Historical survey anomaly found; will help protect groundwater

19 Dec 2019, 1:39 PM

Recent work undertaken by the Northland Regional Council has revealed inaccuracies in bore survey data used to assess the Aupouri aquifer and set allocation limits to prevent saltwater intrusion into its northern parts.

The council’s Acting CEO Bruce Howse says water levels in north-eastern parts of the aquifer may be up to 2.5 metres lower than thought, but conversely up to 1.8m higher in north-western parts.

Mr Howse says that survey datum recorded about 30 years ago (between 1987 and 1991) and which was used in groundwater assessments and recent allocation modelling, is inaccurate.

The issue came to light recently when regional 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 Far North 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 northern-western side,” Mr Howse says.

Remodelling based on the new bore datums would be required to understand the full impact of the newly-surveyed datum levels.

Mr Howse says the council had already allocated roughly half of the water previously presumed to be available in the aquifer’s Houhora subzone, but to date monitoring indicated the existing use was sustainable.

“This potentially means the allocation limit for the northern subzones near Houhora needs to be reduced.”

He says 17 new consents were granted by Environment Court in July this year to take an additional two million cubic metres of water annually from the aquifer.
Those consents are located south of the key area of concern and have 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; essentially where salty water moves further inland and mixes with freshwater in the aquifer.”

Initial indications by checking all Aupouri monitoring bores datums against new LIDAR levels indicated differences ranging from 2.5m lower to 1.8m higher.

However, the key bore in the Houhora waterfront bore has the biggest negative difference and consequently has an increased risk of saltwater intrusion.

Mr Howse says an independent surveyor will be brought in to survey all the monitoring bores used to manage the Aupouri aquifer.

Regional council staff will 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.”

The council will also need to review current allocation limits as set out under its Proposed Regional Plan.

Mr Howse says the council is ‘very aware’ of the concerns many locals in particular had about the sustainability of Aupouri’s aquifer resources and the associated allocations from it.

“The completion of work to set the trigger levels for the resource consents in parts of the aquifer south of Houhora indicated there was an anomaly in bore datum levels which the council’s recently completed regional LIDAR survey helped confirm.”

“Council staff will now work with independent surveyors to rectify it and ensure new robust and accurate bore datums are collected and available as soon as is practical.”