AN OVERVIEW OF SALTWATER INTRUSION

IN THE MEKONG RIVER

By Quang M. Nguyen

June 1999

ABSTRACT

Saltwater intrusion is one of the most important factors affecting the development of the Mekong Delta. Recently, limited data and information indicate that saltwater is intruding into the Mekong River at an alarming rate. For example, the chloride concentration at My Tho, where freshwater had been found all year round, was measured at 5.3 grams per liter (g/l) or parts per thousand (ppt) at the beginning of April 1999 and the trend is toward an increase by the end of the month. This concentration exceeds the standard of 2.0 g/l for drinking water in developing countries, 0.14 g/l for surface irrigation water, and 3.0 g/l for livestock consumption. Saline water, which was reported to intrude 50 km inland in 1995, extended as far as 70 km in 1999, the worst in recorded history. Actual magnitude and extent of saltwater intrusion in the Mekong River may be larger if appropriate data and information can be obtained.

Saltwater intrusion in rivers is a natural phenomenon as a result of the difference in density between freshwater and saltwater. Human activities, however, may exaggerate saltwater intrusion and cause potential adverse impacts. Saltwater moves upstream along the river bottom in the form of a wedge with the leading edge or toe pointing inland. Saltwater intrusion is affected by factors such as the river flow and duration, elevation of thalweg, slope of river channel, tidal magnitude, wind velocity and direction, and water temperature. The dominant factor is the river flow. The most undesirable characteristic of saltwater intrusion is that it is easy to intrude but difficult to retreat.

Topographic and climatic conditions of the Mekong Delta are favorable for saltwater intrusion; therefore, saltwater can easily intrude into the Mekong River when low flows occur for a long duration. Low flows may be caused by prolonged droughts and/or water resources projects for irrigation, water supply, and inter-basin diversion. Direct potential impacts including loss of sole source of water for domestic use and irrigation, destruction of crops and fruit trees, and reduction of fishery production may be severe for areas along the river. More importantly, these potential impacts may spread into larger areas in the Mekong Delta connected to affected reaches by canals. In the long term, saltwater intrusion may destroy the Mekong Delta ecosystem and become an obstacle for the development of the entire region. For those reasons, potential impacts on saltwater intrusion of the water resource projects in the Mekong River basin, including the projects located within the Mekong Delta, should be evaluated and mitigated properly and adequately.

INTRODUCTION

The Mekong Delta in Vietnam is a low and flat area formed by deposited sediments from the Mekong River. Except for sand bars and areas along the river banks, the ground surface elevation of the Mekong Delta does not exceed one meter above mean sea level along the coast and two meters above mean sea level in northern areas. In some areas such as the Plain of Reeds, the ground surface elevation is below mean sea level. The general slope of the Mekong Delta is approximately one percent. In the Mekong Delta, the Mekong River and the interconnected canal system play a key role in domestic water supply, transportation, irrigation, and drainage. The Mekong River and the canal system are also ideal channels for saltwater from South China Sea and Gulf of Thailand to intrude into the Mekong Delta; therefore, saltwater intrusion is one of the most important factors affecting the development of the Mekong Delta. This paper provides an overview of saltwater intrusion in rivers and the Mekong River, discusses potential impacts, and suggests a solution for the saltwater intrusion problem in the Mekong River and the Mekong Delta.

CHLORIDE IN WATER AND ITS EFFECTS

Water tastes salty because of the present of chloride. The chloride concentration is defined as grams of chloride in one liter of water (g/l) or parts per thousand (ppt). Seawater has an average chloride concentration of approximately 19 g/l or 55 percent of dissolved solids. The chloride concentrations in natural waters vary widely. The rivers of humid regions generally have chloride concentrations less than a few milligrams per liter (mg/l). In arid regions, the chloride concentrations in rivers may reach several hundred mg/l [1].

Chloride is one of minerals controlling the taste of water; however, the chloride concentration should not exceed the standard for drinking water. The common standard for chloride in drinking water in developed countries is 0.25 g/l [2]. In many areas of the world, sources with the chloride concentration up to 2.0 g/l are used for domestic purpose [3]. High chloride concentrations are objectionable in drinking water because of possible physiological effects, unpalatable mineral tastes, and higher costs because of corrosion or the necessity for additional treatment.

Chloride is also vital for the growth of vegetation. One of the sources of chloride for vegetation is the chloride concentration in irrigation water; however, high chloride concentrations in irrigation water may have adverse impacts. Results from studies and investigation indicate that impacts are not observed if the chloride concentration in irrigation water is less than 0.14 g/l. The impacts on vegetation vary from slight to moderate for the chloride concentrations raging from 0.14 to 0.36 g/l. The impacts are severe if the chloride concentration in irrigation water exceeds 0.36 g/l [4].

Water with high chloride concentrations may have impacts on livestock and freshwater fishes. Poultry or sensitive animals may show slight effects from prolonged use of water with chloride concentrations above 1.5 g/l. The effects become adverse if the chloride concentration exceeds 3.0 g/l [5]. All species of fish and aquatic life can tolerate a range of chloride concentrations in order to survive under natural conditions; however, water with chloride concentrations in excess of 15.0 g/l is not suitable for most freshwater fishes [6,7].

SALTWATER INTRUSION IN RIVERS

Saltwater intrusion is a natural phenomenon in rivers with bottom elevation below mean sea level. It is caused by the difference in density between saltwater (approximately 1.03) and freshwater (approximately 1.0). Saltwater intrusion is affected by other factors such as the river flow and duration, elevation of thalweg, slope of river

channel, tidal magnitude, wind velocity and direction, and water temperature. Of these factors, the river flow is dominant [8,9].

In the rivers with a long duration of high flows, freshwater is able to prevent saltwater from intrusion into the river channel. If the volume of freshwater is large enough, freshwater can push saltwater away from the river mouth. On the contrary, saltwater can intrude easily into the rivers with a long duration of low flows. Because saltwater is heavier than freshwater, it moves upstream along the river bottom under floating freshwater. Typically, a highly stratified wedge may be formed in deep rivers with high freshwater flows, as shown in Figure 1. The leading edge of the saltwater wedge is well defined; however, some mixing occurs to form a brackish water region along the freshwater-saltwater interface. The shape of saltwater wedge may be changed by other factors, particularly the river flow and tides. In the rivers without freshwater flow, saltwater will intrude into the river channel to the point where the elevation of the river bottom is at the sea level. The most undesirable characteristic of saltwater intrusion is that it is easy to intrude but difficult to retreat.

Although saltwater intrusion is a natural phenomenon and affected by topographic, climatic, and hydrologic conditions, human activities may change these conditions, especially topography and hydrology. The river channels may be deepened and widened for navigation and transportation projects. Hydroelectric, irrigation, and water diversion projects may change the hydrologic conditions including flows and duration. These activities tend to exaggerate saltwater intrusion and may have adverse potential impacts.

SALTWATER INTRUSION IN THE MEKONG RIVER

Topographic, hydrologic, and climatic conditions in the Mekong Delta region are favorable for saltwater intrusion into the Mekong River. The river bottom is below mean sea level and very flat. The tidal magnitude is high, approximately 3.0 m in the South China Sea. During dry seasons, the wind has a southwest-northeast direction, which is opposite to the direction of freshwater flow in the Mekong River. Historically, natural flows in the Mekong River were high for a long duration, especially during wet seasons; therefore, saltwater intrusion was limited in the coastal areas and did not pose any problems for the Mekong Delta.

Currently, saltwater intrusion is one of the most important factors affecting the development of the Mekong Delta. Limited recent data and information indicate that saltwater is intruding into the Mekong River at an alarming rate. For example, the chloride concentration in the Mekong River at My Tho, where freshwater had been found all year round, was measured at 5.3 g/l at the beginning of April 1999 and the trend is toward an increase by the end of the month [10]. This concentration exceeds the standard for drinking water, for surface irrigation water, and for livestock water consumption. Saline water, which was reported to intrude 50 km inland in 1995 [11], extended as far as 70 km in 1999, the worst in recorded history. Actual magnitude and extent of saltwater intrusion in the Mekong River may be larger if appropriate data and information can be obtained. Catching a stingray, a deep-saltwater fish, in the Mekong River at Sa Dec appears to verify the seriousness of the saltwater intrusion problem because Sa Dec is located approximately 120 km from the coast.

Saltwater intrusion is intruding into the Mekong River because the river flows have been reduced, especially during dry seasons. Low flows may be caused by prolonged droughts and/or water resources projects for irrigation, water supply, and inter-basin diversion including the irrigation projects using the Mekong River flow within the Mekong Delta. Reasons for the reduction in the Mekong River flow can only be identified by an extensive investigation using appropriate climatic and hydrologic data and appropriate information relating to the construction and operation of the water resources projects within the entire Mekong River basin.

POTENTIAL IMPACTS ON THE MEKONG DELTA

The Mekong River and the interconnected canal system are the primary source of water supply for domestic use and irrigation in the Mekong Delta; therefore, direct potential impacts of saltwater intrusion on the Mekong Delta would be a loss of that water supply source. A large area along the Mekong River and canal system may not be cultivated because of a lack of freshwater. Agricultural production in areas of high chloride concentrations may be decreased. In areas with very high chloride concentrations, crops and fruit trees may be destroyed and fishery production may be severely affected.

More importantly, these potential impacts may spread into larger areas in the Mekong Delta connected to affected reaches by canals. When reaching these inland areas, saltwater would be difficult to be flushed out and its potential impacts may last for a long time. In the long term, saltwater intrusion may become an obstacle for the development of the entire Mekong Delta.

In addition to the direct potential impacts, saltwater intrusion may have other potential impacts, especially on the environment. In the areas of saltwater intrusion, the ecosystem of the Mekong River and the Mekong Delta may be disturbed and unbalanced. These potential impacts may not be recognized until significant changes in the ecosystem occur.

SUGGESTED SOLUTION FOR SALTWATER INTRUSION IN THE MEKONG RIVER

As mentioned above, the reason for current saltwater intrusion in the Mekong River is simple and understandable. The flows in the Mekong River have reduced to such a low level that freshwater can no longer push saltwater back to the ocean during wet seasons and prevent saltwater to intrude into the river channel during dry seasons. The reduction in the Mekong River flows may be caused by prolonged droughts and/or water resources projects in the Mekong River basin, including the projects in the Mekong Delta; therefore, saltwater intrusion in the Mekong River is truly a basinwide problem. To solve this problem properly and adequately, a basinwide solution is required. This solution does not appear to be simple and easy because of unique natural conditions of the Mekong River and the Mekong Delta and of conflicts of interest among riparian countries within the Mekong River basin. For those reasons, an appropriate and long-term solution for the saltwater intrusion problem in the Mekong River would not be realized without an international cooperation on a basis of equality, openness, freedom, and respect of mutual benefits of all riparian countries.

Feasible water resources projects should be prepared by individual riparian countries and submitted to an international agency, such as the Mekong River Commission, to be incorporated into a master plan for the development of the Mekong River basin (Master Plan). The Master Plan should be evaluated by independent and competent experts using standard procedures based on criteria that are pre-determined by all riparian countries. International boundaries within the Mekong River basin should not be considered in evaluating the Master Plan. The Master Plan should address the issue of saltwater intrusion in the Mekong River and the Mekong Delta through proper evaluation and appropriate mitigation of potential impacts of its projects. All projects in the Master Plan should not be initiated until the Master Plan is verified by all riparian countries.

In Vietnam, a national plan for the development of the Mekong River basin in the Central Highlands and the Mekong Delta (National Plan) should be prepared. This National Plan should be submitted to the international agency to be incorporated into the Master Plan and be used as a basis to negotiate with other riparian countries. The National Plan should optimize social, economic, and environmental benefits using available natural resources. The National Plan should be comprehensive, practical, dynamic, and flexible so that it can be implemented and adjusted under any conditions imposed by the Master Plan. To support the preparation of the National Plan and future investigations and evaluations, a data collection program should be prepared and implemented, as soon as possible, to collect climatic, hydrologic, and water quality data in the Central Highlands and the Mekong Delta. This data collection program should be continued to monitor potential impacts, if any, of the construction and operation of the National Plan projects. The National Plan projects should be prioritized and implemented in phases. At the end of each phase, the National Plan should be reviewed and refined, if necessary, based on data and information obtained from the data collection program and from the construction and operation of the projects in that phase. This National Plan should include appropriate measures to address potential impacts of the Master Plan, if any, on saltwater intrusion in the Mekong River and the Mekong Delta.

Figure 1 General shape of the saltwater
wedge

(Source: “An overview of the
Mississippi River’s Saltwater Wedge.” The U.S. Army Corps of Engineers, New Orleans District. September 1997)

There are several measures that
can be used to address saltwater intrusion in the Mekong River. The first measure is to increase the low flows of the Mekong River within the Mekong Delta, especially during dry seasons. The flows of the Mekong River could be increased by reducing the diversion flow for dry season rice production in low-yield areas or by releasing water from shallow reservoirs built within the Mekong Delta. These reservoirs should take advantages of the topography of the Plain of Reeds and could be operated similar to the natural hydrology of the Great Lake in Cambodia. If feasible, these reservoirs would be multi-purpose facilities not only to prevent saltwater intrusion in the Mekong River but also to maintain and improve the ecosystem of the Plain of Reeds. The second measure is to construct floodgates near the river mouths. This measure is very common and has been used worldwide including the Mekong Delta. However, based on experience in eastern Australia, the floodgates in the Mekong Delta “…will result in extremely poor upstream water quality during the dry season; and secondly, the resulting water quality will cause major decreases in upstream, dry-season irrigated crop production and fish and aquatic production" [12]. The third measure is to construct underwater sills to a proper height above the river bottom to reduce saltwater flow and artificially stop the saltwater wedge, as shown in Figure 2. This measure appears to have limited success to prevent further saltwater intrusion in the Mississippi River in the United States [9]. Another measure is to do nothing. This measure is passive, but might be appropriate under certain conditions.

Figure 2 Underwater sills to prevent saltwater
instrusion

(Source: “An overview of the Mississippi River’s
Saltwater Wedge.” The U.S. Army Corps of Engineers, New Orleans District. September 1997)

An appropriate engineering solution for the
saltwater intrusion problem in the Mekong River, which may be a combination of the above measures, can only be identified through an extensive and thorough study. The primary objective of this engineering solution should be to reduce, to the extent possible, any potential impacts of all water resources projects in the Mekong River basin on the development and the ecosystem of the Mekong River and the Mekong Delta.

CONCLUSIONS

The Mekong Delta in Vietnam is a low and flat area. In some areas such as the Plain of Reeds, the ground surface elevation is below mean sea level. The Mekong River and the interconnected canal system are not only convenient waterways for transportation but also the sole source of water for domestic use and irrigation. The Mekong River and the canal system are also ideal channels for saltwater from South China Sea and Gulf of Thailand to intrude into the Mekong Delta.

Saltwater intrusion is a natural phenomenon in rivers with bottom elevation below mean sea level. This phenomenon is a result of the difference in density between freshwater and saltwater; however, human activities may exaggerate saltwater intrusion and cause potential adverse impacts. Saltwater moves upstream along the river bottom in the form of a wedge with the leading edge or toe pointing inland. Saltwater intrusion is affected by factors such as the river flow and duration, elevation of thalweg, slope of river channel, tidal magnitude, wind velocity and direction, and water temperature. The dominant factor is the river flow. The most undesirable characteristic is that saltwater intrusion is easy to intrude but difficult to retreat.

Topographic, hydrologic, and climatic conditions in the Mekong Delta region are favorable for saltwater intrusion into the Mekong River. Historically, natural flows in the Mekong River were high; therefore, saltwater intrusion was limited in the coastal areas and did not pose any problems for the Mekong Delta. Currently, saltwater intrusion is one of the most important factors affecting the development of the Mekong Delta. Limited recent data and information indicate that saltwater is intruding into the Mekong River at an alarming rate. For example, the chloride concentration in the Mekong River at My Tho, where freshwater had been found all year round, was measured at 5.3 g/l in April 1999. Saline water, which was reported to intrude 50 km inland in 1995, extended as far as 70 km in 1999, the worst in recorded history. Actual magnitude and extent of saltwater intrusion in the Mekong River may be larger if appropriate data and information can be obtained. Saltwater intrusion is intruding into the Mekong River because the river flows have reduced significantly. Low flows may be caused by prolonged droughts and/or water resources projects for irrigation, water supply, and inter-basin diversion including the irrigation projects using the Mekong River flow within the Mekong Delta.

Direct potential impacts of saltwater intrusion on the Mekong Delta would be a loss of the sole source of water supply. Areas along the Mekong River and the canal system may not be cultivated because of a lack of freshwater. Agricultural production in areas of high chloride concentrations may be decreased. In areas with very high chloride concentrations, crops and fruit trees may be destroyed and fishery production may be severely affected. These potential impacts may spread into larger areas in the Mekong Delta connected to affected reaches by canals. In the long term, saltwater intrusion may become an obstacle for the development of the entire Mekong Delta.

Saltwater intrusion in the Mekong River is a basinwide problem; therefore, it can only be resolved by a basinwide solution. This solution does not appear to be simple and easy and would not be realized without an international cooperation on a basis of equality, openness, freedom, and respect of mutual benefits of all riparian countries. Feasible water resources projects should be prepared by individual riparian countries and submitted to an international agency, such as the Mekong River Commission, to be incorporated into a master plan for the development of the entire Mekong River basin. The master plan should be evaluated by independent and competent experts using standard procedures based on criteria that are pre-determined by all riparian countries. The master plan should address the issue of saltwater intrusion in the Mekong River and the Mekong Delta through proper evaluation and appropriate mitigation of potential impacts of its projects. These projects should not be initiated until the master plan is verified by all riparian countries.

In Vietnam, a national plan for the development of the Mekong River basin in the Central Highlands and the Mekong Delta should be prepared to be submitted to the international agency and to be used as a basis for negotiating with other riparian countries. The national plan should be comprehensive, practical, dynamic, and flexible and should optimize social, economic, and environmental benefits using available natural resources. A data collection program should be prepared and implemented, as soon as possible, to support the preparation of the national plan and to provide adequate data for future evaluations and investigations. The national plan projects should be prioritized and implemented in phases. At the end of each phase, the national plan should be reviewed and refined, if necessary, based on data and information obtained from the data collection program and from the construction and operation of the projects in that phase. This national plan should include appropriate measures to address potential impacts of the master plan, if any, on saltwater intrusion in the Mekong River and the Mekong Delta. These measures may include an increase in low flows of the Mekong River within the Mekong Delta by a reduction of irrigation diversion and/or a release of water stored in low reservoirs in the Plain of Reeds, construction of floodgates, construction of underwater sills, or doing nothing.

An appropriate engineering solution for the saltwater intrusion problem in the Mekong River, which may be a combination of the above measures, can only be identified through an extensive and thorough study. The primary objective of this engineering solution should be to reduce, to the extent possible, any potential impacts of all water resources projects in the Mekong River basin on the development and the ecosystem of the Mekong River and the Mekong Delta.

REFERENCES

[1] Chow, V.T. 1964. Handbook of Applied Hydrology. McGraw-Hill Book Company, New York, New York.

[2] The United States Environmental Protection Agency. July 1976. Quality Criteria for Water. U.S. Government Printing Office, Washington, D.C.

[3] Sawyer, Clair N. and P.L. McCarty. 1978. Chemistry for Environmental Engineering. McGraw-Hill Book Company, New York, New York.

[4] The California Regional Water Quality Control Board, Central Coast Region. September 8, 1994. Water Quality Control Plan. San Luis Obispo, California.

[5] McKee, J.E. and H.W. Wolf. 1963. “Water Quality Criteria,” unpublished report to Paul R. Bonderson, California State Water Quality Control Board.

[6] Rawson, D.S. and J.E. Moore. 1944. “The Saline Lakes of Saskatchewan.” Canada Journal of Resources. 22:141.

[7] National Technical Advisory Committee to the Secretary of the Interior. 1968. Water Quality Criteria. U.S. Government Printing Office, Washington, D.C.

[8] American Association for the Advancement of Science. 1967. Estuaries. Publication No. 83. Washington, D.C.

[9] The U.S. Army Corps of Engineers, New Orleans District. September 1997. “An overview of the Mississippi River’s Saltwater Wedge.” New Orleans, Louisiana.

[10] Mai, Truyet T. April 1, 1999. Personal Communication. West Covina, California.

[11] Vo, Minh Q. December 1995. “Use of Soil and Agrohydrological Characteristics in Developing Technology Extrapolation Methodology: A Case Study of the Mekong Delta, Vietnam.” M. Sc. thesis, Graduate School, University of the Phillipines. Los BaOos, Phillipines.

[12] White, Ian, M. Melville, and J. Sammut. December 6-7, 1996. “Possible Impacts of Salinewater Intrusion Floodgates in Vietnam’s Lower Mekong Delta.” Seminar on Environmental and Development in Vietnam. Canberra, Australia.

Sept 1999