Control of Agricultural Nonpoint Source Pollution by Natural Wetland Management

Authors: Shields Jr, Fletcher; Wilson, Casey

Submitted to: Journal of Environmental Science and Engineering
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/12/2010
Publication Date: 4/15/2010
Citation: Shields Jr, F.D., Pearce, C.W. 2010. Control of Agricultural Nonpoint Source Pollution by Natural Wetland Management. Journal of Environmental Science and Engineering. 4(4):62-70.

Interpretive Summary: Remnants of once-extensive natural wetlands occur across the agricultural landscape, and may be managed to yield improved wetland function in terms of trapping and retention of nonpoint source pollutants. Capabilities of such systems must be measured to refine design and management criteria. An existing wetland in a severed meander bend beside the Coldwater River in Tunica County, MS was modified by the construction of weirs equipped with water control structures. Estimates of loads entering and leaving the wetland cell were computed based on concentrations of grab samples collected at the wetland cell inflow and outflow locations and associated flow data. During drier months, when water retention time in the wetland was longer, the wetland retained about 18% of input suspended sediment, 24% of phosphorus, and 29% of nitrogen input from cultivated fields. These results will be useful to other researchers seeking to develop more creative ways to lower loads of nutrients and other nonpoint source pollutants leaving farms.

Technical Abstract: 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 some workers have 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 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. Yields of TN, TP and NO3-N from the ~350 ha of cropland that were drained by the tributary slough were about 0.49, 0.24, and 0.054 tonnes km-2 yr-1, respectively. Water concentrations of sediment and nutrients 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. Examination of monthly flux values indicate that the wetland cell was most efficient during drier months when loading rates were lower and retention times were longer; performance deteriorated during wetter winter and spring periods when most runoff occurred.