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Cao Vån H· Ph.D.

by Cao Vån H·, Ph.D 2/1998


Let start with some thoughts of good will and positive thinking. In lieu of MRC has been perceived as an agency set up to promote the construction of controversial dams and water diversion of the Mekong and its tributaries. MRC has expressed support for a run-of-river plan for large-scale hydropower developments.

These water-related activity programs are to be developed under two cross-sectional themes of environmental/ecological balance and human resource development. On Delta perspectives and context, the strategic focuses and development roadmap can be structured within the BDP framework and suggested on several prongs as detailed in this paper:
Salinity and acidity management in the Mekong Delta The availability of freshwater which is central to salinity and acidity management Watershed management is an issue at basin-wide level, therefore BDP should address all factors and options related to upstream/downstream water sharing

Last but not least, poverty reduction and restructuring the Delta's economic base must be treated as integrated elements of the BDP.

  1. Approach to Mekong Resource Development Within Vietnam’s Economic Framework 1.1. The Mekong Basin in Vietnam
    Vietnam is the last downstream nation of the Mekong Basin. Within Vietnam, the Basin covers 29% or approximately 96,000 square kilometers of the nation’s total area and is represented by two disparate regions, the Mekong Delta and the Central Highlands, excluding a relatively small area in Dien Bien (1,392 km2), upstream of Nam Ou, part of Nam Rom sub-basin.
    The Mekong Delta spans 39,500 square kilometers (12% of country total). Its vast deltaic plains is formed by sedimentation and erosion. The Delta has been constructed over time by overbank flooding and sediment drifting southwest along the coast from the Tien and Bassac nine-channel mouths. Monsoon rains from late May and peak in September and October, in combination with floodwaters from the Mekong, cause annual flood and inundation in about 1.2 to 1.4 million ha from two to six months. Discharge of the lower Mekong during the wet season averages of 39,000 m3/sec. In contrast, in the dry season, low water discharge is 1,700 m3/sec. In the ebb season during February-May period, salinity is most severe, as river flow is inadequate to prevent seawater intrusion.
    The Mekong Delta is crucial to Vietnam as its "rice bowl", the richest agricultural area in the country. It accounts for half the rice production, 40% of total agricultural output, and 27 percent of Gross Domestic Product. Increased agricultural production has fuelled the economic liberalization or "doi moi" since the mid 1980s and the Mekong Delta has been instrumental in this continuing process. The Mekong Delta provides a home for slightly over 15 million people (22% of country total), making it the second most densely populated region in Vietnam.
    The Central Highlands cover the upstream areas of two Mekong tributaries, Se San and Sre Pok rivers. The Se San river originates from the Ngo Clinh Mountains and is the largest eastern tributary of the Mekong. This part of the Basin is typified by steep topography, has large hydropower potential and forestry resources. The monsoon rains in the area last from June to October and the run-off in the wet season accounts for approximately 80 percent of the yearly volume. The Central Highlands are relatively underdeveloped, has the lowest population density in the country with about 3 million people (4% of country total) living in an area of 56,000 square kilometers (17% of country total). It is the home of many ethnic minorities. In recent years, this region has experienced high levels of in-migration with the highest population growth rate in the country of 5.8%. The central Highlands are important for upland agriculture especially for cash crops such as coffee and tea, and hydropower potential. The second largest dam in Vietnam, the 700 MW Yali Falls Dam is under construction since 1994 and will be completed before the end of the century There are many on-going projects supported by MRC for this part of the basin: the 66 MW Vinh Son dam, the 120 MW Pleikrong dam, the Ea Soup multipurpose project and the Upper Sre Pok basin development plan.
    In a capsule, the two regions comprising the Mekong Basin in Vietnam are characterized distinctly by its unique population composition, ethnicity, income levels and other socio-economic factors, topology, and ecosystems. They underscore approaches to natural resources development in Vietnam, a sustainable development with broader concerns for environment. Rapid and sustainable development of the Mekong Delta is central to Vietnam’s sustained growth.
    1.2. Key Concerns: Salinity and Acidity Management
    For Vietnam, key concerns are reduced flows of water from the Mekong River during the dry season and increased flooding during the wet season. Recent increased flooding has been attributed to upstream deforestation. Nevertheless, the yearly overbank flooding is a disguised blessing to the Delta because flooding is helpful to flush the acid soils, to deposit fertile sediment into the floodplains, and to bring nutrients to inland fisheries. As far as hydrological constraints to agricultural possibilities in the Delta, a depth of prolong flooding of 60 cm is the threshold for rice cultivation-- too deep for any varieties of rice, or any upland crops such as pineapples or sugarcane—impractical to construct too high raised beds.
    Above all, reduced flows of water during the dry season are the biggest concern. Salinity intrusions are closely linked to flow rates through the river system and any decrease in flow will cause extensive problems for agriculture and human habitations. The problems posed by acid sulphate soils, which are common in the Delta, are also exacerbated by reduced flows. In other words, the Delta is facing reduced source of water availability to alleviate two major soil constraints, namely soil salinity and soil acidity.

Salinity Intrusion

Salinity problems arise in the Delta, as the tidal effects throughout large areas of the Delta. Salinity intrusion occurs during the dry season from February to May as freshwater flows are reduced (Figure 2). This problem appears to be worsening, and upstream developments within Vietnam and in other riparian countries could exacerbate the situation. Salinity penetration extends into branches of the Mekong ten to forty miles. About 1.6 million hectares of the Mekong Delta are affected by saline intrusions during the dry season. Salinity penetration is more extensive in the smaller tributaries and canals compared to the Tien and Bassac Rivers. The maximum salinity reaches 4.0 p.p.t around April. Even worse, salinity intrusion makes the irrigation water in the canals become saline, i.e.; fertile soils cannot be used for agriculture due to lack of freshwater.
Along the South China Sea, there are coastal areas of Long An, Ben Tre, Hau Giang and Minh Hai provinces, which are permanently saline because they are inundated by seawater more than 9 months of the year. The tidal floodplain covers an area of 216,000 ha (5.5% of the Delta. Salty soils are found along the coast from Genh Hao (Minh Hai Province) to Go Cong (Tien Giang Province), Can Duoc and Can Giuoc (Long An Province). They cover an area of 809.000 ha (21% of the Delta).
Figure 1.1: Salinity intrusion in the Mekong Delta

Acid sulphate soils

Nearly half of the Mekong Delta is susceptible to potential problems of acid sulphate soils (ASS). ASS are those areas where the oxidation of pyrite (iron sulphide) and acidification process take place. . Pyrite is usually found in tidal swamps, but it does not become a problem so long as the soil remains inundated. Acid sulphate is formed when pyrite-rich sediments are exposed to oxygen. ASS have low pH values and high concentration of aluminum, iron, sulphate, and hydrogen sulfide. The depth of the sulfuric horizon (pH below 3.5) is used to characterize the soil acidity: severely acid soil where the scale is within the upper 50 cm; moderately acid soil where the scale is between 50 and 80 cm, and slightly acid soil where the scale is between 80 and 120 cm. Drained ASS produce sulfuric acid, which dilutes into surface water, brings down the soil pH to below 4, attacks clay content of soils, subsequently release aluminum, which is toxic to plant growth. The capillary process makes severely acid soils unsuitable for agriculture.

Potential and actual acid sulphate soils are pre sent in many parts of the Mekong Delta covering a total area of 1,590,000 ha, mainly in the Dong Thap Muoi and Long Xuyen Quadrangle. Acid sulphate soils in the Delta can be subdivided as follows:

Figure 1.2: Acid soils in the Mekong Delta
Basin Development Plan (BDP) Must Embrace Programs to Support Salinity and Acidity Management in the Delta

It is crystal clear that salinity and acidity management are vital issues facing the Delta. They must be incorporated into the BDP. It boils down to one single factor that adequate fresh water availability in the dry season will help to solve or alleviate these two issues.

The salinity control measures and other resource developments in the Delta are set out in the Master Plan for the Mekong Delta (NEDECO 1993). Salinity control measures involving the construction of dykes and canals focus on these smaller watercourses and affect very large areas.

Acidity management is critical to the Mekong Delta as population pressures and programs for agricultural expansion add to intense conversion of acid sulphate soils, mainly for rice production. Dong Thap Muoi or the Plain of Reeds has transformed with new settlement and extensive reclamation. Farmers' experiences in cultivation practices applied to acid sulphate soils produced encouraging results. Dong Thap's rice production amounted to 2.1 million tons in 1994, compared with 600,000 tons in 1980. The government of Vietnam targets to reclaim 101,000 hectares by the year 2010, out of the remainder 120,000 hectares still uncultivated.

Here lays the difference in approach to manage ASS entailed in this paper. As a proposal elsewhere in this paper points to the conversion of deep ASS area into multi-purpose reservoir to provide freshwater in the dry season, instead. Extensive rely on leaching ASS would lead to environmental degradation. Draining of such soils resulting in exposure of severe acid sulphate soils, however, creates major problems for Dong Thap itself and for downstream areas especially at the beginning of the rainy season as acidic water is flushed down towards the sea. The resulting acidity transfers to surrounding land, contaminates surface water and becomes a threat to aquatic ecosystems.

Summarily, management of potential or actual ASS should involve both precautionary action and curative or adaptive interventions where acidity has become a problem. In this regard BDP for the Delta must stress on an integrated management approach. BDP must strike a balance between ASS treatment and environmental protection.

Furthermore, BDP for the Delta must find ways to effectively deal with acid sulfate soils with cursory research on alternative interdisciplinary approaches including the anaerobic treatment process and sulfate burying such as creating multi-purpose large-scale water reservoirs in the Delta's Dong Thap and Long Xuyen Quadrangle to entrap silt buildup and provide source of freshwater in the dry season. The hard fact is as long as freshwater is available for cultivation, almost any types of ASS are manageable for rice cultivation or other upland-type-crops planted on raised beds.

About the author: Ho Van Cao obtained his doctoral degree in Economics from Georgetown University, USA. He was a former Assistant Minister of Finance and Government Commissioner of Industrial Development Bank of South Vietnam. He was a member of the National Petroleum Board and Presidential Cultural and Social Commission. Notably, he was an Administrative Chief of a district peripheral to Dong Thap Muoi (Plain of Reeds), worked and lived with the Mekong delta people whose bare heels tarred with "phen" (sulphate soils). One of his pen name is NGUON PHIEN (Pure Sulphate Nodule). In spite of his pen name he is an advocate of any solution to solve the ASS (Acid Sulphate Soils) problems and hopefully to grow high-yield rice on it, knowing that the current combination of century-old farming methods with recent scientific researches on ASS are the only applications that one could be content with. But his relentless motivation coupled with his professional obsession still drives him toward miraculous solutions of the ASS-related problems.

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