The Release of Endogenous Nitrogen and Phosphorus in the Danjiangkou Reservoir: A Double-Membrane Diffusion Model Analysis

Endogenous contamination from the newly submerged sediment may have an impact on the water quality of the Danjiangkou Reservoir, the water source of the middle route of the South-to-North Water Diversion Project. In this study, in situ sediment samples were collected at different locations (Guojiashan (DJK1), Kongqueyuan (DJK2), Shijiagou (DJK3), Shiqiao Wharf (DJK4), and Songgang Wharf (DJK5) from the north to the south) of the Danjiangkou Reservoir and the upstream tributary using a columnar sediment sampler and statically cultured in laboratory, and the distribution and release of endogenous N and P from the sediment were investigated based on a double-membrane diffusion model. The results showed that the P contents in the main reservoir and the upstream tributary followed the order of inorganic   phosphorus   IP > organic   phosphorus   OP > calcium − phosphorus   Ca − P > iron − phosphorus   Fe − P > labile   phosphorus   LP . IP accounted for the largest proportion (47.10-59.70%) of total phosphorus (TP), while LP accounted for only a small proportion (0.10-0.90%) of TP. There is a significant difference in the spatial distribution of P at different sampling points, especially for the OP content (226.90 mg·kg-1) with a coefficient of variation of 26.90%. The contents of different forms of P and NH4+-N decreased from the upstream tributary to the main reservoir, which was mainly attributed to the land use type. In the vertical distribution, the contents of different forms of P in DJK4 and DJK5 decreased with the increase of sediment depth. The contents of NH4+-N and PO43--P in the sediment interstitial water of DJK4 and DJK5 were higher than those in the overlying water. In addition, the contents of NH4+-N and PO43--P were higher in DJK5 than that in DJK4. The static culture experiments showed that N and P were mainly released from the sediment-overlying water interface to the overlying water. The release rate of NH4+-N and PO43--P ranged from 13.08 mg·(m2·d)-1 to 21.39 mg·(m2·d)-1 and from 3.06 mg·(m2·d)-1 to 6.02 mg·(m2·d)-1, and the release amount calculated based on the double-membrane diffusion model was 1.17 × 10 3  t·a-1 and 0.53 × 10 3  t·a-1, respectively. Thus, endogenous contamination from the newly submerged land is an important factor affecting the water quality of the Danjiangkou Reservoir.