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Pegasus Lake Sub-Committee Report

Updated: Nov 17, 2022

Recap

Over several years, Pegasus Lake has been closed each summer due to the presence of toxic algal bloom.

The lake is 90% fed by nine nutrient-rich natural groundwater sources. These sources have shown a worsening in water quality since the original consenting conditions were established. It is this contaminated water entering the lake which ultimately causes the bloom; our lake conditions (depth, slow-moving flows) provide an optimal environment that nurtures the development of the bloom, especially in warm weather periods.

Unfortunately, this research has shown that no solution is likely to address the algal bloom problem completely.

A realistic expectation may be reducing the period the lake is shut, due to bloom, each year. It is unlikely year-long usage of the lake will be achieved.


The lake owner, Templeton Group (TG), has been working with a range of national and international experts to determine the technical solution most likely to provide improved lake conditions. One was selected and submitted to ECan (details below).


Algal Bloom in the Region

Pegasus Lake is not the only lake or river that struggles with the bloom. Over the last week, Canterbury DHB has put Public Health Alerts in place for Algal Bloom in the following waterways:

  • Lake Forsyth

  • Tributary of Waiau Uwha River

  • Waikirikiri/Selwyn River

  • Wainono Lagoon

  • Kaiapoi River/Silverstream

  • Pegasus Lake

All these areas are closed off for public use whilst the bloom remains.

SUMMARY: WHAT HAS HAPPENED SINCE OUR LAST REPORT?

Templeton Lake Trial Submission to ECan

Upon guidance from their experts, TG has selected the preferred solution to trial. A planning application was submitted to ECan for review. After robust dialogue and review, the consent was approved by ECan in late November 2021.

How it will work

In simple terms, the technology chosen by TG aims to break up the stratification (layering) of water and disrupt the microorganism within the lake by using air bubbles percolating from special hoses laid along the bottom of the lake.

Trial location

The TG trial will split the lake into two sectors by an immersed curtain. The split will essentially follow the path of the suspension bridge dissecting the lake. The area nearest The Good Home and playground will be the trial area, whilst the other sector will act as an untreated control area to assess if any trial intervention makes a difference over the bloom session. The aeration technology will be powered by a generator taking its power from the same source that works the waterfall. The generator will be well insulated for sound dampening. Initially, the equipment will be run continuously but this will be reviewed depending on the results encountered.


Trial timing

As the seasonal algae bloom in the lake has begun, and the lake is already stratified, the experts have recommended that TG start the aeration trial in winter (approx. July 2022). Beginning the trial now might exacerbate the existing algae bloom by distributing algae material around the lake.


Delaying the start of the trial will allow for the aeration to begin once the algae biomass is at an annual low and the lake is not stratified, allowing for a greater chance of success with the trial. This also allows for additional baseline monitoring to be carried out during the bloom this summer. The updated programme schedule also means the aeration period will be longer (10 months) than initially planned, which will provide a longer period over which to assess outcomes.


Ongoing lake maintenance

TG undertakes weekly and monthly maintenance work in line with the approved lake management plan. This continued work has positively contributed to the delay in the bloom starting later this year – last year the bloom started in October. Approximately six tonnes of weed have been removed this year.


Templeton Group drop-in session in the New Year

Templeton Group is looking to arrange an information drop-in session in the New Year to talk with residents about the lake trial. The session will run from the afternoon through till early evening to cater to different residents’ availability so as many can attend as possible. The PRGI will host this event – location and date to be advised.


NEXT STEPS:

PRGI will continue to liaise with Templeton Group and lake stakeholders in monitoring the progress of setting up the trial and keep you informed of any developments.

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