Recent seaweed build-up at Tītahi Bay is due to weather patterns rather than issues at the Porirua wastewater treatment plant, an environmental scientist has confirmed.
Concerns had been raised about a possible link between increased ammonia levels in the plant’s discharge and the large amounts of seaweed washing up on the beach. However, Stantec senior principal environmental scientist David Cameron said the seaweed surge was a natural occurrence.
In an independent assessment, Cameron stated that while ammonia nitrogen levels in wastewater samples had risen since June 2024, the overall risk of ammonia toxicity remained low due to dilution at the Rukutane Point outfall.
“The variation in seaweed biomass on Tītahi Beach is the result of recent weather patterns and does not need any particular management response,” Cameron wrote in an email to the Porirua City Council and Wellington Water.
Despite the plant’s discharge staying within its resource consent limits, a Greater Wellington Regional Council assessment rated the facility’s compliance as “poor” due to multiple breaches of consent conditions.
Issues at the plant stem from reduced oxygen levels in the biological treatment process, which caused a 10-fold spike in ammonia nitrogen levels in treated wastewater from June 2024. The problem persisted over 30 consecutive samples.
A Wellington Water spokesperson said on Monday that additional oxygen is now being dissolved into the treatment process, but it could take at least a month before operations return to normal.
Residents who notice any unusual water quality issues are encouraged to report them to Wellington Water or Porirua City Council.
