Long-term variation characteristics of nutrients in the water and sediments of a surface flow constructed wetland with micro-polluted water sources

This study aimed to evaluate the long-term effects of a large-scale constructed wetland purifying micro-polluted water and explore accumulation characteristics and influencing factors of nutrients in sediments. For this purpose, the water quality and nutrients in sediments of the long-running large-scale constructed wetland of Yanlong Lake were monitored for eight years since the wetland began operation. The results demonstrated that the average removal rates of permanganate index (CODMn), ammonia nitrogen (NH4+-N), total nitrogen (TN) and total phosphorus (TP) were 4.39 ± 12.64%, 41.57 ± 16.36%, 32.61 ± 23.36% and 47.05 ± 20.39% in the constructed wetland from 2013 to 2020, respectively. The CODMn removal effects were the lowest. Sediments of the constructed wetland were moderately polluted and the cumulative level of nutrients was mainly affected by the organic matter content, influent water quality, water depth and plant function. Firstly, TN and TP in sediments are significantly positively correlated with total organic carbon. Besides, the nutrient concentrations in sediments increased along the water flow direction. Furthermore, the nutrient concentrations in sediments increased with water depth, and the carbon was greatly affected. In addition, the overgrowth and lodging of wetland plants could promote the nutrients accumulation in sediments. This research results provide a scientific basis for improving the purification capacity of long-running surface flow constructed wetlands. •Long-term effects of a large-scale CW in purifying micro-polluted water were studied.•Yanlong Lake constructed wetland had relatively certain water purification effects.•TN and TP in sediments are significantly positively correlated with TOC.•Nutrient concentration in sediments increased along the water flow and water depth.•Overgrowth of plants can increase the accumulation of nutrients in sediments.