Scientists To Help Combat Lake Hood's Toxic Dilemma

Tuesday, 19 August 2025, 2:55 pm
Article: Jonathan Leask - Local Democracy Reporter

Monitoring of Lake Hood is due to start next month in an effort to stave off algae blooms that have plagued the lake over the last three summers.

The Ashburton District Council has called in scientists to understand what drives the algal blooms, with the thinking shifting away from simply adding more water.

Chief executive Hamish Riach had declared the lake had “a whole smorgasbord of issues” when the council was considering a 75-page report on the lake in May.

The council decided to investigate its options thoroughly rather than rush into spending ratepayer money on potential solutions.

Infrastructure and open spaces group manager Neil McCann said cyanobacteria expert Professor Susie Wood, Aidin Jabbari from NIWA, and Lincoln University students will help the council to gather information for a hydrodynamic model of the lake, as well as trialling equipment and products that might slow the algal growth over summer.

“The more we learn about the particular species of cyanobacteria in Lake Hood, and how it thrives, the more we believe the solution to Lake Hood’s water quality is way more than increased water flow and circulation,” McCann said.

“We had a significant amount of water this past summer, far exceeding the previous year’s volumes, and most of the water was directed around the north western canals for the first time, and the cyanobacteria still bloomed.

“This suggests there is far more to mitigating the cyanobacteria bloom than increasing the amount of water and creating more circulation, which is why we have engaged experts in the cyanobacteria field.”

In the most recent summer, around 17 million cubic metres of water flowed through the lake, 80% of it via the extension and canals he said.

The average flow over the previous five years was approximately 7 million cubic metres, and ranged from 5.3 to 10.9 million cubic metres.

“We have learnt there are many different species of cyanobacteria which behave quite differently depending on the particular environments they grow in,” McCann said.

“We need to understand more about the species we have in Lake Hood and what sort of nutrients or conditions drive them. That will help determine how best we can mitigate the blooms.”

That monitoring is expected to start next month, and the data will help create a baseline of lake conditions before the water temperature increases.

The hydrodynamic model will simulate water circulation patterns, which are key to understanding how nutrients enter the lake and where they settle and accumulate, which then contribute to the algae blooms.

McCann said council officers were meeting fortnightly with the scientists to plan work, and some Lake Hood residents are volunteering to help with water sampling and maintaining monitoring equipment.

Wastewater plans for Mid Canterbury

Meanwhile, the council is enlisting the help of wastewater engineering experts to develop a wastewater strategy.

It will include assessing current wastewater flows and predicted demand, as well as extending wastewater services into Hinds and Mt Somers, stormwater infiltration into the wastewater network at Tinwald, and disposal options for treated wastewater from the Ashburton scheme.

McCann said the council is asking specialist consultants to register interest in developing the wastewater strategy.

“Ideally, we’d like to identify some projects by Christmas so they can be built into the 2026-27 Annual Plan and budget.”

McCann said the strategy isn’t about assessing existing plants, but making recommendations for the future, including prioritising pipe renewals, keeping up with technology improvements, and how to keep up with anticipated future changes in demand and in consents and regulations.

Ashburton District has three wastewater treatment plants, Ashburton and Rakaia – both undergoing upgrades in recent years, and Methven.

These three schemes service about 65% of the district’s population.

-LDR is local body journalism co-funded by RNZ and NZ On Air.

© Scoop Media

Local Democracy Reporting

Content from the Local Democracy Reporting (LDR) service is published by Scoop as a registered New Zealand Media Outlet LDR Partner.