Wetlands as a solution to water browning: A 3-year efficiency assessment of outdoor mesocosms

Water browning is the phenomenon of increasing discoloration in lakes and water courses due to the leaching of organic matter from soils. This process has ecological, economic, and social implications, such as affecting light penetration, oxygen levels, nutrient cycling, drinking water quality, and recreational use. This study investigated the potential of wetlands as a natural solution to mitigate water browning by reducing organic matter and iron. Six mesocosms in pilot scale were set up and studied for 3 years for their efficiency to reduce water color, with sampling every 14th day. Parameters measured were the changes in water color, dissolved organic carbon, iron, pH, and conductivity over 3 years, under different hydrological and climatic conditions. The results showed that wetlands can decrease water browning by retaining organic matter and iron, especially in vegetated systems. This study showed that long retention times with vegetated shallow systems were needed to reduce the water color by 47%. The decrease of color was primarily due to reduction of iron that decreased by 66%, while the dissolved organic carbon (DOC) concentration decreased by only 6%. These findings highlight the potential of constructed wetlands as a valuable tool for improving water quality and mitigating the impacts of water browning, though further optimization of wetland design and integration with broader land-use strategies is needed to address this complex issue effectively. PRACTITIONER POINTS: Constructed wetlands can mitigate brownification, especially with long retention times and shallow vegetated wetlands. Iron reduction is more strongly correlated with colour reduction than with DOC reduction. Vegetated constructed wetlands reduced the colour concentration of inflow water by 47% after a 14-day retention time. Wetlands need a long retention time to reduce colour, and should be placed far upstream in the watershed. Vegetated systems may use alternative pathways, like biofilm formation, to reduce humic substances, independent of UV exposure.