Submitted to: Journal of the American Water Resources Association
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/15/2008
Publication Date: 12/1/2008
Citation: Shigaki, F., Kleinman, P.J., Schmidt, J.P., Sharpley, A., Allen, A. 2008. Impact of dredging on dissolved phosphorus transport in agricultural drainage ditches of the Atlantic Coastal Plain. Journal of the American Water Resources Association. 44:1500-1511. Interpretive Summary: Concern over nutrient runoff from agricultural soils of the Delmarva Peninsula has focused attention on the role of drainage ditches as conduits of nutrients to the Chesapeake Bay. Ditches are routinely dredged to improve their hydrologic function, but the effect of such an activity on water quality is poorly understood. Dredging exposed materials that were less reactive, so the ditch sediments had a diminished capacity to remove phosphorus from simulated runoff water. Interestingly, approximately 30% of the phosphorus removed from the runoff water was attributed to microbial processes, pointing to dredging as impacting biotic and abiotic processes controlling nutrient runoff. Results point to the potential for ditches to substantially buffer downstream impacts of agricultural runoff, and highlight the role that routine ditch management practices can have in water quality protection.
Technical Abstract: Drainage ditches can be a key conduit of phosphorus (P) between agricultural soils of the Atlantic coastal plain and local surface waters, including the Chesapeake Bay. This study sought to quantify the effect of a common ditch management practice, sediment dredging, on fate of P in drainage ditches. Sediments from two drainage ditches that had been monitored for seven yrs and had similar characteristics (flow, P loadings, sediment properties) were sampled (0-5 cm) after one of the ditches was dredged to remove fine textured sediments (clay = 41%) with high organic matter content (85 g kg-1) and expose coarse textured sediments (clay = 15%) with low organic matter content (2.2 g kg-1). Sediments were subjected to a three phase experiments (Equilibrium, Uptake and Release), in recirculating 10-m-long, 0.2-m-wide to a 5-cm depth flumes to evaluate their role as sources and sinks of P. Under conditions of low initial P concentrations in flume water, sediments from the dredged ditch released 13 times less P to the water than did sediments from the ditch that had not been dredged, equivalent to 24 kg dissolved P. However, the sediments from the dredged ditch removed 19% less P (76 kg) from the flume water when it was spiked with dissolved P to approximate long-term runoff concentrations. Irradiation of sediments to destroy microorganisms revealed that biological processes accounted for up to 30% of P uptake in the coarse textured sediments of the dredged ditch and 18% in the fine textured sediments of the undredged ditch. Results indicate the potential for dredging of coastal plain drainage ditches to potentially impact the P buffering potential of ditches draining agricultural soils with a high potential for P runoff.