Surface Flow Constructed Wetlands as a Drainage Management Tool – Long Term Performance

Subsurface tile drains are used extensively throughout many agricultural areas of New Zealand to reduce excessive wetness from grazed pastures. Along with transporting water, subsurface drains also transport nutrients which have leached from the soil, potentially bypassing natural attenuation processes in shallow groundwaters and riparian zones. These nutrients have the capacity to cause excessive “weedy-ness”, algal growth, oxygen depletion and alterations to natural ecological processes in streams, lake and estuaries. Surface flow constructed wetlands intercepting and treating these drainage waters before discharge into surface waters are one of the tools employed available to farmers to manage nutrient losses. This paper quantifies long-term drain yields and nutrient removals associated with 2 constructed wetlands (situated in Waikato and Southland) planted with native raupo (Typha orientalis) and occupying 0.66–1.6% of their respective catchments. Previous results from these sites (plus an additional irrigated site), quantifying flows, and nitrogen and phosphorus yields from tile drains over continuous 3–5 year periods, and evaluating the capacity of the wetlands to reduce nutrient loads, have been published elsewhere. This paper presents results from an additional year of sampling undertaken 3 years after the earlier periods of study, and compares them to the earlier data. Overall annual drainage water yields ranged from 193–564 mm (16–51% of rainfall), with annual exports from the tile drains of 14–66 kg ha -1 of Total Nitrogen (TN) of which 58–97% was nitrate-N. The two constructed wetlands intercepting these flows removed 30–369 g TN m -2 (7–63%) of influent loadings annually. Annually, the tile drains exported 0.12–1.79 kg ha -1 of Total Phosphorus (TP) of which 33–93% was Dissolved Reactive Phosphorus (DRP). In contrast to the TN results, the phosphorus removal was generally poor in both wetlands, with twice as much TP exported overall than received in some years, although modest removals of up to 32% occurred in other years. These results demonstrate the need for research into reducing nutrient leaching from soils and improved nutrient attenuation within wetlands, particularly for phosphorus. Nutrient removal performance from these and other wetlands, both nationally and internationally, have been used to develop guidelines for constructed wetlands treating agricultural drainage water. Introduction As one of the world’s leading agricultural exporters, New Zealand works hard to maintain its “Clean & Green” image. Over the last 40 years there has been a major reduction in pointsource agricultural pollution from farm dairies. Dairy farmers have either used waste stabilisation ponds to treat effluents before discharge (Sukias & Tanner 2005) or re-applied them to land to reduce direct pollution of surface waters, and utilise the fertilizer value of these wastes. With the overall reduction in direct, or point-sources, of pollution, there is a greater ability to measure the influence of diffuse nutrient losses, and grazing intensification in recent years has tended to exacerbate the problem. In New Zealand, as elsewhere in the