Factors affecting phosphorus retention in small constructed wetlands treating agricultural non-point source pollution

Four surface flow constructed wetlands (CWs) have been intensively investigated for phosphorus retention, from 3 to 7 years in the cold temperate climate of Norway. The aim of this study was to identify factors that affect phosphorus retention from non-point sources. The wetlands were located in first order streams, with surface areas of 0.06-0.4% of the watershed (CW-area 350-900 m(2)). Volume proportional composite samples were taken from inlet and outlet, and sedimentation plates were used in selected areas. The average retention of total phosphorus for the individual CWs was 21-44% of input, despite the high hydraulic load (mean load was 0.7-1.8 in per day). This equals a retention of 26-71 g phosphorus m(-2) surface area per year. A first-order model was fitted to the data giving an average removal constant, k, of 214 in per year. However, the constant increased with increasing hydraulic load due to the simultaneous increase particle settling velocity. Hence, retention increased in spite of increasing hydraulic loads. Moreover, linear multiple regression models showed that retention was influenced by several external variables, e.g. input of phosphorus, season, phosphorus content on suspended solids and phosphorus settling velocity. The results suggest that the first-order model is less suitable to estimate phosphorus retention in similar gravity fed wetlands. The best of the proposed statistical prediction models, reproduced observed data from two independent test-CWs with a deviation of 0.1%. The investigation shows that small wetlands are a useful supplement to best management practice on arable fields. However, the present study focuses on the necessity to investigate how pollutants enter wetlands. Such knowledge can then be used to suggest improvements of wetland layout. (C) 2002 Elsevier Science B.V. All rights reserved.