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Award-winning environmental scientist
Dr Louis Schipper of Waikato University talks about his work in developing the concept of denitrification walls for removing nitrate from shallow groundwater. It was this work, alongside his research into nitrogen saturation of soil and the effects of land treatment of wastes, that saw him win the Kudo for Environmental Science.
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DR LOUIS SCHIPPER
OK, what I've been interested in for a number of years now is ways of managing the amount of nitrogen that comes out of intensive land use. And behind us here, we have land application of wastes, but you can get nitrogen, excess nitrogen, coming out of intensive land uses as well. The trick is the way… how are we going to manage that nitrogen before it hits streams, because it’s moving with the groundwater down from the soil profile here into streams? Now, how are we going to decrease that nitrogen movement? What we have in nature is a group of organisms called denitrifying bacteria, and they convert that nitrate through to nitrogen gases, which is in the atmosphere around us, so we all win really. What we need to do is provide those organisms the right sort of home, and what they need is a food source, organic matter. And so what we did here was we dug a trench, took out the soil and mixed it with a carbon source – we used wood chips – and put that back into the soil. Now, the groundwater with the nitrate in it moves through that trench, and the organisms in the soil – naturally occurring organisms – convert that nitrate through to nitrogen gas. We built the wall back in – this denitrification wall, which is the sawdust wood chip soil mix – back in about 1996, and we monitored it for nearly 10 years, and we basically found that the nitrate concentrations moving into the wall were around about 5–15 milligrams per litre, and on the down slope side of the wall, it was consistently at zero. So it’s a very effective way for removing nitrate from groundwater. Our latest work that we are doing, together with Land Care Research and GNS Science, is to look at other ways of utilising that. What we are doing now is essentially filling large, if you like, swimming pools with wood chips and putting effluent through those wood chips. For example, we've got one up in the Bombay Hills, which is 200 metres long by 6 metres wide by 2 metres deep, filled with wood chips treating effluent from hot houses, and it’s a very small, relatively small, footprint way of getting rid of all the nitrate coming out of these effluent treatment systems.
