Benefits and costs of a constructed wetland on a Wairarapa dairy farm

Abstract

A demonstration wetland was constructed with community support in what was a "wet" 0.75 ha of a Wairarapa dairy farm. This has reduced the level of nitrate-nitrogen leaving the farm, and has also added biodiversity to the farm and the region. Comparison of nitrate-N levels of water flowing in and out of the wetland over three months show this water quality benefit may reduce farm N loss from 14 down to 13 kg N/ha/yr, which equates to an ongoing reduction of around 7% for a one-off investment of at least $55l000. However, other contaminants such as dissolved reactive phosphate may not necessarily be reduced, while counts of Escherichia coli increased in wetland outflow. This project required a range of skills unlikely to be available to an individual farmer without wider community support but now that the wetland is established it will demonstrate what is possible when more complex solutions are implemented for nutrient loss mitigation. Keywords: dairy farm, constructed wetland, denitrification, nitrate-N, water quality, biodiversity