City council to test deep-bore drilling

WASTE: Consent to discharge wastewater into Akaroa harbour expires in 2020. PHOTO: STEVE WILLIAMS

A new method for disposing of Akaroa’s treated wastewater is being tested.

Deep bore injection has been used to dispose of treated wastewater and stormwater in other parts of New Zealand, but has not been used on Banks Peninsula.

The city council’s acting head of three waters and waste Helen Beaumont said it is important to test the method.

“It’s not a new method – it’s been used in other places – but we need to know how it will work in the rocks specific to Banks Peninsula, and whether it’s effective, or even possible,” Ms Beaumont said.

At present, treated wastewater is discharged into the harbour via an outfall pipe out from Green Point.

The city council’s consent application for a new outfall to the harbour was declined in 2015. It has been exploring alternatives since then.

“The topography and erodible soils around Akaroa, combined with the volume of wastewater that needs to be discharged, means that irrigating all wastewater to land is challenging. This is why we’re exploring this new option of deep bore injection,” Ms Beaumont said.

Deep bore injection involves drilling several holes into suitable layers of rock. The treated wastewater then flows through naturally occurring fractures in the rock, and over a long period reaches the sea or harbour.

The treated wastewater is contained within the steel lining of the bore until it reaches the desired dispersal depth, when the lining changes to a mesh.

“During the tests, we will investigate the possibility of releasing treated wastewater into the ground below sea level, to prevent it entering any local springs – which is very unlikely because the treated wastewater would have to travel uphill,” said Ms Beaumont.

It is estimated that about four bores would be needed. These would be spaced 50 to 100m apart, with a flow rate of up to five litres a second down each bore.

“Treated wastewater would enter the bores either by gravity or with the assistance of a low-pressure pump,” Ms Beaumont said.

Before being able to consult the community on deep bore injection as one of the options for dealing with Akaroa’s treated wastewater, the city council needs to collect data to determine whether this method is suitable.

The proposed test drilling site is where the new wastewater treatment plant will be built, at the top of Old Coach Rd, with a second test drilling site across the road, alongside Old French Rd.

The drill will operate between 7.30am and 6pm, six days a week, and the work is subject to the noise limits of the area and has to comply with the conditions of the regional plan.