Saturday, December 9, 2023

Bunds offer phosphorus solution

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Capturing phosphate in water spilling off farm catchments has been made easier thanks to work done by a Rotorua farmer group and a doctoral student who have developed detainment bunds on trial properties.
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A field day later this month gives farmers the chance to look at work that has largely been under the radar but offers a practical, farmer-focused solution to improving water quality. Richard Rennie spoke to the group’s project manager John Paterson.

While nitrogen mitigation has played on the minds of most regional councils and many farmers, phosphorus losses are also required, under the Government’s latest water quality rules, to be measured and curtailed. 

As a nutrient it has tended to attract less attention than nitrogen but in some catchments sediment losses are a greater problem and go hand-in-hand with phosphorus losses.

Mitigating those losses has focused on riparian plantings to absorb the slide of sediment and phosphorus into waterways. 

But the Rotorua Phosphorus Mitigation Project has gone a step further with its efforts to capture the deluge of high-phosphate runoff generated by rain in on-farm detainment bunds, letting it settle back into the soil before releasing the water.

Project manager John Paterson said the Rotorua area has been required to deal with phosphorus alongside nitrogen losses into waterways under Plan Change 10, even though phosphate loss does not have to be quantified.

“Twenty to 30 years ago the Rotorua Lakes District Council was building detainment dams to help reduce peak run-off flows and erosion issues. 

“Nine years ago I started looking at how these might be redesigned to also work for phosphate retention. It’s been my baby since really.” 

The new structures have twice the storage capacity and hold the run-off longer.

Support from the Lake Rotorua Primary Producers’ Collective helped kick-start the project with three farms used for the trials and setting-up measuring equipment on the bunds.

Several years and considerable doctoral input later the project is revealing the relatively simple earth bunds can catch about 60% of phosphate losses and 80% of sediments in stormwater runoff, depending on soil drainage conditions. 

The bunds are built as low earth mounds across gently sloping land on valley floors where stormwater paths occur during intense rain, now quite common in Bay of Plenty. 

The bunds hold stormwater in a ponding area for up to three days, providing time for the trapped water to soak into the soil but not compromising pasture growth. Because sediment particles and their attached phosphorus cannot infiltrate the soil, they settle onto the flooded pasture instead of running off over soil surfaces and going into the lake system.

The Bay of Plenty Regional Council is one of nine project funders and has partnered with farmers to build more than 20 of the bunds. 

Now the bunds are known to catch phosphorus and sediment their ability to capturing E coli needs to be investigated with applied research.

If successful it means the bunds will help mitigate three of the key water quality parameters under the proposed national standards.

Project chairman Lachlan McKenzie says the bunds could make a significant difference to lake water quality.

“This technology has the potential to intercept and treat stormwater for multiple benefits on a whole catchment scale that previously had not been considered possible.” 

It is critical the bunds be built with a suitable storage capacity accurately matched to their catchment area otherwise they will simply spill too readily, defeating their purpose.

He cautions they are by no means a silver bullet solution with trial farmers also working hard on adjusting wintering methods, stocking rates, cropping and fertiliser application into management plans to also retain phosphorus on the farm.

“A proactive management approach is best but a certain amount still gets through and these bunds are just the final scrubbers to intercept and collect that.”

And the bunds are also not suited to all farm landscapes.

“You need a broad valley floor with relatively small catchment areas, less than 50ha or so ideally.”

The group’s work has managed to not only prove the bunds’ value but also devised a technique to read landscape contour and flow paths and to analyse eight attributes through a calculator model that accurately predicts a catchment’s suitability for a bund.

The group is awaiting the final thesis from Massey University doctoral student Brian Levine after which a how-to guide for farmers who want to use bunds will be written and made freely available.

The group is hosting a field day on one of the trial sites for interested farmers on October 30 at Kaharoa.

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