Assessing water quality for urban tributaries of the Three Gorges Reservoir, China

Water quality assessment is essential for water resources management. This paper presents a comprehensive evaluation of water quality conditions in three urban tributaries of the Three Gorges Reservoir, China. The Canadian Council of Ministers of the Environment Water Quality Index (CCME-WQI) and Nemerow Pollution Index (NPI) approach were used in this study. Generally, the assessment results of the NPI approach are consistent with that of the CCME-WQI approach. However, the NPI method overemphasized the influence of the most serious pollutant factor, and thus this method should be used with caution for water resources managers. The CCME-WQI values indicated that the water quality conditions in the Wubu River were quite good during the period 2013–2015. Water quality conditions in the upstream sections of Yipin and Huaxi River are good. However, when the river drains through urban areas, water quality conditions greatly deteriorate due to the excessive release of household and municipal sewage, and industrial wastewater, especially for Huaxi River. Thus, waste water management becomes more and more imperative in urban regions of China. Meanwhile, assessment results indicate that the CCME-WQI approach can provide a reference for decision-makers on water resources management.


INTRODUCTION
In China, the single factor pollution index (SFPI) method is the most commonly used approach to evaluate water quality conditions in surface water bodies (Yan et al. ). In the SFPI method, water quality indexes are normally categorized based on the Environmental Quality Standards for Surface Water in China (GB3838-2002), which was published by the Ministry of Environment Protection of China in 2002. The SFPI approach can easily provide water quality classification for the assessed water quality indexes. However, it uses the worst water quality classification of the assessed water quality indexes as the final water quality classification and gives biased results as a result ( Ji et al. ). Except for the SFPI method, another approach that has been widely used to assess different surface water bodies in China is the Nemerow Pollution Index (NPI) approach. Detailed applications can be found in Liu et al. () and Xu et al. (). In Canada, the Water Quality Guidelines Task group of the Canadian Council of Ministers of the Environment proposed a new water quality assessment approach in 2001 (Canadian Council of Ministers of Environment Water Quality Index, called CCME-WQI hereafter) (CCME ). The CCME-WQI approach provides a flexible index template, and allows researchers to select appropriate water quality objectives according to their own requirements (CCME ; Hurley et al. ). Due to the flexibility of the CCME-WQI approach, it has been applied to evaluate water quality conditions all over the world (Lumb et al. ; Damodhar & Reddy ; Zhao et al. ). Lumb et al. () found that the CCME-WQI method is the most stringent one to grade water quality status for aquatic uses when compared with other assessment approaches. Damodhar & Reddy () concluded that the CCME-WQI method is adequate for evaluating the impacts of industrial effluent on the river water bodies. Zhao et al. () indicated that the CCME-WQI approach can be used to effectively evaluate whether the overall water quality conditions meet the specified water quality objectives. Moreover, the United Nations Environmental Program advised that the CCME-WQI method is a suitable tool for assessing the overall water quality conditions of drinking water globally (Rickwood & Carr ).
The Three Gorges Project has brought substantial social and economic benefits, such as flood control, hydro-power generation, and navigation. However, the Three Gorges Project has long-term impacts on the ecosystem of Yangtze River. For example, severe algae blooms have been reported in many tributary bays since the initial impoundment of the Three Gorges Reservoir (TGR) in 2003 (Ma et al. ). Water quality conditions in TGR have been frequently studied. Previous studies mainly focused on pollution sources and loading estimation (Shen et al. ; Zhang et al. ), and the temporal and spatial variation of the water quality parameters in some rural tributaries of the TGR, especially those that will be dramatically impacted by backwater from TGR, such as Xiangxi River and Da-Ning River (Hu et al. ).
Some studies have focused on water quality variations within the main stem and large tributaries of the TGR over a relatively long time period (Zhao et al. , ).
However, few studies have focused on water quality variations within some urban tributaries in the upper TGR part, especially in the Chongqing section, since these urban tributaries play a significant role in the water pollution of TGR.
The main objective of the study is to apply the CCME-WQI approach to assess the water quality condition of three urban tributaries in the upper TGR from 2013 to 2015. The assessment results of the NPI method were used as a comparison. The results will aid in the optimization of water resources for sound environmental management.

Study area
The Yangtze River is the largest river in China and the third largest in the world. The TGR is located at the end of the upper Yangtze River. It is one of the largest manmade reservoirs in the world with a surface area of 1,080 km 2 , a storage capacity of 39.3 billion m 3 , and a watershed area larger than 1 million km 2 (Huang et al.  (Table 1). Water quality data sets were in the period from 2013 to 2015, with one sampling per month.

Mathematical structure of NPI index
The NPI method is a water pollution index which takes extreme values into account using a weighted environmental quantity index. This index can be calculated by where P is the NPI, n is the total number of water quality parameters, P i is the pollution index of parameter i, (P i ) MAX is the maximum pollution index. P i can be computed by the following formula: For dissolved oxygen: where C i is the measured value of parameter i, C 0 is the desired water quality standard value of parameter i. The NPI method divides water quality conditions into five classifications generally: Class I: P < 0.8, and water is clean; Class II: 0.8 P < 2.5, and water is slightly polluted; Class III: 2.5 P < 4.25, and water is moderately polluted; Class IV: 4.25 P 7.2, and water is heavily polluted; Class V: P > 7.2, and water is seriously polluted.

Mathematical structure of CCME-WQI approach
The CCME-WQI method is based on the following three elements: scope (F 1 ), frequency (F 2 ), and amplitude (F 3 ), which has been well documented in Lumb et al. (). A brief description is presented here.
Scope (F 1 ) is used to measure the number of water quality parameters that do not meet desired water quality objectives. It can be calculated by Equation (4): Frequency (F 2 ) is used to measure how often a water quality objective is not met. It can be calculated by Equation (5): The amplitude (F 3 ) is used to represent the amount by which the failed test values do not meet their water quality objectives. Three steps are needed to calculate F 3 , as detailed below.
First, the number of times by which an individual concentration is greater than or less than the water quality objective should be computed, which is also known as 'excursion'. If the test values exceed the water quality objective, excursion can be calculated by Equation (6): For the cases when the test values fall below the water quality objective, excursion can be calculated by Equation (7): Second, the total amount when the individual tests are out of compliance (nse) should be calculated. Normally, The amplitude (F 3 ) is then calculated by an asymptotic function that scales the normalized sum of the excursions from objectives to yield a value between 0 and 100: Finally, the CCME-WQI index is calculated as is shown in Equation (10): The above formulation generates a CCME-WQI value between 0 and 100. A CCME-WQI value closes to 0 represents very poor water quality conditions. A CCME-WQI value closes to 100 indicates excellent water quality con-

Variations of water quality parameters
According to the monitoring data from eight monitoring stations from 2013 to 2015, there was one water quality parameter (TN) that exceeded the preselected water quality  (Table 3) at least once. The TN concentrations in most monitoring stations were higher than the water quality objectives ( Table 4). The average TN concentrations for monitoring stations in two tributaries, the Yipin River and the Huaxi River, were higher than that in the Wubu River.
Wubu River is listed as a water source protection area for centralized drinking water supply (Table 2) (Table 3) in Huaxi River. Table 5 presents the statistics of the listed water quality parameters in Huaxi River. As is shown in  (Figure 1). As is shown in Figure 1,       Water quality assessment using the CCME-WQI and NPI The CCME-WQI values for each station in the time period 2013-2015 are listed with their corresponding water quality status in River (between NHCK and JLY), it was found that the pollution status in Huaxi basin was quite severe (Figure 2), and strict river management is needed to better manage water resources in this area, even though the overall water quality in the TGR remained stable and was ranked as 'good' (Zhao et al. ).
Generally, the assessment results of the NPI approach were consistent with that of the CCME-WQI approach (Table 6). In some stations, there is a slight difference.
For example, CCME-WQI values are nearly the same for 2013 and 2014 for the JLY station in Huaxi River. However, the NPI values are quite different (Table 6) . Therefore, the comprehensive score will increase dramatically in cases where the index value for one evaluation factor is much larger than those of the others. Hence, there exists a potential issue that the assessment results may disagree with the overall water quality conditions in a certain water body. In this study, there was no obvious variation of water quality conditions in Huaxi River from 2013 to 2014 (Table 5). Thus, the assessment results of NPI in the JLY station may be misleading. This method should be used with caution for water resources managers.
The CCME-WQI approach has many advantages. First, it can be used to effectively evaluate the overall water quality status and health of aquatic systems. Additionally, it conducts water quality evaluation by a variety of typical evaluation indicators rather than using the worst factor, and therefore the assessment results of water quality conditions are dependable. According to the assessment results of this study, the CCME-WQI approach can be

CONCLUSIONS
Water quality conditions in three urban tributaries of the TGR were evaluated with the CCME-WQI and NPI However, when the river drained through urban areas, water quality conditions greatly deteriorated due to the excessive release of household and municipal sewage, and industrial wastewater, especially for Huaxi River. The pollution status in Huaxi basin is quite severe, and strict river management is needed to better manage water resources in this area. Generally, assessment results of the NPI approach consistent with that of the CCME-WQI approach.
However, the NPI method tends to overemphasize the influence of the most serious pollutant factor. Therefore, the assessment result will increase dramatically in situations where the index value for one evaluation factor is much larger than those of the others. This method should be used with caution for water resources managers. For the CCME-WQI approach, it can be used to effectively evaluate the overall water quality conditions, and provide reference for decision-makers on water resources management.