Submitted to: Journal of Soil and Water Conservation
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/15/2007
Publication Date: 8/31/2007
Citation: Smith, D.R., Pappas, E.A. 2007. Effect of ditch dredging on the fate of nutrients in deep drainage ditches of the Midwestern United States. Journal of Soil and Water Conservation. 62(4):258-261. Interpretive Summary: Dredging of the deep drainage ditches of the Midwestern United States is necessary to ensure adequate removal of water from agricultural fields. However this practice may degrade water quality. When nutrient rich water flowed over drainage ditch bed material collected before or after dredging, the Pre-Dredge bed material was able to remove P, NO3-N, and NH4-N more quickly than the bed material present after dredging. Furthermore, when the water with high levels of nutrients was removed and replaced with nutrient free water, the release of P from the Dredged bed material to the water column resulted in greater concentrations of P than the Pre-Dredged bed material. These experiments show the dynamic nature of contaminant transport within these ditches, and demonstrate how the bed material can act as a source or a sink for contaminants. Results from this study represent the immediate impacts of dredging on water quality. The time for the dredged ditch bed to recover from dredging, and the downstream ecological impacts are not yet understood, and should be investigated. Resource managers should consider these findings when planning dredging or other maintenance activities. To minimize water quality impacts, it is suggested that resource managers maintain agricultural drainage ditches during periods of the year when nutrient concentrations are expected to be low. The impact of this research is to inform scientists and resource managers about the immediate impacts of dredging on water quality.
Technical Abstract: Dredging of drainage ditches in artificially drained systems is necessary to ensure that agricultural fields are drained adequately. This study compared the potential impacts of dredging on water quality. Using a fluvarium (stream simulator), bed material collected from drainage ditches prior to dredging was better able to remove P, NO3-N, and NH4-N from water than material collected from the bed of the ditches after dredging. When 17 mg P L-1 water was exposed to Pre-Dredged and Dredged bed material, the concentrations were reduced to 2.3 mg L-1 and 3.4 mg L-1, respectively after 120 hr. Water column NH4-N concentrations were reduced to 0 mg L-1 earlier in Pre-Dredged bed material. Phosphorus and NO3-N processing lengths were lower for Pre-Dredged bed material than Dredged bed material. Resource managers should perform maintenance tasks, including ditch dredging, when nutrient loads are expected to be low, thus minimizing the potential water quality impacts.