Wednesday, November 7, 2007
247-7
The Effect of Time after Ditch Dredging on Phosphorus Transport.
Shalamar Armstrong, Agronomy Department, Purdue University/USDA-ARS-NSERL, Purdue University, 915 West State Street, West Lafayette, IN 47907-2054 and Doug Smith, Agronomy, Purdue University, 915 West State Street, West Lafayette, IN 47906.
Ditch dredging is a critical management practice for agricultural catchments in the Midwestern region of the United States that enhances the removal of water from agricultural fields. Recently, short-term ditch dredge studies have shown that the newly exposed sediments following dredging has a less reactive surface and release a greater concentration of P to the overlying water column that could lead to water impairment. The objective of this study was to determine the effect of time after dredging on P desorption. Sediment samples were collected over a 4 month period from un-dredged and dredged ditches in Northwest Ohio. The sediment samples from each date were analyzed for particle size distribution, labile extractable P, % C and equilibrium P concentration (EPCo ), before being placed into a stream simulator, where P adsorption and desorption experiments were conducted. The adsorption phase was performed by circulating inoculated (0.55 mM P) stream water for 144 hr, followed by a 24 hr. desorption phase with clean distilled water. After 45 days the P adsorption decreased for both the dredged and un-dredged sediments and the P desorption properties of the dredged ditch sediments remain constant. However, net P desorption for the un-dredged sediments increased from 0.2 mg P L-1 to 0.5 mg P L-1 within a time period of 45 days. Results from this study provide conservation managers with the long-term effects of ditch dredging on soluble P transport in drainage ditches.