The Ashburton Water Zone Committee has recommended that Environment Canterbury agree to fund a biochar trial in Carters Creek to see if phosphorus can be filtered from the water before it empties into Lake Hood.

High levels of phosphorus are thought to be contributing to an algal bloom in the lake, which has been under a Te Whatu Ora health warning since early January.

The zone committee recommended ECan grant $10,483 to fund the trial, which involves laying mesh bags of biochar (a granular form of carbon) across the stream before it runs into the lake. The application was made by the Ashburton Aquatic Park Charitable Trust, which is working with the Lake Hood Water Quality Taskforce and Council on potential solutions to the cyanobacteria problem. The funding will be released to the Trust once it has been formally approved by Environment Canterbury.

Technical advisor Les McCracken said the creek was dry at the moment but when it was flowing again, water would be tested weekly for phosphorus levels before and after passing through the biochar.

“The trial will last about two months, depending on how flows go in the creek and the results.”

Water is also tested regularly at the intake from the Ashburton River, in the canals and where it exits the lake into the river.

Mr McCracken said there were other water bodies in the South Island facing similar problems and the results would be of wide interest.

“There is a need for people to do practical things, real things and try to make changes.”

The Water Quality Taskforce is gathering information about phosphorus concentrations to help decide on actions to reduce the risk of algal blooms at the lake. Total phosphorus levels have been highest in the Carters Creek inflows and lake intake.

A report from NIWA on lake management options recommends nutrient controls at the Carters Creek and river inflows by using various products to prevent phosphorus release within the lake or reduce it flowing into the lake.

Other options involve flushing the lake by increasing water flow into and around the lake and undertaking changes to the lake infrastructure to enable this water circulation.

Group Manager Infrastructure and Open Spaces Neil McCann said the report was being digested by taskforce members and another meeting was planned at Lake Hood to keep residents up to date.

“Understanding the water quality issues at Lake Hood and what drivers are contributing to algal blooms is important for the longterm management of the water. Lake Hood is an asset for the whole community and for the thousands of people to visit it to boat or picnic.”

The NIWA report used information from historical water quality testing from about 20 sites, including piezometer data, field measurements, water temperature, clarity, and showing levels of phosphorus, nitrates, chlorophyll and E-coli.

No main contributing nutrient load on the lake was identified.

The NIWA report can be found at ashburtondc.govt.nz.