Council to test deep-bore drilling

Christchurch City Council is preparing to test a new method for disposing of Akaroa’s treated wastewater.

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

The Council’s Acting Head of Three Waters and Waste, Helen Beaumont, said it is important to test the method on Banks Peninsula.

“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.

The Council is investigating methods for disposing of Akaroa’s treated wastewater that avoid discharging into Akaroa Harbour.

The Council is investigating methods for disposing of Akaroa’s treated wastewater that avoid discharging into Akaroa Harbour.

At present, treated wastewater is discharged into the harbour via an outfall pipe out from Green Point. The Council’s consent application for a new outfall to the harbour was declined in 2015 and the Council 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 strata (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.

About four bores will be needed

It is estimated that about four bores would be needed. These would be spaced 50 metres to 100 metres 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. “These bores would be complemented with several nearby monitoring bores, to ensure everything is running as it should.”

Before being able to consult the community on deep bore injection as one of the options for dealing with Akaroa’s treated wastewater, the Council needs to collect data to determine whether this method is suitable for disposing of treated wastewater on Banks Peninsula. Deep bore injection could be a standalone option, or in combination with irrigation to land or non-potable re-use via a ‘purple pipe’.

Ms Beaumont said two test bores will be drilled to discover whether there is a suitable layer of strata at the target depth (below sea level) that is capable of absorbing the required flow, and to monitor where the flow travels and at what rate.

“A range of geophysical and hydrological tests will be conducted during and after the drilling, and the information gained from these will be used in groundwater modelling to assess the likely flow paths and how long it takes the treated wastewater to travel to the harbour,” she said.

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. Both these sites are owned by the Council.

Drilling to begin early next month

“We hope to begin drilling in early June, and expect testing to take eight to 10 weeks,” Ms Beaumont said.

The equipment used is typical of that used to drill water bores throughout Canterbury – a truck-mounted drill rig, she said. This will be supported by a couple of other trucks and a few smaller vehicles.

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.

Once the drilling and investigation work is completed, Council staff will present the findings to the parties to the harbour outfall consent appeal and the Akaroa Wastewater Reuse Working Party for their comment ahead of deciding whether deep bore injection is an option that should be included in the reuse and disposal options to be consulted on later this year.