ENVIRONMENTAL CONTROLS ON THE GEOCHEMISTRY OF A CONSTRUCTED WETLAND

Constructed wetlands are built to meet a number of regulatory requirements, including compensatory mitigation for wetland losses, treatment of wastewater, and reduction of non-point-source pollution. Natural processes in the wetland stabilize the substrate, enhance permeability, remove organic, inorganic and microbiological contaminants and facilitate the flux of gases between the sediments and atmosphere. Here we present the results of an ongoing study to evaluate the nutrient retention efficiency of a constructed wetland at the Mansfield Campus of the Ohio State University, built to mitigate the loss of stream and wetlands resulting from the construction of the Riedl Hall in 2005. The wetland is also used for stormwater retention, draining 190,000 m² of campus uplands. Water samples have been collected monthly since April, 2009 and used for determination of organic and inorganic nutrients. Samples were collected at both the inflow and the outflow. Water temperature, pH, and concentration of total dissolved solids were also measured in situ using portable meters. Although there was considerable variability in the dataset, the mean values for all variables but pH were higher in the inflow compared to the outflow (Total Dissolved Solids = 11% higher; Total Oxidized Nitrogen (NO₃ + NO₂) = 46% higher; NH₄ = 51% higher; PO₄ = 122% higher; Total Nitrogen = 37% higher; Total Phosphorus = 43% higher; pH = 1% lower). These downstream changes in the studied variables were attributed primarily to biogeochemical processes in the wetland, which removes excess nutrients and improves water quality. The nutrient removal efficiency of the wetland varied considerably between seasons. Highest nutrient removal was observed during the summer (~40% efficiency), while the lowest was observed in the fall (~25% efficiency). Higher efficiency was observed in the removal of PO₄ (65% in summer), while TON had the lowest removal efficiency (22% in the fall). There is a positive significant correlation between precipitation and DIN concentrations in both the inflow and outflow stations. Precipitation events explain 73% of the variability in DIN concentrations on the inflow and 87% of the variability on the outflow. On average, a reduction of 14% in the nutrient removal efficiency of the Riedl wetland was observed after rainfall events.