Water quality management for irrigation in the mekong river delta, Vietnam
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......
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: [email protected]
--- 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
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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
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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;
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• 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
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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.
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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.
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