A presentation for today’s full council workshop said a new assessment of land disposal options in the Wakatipu Basin found only the Crown Terrace could provide enough connected land area to make the option technically possible.
It concluded that 288ha of land — about 400 rugby fields — would be required to absorb the estimated 2060 peak wet weather flow of 60,000cu m of treated wastewater a day.
The presentation by council project manager Scott Paterson said the wastewater would need to be pumped to the area through a 15km pipeline up 360m of vertical rise.
The land disposal option, which is the strong preference of iwi, was added to the four short-listed options presented to councillors at a December 5 workshop.
Councillors will choose the final option at a full council meeting on March 19.
Mr Paterson said council staff’s preferred option from a technical standpoint remained direct discharge into the Kawarau River, via a rock-lined channel, at a cost of up to $44m.
It offered the most certainty for dealing with future wastewater flows, had the simplest design and operation, the lowest build and operating costs, and could be completed by the Environment Court’s 2030 deadline.
The Environment Court has given the council until the end of 2030 to deliver a new disposal system to replace the treatment plant’s failed disposal field on the Shotover delta.
The disposal field was taken offline when council management invoked emergency powers to begin discharging directly into the Shotover River last March.
In December, Allied Media reported the other three options in front of councillors.
A "sub-surface" wetland on the Shotover delta, through which treated wastewater would flow before entering a surface channel or pipe to the Kawarau River, was the second-cheapest option with an estimated cost of up to $80m.
Two other options involved discharging treated wastewater into deep bores, or shallower soakholes, drilled in council-owned land on the Frankton Flats, including sports fields and road corridors.
Estimated to cost up to $186m and $206m respectively, they would operate in combination with the wetland because neither would have the capacity to dispose of all the plant’s output.
However, council operations and maintenance manager Simon Mason told councillors in December all four options would have "approximately" the same impact on the Kawarau River, with the impact on water quality expected to be "less than minor" once dilution had occurred.
Meanwhile, a study commissioned by the Otago Regional Council indicated the discharge of treated wastewater into the Shotover River for the next five years would degrade the river’s water quality, but serious harm to aquatic life was unlikely.
