Water quality variation and its conditioning factors in the Three Gorges Reservoir, China

The variation of water quality has been an issue of concern since the impoundment of the Three Gorges Reservoir (TGR). In this study, water samples from the TGR were collected in July and November 2015 and in May and July 2016 to reveal the variations of water quality and its key conditioning factors. The results showed that the concentrations of major ions, nutrients and heavy metals in the TGR’s water body exhibited heterogeneous variations. Especially, the hotspot for major ions and heavy metals emerged at Chongqing downtown, and a decreasing trend from Chongqing toward the TGR dam was spatially observed. The heterogeneous variation of hydrochemistry in the TGR revealed the integrated influences of natural processes and human activities. Especially, the intense rainfall in the spring and early summer of 2016 promoted the transport of pollutants to the TGR, and further magnified the influence of large cities on the spatial variations of the hydrochemistry in the TGR. This study will give more insights into the change of water quality in large reservoirs in the context of the intense rainfall and the human activities.


GRAPHICAL ABSTRACT INTRODUCTION
Dam construction is a global issue that contributes to the loss of river connectivity. So far, more than 63% of rivers in the world have lost the capacity to flow freely, which has altered the hydrological processes, transport of sediment and pollutants, and the water quality in reservoirs (Chen & Chau ; Grill et al. ). At least 3,665 reservoirs (!1 km 2 ) have been built in China during the past 15 years and about one-third of the newly impounded area is located in the upper Yangtze River (Zhu et al. ). The impoundment of the Three Gorges Reservoir (TGR) decreases flow velocity and leads to a lake-oriented alteration of hydrological environment, which has potential effects on the geochemical cycles of key elements and the water quality in the TGR (Bao et al. ). The TGR is also a vital drinking water source for approximately 1.45 million people in Chongqing and Hubei provinces. Therefore, the water quality has consequently become an issue of wide concern in the TGR, and it is necessary to comprehensively understand the dynamic variations of potential pollutants in the TGR water column. Water Resources (CWRC)). As a result, the pollutants may enter the water bodies through the surface runoff (Qiu et al. ), and lead to the variations in hydrochemistry of the TGR. Especially in urban areas, the road dust acts as a key sink of various pollutants, which will migrate through surface runoff and potentially alter the hydrochemistry of the receiving water (Zhang et al. a, b; Zhang et al. ). However, since the full operation of the TGR in 2010, the integrated influence of human activities and rainfall events on the water quality has not been comprehensively explored.
In this study, 14 sites covering the entire TGR were selected to collect water samples for the determination of water physiochemical properties, major ions, and heavy metals. Since some knowledge is lacking in the variations of hydrochemical characteristics and the influence of rainfall events on the water quality in the TGR, the main objectives of this study are to: 1) illustrate the spatial and seasonal variations of hydrochemical characteristics in the TGR; 2) reveal the factors controlling the hydrochemical variations; and 3) clarify the effects of anthropogenic versus rainfall events on the water quality of the TGR.

Study area
The TGR (29 16 0 -31 25 0 N, 106 20 0 -111 50 0 E) is located between Jiangjin District, Chongqing, and Zigui County, Hubei Province (Figure 1). With a mean water depth of approximately 70 m, the TGR has a volume of 39.3 km 3 and a total surface area of 1,045 km 2 . The annual average water discharge recorded by the hydrologic station of Cuntan was 34.5 billion m 3 during 1950-2010 (CWRC).
The TGR region is subject to a subtropical monsoon climate with a humidity of 60-80%. The mean annual precipitation is approximately 1,000 to 1,300 mm, and the annual mean temperature varies from 16.7 to 18.7 C with an extreme maximum temperature exceeding 41 C. More details of the study area are presented by Bao et al. ().

Sample collection
In order to cover most of the counties in the TGR, surface

Sample analysis and data collection
The filters were dried at 105 C to a constant weight for the determination of suspended solids (SS), which was calculated by the difference in weight between pre-and post-filtration. An EXO2 multi-parameter water quality monitor (YSI Co., Ohio, USA) was used for the in-situ measurements of water temperature (T w ), specific conductivity (Spc), total dissolved solids (TDS), dissolved oxygen (DO), pH, oxidation-reduction potential (ORP), turbidity (Turb) and chlorophyll (Chl), and the detection limits are 0.01 C, 1 μS/cm, 0.01 g/L, 0.01 mg/L, 0.01, 0.1 mV, 0.01 NTU, and 0.01 μg/L, respectively.
The concentrations of fluorine (F À ), chloride (Cl À ), nitrate (NO 3 À ), sulfate (SO 4 2À ), sodium (Na þ ), potassium (K þ ), calcium (Ca 2þ ), and magnesium (Mg 2þ ) ions in the filtrates were determined by ICS-90 ion chromatography (Dionex Co., Sunnyvale, USA), with the detection limit of 0.001 mg/L. The analytical precision for Ca 2þ , Mg 2þ , for Na þ , K þ is <2% RSD, and for F À is <5% RSD. Bicarbonate (HCO 3 À ) analysis was carried out via acid titration, and phosphate (PO 4 3À -P) was measured using the molybdenum blue colorimetric method with a Shimadzu UV-2600 spectrophotometer at 700 nm. Dissolved heavy metals, copper The continuously monitored data for discharge at Cuntan, representing the inflow of the TGR, were manually collected from the Yangtze River hydrological network

Water physicochemical characteristics
The water physicochemical parameters of the TGR are shown in Table S2. implied pH, Turb, and DO were higher in the bottom water than in the surface water, and there was no marked difference among the other parameters ( Figure S2).
Variations of the water physicochemical characteristics in different seasons are presented in Figure S3. Two groups of these parameters were classified according to the seasonal variations (Table S3)

Major ions
The concentrations of major ions in the TGR water are summarized in Table 1. The cations were in the order of Ca 2þ > Na þ > Mg 2þ > K þ , and the anions were HCO 3 À > SO 4 2À > Cl À . A Piper diagram illustrates the hydrochemistry type of the TGR water was HCO 3 -Ca, Mg ( Figure S4). The concentrations of Ca 2þ , Mg 2þ , HCO 3 À , SO 4 2À , and K þ increased slightly compared with previous studies in the TGR and the Yangtze River, while the concentrations of Cl À and Na þ were comparable to those in the previous studies (Table 1) (Table 1). Annually, the concentrations of PO 4 3À -P and NO 3 À -N in the TGR water increased after the impoundment of the TGR (Table 1) Xia et al. (2008).

Dissolved heavy metals
The concentrations of dissolved heavy metals for water samples are summarized in Table 2 Table 2). The variation  of heavy metals in different seasons is presented in Figure 4.
The concentrations of Cu, Cr, and Ni were higher in May 2016, and the peak level of Cd appeared in July 2015.
Additionally, the concentrations of Pb and Zn were marked in November 2015.
The spatial variations of the heavy metals illustrated the peak levels at Chongqing (S3) and Wanzhou (S9) (Figure 5), where the chemistry industry plays an important role in the national production. Specifically, the hot spots for Cu, Cd, and Ni appeared only at Chongqing, and the peak level for  Cr was only marked at Wanzhou, however, Zn showed higher levels both at Chongqing and Wanzhou. Furthermore, Pb presented a declining trend from the upstream TGR to the dam. The vertical variation of the heavy metals at S6 also indicated the heavy metals were well distributed in the water column of the TGR ( Figure S6).

Precipitation, discharge, and D-O isotopes
The precipitation and the inflow discharge of the TGR    Our study gave more insights into the change of water quality in large reservoirs both in the context of intensified rainfall and human activities. However, our study was only a short-term investigation of the hydrochemistry, and longterm monitoring and assessment with more hydrochemistry indexes is expected in the future to acquire a full understanding of the change in water quality in the TGR.