Efficiency of a newly established in-stream constructed wetland treating diffuse agricultural pollution

Diffuse agricultural pollution, especially from intensively managed agricultural land is a major cause of eutrophication, therefore it is important to reduce the diffuse load to surface water. Constructed wetlands (CW) are an effective measure for improving water quality and reducing nutrient runoff from agriculture by using natural water treatment mechanisms. We studied treatment efficiency of an in-stream free surface flow (FSW) Vända CW in southern Estonia from 16th of March 2017 till 11th of January 2018. Vända CW has a catchment area of 2.2 km2 and of which approximately 62% is under intensive agricultural management. The CW consists of a sedimentation pond followed by two shallow water wetlands planted with cattail (Typha latifolia). Our analyses showed that the CW retained 17.5 kg phosphorus (P) ha−1 yr−1. During the warm period phosphate removal was up to 41.8% whereas annually it was only 14.4%. Phosphate removal efficiency showed strong negative correlation (R2 = 0.58, p < 0.001) with flow rate and therefore it can be seen that shorter retention time reduces significantly the overall P removal efficiency. The yearly reduction of total organic carbon was up to 3300 kg C ha−1 yr−1 while surprisingly the CW increased total nitrogen up to 1375 kg N ha−1 yr−1. The results demonstrate that the acclimatization period of newly established in-stream FWS CWs in northern countries can be relatively long and after two years of establishment we still cannot see satisfactory treatment processes, especially in nitrogen.