Identification of Dredging Depths Based on Sediment Vertical Distribution Profiles of Total Nitrogen and Total Phosphorus and Their Adsorption-desorption Equilibria

Abstract

A dredging demonstration project in the Baiyangdian Lake included open waters and fishing ponds to reduce the internal release of nitrogen and phosphorus from bottom sediments. The dredging depth design was determined by both the sediment vertical distribution profile of total nitrogen and phosphorus, and the sediment adsorption-desorption equilibrium method. The determined dredging depths were very similar and coincident. The dredging depth for the demonstration area of open waters in Nanliuzhuang was identified as(50±10) cm; and the dredging depths for fishing ponds were(30±10) cm in both the Nanliuzhuang and Caiputai demonstration areas. The equilibrium nitrogen(NH4+-N) and phosphorus(SRP) concentrations at zero net sorption or desorption(ENC0 and EPC0) were significantly positively correlated with both exchangeable and total nitrogen and phosphorus in the sediments. The total nitrogen and phosphorus in the sediments were also used to predict the risk of their release from the bottom sediments to the overlying water column. The sediment layers with ENC0 and EPC0 values greater than the NH4+-N and SRP in the overlying water column indicated the sediments act as a source of dissolved nitrogen and phosphorus to the overlying water column in the Nanliuzhuang and Caiputai demonstration areas. Accordingly, the sediment layers with both total nitrogen concentrations greater than 750 mg·kg-1 and total phosphorus concentrations greater than 500 mg·kg-1 should be identified as dredging layers.