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An investigation is being carried out after a large storage dam at the Omaha wastewater treatment plant became dangerously full and was in danger of collapse after heavy rains last month.
Watercare staff were faced with either risking the 38,000 cubic metre dam bursting its banks or pumping out excess water straight into its tree plantation to save the pond’s structure. They chose the latter.
The dam, which sits next to the public cycleway between Takatu and Jones Roads, is used to store excess treated wastewater over winter, when the ground is too wet for it to be dispersed via the plant’s forestry irrigation fields.
In an email to the plant’s community liaison group, environmental manager Nathaniel Wilson said staff worked day and night to minimise any environmental effects and said they were “hugely disappointed” the incident had occurred at all.
“Inflows into the plant have been abnormally high for a lot longer than usual, to the point where our dam, which we use to store treated wastewater over winter, became dangerously full,” he said.
“We were faced with risking dam collapse or sending some of the dam water into the forest as an overland flow, in addition to our normal irrigation discharges.”
He said the post-treatment pond water had been screened and had nutrients removed, but had not had its “final polishing steps” that would normally be applied before it was used to irrigate the trees.
The “overland flow” pumping operation lasted several days until the dam level dropped sufficiently, with water from the pond flowing into the stormwater system and, ultimately, the Omaha River.
Wilson said Watercare took regular water quality samples throughout and from the start, clarity and bacteria levels were lower downstream from the discharge area than upstream from it.
Wilson said the plant had been handling wet weather flows well until this happened.
Whangateau Harbour Care Group member Elizabeth Foster claimed the incident was avoidable.
“When I was on the Rodney District Council in the 1990s, we were shown a plan for another overflow pond to be constructed adjacent to the current pond. This was never built,” she said.
“I brought this issue up more than once in the early stages of this community group but, once again, no action was taken. Will this happen now?”
Wilson said any need for infrastructure upgrades was something that would be reviewed in the incident investigation.
