Southland an indictor for nitrates

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    Hydrogeologist and aqueous geochemist Dr Clint Rissmann, of Invercargill. Photo: Supplied

    SOUTHLAND is a “crystal ball” for the rest of the country when it comes to reducing nitrates harmful to the environment, hydrogeologist and aqueous geochemist Dr Clint Rissmann, of the University of Canterbury and Lincoln University, says.

    Dr Rissmann’s comment was made in response to a study published last week by the British Geological Survey (BGS) which reported up to 180 million tonnes of nitrates could be stored in underground rock (aquifers) worldwide as a direct result of farming.

    The paper’s lead author, hydrologist Matthew Ascott, of the British Geological Survey (an independent scientific authority) and Lancaster University, said the study included recorded nitrate levels from New Zealand’s Waikato and Waipa river catchment areas.

    When nitrate was present in soils in excess of farmers’ requirements for crop growth, it leached into rock not fully saturated with water, he said.

    Nitrate was also passed through the urine of animals feeding in paddocks boosted with nitrate fertilisers.

    Dr Rissmann, of Invercargill, said the Te Anau, Wendonside and Balfour areas were areas where the kind of nitrate storage described in the global study occurred.

    These areas had slightly longer “lag times” between nitrate being lost from the crop root zone and its drainage to aquifers, he said. Aquifers with elevated nitrate then discharged the nitrate into streams. “We don’t often realise when we look at these streams [during dry spells], often 100% of the water is derived from aquifers. In Southland, on average 50% of the water in streams is from aquifers.”

    Nitrate contained in this water caused increased algae and weed growth which depleted oxygen in rivers, which could kill fish and other wildlife, he said.

    However, the rest of the region had short lag times of two to three years, and leached nitrate moved rapidly through the environment relative to other parts of the country like Canterbury and the Waikato, he said. “This is a real advantage for the region, we could see rapid improvements.”

    With the efforts farmers were already making, such as the use of ‘stand-off pads’ to contain animal urine and effluent so it could be processed in effluent ponds, a difference had already been seen.

    “The Edendale aquifer is a good example – we’re already starting to see a decrease in nitrate levels there.”

    Although this improvement could not be proven to be a result of environmental actions taken by farmers in the area, Dr Rissmann believed it probably was.

    Farmers, industry groups and local and regional councils in other regions were looking to see what worked in Southland so they could apply similar tools and methods, he said.

    “Southland’s short lag times give us a competitive advantage, because changes that are made on the land are seen more rapidly than in other regions, making Southland a key place to understand how reduction in land use losses can improve water quality.”

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