Monitoring and simulating nutrient removal in a constructed wetland

Phosphorus contamination of surface waters is a primary water quality concern in the agricultural watershed of Pike River in southern Quebec. Surface waters from Walbridge creek, a tributary of the Pike River, were diverted into a small constructed wetland consisting of three basins laid out in series to evaluate nutrient (nitrogen and phosphorus) retention within the system. Hydraulic and nutrient loading rates to the constructed wetland were highly variable in time, with peak rates of loading occurring during runoff events in the watershed, with a mean hydraulic loading rate of 25 cm/day. The wetland retained 8.47 kg total phosphorus, which corresponded to 44% of total phosphorus inputs (19.3 kg) and it also retained 132.5 kg nitrates, which represented 13% of nitrate inputs (995 kg) to the wetland, over 4 years (2003-06) of seasonal (May-Nov) operation. Annual mean nutrient retention rates (1.7 g total P m-2 year-1 and 27.3 g NO3 - m-2 year-1) were within the range of values reported in the literature for constructed wetlands treating agricultural runoff. This study therefore provided additional evidence supporting the use of small constructed wetlands as nutrient traps in agricultural watersheds in a moderate Canadian climate. A first generation wetland model was also developed using MATLAB(TM) programming language to simulate phosphorus cycling in the wetland. The model was evaluated as a prediction tool of effluent particulate phosphorus and ortho-phosphate concentrations. Much more work needs to be done to improve the accuracy of the model simulations.