The Waituna Sediment Fingerprint Study by AgResearch scientists Richard McDowell, Matthew Norris, and Neil Cox has been released by the council and is available on its website.
Waituna Lagoon, near Invercargill, is part of the internationally recognised Awarua Wetland, one of the largest remaining wetlands in New Zealand. Recently, an increase in sediment in the lagoon has heightened fears the lagoon may flip to a eutrophic state, so the regional council wanted to identify the source of the sediment.
The number of dairy cows in the area has more than doubled since 2000 with an estimated 20,000-30,000 cows now grazing there. Forestry, sheep, beef and arable are the other land uses in the 20,000ha catchment.
Using soil fingerprinting technology, the report’s authors were able to identify that sediment – reaching the lagoon from one of the three streams which flow into it – was mostly from the streams’ banks.
All three streams – the Carran, Moffat and Waituna creeks – are cleaned by the council every three years. It was not able to be determined whether sediment in water samples from the Carran and Moffat creeks were from topsoil, subsoil or bank sediment.
Sediment in the Waituna Creek was mostly from the creek’s banks, with the rest topsoil. That creek, unlike the others, was cleaned by the council early in 2012 and water samples were taken for the study from April that year until May last year.
Even though water samples later on in the study were from greater flows due to storms, it was those taken in mid-2012 that had the highest sediment levels.
Environment Southland marine and freshwater scientist James Dare said the council was already adopting and promoting new drain cleaning methods (Dairy Exporter, March, p66) and was monitoring its effects on water quality, sediment and fish life.
The council has also released a report from NIWA scientists on building wetlands in the Waituna area to capture sediment and nutrients before they enter the lagoon.
The report, prepared for the council and DairyNZ, looked at 30 wetland options at 14 sites across the catchment, ranging from large main-channel wetlands up to 50ha in size to small wetland areas of 600m2.
Estimated construction costs ranged from $2000 to $3.35 million depending on size and site characteristics. Proposed areas at the bottom of the catchment need large-scale excavation due to the low gradient and there’s also the potential to lessen drainage on adjoining land.
Proposed wetland sites are ranked according for their effectiveness in reducing the number of contaminants entering the lagoon but the report also recommended that other factors such as land value, accessibility and landowner co-operation should be considered.
DairyNZ water quality specialist, David Burger, of Hamilton, said a further study was being done to identify a possible wetland site which could be built as soon as the end of the year in the catchment to enable further research.
“We want to make sure the science is working and that we can apply it.”
Dare said the council was already converting a duck pond into a wetland area in the catchment to observe the effects.
“We’ve worked with the farmer and excavated an area below the duck pond which we’re going to plant in a local native species which has a high uptake of nitrogen,” he said.
“Below that we’re going to have oyster shells and we’re hoping the phosphorous in the water will bind with the calcium carbonate in the shells. We’re building it as a demonstration and hopefully farmers will be able to use these tools to keep farming at the same levels but meet new nutrient rules when they come into effect.”
Burger said another study, funded by NIWA, DairyNZ and the Department of Conservation and Fonterra partnership, was about to start looking at materials which could remove nitrogen and phosphorous in water at the end of tile drains.
“There are a lot of tile drains in the catchment and we’re looking at bioreactors such as wood chips to remove nitrogen and substrates to remove phosphorous.”
The research at Waituna was of interest to the rest of the country as the science could be applied to large chunks of grazing land.
