EIAxpert: An Expert System for screening-level   EIA

Fedra, K., Winkelbauer, L. and Pantulu. V.R. (1991)
Expert Systems for Environmental Screening.
An Application in the Lower Mekong Basin.   RR-91-19. International Institute for Applied Systems Analysis. A-236l Laxenburg, Austria. 169p.

4   Environmental Problems of Water Resources Development
in the Lower Mekong Basin

One of the great rivers of Asia, and ranking twelfth among the world's longest rivers, the Mekong has its source at an elevation of 5000 m close to the Dzanag La pass in the Tanghla Shan mountain ranges, on the northeastern rim of the great Tibetan plateau, in southwestern China. Along it course, the Mekong flows through or along the borders of six countries, China, Burma, Laos, Thailand, Kampuchea and Viet Nam before joining the South China Sea southwest of Ho Chi Minh City. In volume of water discharged into the sea, the Mekong, with an annual average discharge of approximately 475,000 million m³, is the sixth largest river in the world.

Its total drainage basin, including some 160,000 km² in China, is about 783,000 km². The river enters its lower basin at the common Burma--Lao PDR--Thailand boundary point and the distance from there to the ocean is some 2,380 km. It is this stretch of the river that is the subject of the water and related resources development program, sponsored by the United Nations, The Mekong Project. This chapter concentrates on the lower Mekong river and its basin (Figure 4.1).

The lower Mekong basin covers an area of some 611,000 km², or about 77 per cent of the total area of the river basin. It includes nearly the whole of the Lao PDR (202,400 km², the northern tip and the northeast area of Thailand (180,240 km², nine-tenths of Kampuchea (154,000 km² and the western flank and southern tip of Viet Nam (65,200 km²).

4.2 Environmental determinants of development

The distinct environment, geology and climate in the basin together dictate resource patterns and potential for development. The complex geological history of the basin has provided five physiographic units: the Northern Highlands, the Annamite Chain, the Southern Uplands, the Korat Plateau and the Mekong Plain (Pantulu, 1986).

The Northern Highlands

Rainfall is high: 1,200 to 2,000 mm/a. The temperature is generally high, occasionally, however, cold air from Siberia and China penetrates, lowering air temperatures to near zero.

The human population is sparse, averaging 5--14/km² except on the valley floors, as in Chieng Rai (Thailand), where numbers average 57.9/km² however, population density reportedly is increasing.

All the factors described above have a significant influence on resource use. Wet rice cultivation is possible only in the deltas of tributaries. In the uplands, slash and burn cultivation practiced by hill tribes has contributed to considerable loss of natural forest cover and to erosion, which has a significant negative influence on water resource development in the plains. The potential for hydroelectric power development is substantial.

The Annamite Chain

The Annamite Chain, located mainly in the Lao PDR, is 800 km long and has a steep and mountainous terrain in the north and central parts, but forms dissected hills and rolling-to-hilly plateau in the south. The chain extends into Viet Nam and Kampuchea. Of interest is the hilly karstic limestone area---the Khammouane plateau, which is the single most extensive limestone deposit in the basin. This 50--300 km wide and 500--2,500 m high mountain chain divides the western Mekong drainage from eastern South China Sea drainage.

Rainfall is heavy on the south and west flanks which receive the brunt of the southwest monsoon; some inner valleys, however, are drier, with deciduous forests. Once areas which received more than 2,000 mm/a were completely covered by dense rain forest but many of these have been cleared for swidden agriculture. Swidden agriculture still dominates, with less than one per cent of the land under wet rice. The population is sparse ( < 4/km², N; 5-40/km², S), but highly diverse hill tribes are to be found in the region.

The area's potential for agricultural development is limited. Although the tributaries of the Mekong have a more gentle profile than the streams draining into the South China Sea, they are broken by many falls and rapids in the northern sector and are suitable for the development of hydroelectric power. The less accentuated southern sector provides limited irrigation potential in tributary valleys. Vast areas of the chain which are now barren and covered only with grasses with a savannah type of character can be developed for live-stock grazing.

The Southern Uplands

The Southern Uplands consist of the Elephant and the Cardamomes mountains separating the Mekong Plain in Kampuchea from the Gulf of Thailand, and continuing into Thailand. To the east are continuous mountains, while the west comprises rolling, dissected plains, which yield orchard fruit and field crops. The Uplands are at an altitude of 500--1,700 m and except for some steep escarpments, slopes are moderate in the north and steep and eroded in the south.

Rainfall is very high---up to 5,000 mm/a in places---with dense, tropical rainforest and very low human population densities ( < 4/km²). Hill tribes are a negligible proportion of the population, and even swidden agriculture is very limited (though more common on the drier north side). There is little scope for agricultural development.

The Korat Plateau

The Korat Plateau comprises northeast Thailand and adjacent parts of Lao PDR. It is a large (250,000 km²) saucer shaped inter-mountain basin tilted towards the southeast. The altitude of the floor is 100--200 m with the surrounding mountains reaching 1,400 m. The greater part of the plateau consists of relatively flat lands and is underlain by thick, cretaceous salt deposits. Due to the rain shadow effect of the surrounding mountains the area is dry.

Rainfall is erratic and fluctuates between 1,000--1,250 mm. Recurrent floods and droughts afflict the plateau, much of which is now covered with unproductive scrub or grassland vegetation, although it was originally forested. Extensive deforestation has contributed to erosional problems. Several major tributaries of the Mekong in the Lao PDR, the Nam Theun, Se Bag Fai, Se Bang Hieng and Se Done have alluvial valleys in the plateau. In northeast Thailand more than half the plateau is drained by the Mun and Chi rivers; this region experienced some of the earliest development of rice plantation in the basin and judging from archaelogical sites, supported fairly dense prehistoric and early historic human populations. Later populations were thinner, but recent agricultural advances have allowed the population to rise again and much of the plateau now supports between 80--150 people/km².

A number of reservoir sites have been developed mainly for hydroelectric generation and irrigated agriculture. Fisheries are an unforeseen ancillary benefit from the reservoirs. From a purely physiographic point of view the plateau would appear to offer substantial scope for further agricultural development by means of flood control, drainage and irrigation of the more productive soils.

The Mekong Plain

The Mekong Plain is a vast low-lying area, a relatively small portion of which consists of fluviatile deposits of the young Mekong. It comprises most of lowland Kampuchea, the Mekong Delta of Viet Nam and small sections of south Lao PDR and east Thailand. Most of it lies below 100 m, with a few higher outcrops scattered throughout the plain, while much of north Kampuchea comprises rolling and dissected plains between 100--200 m high. The Mekong Plain is the result of erosion and sedimentation; the sediments vary in depth, from at least 500 m near the mouth to only 30 m. At the ``nine mouths'' of the Bassac and Mekong, the combined action of river deposition and the sea has produced a coastal belt of slightly higher elevation. Deposition in the delta continues to extend the Ca Mau Peninsula to the south and west at a rate of 150 m/a in some places.

The plain is the most densely populated part of the basin with more than 450 people/km² in the rice growing regions of the delta (rice is grown on 50 per cent of the land). The richest rice growing areas of Kampuchea are also densely populated, especially south of Tonle Sap and on the Battambang Plain. The north and east savannah, however, are very sparsely populated ( < 4 people/km²). The lowlands, particularly the areas of Holocene alluvium, have historically been the most densely populated and productive agricultural parts of the Lower Mekong Basin, with apparent agricultural and water resource development potential.

4.3 Water resources

The Mekong discharges annually more than 475,000 million m³ of water into the South China Sea. The sources of this surface water are disparate. About 20 per cent of the annual flow comes from the upper basin (i.e., above the Burma--Lao PDR--Thai boundary). Some 70 per cent of the flow is contributed by the Thai--Lao PDR section. The remaining 10 per cent comes from the Kampuchea--Viet Nam sector, excluding the delta.

While snow melt produces a more or less uniform flow in the upper Mekong, the lower Mekong exhibits pronounced seasonal variations reflecting rainfall patterns. The river rises following the onset of the monsoon in May or June, and attains a maximum level in August or September in the upper section of the lower basin and in September and October in the lower section. It then falls off rapidly in December, slowly thereafter, to reach its lowest level in April. There are no mainstream storage structures and those on the tributaries do not have a significant effect on the mainstream flow. Only the Great Lake in Kampuchea significantly affects mainstream flow, largely in the delta.

There are distinct alternating dry and wet seasons in the basin area as a result of the monsoons. While there is a shortage of water during the dry season, large areas are flooded during the wet season. The flooding behavior of tributaries also varies from one part of the basin to another. Tributary basins in Thailand (e.g., Mun and Chi) have relatively small channels but have extensive flood plains up to 10 km wide. These basins, located as they are on the lee side of mountain ranges, receive low rainfall. They usually remain dry for several years, filling irregularly. Highest rainfalls occur along the windward slopes of Annamite mountains and in the Lao PDR and Kampuchea, thus floods of a different magnitude develop in these areas. Stream courses here are generally well defined and accommodate floods which are fairly uniform from year to year.

Resource development constraints

The main foci of water resource development in the Mekong basin are the production of the staple food, rice and fish (the principle source of protein), hydroelectric power for domestic, industrial and agricultural purposes and navigation of the river. Initial estimates place the theoretical potential of hydroelectric power resources of the lower Mekong basin at 58,000 MW installed capacity and 505,000 GWh for annual energy production. The estimated potential of the basin for year round irrigation with the help of storage and flood plain reservoirs is of the order of 6.4 million ha. Development of the resources is sought to be achieved mainly through dam construction and enhanced irrigated agriculture. Due to physiographic limitations, rice cultivation in the basin is possible only in the delta, the Mekong plain, the Korat plateau, the tributary deltas in the Annamite Chain and valley floors in the northern Highlands.

In the natural state, development of rice cultivation is beset with problems of shortage of water in the dry season and flooding of vast areas in the wet season, particularly in the delta and the Korat plateau. Even in the wet season irregular rainfall which causes either dry spells or an over abundance of water, affects plant growth. Furthermore, in the delta inadequate flow in the Mekong for irrigation withdrawal during the low flow period and intrusion of salt water from the sea present additional constraints.

Dams and other water control and regulatory measures would appear, on the surface, to be the logical answer to help overcome the above constraints. However, soil conditions in the Korat plateau, and in the delta, present formidable problems in water management and irrigation development. About 1.8 million ha in the delta are covered by acid sulfate soils and another 2 million hectares in the Korat plateau are influenced by underlying geologic salt deposits. Water control, drainage and irrigation acidify the potentially acidic soils and exacerbate the acid in developed acid sulfate soils. Irrigation of lands underlain with salt deposits results in salinization of top soils and render them unfit for cultivation. Furthermore, salinity control in the delta will affect the important fishery resources which depend on the salinity intrusion for breeding, nursery and forage in the delta wetlands. These problems are described in some detail in the following section.

4.4 Environmental problems

The environmental problems or issues that have direct relevance to water resources development in the basin are listed below:

• Watershed degradation, erosion and sedimentation;
• Acidification of soils in the delta;
• Soil salinization in the Korat plateau;
• Problem soils---danger of desertification as a result of improper exploitation;
• Inundation control effects on fisheries;
• Toxic biocidal levels in edible organisms;
• Waterborne diseases, and
• Potable rural water supply in problem (saline and acid) areas.

The degradation of the Mekong watershed has become one of the main concerns in recent years. Millions of hectares of valuable forests have been degraded to inferior scrub, grasslands or savannah, or have been encroached upon by subsistence agriculture. As a result, soil conditions have deteriorated, with increased water run off and erosion. It is estimated that between 1970 and 1985 alone some 13 million ha of closed forest disappeared in the lower Mekong basin (Table 4.1) through forest encroachment (both legal and illegal), shifting cultivation and agricultural development projects.

Country                  Deforestation in % and 1000 ha
                                1970      |   1985
------------------------------------------|------------
Kampuchea                   11.00 |  7.42 | 239 | 32.5
Lao PDR                     13.00 |  7.91 | 339 | 39.2
Northeast Thailand           5.31 |  2.33 | 199 | 56.1
Southern part of Viet Nam    3.60 |  2.67 |  62 | 25.8
------------------------------------------|------------
Total: Lower Mekong basin   32.91 | 20.33 | 839 | 38.2

A major problem is forest degradation by fire, often started intentionally for reclaiming forest land for shifting cultivation. Forest fires combined with short fallow periods in between lead to soil exhaustion. Grasses such as Imperata cylindrica and Themeda triandra then take over, changing the forest ecosystem from savannah woodland into unproductive grassland. Approximately 8.5 million people are said to depend on shifting cultivation affecting an area of some 17.5 million ha in the lower basin. While slash and burn cultivation, practiced in the traditional way with short cropping and long fallow periods in between may be a sound land use measure, with population increases and the current intensive use, severe soil depletion has resulted. Population increase in the basin and the consequent increase in demand for fuel wood or charcoal have further increased inroads into forests. Except perhaps in the Lao PDR and Kampuchea, there is an acute scarcity of fuel wood in the basin. It is expected that the pressures on remaining forest resources will be extremely high. However, most of the deforestation goes to illegal logging, and effective control to stem this destructive practice seems to be almost impossible for various reasons. The lack of adequately staffed and effective technical organizations, lack of coordination among various agencies, shortage of funds and, at places, unstable political conditions are important contributory factors.

4.4.2 Erosion

The main areas of concern in relation to erosion are the hilly areas mainly in the Lao PDR. By 1972, more than 10 million ha of forest were reported to have been destroyed (Singh, 1972). The annual rate of deforestation for shifting cultivation and through forest fires in the Lao PDR alone is estimated to be 300,000 ha. Generally, in the basin, excessive deforestation is attributed to the enormous increase in population densities in the basin from 16.3 persons per km² some 70 years ago to 66 persons per km² in 1988. As a result, the people living on the plains have encroached on forested hill areas and are reclaiming them for agriculture at a steadily increasing rate. In fact the problem of erosion-induced sedimentation of dams is so serious that power production at two dams, Selabam and Nam Dong, has been adversely affected. It is also apprehended that the rate of sedimentation in the Nam Ngum reservoir has reached alarming proportions. Elsewhere in the basin however, despite the rather drastic changes in forest cover, their erosional effects are not manifest at least in the main Mekong. Sediment yields in the river and tributaries are rather low compared to other Asian rivers (Pantulu, 1986).

4.4.3 Acidification of soils in the delta

An estimated 1.8 million hectares (approximately 45 per cent of the Mekong delta in Viet Nam) is covered by acid sulphate soils and is not readily amenable to agricultural development. These soils are characterized by pyrite deposits at relatively shallow depth, which react to oxygen intrusion with pyrite oxidation and development of sulphuric acid. Soil pH in acid sulfate areas may drop to values below pH 2.0 and, under these conditions, toxic polyvalent cations (metals) are dissolved from the soil minerals. Secondary reactions relate to immobilization of phosphate, inhibition of the nitrogen cycle and potassium deficiency due to leaching. Although farmers in the delta have developed, through trial and error, ingenious water and soil management strategies to overcome these constraints and obtain yields from such soils, large parts of the most severely affected areas lie fallow in spite of the obvious need to reclaim all available land to increase food production in the country. Reclamation of these areas is fraught with difficulties, as inappropriate strategies may lead to enhancement of acidification and even successful strategies may cause damage in other areas, if they result in production of acidic and toxic drainage waters. Such drainage waters and also flood waters which flow over acid sulfate soils are not only unsuitable for all water uses but also cause acidification of adjacent lands and surface water bodies, with often catastrophic effects on agricultural crops and fisheries.

In the earlier days, vast areas of acid soils were covered with Melaleuca forest. However, population pressures have led to reclamation of these lands for irrigated agriculture. Furthermore, defoliation of the Melaleuca forest during the recent war was followed by harvest of the wood and cultivation of the lands for paddy. Lands so converted could only be used for one or two seasons. Thereafter they had to be abandoned because of increased acidity. Even raised-bed cultivation, a method used successfully by farmers, has resulted in the acidification of surface waters, affecting crops and fish in the entire area.

4.4.4 Soil salinization in the Korat plateau

In the whole of northeastern Thailand and parts of the Vientiane plain in the Lao PDR, the recent alluvium is underlain by a typical formation, the Mahasarakam formation. Different strata of this formation are more or less salt-bearing with a lower ``rock salt'' structure, comprising several strata from ``basal salt to upper salt inclusive'', and an ``upper clastic layer'' (Pantulu, 1988). The occurrence of salt-affected soils in the plateau coincides with the area of the Mahasarakam formation and saline ground water. The Korat plateau, as in other parts of the basin, is interspersed with wetlands of various dimensions. In recent years, water resource development activities including dam construction and ``flood plain development'' for irrigated agriculture have resulted in the draining of wetlands and their conversion into irrigated agricultural lands.

Prior to the ``development activities'', agriculture was mostly rain-fed and seasonal, yielding modest returns of 1.5 tons per ha and year of rice. At that time, periodic flooding of the fields by rivers washed out the surface salts, besides providing fish harvests of 10--25 kg/ha for the duration of the flood. Embankment, dam construction and drainage of wetlands and the subsequent development of irrigation in these areas resulted in the elevation of saline groundwater levels, either due to hydrostatic pressure, caused by water storage in the dams, or due to downward seepage from the irrigated fields. This, coupled with capillary rise, has resulted in salinization of surface soils in irrigated areas. This type of secondary salinization in irrigation areas has been reported from many areas.

Examples are the Nong Wai irrigation project area in Khon Kaen, Kampuwapi south of Udorn, the Lam Pao irrigation scheme at Kalasin and Nam Oon irrigation area in Sakhon Nakhon (Arunin, 1984). The progress of surface soil salinization in the irrigated areas is estimated at 10 per cent over a period of 10 years. The areas thus salinized have become unsuitable for any productive use.

4.4.5 Problem soils: danger of desertification as a result of improper exploitation

Problem soils are defined as those which present inherent constraints to productive utilization. Besides the acid and saline soils mentioned above, there are various other problem soils in the basin, such as shallow skeletal soils and sandy surface soils in Thailand and the Lao PDR, and peats and exhausted grey soil in Viet Nam.

Skeletal soils in this context are defined as soils containing 35 per cent lateritic concretions or gravel of more than 2 mm diameter in a given volume of soil. Physical constraints to plant growth are coarse texture and shallow depth, which restrict root growth. Further, the capacity of such soils to retain water and nutrients is generally low.

Peat soils contain at least 20--30 per cent of organic matter in the upper 80 cm of the profile. The main growth-limiting factors are low bearing capacity, shrinkage, irreversible drying, deficiencies of micro and macro nutrients and fungal diseases associated with them. Only peat soils of less than 1 m depth can be brought under cultivation.

The grey soil of the basin remains to be classified and characterized in detail. Its main constraint seems to be low fertility because of nutrient deficiencies.

As in the case of acid and saline soils, ever increasing population pressure in the basin (with the exception of the Lao PDR) and the vast areas the problem soils cover have rendered their reclamation imperative. Therefore, water resource development activities in the basin have to reckon with the problem of utilizing productively the problem soil areas. Unplanned and inappropriate use of these lands has already rendered vast areas irreversibly unproductive. This explains the urgency of addressing this problem in the basin.

4.4.6 Inundation control and its effect on fisheries

Historically, the most productive of all Mekong basin fisheries are those dependent on seasonal flooding; but these fisheries are unusually vulnerable to proposed schemes for the elimination of floods. The floodwater fisheries of the basin hinge on seasonal rains caused by warm humid monsoons from the southwest, which usually begin in May and extend through September, depending on latitude. Along with, and following the monsoon rains, waters of the mainstream and tributaries begin to rise. The timing and effects of this rise differ by river sector but generally floodwaters may cover almost the entire low gradient drainage basin of the Mekong and its tributaries, all the way to the estuarine zone in Viet Nam. The natural, long-time evolution of the reproductive cycle of most freshwater fish of the basin has synchronized maturation of the gonads with the onset of the rainy season and flooding, and has led to extensive migration of these fish into the zones of inundation. These zones not only afford a rich variety of spawning habitats, but also, while inundated, provide nutrient-rich nursery grounds. As the flood water begins to subside following the onset of the dry, cool, northeast monsoons (usually beginning in October), both fingerlings and adults return to the river and its tributaries, and provide rich fishery there: some of the young remain in the wetlands in the flood plains and contribute to year round fish harvests there.

The natural system of high productive potential combined with the opportunity for efficient harvest makes the Mekong floodwater fishery, like those of the other great river-flood inundation zones, one of high catch and value. The seasonal fallowing and drying that follows annual inundation is the key to nutrient release from inundated land for cycling into aquatic production. These events accelerate the breakdown of organic materials, such as plant remains, for rapid transfer via food chains into fish and other aquatic crops during the next flood.

The fisheries of the brackish waters in the Mekong estuary proper and in the adjacent waters of the South China Sea exceed in magnitude the fisheries of the freshwater zone. In these estuarine and coastal waters, shellfish are more prominent than in the inland parts of the basin. Marine organisms predominate, as there is a progressive downstream change from freshwater to marine habitat. Like all estuaries, that of the Mekong is potentially among the most efficient of all aquatic systems for the conversion of solar energy via the food chain into fishery production. This efficiency is reinforced by the shallowness of the waters and the relative nutrient richness of the ecosystem which receives the nutrient-rich silt washed down seasonally during floods, the estuary being situated at the downstream end of the vast drainage basin.

The Mekong inundation zone and estuarial ecosystems contain delicately tuned interactions between the physical environment and the biota. These interactions are highly vulnerable to the alterations in quantity and timing of annual inundation and mainstream discharge implicit in the installation of engineering works and operation of water management systems upstream and in poldering of flood plains. These alterations will impinge upon the life cycles, distribution and abundance of the freshwater zone, estuarine and coastal fishery organisms. Of particular concern may be dispersion of commercially exploitable concentrations of valuable fish, which occur seasonally in the river and off the river mouths.

The fishery yield and value from these ecosystems---the Mekong freshwater zone, estuary and waters of the South China sea under direct Mekong influence---have never been precisely quantified in spite of substantial size and immense economic and nutritional significance at the local level. Extrapolations of existing records and statements by experienced government officials indicate that the annual production of these waters from all types of fisheries (commercial, artisanal and subsistence fishing and from aquaculture) may approximate 500,000 metric tons, valued in 1988 at US$ 225 million.

4.4.7 Toxic biocidal levels in edible organisms

One of the objectives of water resources development in the basin is raising agricultural production from the present 12.7 million metric tons to 37 million metric tons per year. This has required, among other measures, the intensive use of pesticides and herbicides; all the more so because monoculture enhances vulnerability to attack from plant and animal pests. The intensive use of herbicides and pesticides, as is well known, can render the aquatic ecosystems unproductive and even harmful to human populations. In order to avoid these impacts, agricultural development should be made compatible with aquasystem development. The problem of toxic biocides at present is not widespread, though acute in certain locations, particularly in northeast Thailand.

4.4.8 Waterborne diseases

Experiences in different parts of the world have shown that water resource development projects such as those implemented or contemplated in the lower Mekong basin, may result in serious, adverse health consequences. Especially in tropical and subtropical areas, where water and vector borne diseases such as malaria, schistosomiasis (blood fluke disease) and filariasis affect the lives of millions of people, ecological changes induced by water resource projects may directly contribute to the spread, propagation or introduction of such diseases by creating favorable habitats for vectors and intermediate hosts. The incidence of schistosomiasis, for instance, rose dramatically in many arid and semi-arid countries in Africa and the Middle East, after man-made irrigation supported the propagation of snails, which are intermediate hosts. In a southern province of Egypt for example, the prevalence of schistosomiasis reportedly grew from 3 per cent to 42 per cent within 20 years.

Water resource development projects do not only affect habitats of vectors and intermediate hosts but also contribute to the spread and introduction of pathogenic agents by attracting people representing a variety of epidemiological factors; examples are migrant laborers during construction work and settlers after its completion.

Waterborne diseases in the lower Mekong basin could be classified into the following three types:

• Water borne diseases sensu stricto or water transmitted diseases. In this category man or animal is the source of infection and the main host. The agent is discharged into the water with human or animal faeces or urine. Water is a vehicle for infective agents: bacteria, viruses or parasites. Examples are a variety of diarrheal, enterotoxic diseases, escherichia coli infections, shigellosis, salmonelloses, cholera, virus infections, typhoid and paratyphoid, virus hepatitis A, amoebic dysentery, giardiasis, leptospirosis, etc.

• Water transmitted helminthic diseases with involvement of an intermediate host or hosts living in the water. Examples are schistosomiasis, opisthorchiasis and paragonimiasis. Snails are the first intermediate host for parasite development and fish, crabs and plants the second intermediate host for certain parasites. Humans get infected through direct water contact (schistosomiasis) or by consuming uncooked, intermediate hosts;

• Water constitutes the breeding place for the vectors. Examples are malaria, filariasis and Japanese B encephalitis.

Of the variety of diseases, schistosomiasis is the primary focus of the Mekong Committee. The first human case of schistosomiasis originating from the Mekong basin, reported in 1957, was a Laotian living in Paris. Intensive studies showed that schistosomiasis in this area is caused by a then unknown parasite now called schistosoma mekongi. This parasite closely resembles schistosoma japonicum, but its intermediate host is a planorbid freshwater snail, tricula aperta, living in certain parts of the Mekong river. The snail is also abundant in the Mun river, a tributary running through Ubol Province in Thailand, where a major project, the Pak Mun dam is planned. So far, two foci of human schistosomiasis are known in the lower Mekong basin, one at Khong Island in the southern tip of Laos, the second one at Kratie, Kampuchea. No proven case has ever been diagnosed in Thailand, with the exception of refugees from Laos and Kampuchea.

The liver fluke opithorchis viverrini is considered another important potential health problem because of its high prevalence in the population of the northeastern part of Thailand (34.6%) and Laos (46.5% in Vientiane and 39.7% in Khong island). Infection is acquired by the habit of eating raw cyprinoid fish that serve as the second intermediate host for the parasite. Other waterborne helminthic infections such as paragonimiasis, angiostrongylosis and fasciolopsiasis and intestinal flukes, of which detailed data are only available from Thailand, appear relatively less important.

Among vector borne disease, malaria clearly constitutes the most serious health problem in areas of the Mekong basin. Considering the enormous obstacle of drug resistant strains of plasmodium falciparum rapidly spreading over southeast Asia, it is of utmost importance to prevent the creation of new breeding habitats for the vectors. Quite a number of primary vectors are known in the Mekong basin which require different types of breeding sites for efficient propagation, such as slow running, vegetated streams (an. minimus, an. maculatus), paddy fields (an. nivipes), stagnant water in forests (an. dirus) and brackish water (an. sundaicus). Other vector borne diseases in the Mekong basin include dengue haemorrhagic fever transmitted by aedes mosquitoes and Japanese B encephalitis transmitted by culex mosquitoes. Unlike malaria, which is endemic in the area, these two diseases usually manifest themselves as epidemics. In the Mekong delta dengue haemorrhagic fever was one of the leading causes of morbidity during the years 1976--1983. Japanese B encephalitis is usually associated with pig breeding as these animals serve as hosts for the virus.

As mentioned above, diarrheal diseases are known worldwide and are among the biggest killers of children below five years of age. This group of diseases is very common in all three riparian countries and rank high in prevalence and incidence in all age groups, especially in the densely populated Mekong delta where the sanitary standards are low. In water, fecal micro-organism indicators reach medium to high levels in 100 per cent of surface water of the Mekong river, as well as in its branches, canals and ponds. The surveys further showed that 98.5 per cent of dug well water samples were contaminated.

4.4.9 Rural potable water supply in problem areas

In the lower Mekong basin, as in most developing regions of the world, domestic water supply from central water treatment plants is only available for cities and major settlements. Villages and individual households in rural areas are not connected to such facilities. This means that more than 80 per cent of the population have no access to treated water but depend directly on surface water bodies for domestic supply, including drinking water and preparation of food. Thus, public health and hygiene depend to a large extent on the quality of these water bodies and their contamination with pathogenic organisms and chemicals. With increasing population densities and intensification of agricultural land use, increasing amounts of domestic wastewater and agricultural chemicals (fertilizers, pesticides) have reached the surface water bodies. At present, tributaries in northeastern Thailand show signs of eutrophication and in the Mekong delta---where population density is highest---even the main river distributaries show levels of bacterial contamination which render them unsuitable as sources of drinking water. Consequently, there is a high prevalence of diarrhoea type diseases and intestinal parasites in these densely populated parts of the basin.

In addition to these anthropogenic problems resulting from a short-circuit between waste disposal and domestic water supply, two natural problems, namely salt contamination and acid waters, impinge on the quality and potability of water. These problems not only affect surface waters but also the groundwater, which could otherwise be regarded as a comparatively safe alternate source to domestic water supply.

Thus, in large areas of the basin, rain water is the only water source of adequate quality for domestic consumption, but rain water is available only during a part of the year, and safe storage facilities are required to keep a sufficient quantity for the dry season, without risking secondary contamination. Taking 20 litres per person/day as the absolute baseline for the demand of good quality water, a storage tank of about 20 m³ would be required to last a family of 6 persons over the 5--6 months of the dry season. Most families in rural areas are too poor to purchase such a tank, and the smaller tanks which are in use are often open and exposed to secondary pollution. When stored drinking water has been consumed, either water has to be bought, and in some instances transported over several kilometers, or low quality water has to be used, which of course has impacts on public health. In many households of the delta simple filtering techniques are used to make surface water more suitable for domestic purposes. From the above it is obvious that the supply of potable water to rural households in problem areas is at present rather urgent.

4.5 Program of action to solve the problems

The underlying philosophy of the Mekong Committee's environmental program is to cement the environmental dimension into Mekong development projects with a view to ensuring that productivity of primary natural resources (terrestrial, aquatic and human) does not deteriorate as a result of development activities in the basin and that maximum socio-economic benefits can be attained. Therefore, a comprehensive environmental program pervades all the Committee's development activities and includes steps to anticipate, as far as possible, both the undesirable side effects and unaccounted benefits resulting from development activities, and to demonstrate measures to maximize benefits and alleviate the adverse effects through effective management. The most important elements of the program are summarized below:

1. Studies leading to environmental assessment, including identification of problems, and

2. Pilot management, rehabilitation and amelioratory activities to demonstrate measures to enhance benefits and to offset adverse effects of water resource development projects on the environment.

It is now universally acknowledged that investigations of ecological consequences and broadly defined environmental impacts should be central to the planning and design of development projects. Often, quite minor alterations in plans and additional costs in the construction phase can prevent major environmental, economic and social costs.

In international river basin planning, such as that of the Mekong in particular, environmental parameters assume especial importance, since off-site impacts of development actions in one riparian state could manifest themselves in another. For instance, injudicious watershed management in an upper riparian country could have undesirable effects on water use or the viable life of impoundments in a lower riparian state. Unregulated withdrawals of water in upper sections of rivers may adversely influence agriculture and fisheries downstream. Impoundments upstream could alter downstream ecology to such a degree as to seriously affect various facets of river productivity. The Mekong Committee, recognizing in particular the transnational nature of the impacts of river basin development, has given due attention to environmental parameters in development planning.

One of the approaches to incorporate environmental criteria into project planning and assessment at an early, screening-level stage, is the methodology of environmental impact analysis. To develop this methodology and make it available to the staff at the Mekong Secretariat, a study to develop and implement a prototype level expert system for environmental impact assessment was commissioned. The expert systems approach was selected not only to build a tool that is easy to use by project officers with little or no computer experience, but also to provide a common framework and easily accessible repository of environmental knowledge at the Secretariat. The development of the knowledge base of the system requires knowledge contributed by individual experts. The system also provides a mechanism to discuss, review and formalize the environmental policy of the Secretariat.