km aquifer under the Cromwell township and bordered on two sides by Lake Dunstan, as part of its gathering of information to inform future consent decisions.
Mr Rekker said the aquifer received water from rainfall and irrigation excess and infiltration of lake water at Ripponvale.
About 17% of the irrigation water applied over the Cromwell terrace was pumped from bores in the aquifer. Because the lake bordered the aquifer, increases in bore extraction were balanced by additional infiltration of lake water, Mr Rekker said.
"The filling of Lake Dunstan has greatly increased the groundwater availability and further sedimentation of the Kawarau Arm would have an overall beneficial effect on the groundwater resource."
That connection with the lake meant a "relatively flexible" approach could be taken to setting groundwater extraction volumes.
"Because of that strong connection, there is nowhere near the concerns about recharge. In reality there is loads of water."
There was an interim default allocation limit for groundwater which was set at 50% of the mean annual recharge of the aquifer, which for Cromwell was 1.2 million cu m per year.
At present, there were consents granted to take 1.7 million cu m per year, which, under the council's water policy, meant the aquifer was over-allocated.
However, in reality only 400,000cu m was estimated as being taken annually.
Mr Rekker suggested the best way to manage the aquifer's allocation was to bring in a maximum allocation volume of four million cu m per year, possibly combining it with a water-take restriction level decided by the community.
"Four million cu m per year for the Cromwell Terrace Aquifer would provide an adequate protection level for the aquifer, although in order to prevent the adverse effect of a generalised water-table decline, it is suggested that the measure be combined with a water-take restriction zone."
