Increasing risk of water quality deterioration in a typical inland lake of China

Nansi Lake is the largest lake along the eastern route of China's South-to-North Water Diversion Project (SNWDP). It is divided into the upper lake and the lower lake by a dam. By using UV-Vis spectroscopy, synchronous fluorescence (SF) spectroscopy, excitation-emission matrix and parallel factor analysis (EEM-PARAFAC), spatial, and temporal differences in the properties of dissolved organic matter (DOM) were found in the 2 areas of Nansi Lake under different hydrological conditions. A total of 5 fluorescence components were obtained by EEM-PARAFAC, which included 3 humic-like components (C1-C3) and 2 protein-like components (C4 and C5). On the spatial scale, the fluorescence intensities (Fmax) of humic-like substances and the ratio of the fluorescence intensity of humic-like components to the total fluorescence intensity (%Fmax) and degree of humification (HIX) in the upper lake were higher than those in the lower lake. This indicated the strong contributions of terrestrial sources to the upper lake, while DOM properties in the lower lake were more endogenous than those in the upper lake. On the temporal scale, protein-like substances played a more important role in DOM properties in April (Fmax=0.72±0.03 in the upper lake and 1.84±0.13 in the lower lake) and July (Fmax=1.10±0.05 in the upper lake and 1.49±0.04 in the lower lake) than in October. This result might be related to the water transfer of the eastern route of the SNWDP and to the death of submerged plants. However, the contents of humic-like substances (ranging from 55.61±1.23% to 66.56±0.58% for the upper lake and 29.98±1.56% to 61.98±0.99% for the lower lake) and degree of humification (from 2.23±0.06 to 3.10±0.05 for the upper lake and 1.06±0.05 to 2.62±0.08 for the lower lake) in Nansi Lake showed an increasing trend from April to October. In addition, significant correlations and good linear relationships between humic-like components, a254, and DOC in the 3 months reflected the continuous contribution of humic-like substances to DOM properties in Nansi Lake. Rapid changes in the fluorescence signal were largely dependent on changes in water quality. The fluorescence signal could be a tool for the management of water quality in Nansi Lake.

Knowledge of the extent of water pollution along a diversion channel is essential for ensuring the security and effectiveness of water diversion projects. Thus, identifying the trends in the water quality and the factors which influence the trends along a diverted river is a prerequisite to ensuring the safety of the water diversion. As an important control unit of the Eastern Route of the South to North Water Diversion Project, the hilly eastern region of Nansi Lake Basin (ENLB) suffers excess water pollution in the water transferred into Nansi Lake during the flood season. The comprehensive pollution index (CPI), the Mann-Kendall trend test and analysis of variances were used to characterize the spatiotemporal variation of water quality in the three main mountainous rivers of the ENLB. The contribution of different factors which influencing the variation of the water quality parameters may be explained using ridge regression analysis. The results showed that the CPI for the three major rivers decreased significantly from 2000 to 2012. The rivers had problems in compliance, and this was most noticeable in the Guangfu River. The main pollutants exceeding the water quality standards were the chemical oxygen demand and total phosphorus. The GDP per capita was the largest factor which influenced the changes in the regional water quality, with the average contribution being 30%. An increase in the water storage capacity was shown to have a positive effect on improving the water quality. The results provided a scientific basis for establishing a regional pollution control strategy and would give support to the planning of future water diversion projects.

The east route of the South-to-North Water Diversion Project in Jiangsu Province was used as an example to investigate the characteristics of changes in water quality in the area affected by significant water diversion projects. Based on the comprehensive assessment method of the water quality index （WQI）, the M-K trend test, and the geographic information system （GIS）, the spatial and temporal variation characteristics of water quality in the 13 national assessment sections of the east route of the South-to-North Water Diversion Project in Jiangsu Province were evaluated and analyzed from 2013 to 2022. The results showed that the water quality assessment grades in the study area were mainly "medium" and "good." The overall mean value was 74.03, indicating "good" water quality. DO, BOD5, and NH4+-N were the primary indicators of changes in river water quality in the region. Over time, the water quality in the basin had significantly improved from 2013 to 2022. However, there was a rebound in 2021, but this phenomenon was effectively controlled in 2022. Water quality was better during the non-flood season compared to that during the flood season. From a spatial perspective, the water quality in the southern part of the region was superior to that in the north. Furthermore, the rate of improvement was faster in the south. This could be attributed to factors such as precipitation, population density, and the scale of agricultural and livestock activities. However, the water quality of the S11 （Sanduoxi Bridge） section was generally lower than that of other sections and should be given further attention.

Lake Nansi consists of four lakes: the Nanyang,Dushan,Zhaoyang and Weishan Lake.It is a lake in the line of the Eastern Route Project(ERP) of the South-to-North Water Diversion Project(NSBD).After the project supply water in 2013,the water quality of Lake Nansi will be affected significantly.Based on the existing theories and methods,a three-dimensional hydraulic control and water quality model of the lake is presented.The model is developed under the Environmental Fluid Dynamics Code(EFDC) model.It was calibrated and verified to hydrodynamic and water quality data,using two sets of observed data from October 1 to December 31,2006 and January 1 to June 30,2007,respectively.The observed data including water surface elevation,water temperature,chemical oxygen demand,dissolved oxygen and ammonia nitrogen from five stations is used as forcing for simulations.The model results show that temporal and spatial patterns for hydrodynamic and water quality components are consistent with observed data in the reservoir.Because the direction of the water supply and natural flow is converse,the Lake Nansi is divided into the upper-lake and the lower-lake by Erji dam in the Lake Zhaoyang.Two scenarios of whole supplying water term of the lake were simulated using the model.By the analysis of the variations of the hydraulic control and water quality of the upper-lake and lower-lake,This paper proposed three advices of the ERP operation,which can help the project to give full benefits and improve the water quality.

Spatiotemporal characteristics and potential pollution factors of water quality in the eastern route of the South-to-North Water Diversion Project in China

In recent decades, there has been unprecedented development of hydropower in China, especially in the Yangtze River Basin, which has changed the hydrological and hydraulic conditions of natural rivers and has an impact on water quality. However, the spatial-–temporal extent, factors, and the reasons behind the influence of cascade hydropower development are not clear. The six hydropower stations on the main course of the middle reach of the Jinsha River in Yunnan and Sichuan Provinces have been in joint operation for seven years, and the impact of cascade hydropower development on water quality has begun to appear. In this paper, in order to accurately determine the causal relationship between cascade hydropower development and water quality changes on the middle reaches of the Jinsha River and their trends using regression discontinuity analysis, we collected monitoring data on water quality from 2004 to 2019. The results show that cascade hydropower development on the middle reach of the Jinsha River led to a decrease in TP concentration in that section of the river and an increase in the concentration of CODMn and NH3-N. Furthermore, increase in sedimentation following the impoundment of cascade hydropower development is the main reason for the decrease in TP concentration, and the regional economic and social development driven by cascade hydropower development are external sources of the increase in the concentration of CODMn and NH3-N. In addition, influenced by rainfall, the concentrations of CODMn and TP are higher in the rainy season and lower in the dry season, which is directly related to the input of non-point-source pollutants in the basin during the former. This study established a model to accurately judge the causal relationship between cascade hydropower development and water quality changes in the basin, which was then used to assess the impact of cascade hydropower development on water quality. Our results provide a basis for the formulation and implementation of a water quality protection plan for the middle reach of the Jinsha River and can also provide a basis for the development of cascade hydropower in other river basins.

为掌握东平湖浮游植物在南水北调东线工程运行后的群落结构变化特征及探究其环境驱动因子，于通水运行3周年后的2017年分4个季节对东平湖的18个样点进行采样调查，同步开展水环境调查.结果显示：共检出浮游植物191种属（包括变种），其中硅藻门、绿藻门、裸藻门、蓝藻门、金藻门、黄藻门、甲藻门和隐藻门分别为64、57、28、22、8、5、4和3种属；优势种属包括蓝藻门的漂浮泽丝藻（Limnothrix planctonica）和色球藻（Chroococcus sp.），绿藻门的小球藻（Chlorella vulgaris）以及硅藻门的小环藻（Cyclotella sp.）、舟形藻（Nauicula sp.）和针杆藻（Synedra sp.）等（优势度Y>0.02）；全年密度和生物量分别为6.74×10⁶ cells/L和5.71 mg/L，并呈现显著季节与空间差异.浮游植物密度季节变化为夏季 > 秋季 > 冬季 > 春季，生物量季节变化为夏季 > 冬季> > 秋季 > 春季；春、夏、秋、冬季物种数分别为71、78、53和125种，Shannon-Wiener多样性指数（H'）均值分别为1.90、1.84、1.76和2.52，Margalef丰富度指数（D）均值为3.07、3.98、2.79和4.75；冗余分析与Pearson相关性分析结果显示，水体综合营养状态指数、高锰酸盐指数、氨氮和水温是影响东平湖浮游植物群落结构的关键驱动因子.对比南水北调东线工程实施之前及通水初期的结果，东平湖浮游植物优势种群无明显变化，密度则较通水初期稍有升高.本研究分析了东平湖浮游植物群落的时空分布特征，发现调水工程可能通过物质带入影响水质，进而引起浮游植物量增加，湖区养殖的退出及增殖放流的实施降低了富营养化风险，但仍存在浮游植物生物量特别是蓝藻生物量升高的风险，建议在调水管理中加强水域生态监测及生境调控技术研究，保障调水水质安全.;In order to investigate the community characteristics of phytoplankton and the determining factors in Lake Dongping after implementation of the east route of South-to-North Water Diversion Project, seasonal samples of phytoplankton and aquatic environment from 18 sampling sites were collected in 2017. A total of 191 taxa of phytoplankton (genera or species) were identified with 64 taxa of Bacillariophyta, 57 taxa of Chlorophyta, 28 taxa of Euglenophyta, 22 taxa of Cyanophyta, 8 taxa of Chrysophyta, 5 taxa of Xanthophyta, 4 taxa of Pyrrophyta and 3 taxa of Cryptophyta. The dominant taxa were Limnothrix planctonica and Chroococcus sp. of Cyanophyta, Chlorella vulgaris of Chlorophyta and Cyclotella sp., Nauicula sp. and Synedra sp. of Bacillariophyta, etc (Y>0.02). The phytoplankton density and biomass were 6.74×10⁶ cells/L and 5.71 mg/L with the seasonal variation of density being summer > autumn > winter > spring and that of biomass being summer > winter > autumn > spring, respectively. About the phytoplankton diversity in four quarters, the number of species was 71, 78, 53 and 125, while the averages of the Shannon-Wiener index were 1.90, 1.84, 1.76 and 2.52 and Margalef diversity index were 3.07, 3.98, 2.79 and 4.75, respectively. Meanwhile, the results of redundancy analysis and Pearson analysis for the relationship between phytoplankton and aquatic environment showed that the factors such as comprehensive nutrition state index, permanganate index, ammonia nitrogen concentration, and water temperature played a key role in affecting the structure and dynamics of the phytoplankton community in Lake Dongping. Compared with before implementation of the east route of South-to-North Water Diversion Project and the initial stage of it, although the density of phytoplankton showed a rise, there was no obvious response in their dominant genera. This study preliminarily verified the spatiotemporal distribution characteristics of the phytoplankton community in Lake Dongping. Total phytoplankton density was found to have increased with the change of water quality caused by the east route of South-to-North Water Diversion Project. The withdrawal of aquaculture and implementation of fishery enhancement and releasing had reduced the risk of eutrophication, but the proliferation of phytoplankton probably increased the risk of algal-bloom in Lake Dongping. Given the changes of phytoplankton community and water quality after water diversion, it is necessary to strengthen the ecological monitoring and controlling in the management of water quality safety and stability.

Phytoplankton community variation and ecological health assessment for impounded lakes along the eastern route of China's South-to-North Water Diversion Project.

Weishan Lake is one of the four lakes in the Nansi Lake and an important water transport hub for the east route of the South-to-North Water Transfer Project. The water quality study of Weishan Lake is conducive to a comprehensive understanding of the water quality and ensure the smooth progress of the South-to-North Water Transfer Project. This paper uses the water quality comprehensive pollution index evaluation method to analyse the water quality of Weishan Lake, and uses ArcGIS drawing software to spatially distribute the concentration of CODMn, NH3-N, TP, TN and SS in Weishan Lake water quality monitoring. Visualize to analyse the maximum impact factor for each space. At the same time, this paper combines the water quality evaluation results of Weishan Lake and the cause of water pollution, and puts forward the control countermeasures suitable for the pollution control of the basin, which provides a basis for the Weishan Lake watershed management and the realization of the water quality target of the South-to-North Water Diversion Project.

Tracking changes in chlorophyll-a concentration and turbidity in Nansi Lake using Sentinel-2 imagery: A novel machine learning approach

Nansi Lake is an important storage lake in the east route of the South-to-North Water Diversion Project in China, about which there has been serious concern regarding the water quality. In this study, the phytoplankton taxonomic composition, abundance, temporal variations, spatial distribution, and diversity were studied based on a monthly sampling campaign from five sampling stations between January 2010 and December 2010. A total of 159 species (8 phyla, 79 genera), including 74 species of Chlorophyta, 36 species of Bacillariophyta, 19 species of Cyanophyta (including 2 water bloom causative species), 21 species of Euglenophyta, 3 species of Cryptophyta, 5 species of Xanthophyta, 2 species of Pyrrophyta, and 2 species of Chrysophyta, were identified. Average phytoplankton diversity index and evenness values were 4.33 and 0.81, respectively, revealing high biodiversity of phytoplankton community. The phytoplankton abundance averaged at 9.51 × 10(6) cells L(-1) and was much higher than previous investigations carried out in 1983-1984. The dominant species were Bacillariophyta, Chlorophyta in winter and spring, and Chlorophyta and Cyanophyta in summer and atutumn. There were 14 predominant species including Chlorella vulgaris, Cyclotella stelligera, Pseudanabaena limnetica, and Chroomonas acuta. Phytoplankton community structure and environmental variable changed substantially over the survey period. Redundancy Analysis was used to analyze the relationship between them. Temperature was considered to be the key factor driving the change in phytoplankton community composition in Nansi Lake during the 2010 study period.

Under the background of global change, the lake water environment is facing a huge threat from eutrophication. The rapid increase in curly-leaf pondweed (Potamogeton crispus L.) in recent years has seriously threatened the ecological balance and the water diversion safety of the eastern route of China’s South-to-North Water Diversion Project. The monitoring and control of curly-leaf pondweed is imperative in shallow lakes of northern China. Unmanned Aerial Vehicles (UAVs) have great potential for monitoring aquatic vegetation. However, merely using satellite remote sensing to detect submerged vegetation is not sufficient, and the monitoring of UAVs on aquatic vegetation is rarely systematically evaluated. In this study, taking Nansi Lake as a case, we employed Red–Green–Blue (RGB) UAV and satellite datasets to evaluate the monitoring of RGB Vegetation Indices (VIs) in pondweed and mapped the dynamic patterns of the pondweed Fractional Vegetation Coverage (FVC) in Nansi Lake. The pondweed FVC values were extracted using the RGB VIs and the machine learning method. The extraction of the UAV RGB images was evaluated by correlations, accuracy assessments and separability. The correlation between VIs and FVC was used to invert the pondweed FVC in Nansi Lake. The RGB VIs were also calculated using Gaofen-2 (GF-2) and were compared with UAV and Sentinel-2 data. Our results showed the following: (1) The RGB UAV could effectively monitor the FVC of pondweed, especially when using Support Vector Machine that (SVM) has a high ability to recognize pondweed in UAV RGB images. Two RGB VIs, RCC and RGRI, appeared best suited for monitoring aquatic plants. The correlations between four RGB VIs based on GF-2, i.e., GCC, BRI, VDVI, and RGBVI and FVCSVM calculated by the UAV (p < 0.01) were better than those obtained with other RGB VIs. Thus, the RGB VIs of GF-2 were not as effective as those of the UAV in pondweed monitoring. (2) The binomial estimation model constructed by the Normalized Difference Water Index (NDWI) of Sentinel-2 showed a high accuracy (R2 = 0.7505, RMSE = 0.169) for pondweed FVC and can be used for mapping the FVC of pondweed in Nansi Lake. (3) Combined with the Sentinel-2 time-series data, we mapped the dynamic patterns of pondweed FVC in Nansi Lake. It was determined that the flooding of pondweed in Nansi Lake has been alleviated in recent years, but the rapid increase in pondweed in part of Nansi Lake remains a challenging management issue. This study provides practical tools and methodology for the innovative remote sensing monitoring of submerged vegetation.

Abstract. Lakes are under increasing pressure due to widespread anthropogenic impacts related to rapid development and population growth. Accordingly, many lakes are currently undergoing a systematic decline in water quality. Recent studies have highlighted that global warming and the subsequent changes in water use may further exacerbate eutrophication in lakes. Lake evolution depends strongly on hydrologic balance, and therefore on groundwater connectivity. Groundwater also influences the sensitivity of lacustrine ecosystems to climate and environmental changes, and governs their resilience. Improved characterization of groundwater exchange with lakes is needed today for lake preservation, lake restoration, and sustainable management of lake water quality into the future. In this context, the aim of the present paper is to determine if the future evolution of the climate, the population, and the recharge could modify the geochemistry of lakes (mainly isotopic signature and quality via phosphorous load) and if the isotopic monitoring of lakes could be an efficient tool to highlight the variability of the water budget and quality. Small groundwater-connected lakes were chosen to simulate changes in water balance and water quality expected under future climate change scenarios, namely representative concentration pathways (RCPs) 4.5 and 8.5. Contemporary baseline conditions, including isotope mass balance and geochemical characteristics, were determined through an intensive field-based research program prior to the simulations. Results highlight that future lake geochemistry and isotopic composition trends will depend on four main parameters: location (and therefore climate conditions), lake catchment size (which impacts the intensity of the flux change), lake volume (which impacts the range of variation), and lake G index (i.e., the percentage of groundwater that makes up total lake inflows), the latter being the dominant control on water balance conditions, as revealed by the sensitivity of lake isotopic composition. Based on these model simulations, stable isotopes appear to be especially useful for detecting changes in recharge to lakes with a G index of between 50 and 80 %, but response is non-linear. Simulated monthly trends reveal that evolution of annual lake isotopic composition can be dampened by opposing monthly recharge fluctuations. It is also shown that changes in water quality in groundwater-connected lakes depend significantly on lake location and on the intensity of recharge change.

Daily changes in household water access and quality in urban slums undermine global safe water monitoring programmes.

Nansi Lake, located in the southwestern region of Shandong Province, China, consists of four interconnected lakes—Nanyang, Dushan, Zhaoyang, and Weishan—and functions as an essential water conveyance and storage reservoir for the East Route of the South-to-North Water Diversion Project. The construction of the Erji Dam hydraulic complex between 1958 and 1973 resulted in the division of the lake into two distinct sub-lakes: Shangji Lake (upper lake) and Xiaji Lake (lower lake). This development significantly altered the hydrological connectivity and ecological conditions of the area. This research examines the effects of hydraulic engineering on the aquatic environment and ecosystem of Nansi Lake by analyzing water quality parameters and sedimentary diatom communities, with an emphasis on spatial variations between the northern and southern regions. The results demonstrate that Nansi Lake exhibits mild pollution, characterized by mesotrophic to eutrophic conditions. Sediments primarily consist of clay and silt, with minimal sand content, while organic matter originates from both autochthonous and allochthonous sources. A total of 181 diatom species from 37 genera were identified, revealing distinct dominant taxa between Shangji Lake and Xiaji Lake. Redundancy analysis (RDA) indicated that total nitrogen (TN) and Ca2+ were the primary factors influencing diatom communities in Shangji Lake, whereas total organic carbon (TOC) and dissolved oxygen (DO) predominantly governed those in Xiaji Lake. The research indicates that the water quality, nutrient levels, sedimentation patterns, and diatom distributions of Nansi Lake demonstrate considerable spatial heterogeneity, primarily influenced by anthropogenic activities such as hydraulic engineering and aquaculture, leading to marked variations between the northern and southern regions.