Control of Agricultural Nonpoint Source Pollution by Natural Wetland Management

Reduction of nonpoint source pollutants, principally sediment and nutrients moving from cultivated fields to surface waters, is a major challenge. Remnants of once-extensive natural wetlands occur across the agricultural landscape, and it has been suggested that these areas might be managed to yield improved wetland function in terms of trapping and retention of nonpoint source pollutants. An existing wetland in a severed meander bend cut off in the 1940s from the Coldwater River in Tunica County, MS, USA was modified by the construction of weirs equipped with water control structures. The wetland was a segment of old river channel about 500 m long and 20 m wide. Inputs to the wetland cell included sporadic flows due to runoff events from about 350 ha of cultivated fields and less frequent but larger flood events from the river. This type of flood event occurred only once during the study. Concentrations of sediment and nutrients in water were generally lower at the downstream end of the wetland cell than in the major inflow, an ephemeral slough. Mean values of turbidity, suspended sediment concentration, and concentrations of filterable and total phosphorus were 25% to 40% lower at the wetland cell discharge weir than in the slough. Mean concentrations of ammonia were 38% lower, but mean nitrate and nitrite concentrations were essentially unchanged by the wetland cell. Comparison of estimated input and output loads during periods when the wetland cell was not flooded by the river indicated that the wetland cell retained about 18% of input suspended sediment, 24% of phosphorus, and 29% of nitrogen input from cultivated fields. Wetland cell sediment and nutrient retention efficiency was greater for drier months, and declined during wetter periods with frequent runoff events.