PRGI: Pegasus Lake Update June 2021 Author: Matt James, PRGI President and Lake Sub-committee leader
SUMMARY: WHAT HAS HAPPENED SINCE OUR LAST REPORT?
Following the directive of Canterbury DHB, the lake remains closed to recreation users at the time of writing this report.
Templeton Group continues to maintain the Lake Management Plan by use of their contractors.
PRGI made written and in-person submissions to WDC’s Long-term- Plan (LTP) hearings, noting concerns over the worsening lake situation. These submissions were supported by four lake user groups and the Woodend- Sefton Community Board.
The specialist technical report reviewing the status of the lake commission from Golder by Templeton group has been received and reviewed. The report has identified four possible choices of technology that may assist in reducing the period of algal bloom each year.
A meeting to discuss the report was held on 29th April with WDC, ECan, Templeton Group, Golder and PRGI. The key points to note are:
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) just provide an environment that nurtures the development of the bloom, especially in warm weather periods.
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 next step is to narrow down the choice of technology to one or two options to be deployed in a reduced scale trial. Part of this review will be the consideration of costs, both upfront purchasing and ongoing maintenance, as eventually the lake will be vested from Templeton Group to WDC and their ratepayers.
THE CONSENTING CONDITIONS IMPOSED AT THE TIME OF THE ORIGINAL DEVELOPMENT. Four Consent conditions were set for the lake:
Must be well mixed, and
Must have oxygenated bottom waters, and,
Must not produce toxic (cyanobacteria) blooms, and
Must not produce other nuisance or persistent (phytoplankton) blooms
Currently, the vesting criteria to enable eventual ownership transition from the developer (currently Templeton Group) and WDC require all these conditions to be met continuously. Golder monitor these conditions regularly and feed their result to the DHB. The DHB review the results and issue lake closure warning if necessary. As the condition of the water entering the lake has worsened over time, it is questionable if these current conditions can ever be met without the water sources being improved. THE POSSIBLE TECHNOLOGY SOLUTIONS: Simply put, the choices of technology selected for review aim to either break up the stratification (layering) of water, or disrupt the microorganism within the lake, or filter the water entering the lake. The solutions use a mix of varied technology which include aeration (bubbles), oxygenation, soundwaves, and filtering. No option in isolation, or in combination, is a perfect solution. Each has its own pros and cons, with consideration of space, siting, noise, smell, and cost. Hence small-scale trials will be essential to help refine the best solution, balancing all these considerations. The option to convert the lake to extended wetlands was discussed but has been rejected by the stakeholders at this point. NEXT STEPS: PRGI aims to keep focus and momentum on this significant challenge with all the stakeholders involved. Ultimately, this will not be a quick nor simple fix. However, I remain positive that good progress will continue to be made. Hopefully, we may hear about trials soon.