Effect of Dredging an Agricultural Drainage Ditch on Water Column Herbicide Concentration, as Predicted by Fluvarium Techniques

Authors: Pappas, Elizabeth; Smith, Douglas

Submitted to: Journal of Soil and Water Conservation
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/31/2007
Publication Date: 8/31/2007
Citation: Pappas, E.A., Smith, D.R. 2007. Effect of Dredging an Agricultural Drainage Ditch on Water Column Herbicide Concentration, as Predicted by Fluvarium Techniques. Journal of Soil and Water Conservation. 62(4):262-268.

Interpretive Summary: Much of the Midwest US is too wet to farm without draining fields into ditches that carry the water away. Over time, these ditches fill up with sediment from the fields, making field drainage slower. In these cases, it is necessary to dredge the ditches by scooping sediment out of the bottom in order for the ditches to work properly again. When this is done, the newly exposed sediment is different from the sediment that had been there before dredging. Ditch sediments play an important role in the removal of pollutants, such as weed killers, from drainage water, which may eventually become drinking water for people. In this study, a stream simulator was used to evaluate how well ditch sediments can remove herbicides from water before and after dredging. It was found that some herbicides are removed from water more quickly when water is flowing over sediment from a ditch that has not been recently dredged, than when water is flowing over sediment from a ditch that was dredged recently, so it is a good idea to dredge ditches when herbicide concentrations are expected to be lowest. The impact of this result is that ditch managers can use this information to decide when to perform ditch dredging. Ditches should not be dredged immediately before, during, or immediately after the period when farmers are applying herbicides to fields.

Technical Abstract: In artificially drained agricultural areas, dredging of drainage ditches is often necessary to ensure drainage of fields adequate to permit field operations. Fluvarium experiments were performed in order to evaluate the potential of the bed material changes associated with ditch dredging to impact water column concentrations of three herbicides: atrazine, metolachlor, and glyphosate. In the first phase of the experiment, water having initially high herbicide concentrations was allowed to flow across bed sediment collected from a ditch immediately before and after dredging. In the second phase of the experiment, water having initially zero herbicide concentrations was allowed to flow across these bed sediments. Results indicate that the bed sediments remaining after ditch dredging may contribute to generally higher levels of herbicides in the water column in the short term by removing less herbicide from contaminated water and contributing more herbicide to initially uncontaminated water. These results indicate that ditch dredging should be performed when herbicide levels are expected to be lowest.