Luận án TS: Yếu tố ảnh hưởng chất lượng nước hệ thống sông Sài Gòn, Việt Nam

Integrated Study on Factors Affecting Water Quality of the Saigon River System in Vietnam (ベトナム国サイゴン川水系の水質に影響を及ぼす因子に関する統合的研究) By Nguyen Thi Van HA グエン ティ バン ハ A dissertation submitted to the Graduate School of Engineering, The University of Tokyo in partial fulfillment of the requirements for the degree of Doctor of Philosophy Examination Committee: Prof. Satoshi TAKIZAWA (Chairperson) Prof. Keisuke HANAKI Prof. Hiroaki FURUMAI Prof.

Satoru OISHI Assoc. Hiroyuki KATAYAMA Assist. Kumiko OGUMA Department of Urban Engineering Graduate School of Engineering The University of Tokyo Japan December 2009 ABSTRACT The Saigon River System, including Dau Tieng Reservoir and the Saigon River, is not only the vital water resources in the Dong Nai River Basin for supplying water to Ho Chi Minh City (HCMC), but also is the largest irrigation system in Vietnam. In recent years, water quality of the Saigon River System has been deteriorated because of the rapid economic growth of HCMC, Binh Duong and Tay Ninh Provinces in this basin and the higher concentrations of manganese and iron as well as the salinity intrusion.

This unstable and poor quality of the river water has impaired its utilization and increased the health risks for people. The Saigon River system is a complex semi diurnal tidal river which is affected by the water releasing from the Dau Tieng Reservoir, the natural flow, the regulated drainage flow and the tidal waters. The Saigon River extends about 280 km which covers 4,717 km2 with different types of land uses and soil types. It passed through the most developed regions in the southern of Vietnam, i.

HCMC and Binh Duong Province. It is necessary to conduct an integrated analysis on the water quality status in the river system in order to provide a systematic view of the water quality status from the river and its components. Furthermore, it will yield the interactions of various components of river system and provided the scientific foundations for setting up sound policies and strategic management for water resources in the Saigon River Basin. This study was aimed to determine the water quality status of the Saigon River System and to provide the scientific knowledge on natural and anthropogenic factors that affecting water quality of the river system for its strategies and water quality management.

In order to fulfill the above objectives, a study of 6 specific contents was carried out such as: (i) to investigate seasonal water quality variation in the Dau Tieng Reservoir, (ii) to estimate the natural and anthropogenic factors affecting water quality, (iii) to investigate the impacts of fish cage culture on water quality in the Dau Tieng Reservoir, (iv) to identify sources and potential mechanism of manganese and iron inputs into the Saigon River, (v) to elucidate water quality status along the Saigon River, and at the Hoa Phu water intake, and (iv) to apply the artificial neural network to simulate the hourly variations of salinity in the Saigon River at the Hoa Phu water intake. The study combined five research approaches, including: (i) interview of fish cage operators in the Dau Tieng Reservoir, (ii) ad-hoc river survey for on-site and off-site water quality analysis, (iii) batch leaching tests for iron and manganese from soil and sediment samples, (iv) continuously water quality monitoring at the water intake in the -i- Saigon River, and (v) artificial neural network for simulating hourly salinity at the water intake. The monthly water quality monitoring in the Dau Tieng Reservoir was conducted from March 2005 to March 2006. It was found that water in the Dau Tieng Reservoir was monomictic and had seasonal variations of water quality and acidification.

The increased inflows of low pH waters and nutrients, especially nitrogen and phosphorus, caused more acidic, and decreased the water quality in the rainy season. The Dau Tieng Reservoir was divided into three water zones based on the water quality characteristics: the western branch, the eastern branch and the center of reservoir. It was also divided into three layers depending on the water depths: surface (0-5m), middle (6-10m) and bottom layers (deeper than 11 m). Water quality in the Dau Tieng Reservoir varied in the following ranges: pH 4.72, EC 2 - 8 μS/cm, Turbidity 2 – 77 NTU, DO 0 - 12.85 g/L, ORP -169 – 326 mV, BOD5 0.0 mg/L, ammonia nitrogen 0 – 0.77 mg/L, total nitrogen 0.057 mg/L, total phosphorus 0.

coli 0 – 12 CFU, and total coliforms 0 – 295 CFU. The water quality of the Dau Tieng Reservoir met the requirements of surface water quality standards for water supply (TCVN 5942-1995 - Type A), except for 51%, 26%, 20% and 15% of the total water samples that exceeded the permissible ranges for ammonium, dissolved oxygen (DO), nitrite and total coliforms, respectively. The trophic status of the Dau Tieng Reservoir was at the mesotrophic- eutrophic boundary. Phosphorus was found to be the limiting nutrient to algae growth.

The estimated total nutrient loads into the Dau Tieng Reservoir were approximately 4,729 tons of total nitrogen (TN) and 412 tons of total phosphorus (TP) per year. The areal total phosphorus and total nitrogen loads were 1.5g TN/m2 in year 2005, respectively, which were about 8 and 6 times higher than the critical areal loading levels of TP and TN recommended by Vollenweider (Kneale, 1997). The human activities contributed significant portions of the total nutrient inputs into the Dau Tieng Reservoir. Nutrients from the runoffs contributed 73% of the total nitrogen and 24% of the total phosphorus.

Fish cage culture and livestock raising added about 15% and 4% of the total nitrogen, and 39% and 13% of the total phosphorus, respectively, into the reservoir. The fish cage operation at peak time harvested 9,600 tons fish per year but also released to the surrounding water 1,200 tons of TN and 281 tons of TP per year, which increased BOD5, orthophosphate and ammonia nitrogen concentrations in the water in the vicinity of fish cage. The comparison of water quality at fish cage areas before and after the ban showed significant declines of BOD5, ammonia nitrogen, total nitrogen, total phosphorus and orthophosphate in water. Our estimation suggested that limiting the - ii - number of fish cage by 250 cages and stopping animal raise could reduce about 28% of the total phosphorus input, i.

147 tons TP, into the Dau Tieng Reservoir. According to results of the four river water surveys in the dry season (March) and in the rainy season (September) in 2005 and 2006, the Saigon River could be divided into three sections: upstream, middle and downstream sections based on water quality characteristics. The upstream section, from the Dau Tieng Reservoir to the boundary of Tay Ninh Province and HCMC, had a high turbidity; therefore, soil erosion and suspended particles in the discharges should be controlled in water quality management practices. In the middle section, from this boundary to Binh Phuoc Bridge, the impacts of low pH and leachable ions from the acid sulfate soil (ASS), especially manganese and iron, deteriorated the water quality for its supply to HCMC and Binh Duong Province.

In the downstream section, water became more polluted by high concentrations of ammonia nitrogen, total nitrogen and total phosphorus, due to untreated effluents from residential and industrial areas, and dissolved manganese. Both middle and downstream sections were highly contaminated by bacteria of E. coli and total coliforms. The supplementary river and canal water survey was conducted in May 2008 in order to identify water quality in the Saigon River and its tributaries and canals in the middle and the downstream sections.

The acid sulfate soil and sediment samples were also taken for conducting the batch leaching tests and chemical analysis. Those experiments were aimed to identify the manganese and iron sources and transports in the Saigon River Basin and to provide better understanding on factors affecting their release rates. Two major sources of manganese and iron inputs into the Saigon River water were found. In the middle section of the Saigon River, manganese and iron leachings from ASS were the dominant sources.

Iron inputs from ASS were significantly higher than manganese inputs due to much higher contents of iron than those of manganese in ASS. In the downstream river sections, dissolution and reduction of manganese and iron from the deposited Mn-Fe-rich sediments were the major sources. Manganese inputs from sediments became more important than those from ASS leachate, indicating by manganese contents in sediment was about 10 times of those in ASS; and the manganese releasing rate about 14 times of that from leaching of ASS. The Dau Tieng Reservoir, soil erosion and industrial effluents are not major sources of manganese and iron inputs into the Saigon River.

Manganese and iron had similar leaching behaviors from ASS. pH was found to be a determinant factor for manganese and iron leaching from soil. Low pH (less than 3) dissolves the iron-bound manganese and facilitates the pyrite oxidation in ASS, which increases the dissolved manganese and iron concentrations in the drainage waters. Change the pH from 4 to 1.5 could increase manganese leaching from PASS 10 times that of iron 14 times.

In contrast to the manganese and iron leaching from the soils, - iii - manganese leaching from the sediments was independent of iron leaching. Manganese inputs from the sediments was found to be more important in the downstream river section than the leaching from acid sulfate soils, which was evidenced by ten-times higher manganese contents in the sediments than in the ASS and the fourteen-times higher manganese releasing rates from the sediments than the ones from the ASS. In contrast with manganese, iron leaching from sediments was slow and less significant. The sediment leaching test revealed that manganese and iron release rates from sediments depended on pH, redox potential (Eh), their aqueous concentrations and their contents in the sediments, which were of less importance.

Those factors caused the temporal and spatial variations of manganese and iron released. Eh was found to have no direct effect on manganese reduction from the sediments. Because of the acidic nature (pH<6) of Saigon water pH did not show direct effects on manganese leaching from sediment. Manganese contained in the sediments readily dissolved into water.

Meanwhile, decrease of pH and Eh affected the iron releasing rate. Especially, decrease of Eh caused the significant increase of iron releasing from sediments due to the iron reduction process. Both the ad-hoc river survey and the continuous water quality monitoring found that in the middle section of the Saigon River, pH decrease and Eh increase occurred frequently in the rainy season, which facilitated manganese releasing from the sediments, but retarded the iron reduction from the sediments. In the downstream section, acidic water and anoxic water (low Eh) facilitated manganese dissolution from sediments, resulted in the elevated manganese concentrations in water.

When water was mostly anoxic for a long time period, manganese and iron reductions took place, however, the iron reduction were much slower, indicated by the decrease of dissolved iron concentrations in the downstream water. The ASS-derived sediments had the highest release rates of manganese and iron due to its high contents of total manganese, total iron and their readily dissolved forms. The advanced techniques such as the continuous on-site water quality monitoring and the artificial neural network were applied in this study in order to provide (i) baseline data of water quality for water supply in the Saigon River, and (ii) a tool for water quality simulation, respectively. Both have demonstrated their useful application and advantages in water quality monitoring and management.

The three data loggers, including Aquadopp, YSI and CLW, were installed at about 4 m above river bottom at the water intake in the Saigon River for monitoring the water level, water velocity and the physical water-quality parameters at 10 or 30-minute intervals from April 2006 to April 2008.