Constructed Wetlands for Nutrient Attenuation

Abstract

The use of constructed wetlands in New Zealand and their role as an attenuation tool for removing nutrients from agricultural runoff are examined in this study. The need for attenuation is evident through frequent documentation stating that streams flowing through dairy farms are high in nutrients as a result of non-point source pollution from agricultural runoff and fertiliser use. Constructed wetlands are one such attenuation tool that has the potential to ameliorate these water quality issues via a myriad of chemical, biological and physical processes that allows the wetland to act as a filter for polluted water. The objective of this study was to quantify nutrient loads from a small dairy unit at Telford Research Farm to provide baseline data in which to test the efficacy of a constructed wetland. Weekly water samples were collected from the wetland during winter flow and more frequent samples were taken during event flow. These were analysed for total phosphorous (TP), total nitrogen (TN), nitrate (NO3), ammonium (NH4), phosphate (PO4) and total suspended sediment (TSS). Nutrient removal of 63% and 50% for 2011 and 2012 respectively for NO3, as well as consistent TP removal of around 25% indicate that this gully is acting as a wetland. Clear flood peak attenuation is also evident, a further characteristic of a wetland. However, variable results achieved under base flow and event flow by NH4, TN and PO4 combined with continued sediment export as high as 98% under base flow suggests otherwise. Future modifications at this site, such as the planting of macrophytes and creating pools within the wetland will increase attenuation of both N and P forms as well as total suspended sediment. Site modifications combined with best management practices and long term monitoring will ensure Telford farm maintains a high standard of water quality as well as generating valuable information to provide farming communities with knowledge on constructed wetlands and their potential to be used as a water quality management tool within the farming landscape of New Zealand.