Water quality management for irrigation in the mekong river delta, Vietnam

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The Mekong River Delta (MD), the most downstream part of the Mekong river, is known as the biggest "rice bowl" of Vietnam. Currently, 2.4 million ha are used for agriculture. During the rainy season part of the Delta is flooded. Along the 600 kmcoast, the sea tide strongly influences the water quality by sea water intrusion. In addition, an area of 2-million ha is covered by acid sulfate soils. Also, the MD is very densely populated with intense associated water pollution. The combination of the hydrological regime, sea, soil-type and pollution poses original water quality management problems for irrigation. Along the river and......

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Nội dung Text: Water quality management for irrigation in the mekong river delta, Vietnam

WATER QUALITY MANAGEMENT FOR IRRIGATION IN THE MEKONG RIVER DELTA, VIETNAM L.A. TUAN1* , G.C.L. WYSEURE2 , L.H. VIET1 & P.J. HAEST2 1 College of Technology, CanTho University, Campus II, Street 3/2, CanTho City, Vietnam; 2 Laboratory for Land and Water Management, K.U Leuven, Kasteelpark Arenberg 21, B-3001 Heverlee, Belgium, * E-mail: latuan@ctu.edu.vn --- oOo --- ABSTRACT The Mekong River Delta (MD), the most downstream part of the Mekong river, is known as the biggest "rice bowl" of Vietnam. Currently, 2.4 million ha are used for agriculture. During the rainy season part of the Delta is flooded. Along the 600 km- coast, the sea tide strongly influences the water quality by sea water intrusion. In addition, an area of 2-million ha is covered by acid sulfate soils. Also, the MD is very densely populated with intense associated water pollution. The combination of the hydrological regime, sea, soil-type and pollution poses original water quality management problems for irrigation. Along the river and canals in the MD, the water quality parameters are generally related to the use of fertilizers and pesticides in agriculture, of nutrient-rich effluents from aquaculture and animal husbandry and of wastewater from industrial plants and human populations. In the dry season and early rainy period, the polluted water seriously impacts the agricultural cultivation and domestic water supply. Presently water-abstractions from the Mekong river are mainly used to irrigate the rice and upland crops, curb salinity intrusion and leave acid sulfate soil layers with sufficient wetness. These objectives are mainly agricultural. Current irrigation calendars, soil maps and fluctuation of water quality parameters are discussed in this paper. The need for a better water quality management and monitoring network in the river system are presented. Not only irrigation control but also for industrial and population use is considered. The existing discharge and water level recording stations should be the preferred locations for water quality sampling. The classical parameters like pH, salinity, total suspended solid (TSS), total Fe, Ca2+, Mg2+, Cl-, SO42-, Dissolved Oxygen (DO), Biochemical Oxygen Demand (BOD5), Chemical Oxygen Demand (COD), Allumium (NH4+), Nitrate (NO3-), Phosphorous (PO43-), heavy metal, coliform have to be supplemented with bio-indicators like zooplankton and phytoplankton. Initial results show that bio-indicators in the MD give a good indication of water quality. Key words: irrigation, saline intrusion, acid sulfate soil, water quality management. THE MEKONG RIVER DELTA IN VIETNAM IN GENERAL The MD is located in the centre of the Southeast Asian region. It is really a large wetland formed mostly by the alluvium deposition of the Mekong river (Figure 1). It lies between latitudes 104°30' to 107°00' E and longitudes 8°30' to 11°00'N and covers an area of 5.9 million hectares of which 3.9 million in Vietnam. The Delta in Vietnam is bordered to the North by Cambodia, to the west by the Vam Co river, to the south by the Eastern Sea and to the west by the Gulf of Thailand. The whole Delta is flat and low laying except for some low mountains and hills in Chau Doc and Ha Tien. The MD has great potentials for agricultural production with a population of 17 million inhabitants living in 4 million hectares of land. The population of the MD has doubled over the past 30 years and is estimated to grow by another 30 to 50 per cent by the year 2025 (Mekong River Commission - MRC, 2004). Historically and practically the
population has settled densely on the along the river and canal banks, resulting a high concentration of human pollutants along the water bodies in the Delta. Fig.1: The Mekong River Basin and Land forms of the MD in Vietnam The long-term average annual rainfall in the MD varies from 1,400 - 2,200 mm (Figure 2). About 90% of total rain water falls from May to October (Figure 4). From September to December each year, large areas in the South Western part of the Delta are inundated by the Mekong river; especially around the Cambodian border (Figure 3). Due to the effect of tropical monsoon rainfall characteristics, the flows at flooding time are about 25 - 30 times the dry season flows which occur between March and April (Öjendal, 2000). The Long Xuyen Quadrangle and the Plain of Reeds are a poorly drained depression area with an inundation lasting up to 4 to 6 months. The total inundated land of the MD in flooding season is about 1.2 to 1.9 million hectares. The most heavy floods are caused by 3 simultaneous factors: by large discharges originating from Southern Lao and the Great Lake in Cambodia, by long and heavy rainfall in the MD, and by the high tides. Although such floods may cause loss of human life and of properties they also have beneficial consequences: deposition of nutrients, leaching out the field pollutants and toxics as aluminum and iron, removal of acidity from acid sulfate soils, desalinization of water, killing insects and supply of fish. In the dry season, lasting 7 months, the discharge from the Mekong river decreases and leads to more intense seawater intrusion. As a consequence many coastal areas suffer serious shortages of fresh water supply. There are more or less 2.1 million hectares of the MD (Figure 5) affected by salinity from sea water intrusion in open mouths and estuaries. Seawater intrusion in the river branches is very complicated. The main factors are river discharge, local rainfall and runoff, the amplitude of tides in the East Sea and the Gulf of Thailand, the slope of the river bed, the wind velocity and direction and the depth of the estuary. Salt water intrudes the inland from the Hau and Tien rivers' mouths and Ca Mau peninsula's estuaries, strongly from February to April. About 47% of the MD is occupied by acid sulfate soils (more than 1.6 million hectares): mainly in the Long Xuyen - Ha Tien quadrangle, the Plain of Reeds, the West Hau river 2
and in a part of Ca Mau peninsula. This soil is very sensitive to the fluctuations in the river discharge and groundwater table. From March and April, the subsurface water level lowers by approximately 1.0 meter and therefore deep cracks in the soils result in oxidization of the pyrite horizon into acid sulfate. Fig.2 (left): Rainfall distribution in the MD (Yamashita, 2003) Fig.3 (right): Flooding depth and duration in the MD (Yamashita, 2003) Monthly Mean Discharge in Tan Chau (Tien River) and Chau Doc (Hau River) 25,000 Tan Chau Chau Doc 19,296 20,000 Discharge (m 3 /s) 15,000 5733 10,000 2041 459 5,000 0 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Month Fig.4 : Monthly mean discharge of the Mekong river to Vietnam IRRIGATION SYSTEMS IN THE MEKONG RIVER DELTA The socio-economical development in the Delta is constrained by its water resources regime. Agricultural and fishery production has a very high water demand and is a major export earning for Vietnam. The Delta contributes for 27% of the total GDP of Vietnam (Minh, 2003). Approximately 2 million tons of rice, produced in the MD, are yearly exported from Vietnam. Currently, 2.4 million hectares of the MD is used for 3
agriculture and aquaculture highly depending on water from the river and canal systems. The agricultural production currently consumes 85 - 90% of the total water supply. Fig. 5 (left): Status of saline intrusion in the MD (Yamashita, 2003) Fig.6 (right): Density of salinity in surface water (Yamashita, 2003) Rice cultivation occupies almost 90% of the agricultural land. To produce one kilogram of rice requires 3,000 - 5,000 liters of water, depending on the rice variety and type of irrigation used (MRC, 2004). At the same time, aquaculture areas are increasing with an annual rate of 4.1% (General Statistical Office, 2000) due to their higher potential for export earnings as compared to rice. The rice cultivation areas have increased yearly by more than 100,000 ha during the period 1995 -1999. This has lead to higher water demands for irrigation and therefore only a fraction of the rice cultivation lands can be irrigated in the dry season. Water use in the Mekong delta in 1995 was more than 210 million m3 as compared to 534 million m3 used by the whole Vietnam (Su, 1996). Between 1976 – 1990 due to the introduction of high-yielding rice varieties and improved techniques the total rice production doubled while the rice areas only increased by approximately 20%. Even recently from 1996 to 2003, the yearly rice yield of the Delta of 12.8 million has increased to 17.5 million ton. This accounts for 50% national food production in Vietnam and plays an important role in the national food security. Depending on the water supply capacity and land use pattern, there are three major rice cropping systems in the MD (Figure 7): (i) the single rice crop as Mua crop (rainfed rice); (ii) double rice crop as Dong Xuan crop (Winter-Spring) and He Thu crop (Summer - Autumn) or He Thu crop and Mua crop; and (iii) triple rice crop as Dong Xuan crop (Winter-Spring) and He Thu crop (Summer - Autumn) and Mua crop. Many water resources projects have been established and implemented during last 3 decades in order to keep track of the demand and to ensure food security and improvement of the living standards for people of the Vietnam Government. The aims of these water works are: • to extend existing irrigation systems, mainly in the in the middle and upper delta where double and triple rice cultivation each year is dependent on water supply during the dry season; • to prevent salinity intrusion by sluices, water gate, small dams and dikes systems in the coastal areas. These water control works can be closed in the high tides periods for flood protection; 4
• to reclaim the acid surface soil areas by soil washing and acid-neutralization capability of the flood plus rain water and by keeping water table higher than potential acid horizon; • to improve flood water control and drainage by dividing flooding water to the western sides and other local depressed areas; • to extend rural water supply and sanitation, special needs for the remote areas and the saltwater/acid water- affected zone; • to strength water resources management capacity by training and equipping to the water staff; • to install water environmental monitoring systems by establishing water sampling and pollution warning stations along the industrial zones, cities and rivers/canals. Fig. 7: Cropping calendar, monthly rainfall and water demands in the MD Note: VH: very high; H: High; M: Medium; L: Low More than 3,300 billion VND (nearly 210 million USD) was invested in 105 irrigation projects in the Mekong delta in the 1996-2003 period, of which 60 out of 105 projects were completed (Vietnam News Agency, 2003). The main components of these projects are to construct water control sluices, small and medium scale pumping stations, flood control embankments and small dams, together with the necessary primary, secondary and tertiary canal systems connecting to the field. Until now, there are over 7,000 km length of main channels, 4,000 km on-farm systems, more than 20,000 km of protection dikes to prevent early flood (Ministry of Agriculture and Rural Development - MARD, 2003). These canal systems are not only used for irrigation, drainage but also for water supply, fishery, local transport by boat. As general guideline, the systems are designed for an irrigation supply of 1.1 to 1.2 liter per second per hectare (l/s.ha) and for a drainage intensity of 3,3 to 3.5 l/s.ha. They are hydraulically operated by mixing flows from upstream rivers branches in the MD and tides from the East Sea and the Gulf of Thailand. In the fields, the farmers control water level by using small earth-dams, weirs or low-lift pumps according to crop needs. SEASONAL WATER RELATED PROBLEMS IN THE MEKONG DELTA As one of the specific natural characteristics of the tropical monsoon climate regions, there are only two seasons in the Mekong Delta: dry season and rainy season. Dry season: The monthly mean discharge in the Mekong river is lower than 2,500 m3/s, during January to April, together with the lower groundwater table lead to a serious shortage of fresh water for rice cultivation and domestic drinking water. Salinity intrusion areas expand throughout the MD. However, saltwater intrusion also has positive impacts (Miller, 2000). Many coastal farmers apply a rice-shrimp rotation and need saltwater to raise native shrimp. The lower water level in the dry season is also 5
contributing to the oxidization of the pyrite horizon with its acidification of soils. This has a major environmental impact, which not only affects the rice-cultivation by local farmers but also results in limitations of living habits and health by local farmers. Wet season: The wet season in the MD starts with the early rainfalls in May. At the onset of the wet season the rain water induced surface runoff in the river and canals is heavily polluted by flushing solid-wastes and residential waste, by dissolving chemical pollutants from industries and agriculture by leaching acid sulfate soil toxics. The deforestation of Melaleuca sp. in the Plan of Reeds for rive cultivation also lead to the increased of the acidity in upper areas of the Delta. Concentrations of sulfate up to 10 - 70 mg/L have been measured in the floodwater (Morrmann and Van Breeman, 1978). During soil ploughing for rice-field preparation large amounts of toxics are released by acid sulfate soil areas. This flushing out the fields is necessary before planting. In May As a consequence the acidity of the river water increases dramatically in the Plain of Reeds. Water related diseases, especially Anopheles and Dengue mosquitos related epidemics, increase in the early rain season and the end of flooding periods due to stagnant water in the sunken places. WATER QUALITY MONITORING The water quality monitoring data and information has started recently and is limited. The results of 2-year environmental monitoring (1995 - 1996) of some the MD provinces reported that water quality of the MD rivers is not yet heavily polluted by organic wasteforms (Triet et al., 1997). Concern has been raisen about stagnant zones within the canal environment which trap sediments and concentrate pollutants (Haest, 2003). Few data is available, but in August - September the sediment content, considered as total dissolved and suspended solids, would have peak values about 300 mg/l, followed by concentrations less than 50 mg/l (Tuan, 2003). In a 2-year survey by Can Tho University from April 2000 to March 2001 in 6 provinces An Giang, Dong Thap, Vinh Long, CanTho, Tra Vinh and Soc Trang, it was observed that Dissolved Oxygen (DO), Biochemical Oxygen Demand (BOD5), Chemical Oxygen Demand (COD), E.coli and coliform indexes are of sufficient good water quality and sanitary standard (Ve et al., 2002). Seto (2002) based on 37 samples form 6 provinces found high fluctuations for EC, Sulfate, Chloride, Sodium, Phosphate and Coliform between low and high values but in general lower then standards (Seto, 2002).. The salinity in the Mekong Delta for rice cultivation is high (Yamashita, 2003). Typically in the MD is that the concentration of nitrate is very low, often less than 0.1 mg NO3--N/L; the detection limit. The Can Tho river (Table 1) with a strong impact of the city is more polluted. Water quality parameters including water physical, chemical parameters, phyto- and zoo-plankton in the rice fields should that it could be used to cultivate fish and improving farmer's income in the rural areas in the MD (Long et al., 2002). CONCLUSION ON WATER QUALITY MANAGEMENT FOR IRRIGATION Although at this stage water quality is still fairly good, one should carefully monitor the impact of the rapid expansion of intensified agricultural cultivation and urbanization. The water resources are vitally important and pollution will have dramatic consequences for the supply to agriculture, industries and domestic drinking water. The origin of pollution into water resources is complex and may be come from industrial activities, agricultural and fishery chemicals, untreated wastewater, oil leaked from river transportation (Figure 8). Because the river water quality in the MD varies with the season, it is necessary to maintain a water quality monitoring network. This network includes national monitoring stations, experimental sites and local environmental laboratories. 6
Table 1: Water quality monitoring data in Nhi Kieu Bridge, CanTho River Date/Time 19 Sep. 2002 18 Jul. 2002 03 Sep. 2002 14 Nov. 2002 Parameters 7h30 12h30 7h30 12h30 7h30 12h30 7h30 12h30 pH 7.35 7.19 7.37 7.3 6.97 7.01 6.96 6.85 SS (mg/L) 14 25 47 91 33 36 37 46 BOD5 (mg/L) 13 10 12 20 7 7 18 22 COD (mg/L) 23.0 19.5 18 12 15.6 14.0 21.7 28.8 DO (mg/L) 2.87 2.47 1.09 2.39 2.12 2.01 0.78 0.38 Fetol (mg/L) 0.23 0.19 1.16 0.96 0.453 0.067 0.94 2.18 NH3 - N (mg/L) 3.190 1.086 2.110 0.316 1.084 0.676 0.271 0.562 NO3 - N (mg/L) 0.3 0.3 0.3 0.2 0.0 0.1 0.2 0.4 NO2 - N (mg/L) 0.0238 0.0282 0.1104 0.0018 0.0295 0.0183 0.003 0.005 (Sources: CanTho Environmental Monitoring Station, 2003) Upstream flow water Irrigation water (quantity, quality) Agricultural Flooding water wastewater (quantity, quality) River Water supply transportation Fisheries Urban and Industrial wastewater (quantity, quality) Inflow Downstream flow water Outflow (quantity, quality) Fig. 8: An illustration the components to river water pollution Parameters should be collected including: river discharge, water level, water temperature, pH, Total Fe, SiO2, Ca2+, Mg2+, Cl-, SO42-, HCO3, COD, DO, BOD5, NH4+, NO3-, PO43-, Cl-, heavy metals, zooplankton, phytoplankton, oil, and others. Depending on the parameters and budget, sampling frequencies should be sufficient and not just occasional and unsystematic. Data from this network will be analysed or modeled for many cost-effective irrigation systems, prevention of negative effects of agricultural activities by a set of biological, physical and chemical water quality criteria for water users, prevention of soil erosion and sedimentation and education of communities about water environmental protection and food safety. REFERENCES [1] General Statistical Office, 2000. Statistical Yearbook 2000. Statistical Publishing House, Hanoi, Vietnam [2] P. J. Haest, 2003. The Influence of Long Xuyen City on the Water Quality of the Hau river (Mekong Delta). Master Thesis. K.U.Leuven, Belgium. [3] D. N. Long, B. M. Tam, L. M. Lan, D. T. H. Oanh, V. N. Son, N. V. Lanh, N. T. K. Lien, L. T. N. Thanh, 2002. An Investigation on Water Quality Parameters in Wet Season Rice in Related with Integrated Agricultura-aquaculture System Development in the Mekong Delta. Proceedings of the final workshop on "Improvement of environmental education in agricultural sciences". CTU-JICA Mini-project. CanTho University, Vietnam 7
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