Fonterra says it is not dwelling on what its nitrogen (N) leaching rate is on its wastewater farms, but rather on how it can reduce the loading and reduce its environmental impact.
Fonterra has 16 of these farms across the country adjacent to its factories. It uses these farms to dispose of wastewater left over from milk processing.
The usage of these farms as a means of disposing of factory waste came under fire recently after a Radio New Zealand investigation revealed elevated levels of nitrates in the drinking water used by residents who live close to its Hautapu factory.
It supplied 38 water filters to nearby residents to remove nitrates from the drinking water.
That factory has three such farms, which total about 300 hectares.
Fonterra’s general manager of environmental operations Ian Goldschmidt says the amount of N being sprayed onto the three farms varied from 270-460kg N/ha, averaging about 360kg/ha of N.
It is irrigated at 25mm doses using in-ground irrigation with its duration dependent on weather conditions, followed by a 14-day rest period to allow the pastures to absorb the nutrients.
However, the actual amount of N left on the farms that could potentially leach into waterways is around 120kg N/ha.
This is because Fonterra removes nine tonnes of drymatter/ha of pasture by mowing, which is then sold to farmers as grass silage.
Goldschmidt says removing that pasture equates to the removal of 240kg of N/ha, leaving 120kg N/ha if taken from the average.
This is slightly less than half the average N use on dairy farms in Waikato and Bay of Plenty, which amounts to 128kg N/ha, according to a 2019 AgFirst study.
“You would probably get around 120-200kg N there, which is in line with the Government’s requirement of 190kg N,” he said.
When asked if that was enough to cause N leaching into water supplies, Goldschmidt says “that’s a pretty hard question to answer”.
“We’re focusing on where we want to get to, which is getting that leaching rate down to 30kg N, which we know will deliver improvements on the impact that we’re having,” he said.
Fonterra removed the cows a few years ago, turning the land into a cut and carry operation to reduce the N loading. The loadings were similar when cows grazed the farms, but cutting and carrying the grass allowed the co-operative to remove some of N.
“The nitrogen loadings were similar to what they are now, but we have this significant removal of N from that cut and carry operation,” he said.
The wastewater treatment plant Fonterra hopes to build would reduce that loading even further, he says.
“N leaching would be two to three times less than what we are trying to achieve in the future,” he said, adding that overseer modelling would bring it down to no more than 30kg N leached once that plant is built.
It currently varies between 30-70kg N/ha and takes into account the cut and carry operation. The farms also have farm environment plans (FEPs) and irrigation management plans.
Fonterra lodged consents in 2018 for the wastewater plant, which was to be built on one of those farms. However, local residents rejected the proposal and Fonterra is assessing alternative locations for the plant.
“We’ve been engaged with the community around the Hautapu site for a long time,” he said.
Across its other sites, it is undertaking a $400 million upgrade over the next six to 10 years.