NUTRIENT AND SEDIMENT REMOVAL IN FORESTED WETLANDS RECEIVING PUMPED AGRICULTURAL DRAINAGE WATER

The effectiveness of two forested wetland buffer areas at removing sediment and nutrients from pumped agricultural drainage water was evaluated in a two-year field study. The movement of these potential pollutants during pumping events was determined by sampling water quality at 36 stations distributed over each wetland. Automatic water sampling continued after pumping events to determine the nutrient and sediment removal rates in the water left standing on the wetlands. The total volume of water pumped during small events was effectively stored on the wetland until displaced by subsequent pumping or removed by evapotranspiration. Nutrient and sediment concentrations of this stored water, which had several days of residence time on the wetland, were near background levels before leaving the wetlands. Nutrient concentrations leaving the wetlands during these small events ranged from 0.03 to 0.04 mg/L for total phosphorus (TP) and from 0.0 to 0.1 mg/L for nitrate nitrogen (NO3-N). Pumped water completely traversed the wetland during the less-frequent, larger pumping events. Nutrient and sediment concentrations at the wetland outlet were often higher than background concentrations, ranging from 0 to 70 percent of the inflow concentrations. Nutrient concentrations leaving the wetlands during these larger events ranged from 0.06 to 0.12 mg/L for TP and from 0.0 to 4.7 mg/L for NO3-N. Sediment, TP, and NO3-N were removed from drainage water standing on the wetland. This nutrient and sediment removal was described using a first order decay model. Deposition of sediment was observed only within 800 m of the pumps, and was not resuspended during subsequent large pumping events.