Nutrient fluxes regulation in an in-stream ...

The aim of this thesis was to investigate the efficiency of an in-stream free water surface flow, Vända constructed wetland (CW), in a temperate climate in order to reduce agricultural diffuse pollution and study the parameters affecting nutrient reduction and greenhouse gas (GHG) emissions. The Vända CW was established in 2015 and was monitored from March 2017 until September 2021. Water samples were collected biweekly from the inlet and outlet of Vända CW wetlands (Wetland1 and Wetland2). GHG samples were collected from six points in both wetlands. Flow velocity, turbidity, oxygen concentration, electrical conductivity, water temperature and pH were measured on site with portable devices. The long-term results of this thesis show that in in-stream CWs, nutrient re- moval efficiency and GHG emissions are highly dependent on water parameters and wetland design. Vända CW effectively reduced phosphorus, the reduction of which had clear seasonal dynamics. Higher removal efficiency occurred during the warm period and lower values were seen during the cold period. Average annual total phosphorus (TP) removal was 32.1±3.59%, and for PO4-P it was 22.5±4.4%. Higher removal efficiency values for phosphorus were seen when the flow rate was lower. With a lower flow rate, sedimentation and filtration pro- cesses are favoured. The results for nitrogen removal were contrary to expectations because CW acted more as a source of nitrogen. The annual increase in nitrate nitrogen (NO3-N) was 48.1±7.4%, being higher in summer. The addition of nitrogen was suspected to come from groundwater seepage. The concentration of total in- organic carbon (TIC), sulphate (SO42–) and chlorine (Cl–) ions, which are charac- teristic to groundwater, also increased in the wetland, especially during the low flow rate. This suggests a higher groundwater proportion in the wetland that was contaminated with NO3-N during summer with a low flow rate. Vegetation coverage was estimated each year and its spread increased expo- nentially up to 90% and 74% respectively in initially planted and not planted wetland within six years after the establishment thereof. Vegetation helps to remove nutrients in water by sedimentation, filtration and plant uptake, which was seen in Vända CW. Phosphorus removal had a strong positive correlation with vegetation coverage. Based on nutrient concentration in aboveground bio- mass and its translocation after the vegetation period, the optimal time for bio- mass harvesting is in autumn if the purpose is to remove nutrients from the wet- land. Biomass harvesting timing is also crucial if low methane (CH4) emissions are targeted, as its emissions increase after cutting and decrease after a couple of days. During the vegetation period, emissions are higher and vary from month to month, but emissions are lowest in autumn.