Trial tests how far you can bet on a detainment bund

property at Tapora, Kaipara Harbour.

and analysis of the trial sites over the next three years.

A TRIAL to test the effectiveness of detainment bunds on heavy soils is being rolled out on farms at either end of New Zealand.

John Paterson, the project manager with the Phosphorus Mitigation Project, said detainment bund trials to prevent nutrients and contaminants entering waterways have proven successful on free-draining central North Island soils.

Paterson said two studies on farms in the Lake Rotorua catchment showed bunds intercepted about 60% of phosphorus, sediment and contaminants carried by stormwater, preventing it from entering waterways.

That trial is now being extended to heavier, less well-drained soil types and other farming types on Al Body’s farm at Tapanui in West Otago and Tim Short’s

It is backed by a grant from the Ministry for Primary Industries’ Sustainable Land Management and Climate Change fund, the Otago Regional Council and the Auckland Council.

The West Otago trial is under construction now and the Kaipara Harbour catchment site is being completed, with instrumentation being installed.

“Accommodating the trials is a massive commitment by these farmers and shows the depth of their willingness to pursue new methodologies for helping achieve NZ’s water quality goals,” Paterson said.

Construction of a third hill country site is pending, awaiting final funding signoff.

Paterson said a study looking into whether bunds can mitigate the pathogen E coli is also underway, while the Phosphorus Mitigation Project has engaged NIWA to conduct field sampling

The best way to control phosphorus loss is to minimise release to the environment through good application practices, but Paterson acknowledges that is not going to contain it all.

“Even with the best of farm practices in place, some nutrient is still subject to being carried off farm in episodic high-intensity rainstorm run-off events.”

Paterson said the detainment bund concept is proven and a realistic option for farmers operating on well-drained soil types.

“The current ongoing research work will gauge if the exciting performance of detainment bunds on free-draining soils can be repeated in clay soils,” he said. That work will take three years to complete.

The Phosphorus Mitigation Project was established in 2016 and consists of a governance group made up entirely of farmers to initiate and direct applied science on new on-farm methods for environmental mitigation.