Submitted to: Nonpoint Pollution Conference Proceedings
Publication Type: Abstract Only
Publication Acceptance Date: September 1, 2006
Publication Date: November 28, 2006
Citation: Smith, D.R., Warnemuende, E.A., Huang, C. 2006. How does dredging of drainage ditches affect contaminant transport?. In: (D. Wichelns, ed.) Proceedings of the Nonpoint Pollution-Methods, Policies, Programs & Management Conference Proceedings. Rivers Institute at Hanover College, Hanover, IN. November 28-30, 2006. pp. 350-358. Technical Abstract: Tile drainage is an essential component to ensuring the agricultural productivity in much of the Midwestern United States. Water from tile drains is typically conveyed into open ditches, which are dredged every five to ten years to maintain uninhibited drainage into ditches. This study was conducted to determine the impact of dredging on nutrient and pesticide transport in tile-fed drainage ditches. Ditch sediments were collected immediately before and after a ditch was dredged in Northeast Indiana. Sediments were placed in a recirculating flume, and water flowed over the sediments at rates similar to the flow observed at the time of collection. Three separate experiments were performed on ditch sediments, each with an initial phase with ‘contaminated’ water flowing over the sediments, followed by a second phase with ‘clean’ water flowing over the sediments. Water was contaminated with phosphorus (18 mg/L), nitrogen (~20 mg/L), and glyphosate (2,000 ug/L). The sediments exposed by dredging contained greater levels of sand and lower levels of silt, clay and organic matter resulting in lower specific surface area for the dredged sediments. Additionally, dredging exposed gleyed surfaces, indicating the sediments contained reduced iron. For all contaminants tested, the pre-dredged sediments were better able to remove the contaminants from the water column than the dredged sediments. For example, final P concentrations after 120 hr of flow resulted in 1.3 mg/L for pre-dredged sediments and 2.1 mg/L for dredged sediments. When ‘contaminated’ water was replaced with ‘clean’ water, P concentrations were 0.19 mg/L for pre-dredged sediments and 0.30 mg/L for dredged sediments. Similar results were observed for nitrogen and glyphosate. Ditch sediments play an important role in buffering contaminant concentrations. Removal of sediments through dredging results in lower buffering capacity, and thus the potential for greater transport of contaminants to receiving waters. Results from this work indicate that resource managers should plan dredging judiciously (i.e. only during low contaminant loading season), to reduce the potential for negative water quality impacts.