Nutrient removal by a constructed wetland treating subsurface drainage from grazed dairy pasture

Nitrogen and phosphorus budgets over two annual periods are presented for an establishing surface-flow constructed wetland treating subsurface drainage from rain-fed, dairy cattle grazed pasture in the North Island of New Zealand. Drainage flows to the wetland (occupying similar to1% of the catchment area) were highly pulsed, associated with rainfall and soil water status, and differed between years (305 and 197 mm drainage). Flow-proportional sampling of inflow and outflow concentrations were combined with continuous flow records to calculate mass balances for the wetlands. Influent nitrate concentrations were high (median 11 g m(-3)) in both years, but transient loads of organic N were also an important form of N in the first year. Mass removal of total nitrogen (TN) and its main constituent forms nitrate/nitrite and organic N was recorded for all seasons over both annual periods studied. TN mass removal efficiency of 79% (841 g m(-2) per year) in the first year, declined to 21% (40 g m(-2) per year) in the second year, associated with changes in the magnitude, speciation and seasonal pattern of N export from the catchment. Ammoniacal N (NH4-N), which comprised <0.5% of TN loadings to the wetland, was generated in small amounts during passage through the wetland in both years. Total phosphorus (TP) in the drainage waters occurred at median concentrations of 0.1-0.2 g m(-3), mainly in dissolved reactive forms (DRP 92% by mass). TP export rose by 101% (5.0 g m(-2) per year) after passage through the wetland in the first year, but decreased by 12% (0.2 g m(-2) per year) in the second year. The results show that constructed wetlands comprising similar to1% of catchment area can markedly reduce N export via pastoral drainage. but may be net sources of NH4-N, DRP and TP during establishment. Performance of the wetland appeared to be affected by both establishment/maturation factors and year-to-year climatic variations. Longer-term studies, supplemented by process-based laboratory and mesocosm investigations, are required to evaluate sustainable nutrient removal rates over a range of climatic conditions, and identify the key factors regulating performance. (C) 2004 Elsevier B.V. All rights reserved.