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
4.1 The Mekong river and its 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
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
The Northern Highlands, covering northern Lao PDR with only the
western rim in Thailand is a strongly folded mountainous area where
the processes of erosion have carved a highly complex and dissected
relief. There are a few relatively large upland plains, such as the Plain of
Jars on the Xieng Khouang Plateau (Lao PDR).
The river valleys in the Lao PDR usually have small quarternary alluvial
terraces. However, Chieng Rai province of north Thailand has extensive plains,
with 2200 km⊃ of wet rice fields.
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
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
into the South China Sea, they are broken by many falls and rapids
in the northern sector and are suitable for the development of
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.
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
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
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
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
Inundation control effects on fisheries;
Toxic biocidal levels in edible organisms;
Waterborne diseases, and
Potable rural water supply in problem (saline and acid) areas.
4.4.1 Watershed degradation
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
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
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
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
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
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
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
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
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
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:
Studies leading to environmental assessment, including
identification of problems, and
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
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
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.