Rain gardens could help clean lake in Cambridge
Roadside rain gardens could help eliminate contaminants that stormwater systems have piped into a popular Waikato lake for half a century.
A total of 130 rain gardens could be constructed on high-volume traffic roads that had stormwater connections to Lake Te Koo Utu, a 20-hectare reserve in the heart of Cambridge.
The gardens, about $18,000 each, work like a wetland trapping and filtering oil, fuel and rubber residue from road surfaces, before it all flows into the lake via stormwater.
In the near future, more rain gardens may be deployed around other streets of Cambridge and Te Awamutu, to comply with the anticipated higher stormwater treatment levels required by the Waikato Regional Council.
The rain gardens proposal was presented to the Waipa¯ District Council’s service delivery committee as a concept to improve the water quality and health of Lake Te Koo Utu.
Feedback to the council’s concept plan for the lake, completed in May, showed public support to sort out the lake’s ongoing water quality problems.
Regional council monitoring revealed the lake had elevated levels of nitrogen when measured against regional and international guidelines.
Research suggested stormwater from nearby homes and roads for 50 years or more, was to blame for the elevated nutrients and contaminant levels in the lake.
Water services manager Martin Mould, in his report to the committee, outlined a three stage plan to tackle the problem.
Stage 1 was a two-year project to start in 2021, at a cost of $1 million, to construct the first 12 rain gardens. It also involved installing three ‘‘propriety devices’’ connected directly to the stormwater outlet into the lake, as another measure to filter out contaminants.
Mould said there was a budget for stage 1 but stage 2 and 3 would need to be budgeted for in later years.
Those two stages would collectively cost about $1.6m, and involve 118 rain gardens, constructed every 80m along four other high traffic roads.
Councillors liked the idea of the rain gardens and the propriety devices working to remove contaminants heading into the lake.
But some, like councillor Clare St Pierre, were worried about the high cost of the project and how it could be monitored to show the public it was working.
‘‘I was surprised at the cost of the rain gardens, $18,000 each,’’ she said.
Councillors heard the gardens would have to be retrofitted into the existing road networks, which generated a higher cost, compared to installing them as part of a new roading development.
The gardens would have to be removed and replaced every 5-10 years and the cartridge filter inside the propriety devices would have to be cleaned regularly.
Mould said the new system would be monitored pre- and post-installation as part of the council’s work to apply for a new resource consent from the regional council to manage stormwater.
Committee chairman Graeme Webber asked whether the council could apply for funding for the project through the Waikato River Authority.
Service delivery group manager Dawn Inglis said there were opportunities to pursue some funds.