Assessment of Cau River water quality assessment using a combination of water quality and pollution indices

This research aims at using a combined water quality index (WQI) and pollution index (PI) to assess and characterize river water quality of Cau River which is one of the longest rivers in the north of Vietnam. Five different water quality and water pollution indices were used including the Water Quality Index (WQI), Comprehensive Pollution Index (CPI), Organic Pollution Index (OPI), Eutrophication Index (EI), and Trace Metal Pollution Index (TPI). The combined water pollution indices show more serious pollution towards the river downstream. In particular, CPI and OPI reveal a high risk of eutrophication. Cluster analysis was applied to classify water monitoring points into different quality groups in order to provide a better understanding of the water status in the river. This study indicates that a combined water quality analysis could be an option for decision making water use purposes while its single index shows the current situation of water quality. This is an Open Access article distributed under the terms of the Creative Commons Attribution Licence (CC BY 4.0), which permits copying, adaptation and redistribution, provided the original work is properly cited (http://creativecommons.org/licenses/by/4.0/). doi: 10.2166/aqua.2020.122 om http://iwaponline.com/aqua/article-pdf/69/2/160/708485/jws0690160.pdf er 2021 Cao Truong Son Nguyen Thi ̣ Huong Giang Nguyen Hai Nui Nguyen Thanh Lam Vo Huu Cong (corresponding author) Vietnam National University of Agriculture, Trau Quy, Gia Lam, Hanoi, Vietnam E-mail: vohuucong@gmail.com Trieu Phuong Thao Centre For Environmental Monitoring Portal, 556 Nguyen Van Cu, Long Bien, Hanoi, Vietnam

water shortage country due to three main causes. About 72% of the river basins are distributed outside the country, which accounts for 63% of the total national water source (equal to 520-525 billion cubic meter) and the volume of water coming from inside the country area only is 37%, around 310-315 billion cubic meters (MONRE ). Vietnam water resources are distributed unequally throughout geographical location and seasons. The Cuu Long River delta basin has the largest water proportion at 60%, the Hong and Thai Binh rivers have nearly a fifth of the national water volume at 16%, and less than a quarter of the total water volume belongs to other rivers. Vietnam also has seasonal precipitation in which the rainfall is concentrated mostly in the summer season and the other seasons have less than a third of the total rainfall. Therefore, many areas suffer drought for several months, especially in the winter (MONRE ). The other reason is water pollution. The water quality in Vietnam recently has been declining significantly under the pressure of population and economic growth.
Currently, the average water consumption is 9,560 cubic meter per capita annually. It is slightly lower than the middle water consumption country according to the average level of the International Water Resource Agency (IWRA). However, Vietnam accounts for a half (ca. 4,000 cubic meters) while other sources come from neighboring countries (MONRE ). The Vietnamese government has implemented various national water monitoring programs to protect the water environment. From 2008 to 2018, the general water monitoring programs of ten main river basins were established and implemented. In this study, we use a monitoring database in Cau River basin (from 2008 to 2018) to calculate the quality index and pollution index. The results of this study provide clear scientific evidence to regulate the situation of water quality of this river. The case of Cau River water evaluation could propose a solution for the Vietnamese government in applying water quality indices to managing and monitoring water sources in the future.

RESEARCH METHODOLOGY Study area
Cau River is the longest river branch of the Thai Binh River system at 288 km and its basin area covers 6,030 km 2 ( Figure 1). The Cau River flows through four provinces including Bac Kan, Thai Nguyen, Bac Ninh and Bac Giang before being discharged into the Thai Binh River to the sea. The river's width changes following the flood season and drought season from 100 to 50 m, respectively.
The river's bed is larger and the slope reduces to 0.05%.
The downstream river is located in two provinces, Bac Giang and Bac Ninh, with an average elevation of 10-20 m (MONRE ).
According to the Center for Environmental Monitoring of Vietnam (CEM) water monitoring report (2018) persons per square kilometer. This area has ten pollution sources mostly related to agriculture production and residential activities. The middle part of the river flows through Thai Nguyen province, and it has a high population density as well as heavy industrial activities. CEM's water monitoring program at Cau River Basin has identified 16 sources of pollution in this area. The downstream of the river crosses two provinces, Bac Giang and Bac Ninh.
These are populated places with diverse economies: agriculture, industrial and craft villages. The total waste water discharge into the river is estimated at 138.192 cubic meters per year (Table 1). The location of these pollution sources is the foundation for the establishment of water monitoring points.  (VEA ). The water quality index is represented as follows: where WQI a represents the chemical parameters (DO, BOD, COD, amonium and phosphates); WQI b represents the physical parameters (TSS and turbidity); WQI c represents the biological parameter of coliform; WQI pH represents the pH.
Based on the calculated score of WQI, water quality is classified into five categories: 1. Level 1: WQI score obtained from 0 to 25: Water is extremely polluted, emergency treatment is required before reuse.
2. Level 2: WQI score obtained from 26 to 50: water quality is suitable for transportation and equivalent purposes.
3. Level 3: WQI score obtained from 51 to 75: water quality is suitable for irrigation and equivalent purposes.
4. Level 4: WQI score obtained from 76 to 90: water quality is suitable for domestic usage.
5. Level 5: WQI score obtained from 91 to 100: water quality is suitable for domestic water supply.

Comprehensive pollution index (CPI)
The CPI is used to access the level of pollution in a specific watershed by using monitoring statistics (Liu & Zhu ).
The formula to calculate CPI is presented as follows: PI i is calculated according to the following equation: where Ci ¼ measured concentration of parameter number in water; Si ¼ permitted limitation of parameter number according to environmental standard. TPI is used to evaluate the trace heavy metals in the water (Reza & Singh ). TPI is calculated according to the following equations: The TPI value is categorized into two groups: 1. Group 1: 0 < TPI 1none pollution 2. Group 2: TPI > 1pollution In this study, TPI is calculated based on the average concentration of four trace heavy metals: Fe, Cu, Zn and Cd.
These are four monitoring indicators in the general water monitoring program of Vietnam.

Data analysis
T-test analysis was used to evaluate the significant difference of water indices between the rainy season and the dry season in Cau River. ANOVA was used to identify the significant difference between upstream, midstream and downstream. Cluster analysis was applied to evaluate the water quality and pollution level between monitoring points.
The points which have similar polluted levels were gathered into one group. The results of cluster analysis could assist managers to conduct better water monitoring and management plans.

RESULTS AND DISCUSSION
Water quality in Cau River The average values of ten consecutive years of water quality show that TSS and COD exceed the regulated level indicated in Vietnam National Standards QCVN08:2015/ BTMT, column A1standard for domestic water usage (

The water quality index
For overall quality assessment, data from 22 sampling points was calculated (Table 6). The average WQI scores were 67.52 and 69.67 in the dry and rainy season, respectively, indicating sufficient quality for irrigation supply. The WQI

Water pollution indices
The results of water pollution indices calculation in both the dry and rainy season are summarized in Table 7.

Comprehensive pollution index (CPI)
The CPI data show the value of the entire river with no significant difference between dry and rainy seasons. In the dry season, the CPI of Cau River ranged from 0.50 to 1.57 with an average value of 1.08. According to the CPI's classification, this river was slightly and medium polluted. In the rainy season, the CPI of Cau River reached 0.66-1.37 with an average score at 0.96 and its quality was classified as the same level as the dry season.
Although the water quality of each water monitoring point tended to be better during the monsoon period, this difference was not statistically significant. However, the CPIs were different among the monitoring points upstream, midstream and downstream. The midstream had lower CPIs during the monsoon period in comparison to the dry season. We used the ANOVA test to analyze the significant difference levels of CPI in three parts of the river and the t-test to analyze the significant difference between the two seasons of the year. The results showed that the difference of CPI in three parts of the river was not statistically significant.

Organic pollution index (OPI)
Similar to the CPI, the OPI in the dry season was higher than the rainy season (1.55 compared to 1.18). This result was significant with α ¼ 0.1 (P ¼ 0.0538). The OPIs of Cau River could be classified into two groups: Good (0 < OPI < 1) and Polluted (1 < OPI < 4). Regarding spatial location, the OPI of the upstream was 0.92 and 0.63 in the dry and rainy season respectively. These scores were in the good quality category (0 < OPI < 1). However, this difference was not statically significant. In the midstream of the river, the OPI was substantially higher in the rainy season (1.35 compared to 0.79 in the dry season) and this difference was statistically significant with α ¼ 0.01 (P ¼ 0.007559). The average OPI of the downstream in the dry season was 2.12, which was equal to the extremely polluted level, much higher than the OPI in the rainy season (1.83).
However, the difference of OPI between the two seasons was not significant. Similar to the results of the CIP analysis, the OPI of the upstream had the highest quality and the downstream had the worst quality ( Figure 3). However, the difference of OPI among three geographical areas was not significant.

Eutrophication index (EI)
The EI of the entire river was obtained in the range of about 100-400 for both dry and rainy seasons. This indicates that the river was at high risk of eutrophication. However, the EI in the dry season was much higher than the rainy season The EI in the dry season was always higher compared to the rainy season. However, this difference was only significant at midstream with α ¼ 0.05 (P ¼ 0.019287).

Trace heavy metal index (TPI)
In contrast to other pollution indices, the TPIs of Cau River were low which clarified that this river was not polluted by trace heavy metals according to the TPI's classification. The rainy season also had higher TPI compared to the dry season, although this difference was not significant.
The TPI also slightly differed according to geographical computation categorized 22 points into five groups as summarized in Table 8.
According to  River achieved the standard for irrigation purposes in both the dry season and rainy season. However, further study on bearing capacity of the river will be needed for water supply purposes.
The water quality indices varied depending on location and monitoring time. In the dry season, the water quality of the upstream was better than other parts of the river, followed by the midstream and downstream respectively.

Pollution Indices calculation indicates that the Cau
River is polluted in different geographical conditions.
Many locations of the river are contaminated by organic pollution with OIP > 1. The river water was at high risk of eutrophication as EI was above zero. The TPI of Cau River is in the safety level. All pollution indices in the Cau River tend to increase from upstream to midstream, then downstream. Cluster analysis grouped the water monitoring points into five groups with the quality reducing gradually from the first group to the fifth group. The classification of clustering analysis provided meaningful support for water pollution monitoring and appropriate solutions for the treatment of water for water supply.