Evaluation of treatment performance for one constructed wetland in a Swedish forest: A case study with emphasis on brownification mitigation

Brownification refers to the increase in water colour reported in many freshwater systems. Constructed wetlands (CW) are commonly used for pollutant mitigation, including the causes of brownification. This study aimed to investigate the effects of a CW established in a forest area with high colour and low pH on its capability to reduce nutrient and colour. The inflow and outflow colour (mg Pt/L), total organic carbon (TOC), and iron (Fe) were assessed daily over a period of recorded dynamic flows during spring and autumn 2019 (15 and 19 days, respectively). Total nitrogen (Ntot) and phosphorus (Ptot) were also assessed to verify the removal efficiency of this mature constructed wetland. The prevailing hydraulic retention time (HRT) was assessed using conservative tracing. The results showed that the colour, TOC, and Fe were not reduced during the evaluation period. In contrast, while colour did not change (4–8% increase) over the assessed periods, TOC and iron significantly increased by 7.7% and 29.5%, respectively, during spring (4.2% and 10.6% during autumn). As Ntot and Ptot increased by 14.8% and 120%, respectively, during spring (7.4% and 8.8% during autumn), we concluded that constructed wetlands designed for nutrient control offer little potential for the mitigation of brownification, as expressed by colour, under low pH conditions.