Hindawi Journal of Sensors Volume 2021, Article ID 6610178, 11 pages https://doi.org/10.1155/2021/6610178 Research Article The Release of Endogenous Nitrogen and Phosphorus in the Danjiangkou Reservoir: A Double-Membrane Diffusion Model Analysis Zhiqi Wang,1,2,3 Hongxin Ren,2 Zhaolong Ma,3,4,5 Zhihong Yao,2 Pengfei Duan,6 and Guodong Ji 1 1Key Laboratory of Water and Sediment Sciences, Ministry of Education, Department of Environmental Engineering, Peking University, Beijing 100871, China 2School of Surveying, Mapping, and Geographic Information, North China University of Water Resource and Electric Powder, Zhengzhou 450000, China 3River and Lake Protection Center of Ministry of Water Resources, Beijing 100038, China 4Department of Hydraulic Engineering, Tsinghua University, Beijing 100084, China 5China Institute of Water Resource and Hydropower Research, No. 20 Chegongzhuang West Road, Haidian District, Beijing 100044, China 6Key Laboratory of Ecological Security in Water Source Area of the Middle Route of South-to-North Water Transfer Project in Henan Province, Nanyang Normal University, 473000, China Correspondence should be addressed to Guodong Ji; [email protected] Received 9 November 2020; Revised 12 January 2021; Accepted 25 January 2021; Published 13 February 2021 Academic Editor: Yuan Li Copyright © 2021 Zhiqi Wang et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. 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 ðIPÞ > organic phosphorus ðOPÞ > calcium − phosphorus ðCa − PÞ > iron − phosphorus ðFe − PÞ > labile phosphorus ðLPÞ. 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) ffi ff + with a coe cient of variation of 26.90%. The contents of di erent 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 ff + 3- di erent forms of P in DJK4 and DJK5 decreased with the increase of sediment depth. The contents of NH4 -N and PO4 -P in the sediment interstitial water of DJK4 and DJK5 were higher than those in the overlying water. In addition, the contents of + 3- NH4 -N and PO4 -P were higher in DJK5 than that in DJK4. The static culture experiments showed that N and P were mainly + 3- released from the sediment-overlying water interface to the overlying water. The release rate of NH4 -N and PO4 -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 × 103 t·a-1 and 0:53 × 103 t·a-1, respectively. Thus, endogenous contamination from the newly submerged land is an important factor affecting the water quality of the Danjiangkou Reservoir. 2 Journal of Sensors 1. Introduction the total amount of sediment nutrients remains unchanged. It avoids the influence of complicated environmental factors Reservoir sediment is considered to be an important sink of and allows for the estimation of the release of sediment nutri- various nutrients in the overlying water [1] and thus even a ents in a large reservoir area. The K value is calculated from small change of reservoir sediment can have a substantial the data obtained from the simulated release experiment, impact on the quality of the overlying water [2]. The nutri- which is the net release coefficient derived from the actual ents accumulated in the sediment such as nitrogen (N) and sediment adsorption, precipitation, and microbial action phosphorus (P) can be released from the sediment back to [14]. the overlying water through the interstitial water as a result In this study, in situ sediment samples were collected at of convection, diffusion, and resuspension, causing second- different locations of the Danjiangkou Reservoir and stati- ary pollution of the overlying water [3]. For instance, the cally cultured in laboratory, and the distribution and release + release rate of NH4 -N from the sediment is still as high as of endogenous N and P from sediments were investigated 3:31 × 103 t·a-1 in the Dianchi Lake of Yunnan Province, based on the double-membrane diffusion model. The results China, after dredging and sewage interception [4], and resus- of this study may provide important insights into how to pension of nutrient-rich sediment causes eutrophication for improve the water quality of the reservoir. several decades even when external nutrient loads are cur- tailed [5]. In this case, endogenous release of nutrients from 2. Materials and Methods the sediment is the main factor affecting the water quality [6]. Therefore, a better understanding of the distribution of 2.1. Study Area and Sampling Sites. The Danjiangkou Reser- nutrients in the sediment and their release fluxes is essential voir is located in the northwest of Hubei province and the to take steps to reduce endogenous nutrient load and southwest of Henan province and consists of Hanjiang and fl improve reservoir water quality [7]. Danjiang reservoirs. The annual average in ow is about 3 The Danjiangkou Reservoir is the main water source of 39.48 billion m from the Hanjiang River and its tributary the middle route of the South-to-North Water Transfer Pro- Danjiang River, and the terrain is high in the northwest but ject of China, and thus the water quality has been maintained low in the southeast. The reservoir was put into operation fl at a high level for years. However, evidence has emerged that in 1973, and the over ow weir crest was heightened in fi there is an increasing risk of endogenous contamination that 2012. Now, it is the largest arti cial freshwater lake in Asia, can seriously affect the water quality of the reservoir [8]. The with a water area of about 102275 ha and a storage capacity 3 overflow weir crest of the Danjiangkou Reservoir was height- of about 29.05 billion m [15]. fi ened in 2012, resulting in an increase in the storage capacity In this study, ve representative sampling sites were from 17.45 billion m3 to 29.05 billion m3 [9] and conse- established at Guojiashan (DJK1; 33°02′39″N and 111°28′ quently inundation of 17333 ha of farmland [10]. The newly 46″E), Kongqueyuan (DJK2; 33°0′51″N and 111°27′11″E), formed sediments contain a higher amount of organic matter Shijiagou (DJK3; 32°54′42″N and 111°30′47″E), Shiqiao compared with the original sediments, which implies a Wharf (DJK4; 32°49′23″N and 111°34′28″E), and Songgang higher risk of endogenous release and contamination. N is Wharf (DJK5; 32°46′02″N and 111°38′50″E), as shown in more likely to be released from the newly submerged farm- Figure 1. These sampling sites were about 0.5 km away from land and orchard of the Danjiangkou Reservoir [11], and P the reservoir shore with a water depth of about 2 m. The loss is more prominent in the new riparian zone compared overlying water and sediment samples were collected in with that in the existing riparian zone [12]. Thus, endoge- March and August, 2018. The longitude and latitude of the nous release of nutrients such as N and P from the sediment sampling sites were determined by GPS (see Table 1). AD, in the newly submerged area may pose a threat to the water DJK 4, and DJK 5 were continuous farmland since 2012 quality of the reservoir. A number of studies have been con- where a large amount (188-356 kg·ha-1) of chemical fertil- ff ducted to investigate the factors a ecting the release of nutri- izers (mainly potash and phosphate fertilizers) were used ents from the sediment, such as existing states and seasonal with a maximum of 646 kg·ha-1 [16]. Thus, the double- variations of nutrients, dissolved oxygen, and temperature membrane diffusion model was used to investigate the effect [13], but little is known about the release of nutrients from of the release of endogenous N and P on the overlying water the sediment in the newly submerged area. at these two sampling sites based on the data collected in A number of models have been proposed to describe the March, 2018. release of nutrients from the lake sediment, such as the per- meation model, the double-membrane diffusion model, and 2.2. Sample Collection and Analysis. Columnar sediment the surface renewal model. Among them, the double- samples were collected using a columnar sampler with an membrane diffusion model is particularly suitable to charac- organic glass tube (90 mm × 500 mm), and three parallel terize the flux of endogenous nutrients from the kinetic per- samples were collected at each sampling site for static culture spective, as it uses mathematical conversion to obtain the experiments.