Work undertaken by the Northland Regional Council has revealed inaccuracies in bore survey data used to assess the Aupōuri aquifer and set allocation limits to prevent saltwater intrusion.
Acting CEO Bruce Howse said water levels in north-eastern parts of the aquifer may be up to 2.5m lower than thought, but up to 1.8m higher in the northwest.
That survey datum recorded between 1987 and 1991, and which was used in groundwater assessments and recent allocation modelling, was inaccurate had come to light when council staff were undertaking preliminary surveys of bores to establish 'trigger' levels for resource consents in parts of the aquifer south of Houhora.
The bore datum was also used to assess the effects of 24 current consent applications by avocado growers seeking groundwater from the aquifer.
"In a nutshell, this discovery potentially means there's less water available in the north-eastern part of the aquifer than previously thought, although there may actually be more available on the northwestern side," Mr Howse said.
Remodelling based on the new data would be required to understand the full impact of the newly-surveyed datum levels.
Mr Howse said the council had already allocated roughly half the water previously presumed to be available in the aquifer's Houhora subzone, but monitoring indicated existing use was sustainable, potentially meaning that the allocation limit for the northern subzones near Houhora needed to be reduced.
Seventeen consents were granted by the Environment Court in July to take up to an additional two million cubic metres of water annually from the aquifer south of the key area of concern, with stringent conditions requiring monitoring bores located near the coast and further inland.
"There are also trigger levels to be set under the consents to avoid saltwater intrusion," Mr Howse said.
Initial indications by checking all Aupōuri monitoring bore datums against new LIDAR levels indicated differences ranging from 2.5m lower to 1.8m higher. The Houhora waterfront bore had the biggest negative difference, and consequently an increased risk of saltwater intrusion.
Mr Howse said an independent surveyor would survey all the monitoring bores used to manage the aquifer. Council staff would also be obtaining water level and water quality information for an additional 60 bores over the next two months.
"We'll then need to rerun allocation models with the new datums, including results from the new bore survey and more recently obtained data," he said.
The council would also need to review current allocation limits, as set out under its proposed regional plan.
The council was "very aware" of the concerns many local people had about the sustainability of the aquifer and allocations of the water. Council staff would work with independent surveyors to ensure new robust and accurate bore datums were collected and available as soon as was practical.