Ultra-filter brought in to deal with over-full dam

Watercare is having to take extreme measures to stop a large storage dam at the Omaha wastewater treatment plant from bursting its banks after more than a year of exceptionally heavy rains.

To save the 38,000 cubic metre dam, there have already been three emergency discharges of partially treated water into the wastewater system and, ultimately, Whangateau Harbour – the first a year ago, then two more after the January floods and cyclone in February.

Watercare is now saying that, even if the weather improves, the dam will still be at significant risk of overflowing over the next 18 months.

“A breach of the dam crest could damage the dam’s integrity, with a risk of catastrophic failure,” environmental scientist Laurent Daghdevirenian and environmental care manager Nathaniel Wilson said in a letter to the treatment plant’s community liaison group, Ngāti Manuhiri Settlement Trust and Rodney Local Board.

“There is limited storage. Watercare has been running the treatment plant at full capacity since July 2022, but even at this rate, dam levels have not significantly dropped.”

In a bid to prevent the dam’s failure, Watercare has brought in a temporary ultra-filtration plant to treat the stored water further, so more of it can be released safely into the wastewater system, and thereby lower water levels. Normally, any wastewater in the dam has been filtered and had nutrients removed, but not had “final polishing steps” that would be applied before being used to irrigate gum tree plantations off Jones Road and parts of Omaha Golf Course.

Watercare said it took regular water quality samples during the emergency discharges, and clarity and bacteria levels had been more than acceptable. However, bringing in the ultra-filtration plant from now until October would ensure the wastewater was even cleaner.

Daghdevirenian and Wilson said although the works may trigger a short-term breach of its resource consent for permitted volumes discharged to the Jones Road plantations, they believed any environmental effects would be “very small, if not negligible”.

They said Watercare would increase its monitoring, sampling and analysis from quarterly to monthly until six months after the discharge volumes returned to below the consented volume limits.

“In the unlikely event a meaningful change in groundwater quality is evident in the data, Watercare will contact Auckland Council immediately to discuss potential mitigation options,” they said.

“Watercare considers this approach the best means to mitigate the unprecedented rainfall experienced over the last year.

“The potential breach of the rolling

12-month average on the Jones Road forestry blocks is justified against the possible uncontrolled overtopping of the storage dam, which could led to the failure of the dam itself.”

Levels have remained dangerously high in the Omaha dam, which is next to the cycleway between Takatu and Jones Roads.

The wastewater treatment plant community liaison group’s annual meeting with Watercare is due to take place on August 8 at Point Wells Bowling Club, 4 Kowhai Avenue, starting at 5pm.

Choir invitation

The Matakantata Choir is inviting new singers to join its ranks. The choir, which formed 14 years ago, is a friendly and social team of choristers. It performs locally several times a year, presenting programmes with a wide range of musical styles. It has a new programme under rehearsal, so new singers need to sign up by August 17. The choir is directed by Jenny Eirena and rehearses in the Matakana Hall on Thursday evenings at 7pm. Enquiries: join@matakantata.org.nz or phone 027 436 4363.