2006b. Dredging of drainage ditches increases short-term transport of soluble phosphorus

Managed drainage ditches are common in the midwestern United States. These ditches are designed to remove water from fields as quickly as possible, and sediment buildup necessitates dredging, to ensure adequate water removal. This laboratory study was conducted to determine the impact of ditch dredging on soluble phosphorus (P) transport. Ditch sediments were collected from a drainage ditch in northeastern Indiana immediately before and after dredging. The sediments were placed in a stream simulator, and stream water was loaded with 0.55 mM P for 5 d (adsorption experiment). Water was then removed, and ‘‘clean’ ’ water (no P added) was used for a desorption experiment, lasting 1 d. During the adsorption experiment, pre-dredged sediments were able to remove P from the water column quicker, and P concentrations 120 h after introduction of high P water were lower for the pre-dredged sediments (0.075 mM P) than the dredged sediments (0.111 mM P). During the desorption experiment, P was released to the water column slower in the pre-dredged treatment than the dredged treatment (instantaneous flux at t5 0 was 0.205mM P h21 for pre-dredged and 0.488mM P h21 for dredged). This occurred despite higher Mehlich 3–extractable P in the pre-dredged sediments than the dredged sediments. Equilibrium phosphorus concentrations (EPCo) were lower in the pre-dredged sediments during both adsorption and desorption experiments. Transport of soluble P immediately after dredging will likely increase in drainage ditches; however, dredging is a necessary management tool to ensure adequate discharge of water from surrounding fields. ALGAL PRODUCTION in many U.S. surface waters is lim-ited by phosphorus (P) concentrations, and thus small inputs of bioavailable P from anthropogenic sources can result in accelerated eutrophication (Schind-ler, 1977). Agriculture has been identified by theUSEPA as one of the primary contributors of nonpoint-source P to surface water (USEPA, 1990). A large amount of research has been conducted on ways to decrease or eliminate P discharges from agri-cultural lands. Efforts to decrease P losses at the field scale have included treatment of manures with chemi