The catchment, which is part of the internationally significant Awarua Wetlands, has been in the spotlight since early 2011 when Environment Southland became concerned the lagoon might flip to a de-vegetated and turbid state. There are about 80 farms in the area, half of them dairy.
The study, a partnership between DairyNZ, Fonterra and the Department of Conservation’s Living Waters Partnership, National Institute of Water and Atmospheric Research (NIWA), Environment Southland and local farmers, is investigating whether filters at the end of tile drains, which are under most Southland farms, can remove nitrate and phosphorus from water. High levels of the two, plus sediment, have been identified as a risk to the health of the lagoon.
“We are trying to develop effective and practical tools that farmers can use on their farms to help remove nitrates and phosphorus from their drains,” project co-ordinator and DairyNZ water quality specialist Dr David Burger said.
“Ideally these tools have to be something they can build and maintain themselves and they have to work under a variety of environmental conditions.”
The nitrate filter has been working since November on Drakes Hill Farm which is on Mokotua Brown soils which contain clay with little gravel and are poor-draining.
Water flows from a tile drain into a sediment trap and then into the filter, which is a 10m by 10m square plastic liner filled with wood chips 1m deep. The bacteria in the filter convert the nitrate in the water to nitrogen gas which is released into the air. The water flows back into the farm’s tile drain system.
“The bacteria occur naturally in the filter, we don’t put them there,” Burger said. “They’re using the carbon in the woodchip as a food source and the nitrate in the water as part of their respiration process.”
NIWA has set up the filter and monitoring equipment, which captures live data of water flow rate and temperature and automatically takes samples of the water before and after for analysis.
Monitoring will be done over the next two years to assess the effectiveness of the system under different conditions.
“We’re not sure how temperature will affect removal rates. We think that low temperatures in winter will slow the bacteria’s activity so fewer nitrates will be removed but we will have to wait and see if that happens.”
The filter is safe to walk on and overseas studies have shown it could have a life of 15 years or more before the woodchips need replacing.
Burger estimates the cost at $5000- $12,000, depending on the filter size and volume of water to be treated.
“But it could be a lot less if the farm has a digger and a tree that can be chipped.”
“It looks very simple and it is very simple. We just need to see if it actually works in these kinds of landscapes.”
Farm partner Ewen Pirie was impressed with it.
“It’s an interesting experiment but we have to see how it will work practically. We have 130 separate tile drains that flow into the Waituna catchment.”
He said the farm, which milks 585 cows, was converted to dairy about seven years ago, just as problems with the lagoon were starting to be realised.
“When we converted we did everything possible to stop nutrients leaving the farm. We fenced off all the wet areas and we had the first low-rate application effluent system in the catchment.
“We’re interested in anything that we can do to improve the health of the lagoon.”
The phosphorous filter on Craig and Heidi Williams’ farm, which is only 4km from the lagoon, is still being set up and looks like a large plastic box of kitty litter, rather than cutting-edge science.
“It’s actually modified zeolite,” Burger said. “Zeolite is also used as kitty litter and absorbs ammonia. Modified zeolite absorbs phosphorus.”
The phosphorus filter isn’t limited by temperature.
“The phosphorus becomes permanently bound to the zeolite so there is no risk of it being released back into the water. It’s quite stable and it’s not toxic.
“The zeolite comes from the North Island so we would like to see in the future if a naturally occurring substance from Southland, such as crushed oyster shells or limestone, would do the same thing.
“Ideally we would like it the size of a suitcase and farmers then could just lift it in and out with the tractor.”
DOC project leader for Waituna, John McCarroll, said the two filters could be important tools for farmers in the catchment in the future.
“They won’t be the only tools but we’re keen to prove the science and help support farmers to use them if they work,” McCarroll said.
“We’re excited about proving the integrity of the science and if they work here they could work elsewhere in the country as well.”
Craig Williams milks 900 cows on the farm where the phosphate filter is being set up. It was converted to dairying about 22 years ago.
“Since we became aware of what was happening in the lagoon we’ve changed a lot,” he said.
“We’ve done more riparian planting and now use low-rate application K-lines for effluent from the dairy over a much larger area of the farm.
“We’ve also split our fertiliser applications so we’re putting on lower rates but more often.
“We really noticed a difference when we soil-tested. We hardly have to put any phosphorus on now at all, only sulphur.
“The fertiliser company said we can’t just put sulphur on, they have to mix it with something. We laughed.”
