Article pubs.acs.org/est
Characterization of Mixing Processes in the Confluence Zone between the Three Gorges Reservoir Mainstream and the Daning River Using Stable Isotope Analysis Yunyun Zhao,†,‡ Binghui Zheng,*,†,‡ Lijing Wang,‡ Yanwen Qin,‡ Hong Li,‡ and Wei Cao‡ †
School of Environment, Tsinghua University, Beijing 100084, China State Environmental Protection Key Laboratory of Drinking Water Source Protection, Chinese Research Academy of Environmental Sciences (CRAES), Beijing 100012, China
‡
S Supporting Information *
ABSTRACT: Understanding the interaction processes between the mainstream and its tributaries and detailing the rates of contribution of water and nutrients from two different waterbodies in the confluence zone are essential for water management in the Three Gorges Reservoir (TGR). The stable isotope ratios of hydrogen (δD) and oxygen (δ18O) were applied to explore the interactions between the TGR mainstream and a typical tributary, the Daning River. The results of the model calculations showed that approximately 78.9% of the water and 88% of the nitrate in the confluence zone were from the TGR mainstream. The dynamic vertical distributions of the mixing ratios, major ion contributions, and flow velocities indicated that the water mass from the Yangtze River mainstream flowed backward from the confluence zone up to the tributary along the surface and upper-middle layers, whereas water from the tributary flowed into the mainstream through the lower-middle and bottom layers. This study demonstrates the value of hydrogen and oxygen isotope tracers in accurately describing water mass mixing processes and estimating the rates of contribution of different nutrient sources in the confluence zone, which will provide valuable information for controlling algal blooms in the future.
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velocity.11 Therefore, the identification of the mixing ratio and contribution rate of nutrients of the impounded Yangtze River is critical for controlling algal blooms in the TGR. Until recently, the published studies did not seem to accurately quantify the contribution rates of different sources of nutrient. Several studies have focused on the interaction processes of river waters between the TGR mainstream and its branches in the confluence zone.12−14 The mainstream of the TGR was found to flow backward into Xiangxi Bay as a density current with different plunging depths, which were initially at the bottom, later at the middle depth, and finally at the surface. These density currents were shown to be caused by water temperature and turbidity differences between the mainstream of the TGR and the tributaries.12 Cl− was used as a tracer to estimate the water composition in the Daning Bay and the Jialu River basin.13,15 The results showed that on average 76 and 73% of the water in the Xiangxi River and the Daning River, respectively, is from the TGR mainstream.13
INTRODUCTION The Three Gorges Dam (TGD), which impounds the Yangtze River to provide drinking and irrigation water, electrical energy, flood control, infrastructure, and economic benefits, is one of 45 000 large dams throughout the world.1−3 The impacts of the operation of the Three Gorges Reservoir (TGR) on the ecosystem and environment have been widely discussed,1,4 but generating accurate predictions of such impacts for the young reservoir remains difficult.4 The eutrophication of secondary rivers in the TGR is one of the most severe environmental issues in China.5,6 Since the TGR’s initial impoundment in 2003, nutrient concentrations have not changed significantly in the Yangtze River’s main channel, and the water quality has generally remained stable.7 However, due to the impoundment of large quantities of water with high nutrient loads, dramatic changes in both nutrient concentrations and the hydrodynamic conditions have been observed in mainstream−tributary interactions. These changes include increased water levels, extended water retention time, increasingly serious eutrophication in the secondary rivers, and multiple occurrences of algal blooms.8−10 Most studies have shown that microscopic algae grow rapidly under favorable environmental conditions, including abundant nutrients, appropriate light and temperature, and low flow © XXXX American Chemical Society
Special Issue: Critical Materials Recovery from Solutions and Wastes Received: March 11, 2015 Revised: August 23, 2015 Accepted: August 30, 2015
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DOI: 10.1021/acs.est.5b01132 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
Article
Environmental Science & Technology
Figure 1. Map of the study area and sampling sections.
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STUDY AREA AND METHODS Study Area. The Three Gorges Dam, located in Yichang City, China, is on the Yangtze River. Impoundment began in 2003, and the water level was initially brought to 175 m in two stages between September and October 2010. The length of the reservoir mainstream is approximately 660 km when the water level is 175 m. The TGR operational cycle can be divided into four stages according to water level and discharge (Figure S1): dry season (Nov−April), drawdown (May−early June), flood season (June−Aug) and impoundment (Sept−Oct).22 During the impoundment stage, the water level rises considerably, and water from the TGR mainstream flows into the tributaries, changing the water quality and hydrodynamics. The Daning River is one of the 40 Yangtze River tributaries that enter the TGR through Wushan Lake, which is located in the middle of the TGR at a distance of 123 km from the dam. The Daning River has a length of 162 km and a watershed area of 4170 km2, which covers both Wushan and Wuxi Counties. Additionally, the economy in this watershed is underdeveloped and has no notable industry. Therefore, this watershed is less affected by human pollution and is a typical case for the study of natural interaction processes.3,9,23 The confluence zone (or backwater area) formed when the water mass from the Yangtze River mainstream flowed back to the Daning River after the initial impoundment in 2003.16 The upstream part of the confluence zone of the Daning River is defined as the upstream of the confluence zone (UCZ) in this study. The length of the backwater area is approximately 60 km when the water level is 175 m.24 For the 2003−2012 period, despite rather stable social and economic activities, frequent algal blooms occurred in this area, which may be related to the low flow velocity, high nutrient concentrations, and long hydraulic retention time after the impoundment.25 Water Sampling. There are two sample sections, Changjiangxia (CJX) and Hongshiliang (HSL), in the TGR
The routine water quality parameters (temperature, turbidity, chlorophyll concentration, colored dissolved organic matter (CDOM), dissolved oxygen, major irons, pH, and conductivity) and velocity distributions used to explore the interactions between two rivers in previous studies16 are typically nonconservative indicators shaped by physical (turbidity) and biological (chlorophyll, CDOM, DO, pH) processes. The stable isotopic composition of H2O can provide useful information for tracing the source of water in rivers according to the distinct isotopic signatures of the different water sources, such as the Mekong River and the Tonle Sap River.17 Studies of many large river basins around the world, including the Danube, the Amazon, the Rhine, and the Rio Grande, have used stable isotope tracers to investigate hydrologic processes.18 In the TGR, hydrogen and oxygen stable isotope tracers have seldom been used to study the water mixing ratios of different waters and the mixing processes in the backwater area, and few studies have investigated the stable dynamic interaction processes. These stable isotope tracers have been demonstrated to be effective tools for clarifying the complex interactions among different sources.19−21 In the current study, stable oxygen and hydrogen isotope tracers and numerical models were used to estimate the spatial−temporal mixing ratios in the confluence zone. During sampling, a 3D acoustic velocimeter was used to determine the flow velocity in the confluence zone, helping to demonstrate the water mass movement. Major inorganic ions were analyzed and revealed the chemical characteristics of the water mass in the confluence zone during the interaction process. The results of this novel approach illustrated large-scale movement patterns of water masses and nitrate loads throughout the Daning River backwater. The method can potentially be utilized to quantify the mixing ratio even in more complex water interactions such as when more than two types of water masses interact. B
DOI: 10.1021/acs.est.5b01132 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
Article
Environmental Science & Technology
Figure 2. Vertical temporal distributions of flow velocity in BSH, SL, and SPY during the sampling period. The red arrows represent the reverse flow velocity, and the black arrows represent the normal flow velocity.
mainstream; eight sample sections, Caiziba (CZB), Longmen (LM), Baishuihe (BSH), Shuanglong (SL), Dongpingba (DPB), Maduhekou (MDHK), Shoupayan (SPY), and Dachang (DC), in the confluence zone; and two sample sections, Huatai (HT) and Longxi (LX), in the UCZ. The sample sections were distributed in the TGR mainstream, the confluence zone, and the UCZ to estimate the contribution rates of two driving forces to the backwater area. The data and locations of the sample sections are shown in Table S1 and Figure 1. The impoundment stage of the TGR is from September to October, and the water level rises strongly during this period, causing intense interactions between the mainstream and the tributaries in the backwater area.26 Thus, this study ran from September 16 to September 23, during which the water level rose approximately 3 m (the water level ranged from 163.43 to 166.42 m). We performed in situ and inline water flow velocity measurements six times with the 3D acoustic velocimeter. Samples were taken from nonstagnant, moving river water with a water sampler. Five layers per section (surface, 0.2H, 0.6H, 0.8H, and H; where H represents the section depth) were sampled at the two TGR mainstream sample sections (CJX and HSL) and eight confluence zone sample sections (CZB, LM, BSH, SL, DPB, MDHK, SPY, and DC). Due to the shallow depths of sections HT and LX, only surface samples were taken at these sections. River water was collected in airtight glass bottles (20 mL) before filtering with 0.45 μm polycarbonate filters to remove large particulates. The caps were then sealed with paraffin film. The water samples were preserved in a refrigerator (4 °C) and promptly analyzed. The Yangtze River mainstream was so wide that we sampled three points of the mainstream section (left, right, and middle water columns), whereas only the middle water column was sampled for the sample sections in and around the confluence zone. A total of 286 samples were collected during the sampling period. Measurement of Stable Hydrogen and Oxygen Isotopic Ratios. The hydrogen and oxygen isotopic compositions of the water samples were determined using a liquid water isotope analyzer (LWIA, DLT-100, Los Gatos Research, Inc., Mountain View, CA). In nature, stable isotope composition changes are small, and δ values are generally used to represent the value of the isotopic composition of an element. The δ value is the ratio of stable isotopes in a sample
relative to a standard sample isotope ratio multiplied by a factor of 1000. The oxygen and hydrogen isotopic compositions can be expressed as follows, respectively: ⎛ R sample ⎞ δ18O (‰) = ⎜ − 1⎟ × 1000 ⎝ R standard ⎠ ⎛ R sample ⎞ δ D (‰) = ⎜ − 1⎟ × 1000 ⎝ R standard ⎠
where Rsample is the stable isotope ratio (18O/16O or D/H) of the sample, and Rstandard is the stable isotope ratio of Vienna Standard Mean Ocean Water (VSMOW, 0‰); in VSMOW, D/H and 18O/16O were 1.5576 × 10−4 and 2.0052 × 10−3, respectively.27 All of the stable isotope ratio results were expressed as parts per thousand deviations from VSMOW with analytical precisions of ±0.15‰ (δD) and ±0.02‰ δ18O. Measurement of Major Ion Concentrations. The concentrations of major ions (Na+, K+, Mg2+, Ca2+, F−, Cl−, SO42−, and NO3−) were measured with a Dionex ion chromatograph. The relative standard deviation (RSD) for all ions was
