Assessment of surface water quality by water quality index (WQI) at the Cai Sao canal, An Giang province, Vietnam

Trinh Thi Lan and Nguyen Phuoc Long*

An Giang University, Vietnam
ttlan@agu.edu.vn
* Center for Monitoring and Resources Technology – Environment,  An Giang, Vietnam

Abstract

Assessment of surface water quality by the water quality index (WQI) at the Cai Sao canal, An Giang province was conducted to provide a database for planning and monitoring water quality in this region. The monitoring was carried out from February/to August/2009, taking e water samples on a monthly basis and at 8 sites along the canal.

The water quality indices (WQI) were in the range 39-29 indicating mild pollution at the two sites close to the joining of the canal with the river, increasing to severe pollution along the rest of the canal. It is recommended that the water from the canal should not be used for human activities before being treated by appropriate methods. The formulae used to caluclate the water quality index (WQI) are easy to use, thus it is a valuable tool for observation of the water environment, and for monitoringof the pollution

Key words: Agriculture, aquaculture, garbage, irrigation, waste

Introduction

The traditional way to assess water quality is measure distinctive parameters of water quality, then compare each of these parameters with the values permitted by the National Technical Code or an International Standard. However, it is impossible to follow the variation in water quality of a river (or section of a river) through each distinctive parameter of assessment of water quality. It is also difficult to compare water quality in each section of a river, between one river and another river, between one period and another period (in months or seasons), between the current water quality and the past one.  On the other hand, only scientists or experts can interpet the distinctive parameters of water quality assessment, so it is difficult to inform local communities and State management agencies  about the quality of the water in a particular locaation, so that they can make appropriate decisions on the exploitation and protection of the water source. In the assessment of water quality, use of statistics and classifications of water quality, creates many obstacles because this information is very abstract and not easily understood by decision makers. Meanwhile, the classification of the water source is very important as a guide to its exploitation and use. 

The Water Quality Index (WQI) is popularly used by many countries and experts for the analysis and assessment of water quality. There are many forms of WQI that are being used in differfent parts of the world. However, the procedure to calculate the WQI that is used in Canada is relatively simple to follow. For this reasone we have selected the WQI as used in Canada for our research.

The Cai Sao canal passes through Long Xuyen City, Thoai Son District and territory of Kien Giang Province. This is a natural canal which was dredged in 1997. The width at the bottom is 12m and the depth 1.7m (An Giang Department of Water Resources 2005). The water in this canal is used mainly for housewhold purposes and for agriculture. In recent years, many farmers have changed from crop-based agriculture to intensive production of Striped Catfish. However, the recycling to the Cai Sao canal of the waste water from the fish ponds has created problems of pollution, as the waste waters were not treated or were treated inadequately.  The present study aimed to monitor the changes in water qualiity in different sections of the Cai Sao canal and at different times of year.

Methodology

Sampling sites

On the basis of a survey of the Cai Sao canal, 8 sampling sites were identified along a 20 km stretch, beginning from the point contiguous to the  Hau river (Photo 1).

Photo 1: Sampling sites along the Cai Sao canal

Frequency of Sample Collection

Samples of water from the canal were collected in the morning on the 20^th day of each month from February 2009 through August, 2009. The parameters used in the analysis included:  pH, dissolved oxygen (DO), nitrate nitrogen (N-NO₃^-), soluble phosphorus (P-PO₄^3-), nitrite nitrogen (N-NO₂^-), chemical oxygen demand (COD), total suspended solids (TSS), biochemical oxygen demand (BOD5) and total Coliforms. The samples were collected and analyzed according to current standards in Vietnam. 

WQI calculation

                           
The results were assessed according to:

Results and discussion

WQI at each monitored site 

As was to be expected, the water quality deteriorated sharply with distance from the Hau river up to site D3 after which  the values remained constant and at levels indicating severe pollution (Figure 1). Even at site D1, where the highest WQI was recorded, the water quality was at the "limiting" point, ie bordering on "polluted". At site D2 the WQI indicated that the water was "polluted" with an even greater degree of pollution at the subsequent sites.

At the D3 site there were 13 aquaculture households and a big aquaculture zone (NTACO) with a total water surface are of more than 40-60 hectares, so the impact of waste water from fish-raising ponds on water quality was considerable. At the D4 site, the Cai Sao canal received waste sources from other interior-field canals (intersections). The D5 site also coincided with a big aquaculture zone (4 households and 1 farm of the Southwest seafood import-export company) and a flooding shelter for many people, so untreated waste sources from the aquaculture and households also impacted on water quality in this sector. The pollution at site D6 was because this sector received large quantities of waste water from interior-field canals (which served for agricultural production), from the aquaculture activities of 3 households, from 2 rice mills along the canal banks, and from floating gasoline stores. At site D7 there was a dense residential zone and Vinh Khanh market along both banks of the canal, so untreated garbage and waste water was a major source of pollution in this sector.  Site D8 was the furthest from the Hau river and was also influenced by agricultural production activities (especially two-crop rice cultivation).

Figure 1. The variation of WQI at the different sites (mean values for February through August)

WQI from February through August 2009

Rainfall appeared to have no consistent effect on the WQI as the highest value was recorded in March which is approaching the end of the dry season. There was an indication (P=0.14) that the WQI increased slightly as the rainy season progressed from April through August.

Figure 2. The variation of WQI from February through August (mean values for the different sites)

From the reasearch results on water quality, , it could be seen that water quality in the canal was bad and polluted. The causes of this phenomenon were because many households living along the two Cai Sao canal banks were not aware of the need for protecting the environment and the infracstructure in the region was under-developed. Therefore, people routinely dumped untreated wastes into the canal. In addition, the water source also had to receive untreated garbage from markets, and untreated waste water from aquaculture zones, two-crop rice cultivation zones, rice mills and brick kilns.

The observed pollution undoubtedly influenced not only the people’s daily life and health but also the socio-economic development of the region, because water quality in the Cai Sao canal directly influences the aquaculture project zone of the province. Hence, it is necessary to recommend people in the region about the current pollution of the water source as well as to let them know about the importance of the Cai Sao canal and its benefits.

Conclusion

• The water quality indices (WQI) were in the range 39-29 indicating mild pollution at the two sites close to the joining of the canal with the river, increasing to severe pollution along the rest of the canal.

• It is recommended that the water from the canal should not be used for human activities before being treated by appropriate methods.

• The formulae used to caluclate the water quality index (WQI) are easy to use, thus WQI is a valuable tool for observation of the water environment, and for monitoring the pollution. 

References

Ministry of Natuaral Sources and Environment 2008 National Technical Code of Surface Water Quality.

Canadian Council of Ministers of the Environment 2001 Canadian water quality guidelines for the protection of Aquatic life: CCME water quality index 1.0.

Lê Trinh 2008 Nghiên cứu phân vùng chất lượng nước theo các chỉ số chất lượng nước (WQI) và đánh giá khả năng sử dụng các nguồn nước sông, kênh rạch ở vùng TP Hồ Chí Minh. Sở khoa học công nghệ TPHCM. (The partition of water quality and use of the water quality index (WQI) to rate the ability to use the river water, and the canals in Ho Chi Minh city. HCMC Department of Science and Technology

Received 9 April 2011; Accepted 28 May 2011; Published 1 July 2011

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