Impact of Dredging on Phosphorus Transport in Agricultural Drainage Ditches of the Atlantic Coastal Plain¹

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 years and had similar characteristics (flow, P loadings, sediment properties) were sampled (0‐5 cm) after one of the ditches had been dredged, which removed fine textured sediments (clay = 41%) with high organic matter content (85 g/kg) and exposed coarse textured sediments (clay = 15%) with low organic matter content (2.2 g/kg). Sediments were subjected to a three‐phase experiment (equilibrium, uptake, and release) in recirculating 10‐m‐long, 0.2‐m‐wide, and 5‐cm‐deep 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 mg dissolved P. However, the sediments from the dredged ditch removed 19% less P (76 mg) 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 that dredging of coastal plain drainage ditches can potentially impact the P buffering capacity of ditches draining agricultural soils with a high potential for P runoff.

Hydrology and Soil Manipulations of Iron‐Rich Ditch Mesocosms Provide Little Evidence of Phosphorus Capture within the Profile

“…Our finding that dredging lowered effluent P relative to the control when hydrology was held constant is consistent with Smith and Huang (2010) and Shigaki et al (2008). Disrupted materials from dredged and undredged ditches from the same site where our intact soil mesocosms were taken were compared for their P‐release potential by Shigaki et al (2008).…”

“…In a field study of Midwestern collection ditches, Smith and Huang (2010) observed decreased P exports in dredged ditch reaches versus undredged reaches for up to 1 yr after dredging (similar to Shigaki et al, 2008, and the present study; Fig. 2b).…”

Phosphorus Fate, Management, and Modeling in Artificially Drained Systems

“…This can result in increased bank erosion (hence particulate P loss) and removal of bed materials with a high P sorption capacity leading to ditches becoming a P source rather than a P sink Sharpley et al, 2007;Smith and Pappas, 2007;Shigaki et al, 2008). In some instances, however, short-term increases in P retention have been observed following ditch dredging (Smith and Huang, 2010).…”

Drainage water management for water quality protection

“…Considerable research documents the role of drainage ditches as conduits of field pollutants (e.g., Kleinman et al 2007) and Figure 1 the effect of routine ditch maintenance practices such as dredging in disrupting the natural buffering ability of ditches Pappas and Smith 2007;Shigaki et al 2008). However, a growing body of research suggests the potential exists for using vegetated open ditches as best management practices for mitigating potential agricultural contaminants.…”