Transboundary Water Quality Issues in the Mekong

Transcription

Mekong River Commission
Transboundary Water Quality Issues
in the Mekong River Basin
Barry T. Hart
Water Studies Centre, Monash University
Melbourne, Australia
Michael J. Jones & Gabrielle Pistone
NSR Environmental Consultants
Melbourne, Australia
November 2001
Mekong River Commission
Transboundary Water Quality Issues in the
Mekong River Basin
November 2001
975_1_v2
Prepared by:
Water Studies Centre
Monash University
In association with:
NSR Environmental Consultants Pty Ltd
124 Camberwell Road
Wellington Rd, Clayton Victoria 3800
Australia
Tel: 61 3 9905 4070 Fax: 61 3 9905 4196
e-mail: [email protected]
Hawthorn East, Victoria 3123
Australia
Tel: 61-3-9882 3555 Fax: 61-3-9882 3533
e-mail: [email protected]
Transboundary water quality issues in Mekong River
i
Executive Summary
Deteriorating water quality in the lower Mekong River basin has been identified as a priority
transboundary issue by each of the four member countries. Three particular transboundary
water quality issues are considered in this report:
•
•
•
the potential effects of municipal and industrial wastewater from Phnom Penh on both
downstream Vietnam and fish migration in Tonle Sap River;
the potential effects of municipal and industrial wastewater from Vientiane on both
neighbouring Thailand and fish migration in the Mekong River;
the influence of upstream water on the degraded water quality in the Mekong Delta.
A risk assessment framework has been used to assess these issues. In particular, the risk of
adverse effects on three key values of the Mekong River – ecosystem health (characterised by
eutrophication, toxicity due to dissolved oxygen and toxicants, and ecosystem processes), fish
migration and human health (drinking, recreation) - have been assessed. The risk to irrigation
water quality was also assessed for the third transboundary issue above.
Risk assessment is concerned with estimating the likelihood or probability of an undesired
event occurring and the consequences if that event does occur. The risk assessment process
seeks to:
•
identify the key (ecological) issues and key stressors;
•
identify the linkages between the key stressors (drivers) and each ecological consequence
(conceptual model or quantitative ecological model), and from this provide information on
which drivers are most sensitive to management or controls;
•
assess the risks associated with each issue as quantitatively as possible (it is important here
to identify measurable end points for each issue);
•
identify (and where possible quantify) all major uncertainties so the decision maker can
decide on the confidence that should be placed on the final assessment;
•
assist in establishing a robust monitoring & assessment program;
•
identify the key knowledge gaps.
Recommendation 1:
that MRC adopt the risk-based approach to assess transboundary
and basin-wide environmental and human health issues, and to
prioritise the management actions required to reduce the risk due to
each important issue.
Unfortunately, the data currently available is inadequate to fully assess the risk of
transboundary water quality issues in the Mekong River basin. Assessment of the current
database identified the following deficiencies:
•
Physico-chemical data – the Mekong water quality network is similar to many other such
networks around the world in that it is collecting inadequate data. For example, many of
the indicators currently being measured are inappropriate and should be replaced with
more appropriate indicators. Additionally, samples are being collecting at inappropriate
site locations and, for a number of indicators, at inadequate frequency. In all cases, the
ii
sampling design was such that there was essentially no statistical power in the data to
detect any significant transboundary changes.
•
Toxicant data – the pesticide and heavy metal data were either non-existent or insufficient
to be used to assess transboundary or basin-wide toxicity issues.
•
Biological data – there is no on-going biological monitoring program for the Mekong
River. In the time available we were able to access only a small amount of biological data
relevant to the Mekong, which would include fish, macroinvertebrates, algae and
ecosystem processes. Efforts should be made to collect all published and unpublished
information on the biology and ecology of the Mekong River and its tributaries, and to
prepare a synthesis of this information that summarises current knowledge in this area.
•
Urban contaminant loads – the loads of contaminants discharged from the urban centres
of Vientiane and Phnom Penh are poorly known. We estimated likely loads in order to
make a preliminary assessment of the transboundary risks due to these discharges. To
improve on the very preliminary risk assessment reported here, a more detailed
understanding of the wastewater systems in each city needs to be developed, and both the
quantity and quality of the wastewater discharges needs to be determined.
Recommendation 2: that the current review of the physico-chemical monitoring network
consider in particular the optimum location of sampling sites, the
frequency of sampling, the need for depth sampling in some cases, the
indicators being analysed and the power of the data collected to detect
changes.
Recommendation 3: that MRC undertake a preliminary risk assessment to identify possible
transboundary or basin-wide toxicity or bioaccumulation problems due
to organic contaminants and/or heavy metals, and if problems are
identified, the type of investigations (including monitoring) that should
be undertaken to better characterise the risk.
Recommendation 4: that the MRC establish a project to assess the feasibility of establishing
a biological monitoring program for the Mekong River basin. The
following biota should be considered – fish, macroinvertebrates, algae
and macrophytes.
Recommendation 5: that MRC collect all published and unpublished information on the
biology and ecology of the Mekong River and its tributaries, and
prepare a synthesis of this information that summarises current
knowledge in this area.
Recommendation 6: that MRC obtain a more detailed understanding of the wastewater
systems (including information on the quantity and quality of the
wastewater discharges) in the two major urban centres – Vientiane and
Phnom Penh.
A summary of the preliminary risk assessment for the three transboundary water quality issues
is given in the table below. The present risks are low in all cases where they could be
assessed. It was possible to undertake a reasonably quantitative assessment to assess the risks
from eutrophication and the adverse effects of low dissolved oxygen concentrations caused by
wastewater discharges from Phnom Penh. However, for the other effects we were forced to
iii
assess risk on the basis of either a comparison of the loads of contaminants discharged from
Phnom Penh and Vientiane with those transported “naturally” by the Mekong River, or the
degree of dilution achieved on discharge of the wastewater.
Issue
Effect
Issue 1
(Phnom Penh)
Issue 2
(Vientiane)
Issue 3
(Delta)
Low-moderate risk1
Low risk
Low risk
Low risk
Low risk
Low risk
Not assessed
Not assessed
Not assessed
Uncertain
Uncertain,
likely to be low
risk
Uncertain
Uncertain
Uncertain
Uncertain
Uncertain
Uncertain
Uncertain
Ecological
Eutrophication
Algal blooms
2
Toxic effects
Fish/invertebrate kills
Ecosystem function
4
Fish migration
3
To be determined
Adverse effects on fish
movement upstream,
downstream or onto
floodplains
Human health5
Drinking water
Recreation
Microbial contamination
causing sickness
Agriculture
Irrigation
1.
2.
3.
4.
5.
Increased salinity
Low risk
More likely low risk since only nutrient concentration were used in the assessment; high turbidity and high flow
would also reduce the chance of algal problems.
Risks based on toxic effects due to low dissolved oxygen concentrations. It was not possible to assess toxicity due to
toxicants (heavy metals, pesticides) because of the lack of data.
No information is available at present, but should be developed in the future.
Lack of data to make assessment, present water quality sampling network cannot provide the required information.
Lack of data to make assessment. Risk likely to be low-moderate due to large dilution (also expect significant
microbial die-off during transport to Vietnam in case of Issues 1 & 3).
Recommendation 7: that MRC establish a project to undertake a more detailed assessment
of the transboundary ecological and human health risks due to the
discharge of wastewater from both Phnom Penh and Vientiane. Such a
project would provide an ideal opportunity to “train” relevant National
Mekong Committee members in the risk assessment methodology.
Recommendation 8: that MRC establish a project to investigate the key ecological processes
occurring in the Mekong River basin, including those associated with
deep pools in the Mekong River mainstream. The objective of this
project should be to develop a number of sensitive ecosystem process
indicators that can be used to assess the ecological “health” of the
Mekong River.
Another potential transboundary issue not covered in the objectives of this report, but which
appears to require assessment, is the apparent higher salinity (conductivity) in the river Nam
Mun that drains the extensive agricultural region of northern Thailand.
While the assessment reported herein indicates that the transboundary risks due to water
quality are low, this is not the case for local effects. For Phnom Penh in particular, our
preliminary assessment suggests that there are moderate to high risks of adverse ecological
iv
and human health effects in Chaktomuk, Tonle Sap River and the upper reaches of the Bassac
River. The relevant Cambodian authorities may wish to further investigate this situation.
Analysis of the present water quality monitoring network showed that it is unable to detect
any transboundary changes due to the discharge of wastewater from either Vientiane or
Phnom Penh. The network design has insufficient statistical power to detect realistic changes
in physico-chemical water quality. Additionally, since no biological indicators are measured,
there is no possibility of detecting transboundary or basin-wide changes in ecosystem health.
Recommendation 9:
that MRC develop a new and more robust environmental assessment
program designed to identify and assess the risks from a broader
range of current and future transboundary and basin-wide issues.
The process to achieve this new assessment program should be done
in collaboration with the member countries and would involve:
• using the (ecological) risk assessment technique to underpin the
process, with the first task being to scope the full range of existing
and possible future transboundary issues (and their priority);
• running a number of workshops (involving each country) to
develop the conceptual models and decide upon the target areas,
the assessment endpoints and the best indicators to measure;
• undertaking a program of short-term, targeted investigations to
provide essential information on specific aspects of the system that
will enable the main program to be better designed;
• preparing a full program proposal, obtaining funding and
implementing the program.
1
EXECUTIVE SUMMARY .................................................................................................i
1.
Introduction................................................................................................................. 4
2.
Project objectives ........................................................................................................ 6
3.
Risk assessment approach........................................................................................... 7
4.
5.
6.
7.
3.1
RISK ASSESSMENT FRAMEWORK ................................................................................................... 7
3.2
UNCERTAINTY............................................................................................................................... 10
Mekong River system and potential transboundary water quality issues............... 12
4.1
MEKONG RIVER ECOSYSTEM ....................................................................................................... 12
4.2
ENVIRONMENTAL VALUES ........................................................................................................... 16
4.3
KEY TRANSBOUNDARY ISSUES AND MAJOR STRESSORS ............................................................ 16
Analysis of present environmental monitoring data ................................................ 20
5.1
PHYSICO-CHEMICAL WATER QUALITY MONITORING .................................................................. 20
5.2
TOXICANT (PESTICIDES, HEAVY METALS) MONITORING ............................................................ 28
5.3
BIOLOGICAL MONITORING ............................................................................................................ 29
5.4
URBAN WASTEWATER LOADS ...................................................................................................... 30
5.5
USE OF EXISTING DATA TO ASSESS TRANSBOUNDARY ISSUES................................................... 31
Assessment of transboundary water quality issues .................................................. 33
6.1
PHNOM PENH ................................................................................................................................ 33
6.2
VIENTIANE..................................................................................................................................... 45
6.3
WATER QUALITY IN THE MEKONG DELTA .................................................................................. 48
Monitoring network required to assess transboundary issues................................. 51
7.1
ASSESSMENT TOOLS ..................................................................................................................... 51
7.2
MONITORING NETWORK DESIGN.................................................................................................. 51
7.3
TOWARDS A NEW ENVIRONMENTAL ASSESSMENT PROGRAM ................................................... 53
7.4
RECOMMENDATIONS .................................................................................................................... 56
8.
Conclusions & Recommendations ............................................................................ 58
9.
References.................................................................................................................. 62
2
Tables
Table 1
Estimated contaminant loads (tonne/year) discharged from Vientiane and Phnom
Penh (see Appendix A for details). In brackets is the contribution of each city as a
percentage of the total load transported by the Mekong River at that point.......... 31
Table 2a Summary of the transboundary risks from Phnom Penh wastewater discharges on
the Vietnam Delta region .................................................................................... 36
Table 2b Summary of the transboundary risks from Vientiane wastewater discharges on
Thailand near Vientiane and downstream............................................................. 37
Table 2c Summary of the transboundary risks from upstream degraded water on the Vietnam
Delta region ........................................................................................................ 38
Table 3 Trigger values for the concentrations of filterable reactive phosphorus (FRP) that
could cause algal problems in the Mekong River, and the likelihood that these will
be exceeded in the vicinity of Phnom Penh .......................................................... 40
Table 4 Trigger values for toxic effects due to low DO concentrations in the Mekong River,
and the likelihood that these will be exceeded in the vicinity of Phnom Penh........ 41
Table 5 Logic of environmental decisions. Monitoring programs are generally established to
conclude that projects or activities have or have not had an impact...................... 51
Table 6 Analysis of the power of the present water quality monitoring network to detect
changes of greater that 25% in conductivity, SPM and Total-P concentrations due
to wastewater inputs from Phnom Penh............................................................... 52
Figures
Figure 1 Location map of the lower Mekong River catchment............................................. 5
Figure 2 Schematic of the ERA process. ............................................................................ 8
Figure 3 Risk assessment curve showing the cumulative frequency of dissolved oxygen
concentrations in the Mekong River at Phnom Penh and the Tonle Sap River at
Phnom Penh Port (all data and dry season data shown). Also shown on the graph
are the low (1.5 mg/L) and high (6.0 mg/L) trigger values for adverse effects..... 10
Figure 4 Mean monthly flows in the Mekong River at Vientiane and Phnom Penh. ............ 13
Figure 5 Conceptual models showing the potential transboundary water quality issues caused
by wastewater discharges to the Mekong River from (a) Phnom Penh and (b)
Vientiane, and (c) in the Delta region. ................................................................. 18
Figure 6 Box plots of the key water quality indicators over the length of the lower Mekong
River. Data for Lao PDR, Thailand and Vietnam collected in period 1985-2000,
for Cambodia between 1993-2000....................................................................... 21
Figure 7 Conductivity vs time for the Mekong River at Tan Chau. .................................... 25
Figure 8 Plot of the monthly and annual SPM loads transported by the Mekong River at
Vientiane, Pakse, Kratie and Phnom Penh. .......................................................... 26
Figure 9 Plots of nutrient concentrations in the vicinity of Phnom Penh (a) mean (s.d.) FRP
& Total-P concentrations (µg/L), (b) mean (s.d.) NOx-N & NH4-N concentrations
(µg/L), (c) distribution of dry season FRP concentrations at Phnom Penh, Ta
Khmao and Chau Doc (mg/L). ............................................................................ 39
Figure 10 Plot of the distribution of dry season DO concentrations at Phnom Penh, Ta Khmao
and Chau Doc. .................................................................................................... 42
3
Figure 11 Sample design to test effect of wastewater discharged from Phnom Penh on the
Bassac River and downstream at the Vietnam Border. ................................................ 55
Appendices
Appendix A
Stormwater and wastewater pollutant load estimates for Phnom Penh and
Vientiane
1.
4
Introduction
The lower Mekong basin (Figure 1) has a population of 60 million people (MRC, 1997) who
exist mainly by subsistence agriculture based on rice and wild-caught fish. Since the late
1950s the four countries that make up the lower basin (Cambodia, Lao PDR, Thailand,
Vietnam) recognised the need to cooperate to develop the basin’s resources sustainably if
they were to avoid future conflict. Prior to 1995, the Mekong River Commission (MRC)
managed a number of projects focused largely on water resources development within the
basin. Following the 1995 Agreement of Cooperation for Sustainable Development of the
Mekong Basin, the focus shifted to address basin-wide and transboundary issues through a
program approach.
The MRC has three core programs – the Water Utilization Program (WUP), the Basin
Development Program (BDP) and Environment Program (EP) (MRC, 1999). The WUP,
supported by the World Bank through the Global Environment Facility, has established three
working groups within which member countries can develop solutions to basin-wide
problems. The working groups are addressing modeling, rules and protocols and
transboundary issues. This last working group identified water quantity and quality as two of
their three highest priority transboundary issues. Fish production, river bank erosion and
sedimentation were also identified as priority issues (Sukhsri, MRC, pers. comm.).
Water quality monitoring is undertaken throughout the basin under the Environment Program
of the Mekong River Commission (MRC, 1998). The program was initiated in 1985 in Lao
PDR, Thailand and Vietnam, and in 1993 in Cambodia. This program was not established
specifically to deal with transboundary issues and is now undergoing a major review in order
to revise and refocus the entire program. Additionally, there is a dearth of information on
what the transboundary water quality issues are (or could be), especially as the main stem of
the Mekong remains in generally good condition.
Water quality issues identified by the member countries as being of transboundary concern,
and which form the basis of this report, include:
• potential effects of municipal and industrial wastes from Phnom Penh (the capital city of
Cambodia);
• potential effects of municipal and industrial wastes from Vientiane (the capital city of Lao
PDR);
• whether the degraded water quality in the Mekong Delta is caused by transboundary
transfers of poor quality water.
In the case of Phnom Penh and Vientiane, the presumed threat is not supported by any
analysis of existing data that has drawn a definitive conclusion. For the Delta region, a
number of studies have shown that significant water quality degradation now occurs,
particularly during the dry and early wet seasons (Tin & Wilander, 1995; Minh et al., 1997;
Joy et al., 1999), but this may be more a product of local land use and seasonally low
transboundary river flows than due to degraded water from upstream.
The Mekong River Commission commissioned the Water Studies Centre, Monash
University, in collaboration with NSR Environmental Consultants Pty Ltd, to undertake the
Transboundary Water Quality Study Start-up Project. The aim of this project was to carry
out a desk audit of existing information, together with discussions with local officials on
future developments, and to advise Mekong River Commission on the three issues above.
Further, if the desk audit indicated reason to be concerned, the Mekong River Commission
required that the specific studies needed in order to determine the magnitude of the risk be
clearly identified.
Figure 1
5
Location map of the lower Mekong River catchment.
2.
6
Project objectives
The objectives of this study are to:
1. Review existing data and information to determine if real or potential downstream and
transboundary impacts exist from effluent discharges from the urban areas of Phnom
Penh and Vientiane, and to determine if degraded water quality in the Mekong Delta is
caused by transboundary transfers of poor quality water.
2. Recommend specific studies or actions that should be carried out to make a definitive
assessment, if the present assessment remains inconclusive due to lack of sufficient data
or information.
3. Advise the Mekong River Commission on steps that could be taken to monitor real or
potential transboundary issues arising from this analysis.
3.
7
Risk assessment approach
3.1
Risk assessment framework
A risk assessment framework has been used to assess the perceived water quality issues. In
particular, the risk of adverse effects on three key uses or values of the Mekong River have
been assessed, these being:
• ecosystem protection (specifically eutrophication and toxic effects on biota due to low
dissolved oxygen or chemical contaminants);
• fish migration;
• human use (drinking, recreation).
Risk assessment is a general term used to describe an array of methodologies and techniques
concerned with estimating the likelihood and consequences of undesired events. Risk is
often defined as the product of the probability or likelihood of a hazard and the consequence
if that hazard occurs. Risks are characterised in terms of probability (the likelihood of some
event occurring), consequence (the severity if that event occurs) and the sensitivity to
management interventions.
More specifically, ecological risk assessment (ERA) is a relatively new technique that is now
available for assessing the level of risk to the health of river ecosystems posed by multiple
stressors1. Ecological risk is defined as:
Ecological risk =likelihood of ecological effect x consequence of that effect.
To date, most applications of the ERA process in North America (Renner, 1996; USEPA,
1998), Europe (Calow, 1995) and Australia (Hart et al., 2001) have focused on toxic
chemicals. In these cases risk assessment includes a consideration of both the severity (or
hazard) and frequency (or exposure) of the issue. For example, an extremely toxic chemical
(e.g. mercury) may be a high or low risk, depending on its potential exposure to the
ecosystem. A less hazardous material, such as orthophosphate, may represent a low risk if
released in relatively small quantities, but can pose a high risk if released in quantities that
allow toxic cyanobacterial blooms to occur.
There are now a number of initiatives aimed at further developing the ERA technique to
provide a framework for considering a wider number of interacting stressors (e.g. nutrients,
environmental flows, habitat, sediments, exotic species) within a catchment or river basin
context. Thus, the ERA process is an attractive tool that can assist in assessing the impacts
of multiple stressors on complex ecosystems, the situation that exists within the Mekong
River system.
In this study, the focus of the ERA process is to identify the risks of transboundary water
quality issues occurring. Member countries may also find the process useful in assessing the
risk of environmental problems occurring within each country.
Ecological risk assessments generally involve three steps discussed below (see also
Figure 2).
Problem formulation
This is a planning and scoping process that establishes the goals, breadth and focus of the
risk assessment. The end products of the problem formulation phase are:
1
Stressors or drivers are physical, chemical or biological factors influencing the system, for example,
toxicants, nutrients, salinity, temperature, acidity, flow, habitat and exotic fish species.
•
•
•
•
•
8
an outline of the assessment process that provides confidence that this process is
transparent and credible;
identification of the important ecological issues and key stressors;
identification of the appropriate spatial and temporal scales for evaluating the risks;
a conceptual model2 for each of the undesired issues, where the key stressors are linked
to the ecological effect;
identification of the assessment endpoints3.
Figure 2 Schematic of the ERA process.
Analysis and assessment
During this phase, information relevant to each key issue is gathered on the two risk
components - environmental exposure and severity of the effects.
The purpose of the likelihood characterisation is to predict or measure the spatial and
temporal distribution of the stressor(s) and the co-occurrence or contact with the ecological
components of concern. The purpose of the ecological effects characterisation is to identify
and quantify the effects caused by the stressor(s) and, to the extent possible, to evaluate
cause-and-effect relationships.
Ideally, these assessments should be as quantitative as possible (Hart et al., 2001). However,
with many systems, the Mekong included, it is often possible only to make semi-quantitative
(e.g. low, medium, high) ratings of these two components of risk.
Risk characterisation
Here the likelihood and effects profiles are integrated to provide an estimate of the level of
risk. In many circumstances, this step is reduced to assessing the risks associated with a list
of hazards in which risks are ranked relative to one another.
2
3
These conceptual models form the basis for more quantitative ecological models in systems where
there is both sufficient knowledge about the linkages and sufficient data to quantify these linkages.
These are explicit statements of the environmental values to be protected.
9
The final assessment of the level of risk should include some estimate of the uncertainty in
the predictions. For example, the final risk assessment might be that given a particular set of
conditions, there is a 60% likelihood of a blue-green algal bloom occurring in a waterbody in
the next three weeks. However, the manager would treat this result differently if he knew
that the upper and lower bounds to this prediction were, say, 50% and 70% respectively,
compared with another situation where the prediction was less certain, say 20-80%, with the
most likely being 60%.
The final risk assessment should include a summary of the assumptions used, the scientific
(and other) uncertainties, and the strengths and weaknesses of the analyses.
Summary
In summary, the risk assessment process seeks to:
•
identify the key ecological and human health issues and key stressors;
•
(conceptual model or quantitative ecological model), and from this provide information
on which drivers are most sensitive to management or controls;
•
assess the risks associated with each issue (i.e. the likelihood that the issue will occur and
the consequences if it does occur) as quantitatively as possible;
•
decide on the confidence that should be placed on the information;
•
assist in establishing performance monitoring & assessment programs;
• identify the key knowledge gaps.
The ERA process therefore is an ideal tool for assessing the risk of transboundary impacts on
the Mekong River from effluent discharges from the urban areas of Phnom Penh and
Vientiane, and within the Delta region.
Unfortunately, there is a dearth of information on the Mekong River that makes a detailed
and quantitative risk assessment impossible at this stage. Thus, it has been necessary to
revert largely to qualitative assessments, although these have been backed with quantitative
data where this was possible. Recommendations are made in Section 7 on the types of
investigations and data collection that are needed to make more quantitative transboundary
risk assessments possible.
For the preliminary assessment considered in this report, the risk associated with each issue
has been assessed by combining information on the severity of the effect and the likelihood
that the effect will occur. Each of these two components (severity and likelihood) will be
separated into a three scale rating scheme - low, moderate or high.
Where data was available, we have used the full data distribution to assess the probability
(likelihood) that the system will be in excess of certain effect levels (or trigger values)4. This
is shown in Figure 3 where the high and low severity trigger values for adverse effects to
aquatic biota from low dissolved oxygen concentrations are marked on the cumulative
frequency distributions of the concentration data for a particular site. The points at which
the cumulative curve crosses the trigger values can be used to assess the likelihood
(probability) that low, moderate and high DO toxicity will occur at this site. For example,
for the Mekong River at Phnom Penh, there is a ca. 7% probability of moderate effects due
to low DO (see Figure 3).
4
In a number of cases we used data for particular periods (e.g. dry season) when the problems are
more likely to occur.
10
For the issues assessed here, the trigger values were mostly obtained from relevant water
quality guidelines (e.g. USEPA, 1986a,b,c; 1989; 2000; CCME, 1999;
ANZECC/ARMCANZ, 2000). Unfortunately, almost all of the available water quality
guidelines have been established for temperate regions, and may be less appropriate for
tropical regions.
Figure 3
Risk assessment curve showing the cumulative frequency of dissolved
oxygen concentrations in the Mekong River at Phnom Penh and the
Tonle Sap River at Phnom Penh Port (all data and dry season data
shown). Also shown on the graph are the low (1.5 mg/L) and high
(6.0 mg/L) trigger values for adverse effects.
3.2
Uncertainty
One of the reasons why qualitative risk assessments fail is because they do not make the
treatment of uncertainty explicit. Risk assessments must deal with four types of uncertainty
(Regan et al., 2001):
•
Parameter uncertainty (e.g. measurement error or natural variation. This is the type of
uncertainty most commonly considered. Science deals with this kind of uncertainty by
using confidence intervals in statements such as “the size of the change is 54 with 95%
confidence limits of 23”).
•
Structural uncertainty (this is where an incorrect model for the system being studied is
used).
•
Shape uncertainty (this is uncertainty about the distribution of the data being considered).
• Dependency (relates to possible correlations between parameters).
For the Mekong, the main uncertainties are the lack of appropriate water quality and
biological data relevant to assessing transboundary issues, and a lack of understanding of
how the system functions (e.g. fish spawning and migration). These are addressed in Section
4.
11
Risk assessments also need to ensure that semantic uncertainties, including ambiguous
statements and vague definitions (e.g. concepts that permit borderline cases to occur), are
kept to a minimum. An example of linguistic ambiguity is “there is a 70% chance of rain” –
does this mean rain during 70% of the day, or over 70% of the area, or a 70% chance that it
will rain at a particular point (the weather station)? Equally, vague statements or definitions
are common in ecological risk assessments. For example, it is common to read statements
that a certain hazard will pose a “low risk”. However, some hazards may be considered
definitely low, whereas others may be low but tending towards moderate. Such vague
statements can be strengthened if the definitions of low, moderate and high are quantified.
4.
12
Mekong River system and potential transboundary water
quality issues
4.1
Mekong River ecosystem
General
The Mekong River rises in China (ca. 5,000 m altitude) and flows some 4,880 km through
five other countries (Myanmar, Lao PDR, Thailand, Cambodia, Vietnam) before discharging
into the South China Sea (see Figure 1). The Mekong Basin is 795,000 km2 in area, and has
an annual flow of 475,000 million m3 (mean flow – 15,060 m3/s) which makes it the tenth
largest river in the world (MRC, 1997).
A good description of physiography of the Mekong basin is given in MRC (1997). North of
Vientiane, the Mekong flows through a rather narrow valley in hilly and mountainous
country. In the region of Vientiane, two large tributaries (Nam Ngum and Nam Lik) form a
broad alluvial plain along the north bank of the Mekong, which then flows through a wide
valley for around 560 km before entering the Khemarat rapids just south of the confluence
with the Se Bang Hieng. The river continues through a rocky gorge for a further 160 km
before emerging onto the plains above Pakse. The river Nam Mun (that drains the extensive
agricultural area of northern Thailand) also enters the Mekong approximately 40 km above
Pakse at Khong Chiam (see Figure 1). From Pakse, the Mekong flows across lowlands to
the Khone Falls on the Cambodian border, located approximated 100 m above sea level.
Below Khone Falls, the Mekong winds its way across the Cambodian lowlands to Phnom
Penh, where it splits into three – the main Mekong channel, Bassac River and Tonle Sap
River. The behaviour of Tonle Sap River is described below. The Mekong and Bassac (ca.
5-20% of the total flow) continue for 330 km through the highly agricultural Vietnam delta
region before entering the South China Sea
The hydrological regime of the Mekong River is controlled by alternating wet and dry
monsoon seasons. The annual flows in the Mekong are relatively predictable, with the wet
season extending from June-July to November, and the dry season from December to May
(MRC, 1997). This pattern is well illustrated by the long-term flow regimes measured at
Vientiane and Phnom Penh (Figure 4). In general, the wet season accounts for around 8590% of the total annual volume, with September being the peak flow month (contributing ca.
20-30% of the annual flow). The Mekong’s flow increases with distance downstream due to
tributary inputs.
For example, the mean annual flows increase from around
3
4,500 m /s at Vientiane to 10,100 m3/s at Pakse and 14,600 m3/s at Phnom Penh (MRC,
1997).
Extensive areas of the lower Mekong Basin are flooded each year. In most years, over
50,000 km2 is flooded, extending from around Kratie down to Phnom Penh, upstream to
Tonle Sap Great Lake and downstream to the delta region (see Figure 1). These floods are
very important in maintaining the high agricultural productivity of these lands by depositing
alluvial sediments. Also, as discussed below, the annual floods are a key factor in the very
high fish productivity in the Mekong.
The population of the lower Mekong Basin is very poor and growing rapidly. The
population in the lower Mekong has doubled in the past 30 years, and is predicted to increase
to around 108 million by 2020 (R. Corsel, MRC, pers. comm.). The current population
growth rate is 2.0% for the entire lower Mekong Basin, with higher rates (2.5%) in Lao PDR
and Cambodia, and somewhat lower rates in Vietnam and Thailand (Kristensen, 2000).
Figure 4
13
Mean monthly flows in the Mekong River at Vientiane and Phnom Penh.
The Mekong and its tributaries are vital to the 73 million people living in the lower part of
the catchment, who depend upon the river for water, food and transport. Most of the
population are engaged in agriculture and produce rice. It is estimated that food demand
from the Mekong River basin will increase by 25-50% in the next 25 years, with a
corresponding increase in water demand (Kristensen, 2000).
Rice and fish make up the main diet of the Mekong population, with fish being the single
most important source of protein. People of the Mekong consume between 28 and 67 kg
fish per capita annually (Jensen, 2000). The Mekong River is very rich in fish. The size of
the inland fisheries is large with the total annual catch estimated at around 2 million tonne,
distributed over more than 1,500 different fish species. This is supplemented by a rapidly
growing aquaculture industry that currently produces around 230,000 tonne annually (MRC,
2001).
MRC (1997) summarised the large number of land use changes and other activities that have
occurred within the lower Mekong River basin in recent years, and the environmental
concerns associated with these activities. Some of the most serious are:
•
excessive logging in all countries, leading to deforestation, degradation of terrestrial
ecosystems and soil erosion, with consequent elevated sediment loads and sedimentation;
•
clearing of important floodplain forests for agricultural land5, leading to a significant loss
of important fish breeding areas;
•
conversion of wetlands to rice farms or aquaculture, leading to loss of aquatic habitat;
•
agricultural expansion and shifting cultivation;
•
major irrigation developments in the Korat Plateau of northern Thailand, leading to
increased salinisation, sediment transport and water quality degradation;
•
water quality and land degradation in the Delta region caused by agricultural activities on
acid sulfate soils.
5
A large proportion of the 5,000 km2 of flooded forests around Tonle Sap Great Lake have been
cleared.
14
Fisheries and fish migration
As noted above, caught fish are a very important food source for the people of the lower
Mekong Basin. Studies in the lower Mekong have classified two groups of freshwater fish,
based on their spawning behaviour and environmental tolerance (MRC, 1997). “White fish”
(principal families Cyprinidae and Schilbeidae) migrate into the main channels during the dry
and early wet season. They spawn in relatively sheltered waters after the peak of the
innundation. “Black Fish” (principal families Clariidae, Siluridae and Ophiocephalidae) are
more widely distributed than the “white fish”, more environmentally tolerant, are mainly
bottom dwellers and have a broad range of spawning behaviours.
The biology and ecology of many of the fish species in the Mekong is poorly known (Poulsen
& Valbo-Jorgensen, 2000), although the MRC Fisheries Program is seeking to address this
situation through its Assessment of Mekong Fisheries Component (AMFC).
From those studies that have been undertaken, it seems apparent that four aspects are
important to the maintenance of viable fisheries in the Mekong:
•
the Khone Falls region in southern Lao PDR;
•
dry season refuges in the main channel;
•
the extensive floodplain region further downstream of Khone Falls, and particularly Tonle
Sap Great Lake and its floodplain;
•
maintaining adequate water quality, environmental flows, habitat, fish migration patterns,
fishing pressure and introduced species.
This report considers the risk of pollution from Phnom Penh and Vientiane adversely
affecting fish migration patterns. However, it is clear that maintaining a viable fisheries in the
Mekong will involve more than just the management of urban pollution. Basin-wide
management of water quality, habitat condition and flow regimes will also be needed.
Khone Falls
The Khone Falls region in southern Lao PDR has emerged as an important area for fish in the
Mekong, and not surprisingly has been the focus of a number of fish studies (Roberts &
Warren, 1994; Roberts & Baird, 1995; Singanourvong et al., 1996a,b). Khone Falls does not
appear to be a physical barrier for most of the described fish species (e.g. most species live
both above and below the Falls), although the migratory patterns differ significantly below
and above the Falls (Poulsen & Valbo-Jorgensen, 2000). Many species migrate upstream
from southern regions of Vietnam and Cambodia to Khone Falls during the dry season, and
then migrate downstream with the onset of the wet season floods. Interestingly, many of
these same species above the Falls have the opposite migratory behaviour, migrating
upstream with the onset of the flood season. For most species, whether above or below the
Falls, the timing of the migrations seems to coincide with the main spawning periods
(Poulsen & Valbo-Jorgensen, 2000).
Dry season refuges
A small amount of evidence now exists pointing to the importance of deep pools in the
Mekong mainstream as dry season habitat for many fish species (Poulsen & Valbo-Jorgensen,
2000). Deep pools exist both upstream and downstream of Khone Falls. In particular, the
region from the Falls to Kratie in Cambodia (see Figure 1) contains a number of deep pools.
We have no information on the existing water quality in the pools during either the dry or
wet seasons. However, these deep pools would be vulnerable to upstream pollution and to
reductions in river flow.
15
In view of the potential importance of the deep pools as dry season habitat, we recommend
that the MRC commission a study to determine their extent, importance and risk from future
land use changes and water resources management options.
Floodplains
The extensive areas of floodplain that dominate the lower Mekong Basin during the wet
season are the basis of the high fish productivity. Poulsen & Valbo-Jorgensen (2000)
reported that all 50 priority fish species covered in their survey of fish migration patterns
spent some stage of their lifecycle on flooded areas, this being particularly so for larval and
juvenile stages. It seems that many species time their longitudinal migrations so that their
offspring can access the highly productive flooded areas, where they can capitalise on the
food resources associated with these floodplains.
The floodplains extend from upstream of Khone Falls to the Mekong Delta region in
Vietnam, some 900 km downstream (see Figure 1). However, there are major differences
between these floodplain systems. Upstream of Khone Falls in the middle Mekong, the
floodplains are mainly associated with tributaries, while downstream of the Falls they are
directly connected to the main channel. The migratory behaviour of the fish species reflect
these differences in floodplain location.
Tonle Sap Great Lake, thought to be the area of highest fish production in the Mekong, is a
very important part of the lower floodplain system. This large lake, situated ca. 125 km
north west of Phnom Penh (see Figure 1), varies dramatically in area from around 2,500 km2
in the dry season to 14,000 km2 in the wet season (MRC, 1997). During the wet season, the
lake is filled by water from the Mekong main channel flowing up Tonle Sap River to the lake
(Rainboth, 1996). Large numbers of young fish are brought into the lake with these
floodwaters. During the dry season (December to February), the flow reverses and water
flows out of the lake back into the Mekong main channel. With these receding floodwaters
the fish migrate out of the lake back to their spawning grounds. During this annual migration
period, huge numbers of fish are caught in the set-bag-net fisheries (dai fisheries) in Tonle
Sap River.
Fish migration
Fish migrations are an important feature of river ecology in most major tropical rivers, and
are adaptations to life in running waters. Three types of migrations are observed:
“longitudinal migration” that occurs within the main channel and larger tributaries, “lateral
migration” where fish move from the main channel to the floodplain and back again, and
“larval drift” where, during the flood season, fish larvae can drift downstream from upstream
spawning areas to downstream nursery areas in the flood plain.
In their extensive study of fish migration patterns in the Mekong River, Poulsen & ValboJorgensen (2000) separated the migrations into three time periods: the late wet to early dry
season (October to February), the late dry to wet season (May to September), and the dry
season (March to May). It is during these times of migrations that a substantial number of
the fish are caught. For example, the dai fisheries in Cambodia (Lieng et al., 1995) and the
Khone Falls fishery in Lao PDR both exploit times of major fish migration (Singanouvong et
al., 1996a,b).
The fish of the Mekong are a transboundary resource. As such, any land use change, water
resource development or urban pollution that interferes with their migration patterns is a
transboundary management issue. In this report, we cover one aspect of the wider issue, the
possible risks to fish migration due urban wastewater discharges from Phnom Penh and
Vientiane.
16
4.2
Environmental values
Most water quality management strategies or plans require that the environmental values6 of
the water resource should be explicitly determined in order that management of the resource
can be properly focused to protect these values (Hart et al., 1999; ANZECC/ARMCANZ,
2000). For example, the Australian National Water Quality Management Strategy
(ANZECC/ARMCANZ, 1994) recommends that the following environmental values be
considered: ecosystem protection, drinking water, water for agriculture (irrigation, livestock
drinking water, aquaculture), and water for recreation and aesthetics.
Somewhat surprisingly, we have not been able to find any MRC documents that provide an
explicit statement of a shared vision the four countries have for the Mekong River basin, nor
indeed any statement of the environmental values they wish to protect in this system.
However, it seems clear that the four countries with the MRC are managing this river system
to protect the following values:
•
the riverine and floodplain ecosystems;
•
native fisheries production;
•
water for drinking;
•
irrigation (mainly rice);
•
aquaculture;
• recreation and aesthetics.
This report is restricted to an assessment of the impact of transboundary water quality
changes, focussing on three environmental values:
•
•
fish migration;
• human health (drinking water & recreational use).
It should be noted that our focus on fish migration is only one component of what should be
a much larger study to assess the risk to native fish production in the Mekong River system.
This larger study would require the inclusion of information on at least three key stressors –
water quality changes, habitat changes and possible reduction in flows and flooding.
Changes in the total flows and in key components to the flow regime would be particularly
important, probably more important than water quality changes.
The MRC is currently considering a major study of the environmental flow requirements to
ensure that the ecological “health” of the Mekong River system is maintained into the future.
It should be noted that determination of environmental flow regimes will require considerably
more knowledge about how this system functions ecologically, including what factors control
the vitally important Mekong fisheries.
4.3
Key transboundary issues and major stressors
The three transboundary water quality issues considered in this assessment are:
•
potential effects of municipal and industrial wastes from Phnom Penh, both downstream
and on fish migration near the outfalls;
•
potential effects of municipal and industrial wastes from Vientiane, both downstream and
on fish migration near the discharge point;
6
Environmental values are also referred to as “beneficial uses” or “functions of water”.
17
•
whether the degraded water quality in the Mekong Delta is caused by transboundary
transfers of poor quality water.
Below we outline a conceptual model for each situation in which the spatial scale is
addressed and the key stressors are identified. In each case we have tried to keep separate
the transboundary issues (i.e. adverse effect on downstream or adjacent country) from those
arising entirely within one country.
Phnom Penh
A conceptual model for the potential water quality issues resulting from municipal and
industrial wastes from Phnom Penh is shown in Figure 5a. The details are discussed in
Section 6.1 and include:
•
wastewater is discharged to the Tonle Sap and Bassac Rivers via a number of drains
(with very little treatment);
• impacts are possible in the immediate vicinity of the outfalls and further downstream on
entering Vietnam;
• there are two potential transboundary issues: (a) transport of contaminants to Vietnam,
and (b) effects on fish migration both upstream and downstream of Phnom Penh;
• wastewater discharged to the Bassac River will have the greatest potential to reach
Vietnam7.
Vientiane
A conceptual model for the potential water quality issues resulting from municipal and
industrial wastes from Vientiane is shown in Figure 5b. The details are discussed in Section
6.2 and include:
• wastewater is discharged to the Mekong via a wetland (hence some treatment);
• impacts are possible in the immediate vicinity of the discharge and further downstream,
• since the Mekong River near Vientiane is the border between Lao PDR and Thailand,
potential transboundary issues could presumably occur both in the immediate vicinity of
the discharge and further downstream;
• the wastewater discharge could potentially interfere with the migration of fish upstream
and downstream in the Mekong River.
Mekong Delta
The conceptual model for the degraded water quality in the Mekong Delta is shown in Figure
5c. The details are discussed in Section 6.3 and include three potential sources of
contamination:
• transboundary transfers of poor quality water from upstream Phnom Penh;
• pollution from the intensive agricultural areas within the Delta region;
• intrusion of seawater into the Delta region8.
7
8
Transport of contaminants from Tonle Sap River is likely to occur only during periods when water
drains from Tonle Sap Great Lake back into the Mekong (Dec-Feb).
This presently occurs during the dry season, and is predicted to increase if the major water resources
developments occur in upstream countries.
18
(Figure 5a)
Figure 5
(Figure 5b)
Conceptual models showing the potential transboundary water quality
issues caused by wastewater discharges to the Mekong River from (a)
Phnom Penh and (b) Vientiane, and (c) in the Delta region.
19
(Figure 5c)
Figure 5 continued.
5.
20
Analysis of present environmental monitoring data
5.1
Physico-chemical water quality monitoring
The MRC supports an extensive physico-chemical monitoring network with 103 sites
sampled monthly. This program commenced in 1985 (1993 in Cambodia) and was reviewed
in 1997. Currently, the program is being reviewed again, this time with a focus on whether
the existing network can adequately characterise transboundary and basin-wide water quality
issues (E. Ongley, MRC Consultant, pers. comm.).
The network consists of 16 stations along the mainstream, 35 along tributaries, 46 stations in
the Vietnam Mekong delta, and 6 stations in wetlands along the Mekong corridor. Each
sample is analysed for pH, DO, conductivity, SPM (TSS), Ca, Mg, Na, K, alkalinity, Cl, SO4,
Tot-Fe, Tot-P, FRP, Tot-N, NOx-N, NH4-N, Si and COD.
We have analysed part of this water quality database in an attempt to provide information on
the three transboundary issues. We have used the data as provided and have not done any
checks for accuracy or quality; presumably, this will be done as part of the above mentioned
review.
We found that much of the available data was of limited value in addressing these
transboundary issues. There were three main reasons for this:
• the sampling sites were not well located to address these issues;
• only surface samples are taken and then only at monthly intervals;
• many of the indicators measured regularly (e.g. Na, K, Ca, Mg, Cl) are of limited use in
assessing water quality issues. Further, indicators we would like to have had information
on were not measured (e.g. Chlorophyll-a, toxic contaminants, depth-integrated SPM
concentrations, diurnal DO concentration changes, total and dissolved organic carbon).
We expect that the current review will recommend substantial changes to the present water
quality monitoring network. In brief, there was 15 years (1985-2000) of data for sites in Lao
PDR, Thailand and Vietnam, and 7 years (1993-2000) of data for sites in Cambodia.
The following subset of the full database was used:
• SPM (TSS)
indicator of land disturbance and erosion
• Conductivity (EC)
indicator of salinity increases
• pH, alkalinity
indicators of buffer capacity and possibly
autotrophic/heterotrophic status
• Tot-P, FRP, NOx-N, NH4-N (Tot-N) indicators of eutrophication potential
• DO, COD
indicators of organic pollution.
The full water quality database is held by the Mekong River Commission. The subset
analysed for this report is available from the authors on request.
General observations
Figure 6 contains box plots of the key indicators (EC, SPM, pH, alkalinity, DO, COD, Tot-P,
Tot-N) over the length of the lower Mekong River from Chang Saen (at the Thai/Lao
PDR/Myanmar border) to the Delta. The water quality of major tributaries (at Ubon, Tonle
Sap River at Phnom Penh Port, Prek Kdam (that enters Bassac River just downstream of
Phnom Penh), and Bassac River at Ta Khmao, Chau Doc and Can Tho) are also included on
Figure 6 (and are identified by an asterisk (*)). The interpretation of these plots is as
follows: the box spans the 25 to 75 percentile concentrations with the middle dash being the
median concentration; the whiskers span the 10 to 90 percentile concentrations.
21
(Figure 6a)
Figure 6
Box plots of the key water quality indicators over the length of the lower
Mekong River. Data for Lao PDR, Thailand and Vietnam collected in
period 1985-2000, for Cambodia between 1993-2000.
22
(Figure 6b)
Figure 6 continued.
23
(Figure 6c)
Figure 6 continued.
24
(Figure 6d)
Figure 6 continued.
25
Two general observations can be made:
•
the water quality indicators are all subject to considerable variation, with typical relative
standard deviations (RSD) ranging from 20-30% for conductivity to over 100% for
SPM. This variability has major implications for the number of samples required to
detect statistically valid changes between locations and with time;
•
a major change in most of the water quality indicators seems to occur between Pakse
(869 km) and Kratie (545 km) – this may correspond to the large change in river gradient
between these two locations (MRC, 1997).
Conductivity
The Mekong River contains low conductivity waters (Figure 6a). At all sites there was an
obvious annual pattern with low conductivity recorded during the wet season and
correspondingly high conductivity during the dry season. Figure 7 shows the record at Tan
Chau and this is typical of all sites. Interestingly, the conductivity of the Mekong actually
decreased with downstream distance, from a median of ca. 24 mS/m at Chang Sean to ca.
14 mS/m at My Thuan (Figure 6a). Between Pakse and Kratie, the median conductivity
appeared to decrease by around 20% and then remain relatively constant between Kratie and
the lower Delta. An explanation for this observation is not immediately obvious. It is
apparent that higher conductivity water is entering the Mekong from the Nam Mun
catchment in northern Thailand (see Ubon site in Figure 6a). This probably reflects the
increased salinity reportedly occurring in this highly agricultural catchment (MRC, 1997).
There was no noticeable increase in conductivity at the two sites closest to the sea (My Than,
Can Doc), as would have been expected had seawater intrusion reached these points.
Figure 7
Conductivity vs time for the Mekong River at Tan Chau.
26
Suspended particulate matter
The Mekong River transports high concentrations of suspended particulate matter9 (SPM,
Figure 6a). The SPM concentrations are highly variable and are correlated with flow.
Interestingly, the SPM concentration appears to reduce with distance downstream. For
example, over the 1,500 km between Luang Prabanh (2010 km) and Kratie (545 km), the
median SPM concentration reduced from ca. 175 mg/L to 74 mg/L. There also appears to
be a significant decrease in SPM concentration between Pakse and Kratie (Figure 6a). This
may be due to sedimentation of SPM during the wet season when much of the Mekong flow
is dispersed over a very extensive floodplain area.
Considering both the high SPM concentrations and the very large flows, it is hardly
surprising that the Mekong River transports massive loads of SPM, with most of the
transport occurring during the wet season (Figure 8). Using the available (inadequate) data,
it is estimated that the annual loads are in the range of 65-120 million tonne, but could be as
high as 200 million tonne/year10. Figure 8 shows the monthly and annual load calculated at
Vientiane, Pakse, Kratie and Phnom Penh. It is tempting to suggest from these data that
there is a large loss of SPM between Pakse and Kratie; however, the uncertainty in these
estimates (ca. 100%) makes such interpretation speculative at best.
Figure 8
Plot of the monthly and annual SPM loads transported by the Mekong
River at Vientiane, Pakse, Kratie and Phnom Penh.
These SPM load estimates are similar to those reported elsewhere. For example, Milliman &
Meade (1983) and Milliman & Syvitski (1992) estimated that the Mekong discharges around
180 million tonne/year, while a mean annual discharge rate of 120 million tonne is calculated
9
10
We have used SPM rather than TSS in this report.
The SPM data base is inadequate because the SPM concentrations are only measured once per month
and then only at the surface. Reasonably accurate load estimates would require a significantly
increased sampling frequency, particularly during the high flow period, and depth-integrated
sampling instead of surface sampling.
27
using the mean SPM concentration of 250 mg/L reported by Wolanski et al. (1996) and a
mean flow of 15,060 m3/s (MRC, 1997).
To put these figures into perspective, the annual sediment yield (load) for the Mekong is
around 130 tonne/km2 (load = ca. 100 million tonne/year), making it comparable with the
Mississippi (120 tonne/km2, 210 million tonne/year) and the Amazon (190 tonne/km2, 1,200
million tonne/year), but very much less than the Ganges/Brahmaputra (1,670 tonne/km2,
2,180 million tonne/year) and the Fly River (1,500 tonne/km2, 116 million tonne/year)
(Wolanski et al., 1996).
pH
The Mekong River waters are slightly alkaline (Figure 6b), with median pH values ranging
from ca. 7.9 in the upstream reaches (Chang Sean - Nakhon Phanom) to ca. 7.5-7.6
downstream of Kratie.
Alkalinity
The Mekong River has noticeably higher alkalinity water in the upstream reaches around
Vientiane (ca. 100 mg/L (as (HCO3), i.e. approx. 1.64 meq/L) than in the downstream
reaches below Kratie (ca. 60 mg/L, i.e. approx. 0.98 meq/L). Figure 6b also shows the
somewhat lower alkalinity waters entering the Mekong from the Nam Mun catchment
(Ubon) and Prek Kdam (near Phnom Penh). The lower alkalinity water in the Bassac at Ta
Khmao probably reflects the influence of Prek Kdam which enters very close to this site.
Nutrients
Phosphorus – total phosphorus concentrations are plotted in Figure 6c. Except for the lower
sites in Vietnam, the median concentrations are all reasonably low (<50 µg/L). The four
lower sites in the Delta region are all noticeably higher with median concentrations around
100 µg/L. There are two possible explanations for these higher results, i.e. either they are
correct and represent additional nutrient inputs in this region, or there is a difference in the
analytical procedures between the Vietnamese and Cambodian laboratories.
Nitrogen – total nitrogen concentrations are not determined at all sites. However, for those
sites where data are available (Figure 6c), the trend in median concentrations is similar to
total phosphorus, namely relatively low concentrations in upstream sites (ca. 450 µg/L) and
slightly higher concentrations at the four lower sites (ca. 600 µg/L).
Organic matter
Unfortunately, chemical oxygen demand (COD) is the only measure of organic matter
available for the Mekong River. We would have preferred to have had data on biochemical
oxygen demand (BOD), which provides a better measure of the biologically available organic
matter, or total organic carbon (TOC). The median COD concentration approximately
doubles between the upstream reaches and downstream of Phnom Penh (Figure 6d). At
Ubon the COD concentrations are noticeably higher than in the Mekong, and this probably
reflects organic pollution either from the city of Ubon or more generally from the Nam Mun
catchment in northern Thailand. Further downstream, the relatively high values in the two
sites close to Phnom Penh (Prek Kdam – a tributary stream that drains the southern part of
Phnom Penh to the Bassac; Ta Khmao – a site in the Bassac just downstream of Phnom
Penh) almost certainly reflect organic pollution from Phnom Penh.
Dissolved oxygen
The median dissolved oxygen concentrations decreased by around 20% between Chang Sean
and My Thuan (Figure 6d). This change probably reflects the general increase in both the
inputs of organic matter (see COD graph) and temperature over the length of the Mekong.
28
The lower median DO concentrations at Prek Kdam and Ta Khmao probably reflect the input
of stormwater and sewage from Phnom Penh.
5.2
Toxicant (pesticides, heavy metals) monitoring
Pesticides
MRC (1997) report that pesticides have been monitored in Lao PDR (5 stations) and
Vietnam (15 stations) since 1991 and in Thailand (11 stations) since 1994. At each station,
fish (5 species) and water are sampled and analysed twice a year, once in the dry season and
once in the rainy season. Over 26 pesticides are analysed, including HCB, aldrin, dieldrin,
DDT, parathion, dichlorvos and endosulphan. The databases for water samples and fish
tissues provided by the MRC had significantly fewer analytical results than the above
sampling regime should have produced.
The database for pesticides in water shows that a total of 117-166 water samples were
analysed by the three countries from a total of 35 sites11 in the period 1994-1997. As
expected, pesticide concentrations were very low in those water samples that had detectable
concentrations, although on some occasions elevated levels of DDT (max – 220 µg/L),
endosulphan (max – 180 µg/L) and diazinon (max – 1,160 µg/L) were measured. We have
no information on the quality of the pesticide analysis data because no quality assurance
information was provided.
Pesticide analysis in water samples from the Mekong Basin is questionable given the very low
levels generally recorded, the difficulties in undertaking these types of analyses and the fact
that it does not appear these analyses have been undertaken since 1997. We recommend that
the MRC review the effectiveness of this part of the pesticide analysis program.
The fish tissue analysis database also contains considerably less data than suggested by the
MRC’s information. Since 1994, there have been 580-800 fish tissue samples analysed for
pesticide contamination. The numbers of samples analysed have varied significantly between
countries, e.g. Cambodia - 3 sites, 14 samples; Lao PDR – 5 sites, 25 samples; Thailand – 12
sites, 198 samples; Vietnam – 19 sites, 564 samples. As expected, more fish tissue samples
were found to contain pesticide residues because of the potential for pesticides to be
concentrated in the fatty tissues of fish. For example, quite high concentrations of parathion
(max – 100 µg/kg), DDT (150 µg/kg), dieldrin (22 µg/kg), endosulphan (74 µg/kg) and
diazinon (600 µg/kg) were recorded in the period between 1994 and 1997. Approximately
10% of the fish tissues analysed for parathion (578 samples) and 15% of those analysed for
DDT (801 samples) contained higher than 5 µg/kg of the pesticide. The highest tissue levels
were recorded in Vietnam, presumably reflecting the intensive agriculture occurring in the
Mekong Delta region. Again, we have no information on the quality of the pesticide analysis
data because no quality assurance information was provided.
Monirith et al. (1999) reported generally low concentrations of persistent organochlorine
pesticide12 residues in 27 species of freshwater and marine fish from Cambodia. DDT and its
derivatives were detected in all the fish analysed (mean – ca. 10 µg/kg (wet wt), range – 0.325 µg/kg). The other organochlorines were present in very much lower concentration (mean
– ca. 0.1 µg/kg). Kannan et al. (1995) also reported quite high concentrations of DDT and
chlordanes in fish from Thailand (DDT: 0.48-19 µg/kg; chlordanes: 0.1-15 µg/kg) and
Vietnam (DDT: 3.9-76 µg/kg; chlordanes: <0.01-0.35 µg/kg).
11
12
Lao PDR – 5 sites sampled once only; Thailand – 11 sites sampled between 1994 and 1997; Vietnam
– 19 sites sampled between 1994 and 1997.
Polychlorinated biphenyls (PCBs), DDT compounds, hexachlorocyclohexanes (HCH) and chlordane
compounds.
29
Unfortunately, it has not been possible in the time available to use the available pesticide
database to assess the transboundary risks from these toxicants in the Mekong Basin. This
would require a further dedicated study that could be undertaken if MRC wished.
Heavy metals
We have been unable to obtain any data on heavy metal concentrations in water, sediments or
biota in the Mekong Basin. Typical sources of heavy metals are mining activities, certain
manufacturing industries and urban areas. Currently, there are relatively small numbers of
these activities in the lower Mekong basin.
Thus, the risk of toxic effects from excessive heavy metal concentrations is likely to be low,
except perhaps for localised effects close to poorly managed mining activities.
5.3
Biological monitoring
Benthic macroinvertebrates
Benthic macroinvertebrates are being used in a number of countries to assess river “health”
(Bailey et al., 1998; Norris et al, 1995, 2001; Rosenberg & Resh, 1993; Simpson & Norris,
2000; Wright et al., 2000).
According to MRC (1997), benthic macroinvertebrates are sampled seasonally in Lao PDR
(9 stations), Cambodia (9 stations) and Vietnam (unknown number of stations). However,
we were able to locate only one report (Smith, 1988) on macroinvertebrate sampling. This
reports a major study to sample the benthic invertebrates in the period April to June 1988,
and also some less intense sampling in 1987. Apparently, another sampling run was
undertaken in 1989 or 1990 but we have not been able to locate this report.
In April to June 1988, benthic macroinvertebrates were sorted from sediment samples
collected at 11 sites in the Mekong main channel, 17 sites in main tributaries and 10 in the
Plain of Reeds in the Mekong Delta region. This study reported difficulty in being able to
sample consistent habitat type over the length of the Mekong. Much of the sediment in the
main stream consists of sand and silt, a poor habitat for benthic invertebrates. As a
consequence, only 4-8 species were present, the dominant fauna being oligochaete worms
and chironomids. A larger number (ca. 30) of species was found in the lower Delta region of
the Mekong. Smith (1988) suggested that the four main channel locations in the Delta region
were suitable, but the upstream main channel was unsuitable.
Given the major advances that have been made over the past decade in the use of benthic
macroinvertebrates to assess the biological condition of rivers, we recommend the MRC
establish a project to assess the potential use of these biological indicators in the Mekong
River.
Fish
Fish have also been extensively used as indicators of the biological condition of rivers. We
have no information on the use of fish as a biological (or biodiversity) indicator in the
Mekong River. However, the large number of fish species known to be present in this
system suggests there is great potential to use these organisms to provide a robust measure
of river condition.
We recommend the MRC establish a project to assess the potential use of fish as biological
indicators in the Mekong River. The investigations that would be necessary in such a project
would also provide excellent information on the ecology of this important resource.
Algae
Member countries have identified eutrophication (excessive growth of aquatic plants) as a
potential problem in the Mekong Basin. Given the rather poor management of stormwater
30
and sewage effluent in urban regions, and the equally poor management of agricultural
activities, it seems highly likely that many aquatic systems within the basin are, or will
become, eutrophic. However, as indicated in Section 6, we have assessed the risk of
transboundary eutrophication problems as low.
With the present information it is possible to provide only a preliminary assessment of the
potential for eutrophic conditions to exist, both within each country and at the borders. The
current monitoring program does not collect any information on algal species composition or
biomass (e.g. Chlorophyll-a concentrations).
We recommend the MRC consider including algal monitoring as part of a biological
monitoring network for the lower Mekong River.
Ecological processes
As noted above, biological monitoring programs based on macroinvertebrates, fish and algae
have been introduced in a number of countries over the past decade. These make use of
changes in the pattern of these communities, using indicators such as abundance, richness and
species composition compared with known reference systems. For example, Australia now has
a national program based on the AUSRIVAS system (AUStralian RIVer ASessment System)
(Norris et al., 1995; 2001; Simpson & Norris, 2000).
However, Bunn & Davies (2000) have argued that because changes in pattern do not always
equate to changes in ecological integrity, measures of key ecological processes should also be
included in programs to assess the health and integrity of ecosystems. Ecosystem process
measures that are now starting to be used include benthic metabolism, gross primary
production, respiration, nitrification and denitrification. Over the next few years we should see
the development of more robust ecosystem process measures that can be incorporated into
existing biological monitoring programs.
We recommend that the MRC consider establishing a project to investigate a range of
ecological processes in the Mekong River. The ultimate objective of this project should be
to develop a number of ecosystem process indicators that can be used to assess the
ecological “health” of the Mekong.
5.4
Urban wastewater loads
A key objective of this project was to assess whether real or potential transboundary impacts
result from the wastewater discharges from the urban areas of Phnom Penh and Vientiane.
An essential component of such an assessment is good information on the quantity and
quality of these wastewater discharges.
It seems that there is very little routine monitoring of the wastewater from either city, mainly
because of a lack of resources. We were able to meet the relevant officials in Phnom Penh
who were very cooperative and provided a great deal of very useful information on the
general wastewater collection and treatment system (Section 6.1). We were also able to
obtain similar information for Vientiane, although we did not visit that city (Section 6.2).
However, even with this information we still lack detailed knowledge on the collection
systems, treatment facilities, discharge locations and the magnitude of the contaminant loads
entering the Mekong.
This information must be obtained if MRC is to better assess the transboundary risks from
these urban areas. We recommend that MRC obtain considerably better information on the
wastewater systems in the two major urban centres – Vientiane and Phnom Penh.
Because it was not possible to obtain sufficient good quality information on the total load of
wastewater and contaminants discharged from each city in the time available, Assoc. Prof.
Tony Wong (Ecological Engineering Pty Ltd, Melbourne) was commissioned to provide an
31
estimate of the likely loads of SPM, BOD, Total-P and Total-N from each city (see Appendix
A for report).
The estimates of loads of sediment, organic matter and nutrients from both stormwater and
sewage for both cities are listed in Table 1. The various assumptions are listed in the
footnote of the table. These estimates are likely to be accurate to an order of magnitude
only, given the lack of knowledge about the generation of stormwater and sewage, and the
effectiveness of treatment. Also shown in Table 1 is the relative annual load from each city
as a percentage of that transported by the Mekong at that location. This latter figure allows
an estimate of the relative contribution of contaminants to the river from the two cities.
On an annual basis, each city contributes relatively small amounts (0.1-5%) of sediment,
organic matter and nutrients to the river. However, the relative contribution at particular
times of the year (e.g. dry season, low flow) could be higher than this.
Table 1
Estimated contaminant loads (tonne/year) discharged from Vientiane and Phnom
Penh (see Appendix A for details). In brackets is the contribution of each city as
a percentage of the total load transported by the Mekong River at that point.
City
Vientiane
SPM
24,000
(0.1%)
BOD
6,500
(4%)
TP
300
(4%)
TN
1000
(1%)
Phnom Penh
82,000
(0.1%)
15,000
(2%)
600
(5%)
2,000
(2%)
Assume urban area: Vientiane = 60 km2; Phnom Penh = 375 km2
Assume populations: Vientiane = 580,000; Phnom Penh = 1,000,000
5.5
Use of existing data to assess transboundary issues
This section reviews the capacity of the existing data set (summarised in the above sections)
to assess transboundary issues.
Physico-chemical water quality data
Despite the fact that the MRC has 7-15 years of water quality data at 16 sites along the main
Mekong channel, these are of limited use in assessing transboundary issues. The main
limitations in the database include:
Inappropriate indicators – a wide range of traditional indicators are measured (see
Section 5.1). However, many of these are inappropriate for assessing important ecological
and human health changes in the system. We have focused on 11 indicators - SPM (TSS),
conductivity, pH, alkalinity, Tot-P, FRP, NOx-N, NH4-N, (Tot-N where available), DO and
COD. Additionally, we have identified important indicators (e.g. Chlorophyll-a, TOC, DOC)
that are not currently measured but should be.
Inappropriate site locations – when the water quality network was designed it seems
obvious that assessment of transboundary issues was not one of the objectives for which the
data was being collected. The sites are located considerable distances apart (see Figure 1)
and at best can be used to assess broad overall changes in water quality over the length of the
Mekong. The situation for Vientiane illustrates the poor network design for transboundary
issues. Wastewater is discharged approximately 15 km downstream of Vientiane and could
affect environmental values in Thailand which shares the river with Lao PDR in this region.
32
However, there are no sample locations immediately downstream of the effluent discharge.
Below Vientiane, the next sample location is some 300 km downstream at Nakhon Phanom.
Inadequate sample frequency – current sampling occurs monthly and is inappropriate to pick
up changes that occur more rapidly than this, e.g. algal growth. Dissolved oxygen
fluctuation will occur over a day, and taking a single measurement at one time once a month
is unlikely to detect any major changes in this important indicator.
Surface samples – only surface water samples are taken at present. This is inappropriate for
indicators that vary with depth (e.g. SPM, DO). Additionally, the dissolved oxygen status of
deep pools in the main channel cannot be assessed with surface samples.
Statistical power – the present sampling design has very poor statistical power to detect any
significant changes (see Section 7).
Toxicants (pesticides, heavy metals)
A small amount of pesticide data exists, but no heavy metal data. These data are insufficient
to assess possible transboundary toxicity issues.
Biological
Biological assessment is increasingly being used to assess the condition of rivers worldwide.
Unfortunately, there is no on-going biological monitoring program for the Mekong.
Additionally, in the time available we were able to access only a small amount of biological
data relevant for the Mekong. We are told (J. Lacoursiere, Univ Lund, pers. comm.) that a
considerable amount of relevant biological information does exist. A large number of
international (e.g. WHO, FAO, WWF, IUCN) studies have been funded over the years, and
these have been complemented by studies undertaken by national institutions in all member
countries. However, reports that do exist have not been collected in any central location
(e.g. MRC) and the information has not been well synthesised.
We recommend that MRC make efforts to collect all published and unpublished information
on the biology and ecology of the Mekong River and its tributaries, and prepare a synthesis
of this information that summarises current knowledge in this area.
Fluxes and loads
In view of the deficiencies in the water quality database, we have sought to obtain
information on the amounts or loads of contaminants discharged into the Mekong,
particularly from Vientiane and Phnom Penh. Comparison of these human-generated loads
with those transported “naturally” by the Mekong provides a first broad assessment of the
possible impacts on the system.
These load estimates still have considerable uncertainties. For example, it was difficult to
obtain daily flows for some sites to use in calculating daily loads. Additionally, we had
limited concentration data (monthly values) and had to make assumptions about what was
happening over the rest of the time when samples were not available.
Ecosystem processes
A number of contaminants can undergo significant transformations after they are discharged.
For example, organic matter is decomposed by the natural bacterial populations and
phosphorus can be adsorbed by particulate matter. We have no information on such
processes. Such information will only be obtained by undertaking special studies aimed at
answering specific questions.
6.
33
Assessment of transboundary water quality issues
6.1
Phnom Penh
Background
The current population of Phnom Penh is ca. 1 million, with approximately 57% living in the
urban areas of the city and 43% living in semi-rural areas on the outskirts of the city13
(General Population Census of Cambodia, 1998).
The central part of Phnom Penh has a combined sewage system (domestic, industrial,
stormwater), but this is in poor condition having been poorly maintained since the 1980s14.
The remainder of the city has no sewerage system, with the majority of the population in
these areas using latrines or septic tanks.
A network of street drains conveys water and sewage by gravity to retention ponds where
some “treatment” of the wastewater occurs. Wastewater from these ponds is either pumped
over embankments directly into the Bassac or Tonle Sap Rivers, or into a system of natural
ponds and canals that eventually drain into these rivers. A total of 14 sewers discharge to
these two rivers, but we have no information on the relative pollution loads from these
sewers.
There are a small number of industries in Phnom Penh (e.g. dye, galvanising & tanning
factories, paper mill, beer production), but few of these (8 of 148) undertake any treatment
before discharging their wastewater directly into the sewerage system or to nearby creeks.
Unfortunately, there is little monitoring of either the quantity or quality of Phnom Penh’s
wastewater. A recent survey of the effluents from a small number of factories in Phnom
Penh found these discharges had quite high BOD concentrations (100-600 mg/L; Chrin
Sokha, Cambodian Ministry for Environment, internal report).
We have estimated that 75 million m3 of sewage and 470 million m3 of stormwater is
discharged annually to the Tonle Sap and Bassac Rivers (assume approximately equal
amounts to each river). The loads of sediment, organic matter and nutrients are listed in
Table 1. These loads comprise a relatively small proportion of the total loads transported by
the Mekong near Phnom Penh. However, this comparison is hardly relevant because these
pollutants are not discharged to the main Mekong River channel, but to the Tonle Sap and
Bassac Rivers (see Figure 5a).
The potential local impacts from these discharges are likely to be greatest during the dry
season when flows in these rivers are at their lowest, and during no-flow periods in Tonle
Sap River. Flow data collected during 2000 as part of the Chaktomuk Project (2001),
provides a useful picture of the changing flow conditions around Phnom Penh during the
period August to November. In this year, the Mekong flow increased rapidly upstream of
Phnom Penh around late July (flow increased from 5,000 m3/s to 30,000 m3/s in a few days).
Before this rapid increase, the flow in both Tonle Sap and the Bassac was essentially zero.
With the high flow in the Mekong, flows increased to around 3,000 m3/s in the Bassac and
6,000 m3/s in Tonle Sap for most of August and September. By the end of September the
flow north in Tonle Sap had started to decrease and by the end of October had actually
changed direction so that it was flowing out of Tonle Sap at around 5,000 m3/s. At this time
the flow in the Bassac River had increased to around 8,000 m3/s.
13
14
The population increase in Phnom Penh has been quite rapid, e.g. in 606,800 in 1989, 718,400 in
1993, and 999,800 in 1998.
Mr Chrin Sokha (Chief, Office of Water & Soil Quality Management, Cambodian Ministry of
Environment) assisted us in assessing the sources of pollution from Phnom Penh.
34
There is some evidence (DO, COD, NOx, FRP concentrations – see Figure 6) that the
discharges to the Bassac River (and Prek Kdam) are having a local effect with higher
concentrations of COD and nutrients and lower concentrations of DO measured at times of
lower flows. Additional information on water quality in Tonle Sap River (at Phnom Penh
Port) also shows that, at certain times of the year, and particularly when the flow is very low,
the water quality is degraded (see Figure 6).
Conceptual model
To assess the risk of transboundary water quality issues arising because of pollution from
Phnom Penh, we have adopted the following conceptual model (see Figure 5a):
•
wastewater from Phnom Penh is discharged to the Tonle Sap and Bassac Rivers via a
number of discharge points - we have no information on how much of this wastewater
enters the main channel of the Mekong, but have assumed it is very low;
•
wastewater discharged to the Bassac River will be transported downstream, while that
discharged to Tonle Sap will be either transported downstream (Dec-Mar), upstream
(Jul-October) or remain in the immediate vicinity of the outfalls (remainder of the year);
•
the quality of this wastewater will range from very poor (during the dry season and the
first flush of the wet season) to poor, since it receives essentially no treatment;
•
the urban regions in Phnom Penh have little paving, which means that the SPM loads will
be high, particularly during the wet season;
•
there is very poor information on both the quantity and quality of this wastewater, since
there is no regular monitoring program;
•
adverse effects due to these wastewater discharges may occur (a) in the immediate vicinity of the discharges (Tonle Sap, Bassac),
(b) further upstream in Tonle Sap River (and possibly even in Tonle Sap Great Lake),
(c) further downstream on entering Vietnam (transboundary issues).
Issues
As noted in Section 4.2, assessment to the possible transboundary water quality effects will
focus on three environmental values:
•
•
fish migration;
For the first environmental value, ecosystem protection, we have attempted to assess the risk
of adverse changes occurring to three key biological components:
•
eutrophication or algal blooms due to excessive nutrient concentrations;
•
toxicity to biota from either low dissolved oxygen concentrations or toxic compounds;
•
ecosystem processes (e.g. changes to the light regime due to increased SPM
concentrations, with consequent changes to primary productivity).
This transboundary assessment focuses on two regions:
•
in the immediate vicinity of Phnom Penh where pollution may interfere with fish
migration in the Bassac and Tonle Sap Rivers;
•
in the Delta region of the Mekong, where pollution from Phnom Penh could result in
unacceptable risks to the aquatic ecology and to human health.
35
Local issues in the Tonle Sap and Bassac Rivers, such as the impact on the ecology or human
health during the dry season, are not considered to be transboundary issues and are therefore
not covered in this report.
Risk assessment
Table 2 contains a summary of the information provided below for each issue considered.
Where appropriate, three lines of evidence have been used to assess each issue.
•
The existing water quality data both in the vicinity of Phnom Penh and at the border was
compared with appropriate trigger values to determine the proportion of time these
trigger values were exceeded.
•
The existing water quality data was used to discern any influence of Phnom Penh on the
delta region. A typical upstream/downstream experimental design was adopted, with the
major influence being the wastewater discharges from Phnom Penh (see Figure 5). Thus,
we compared the water quality data from the two (three) upstream sites – Kratie,
Kompong Chan and Phnom Penh – with the downstream sites in both the Mekong (Tan
Chau, My Than) and the Bassac (Ta Khmao, Chau Doc, Can Tho). As discussed in
Section 7, the available data are too sparse and too variable to show statistically whether
Phnom Penh is having an effect in Vietnam.
•
The loads of key contaminants likely to be discharged to the system from Phnom Penh
were compared with those transported “naturally” by the Mekong. Somewhat arbitarily,
we have assumed the following categories of risk of adverse impacts:
Low risk
If ratio of load from Phnom Penh to natural load
<5%
Moderate risk If ratio of load from Phnom Penh to natural load
5-50%
High risk
If ratio of load from Phnom Penh to natural load
>50%
The following points are relevant in making the assessments below:
•
we have assumed that the bulk of Phnom Penh wastewater transported to the Delta will
occur via the Bassac River;
•
the distance from Phnom Penh to the Vietnam border is ca. 110 km;
•
the water temperature is high (ca. 29oC) meaning that the rate of decomposition of any
organic matter discharged from Phnom Penh will be relatively rapidly broken down.
Ecosystem protection
Eutrophication
Algal growth is primarily influenced by three factors:
•
nutrients - available nutrient concentrations are likely to be greatest in the dry season;
•
light - in view of the high SPM concentrations, the Mekong system is likely to be light
limited for much of the time, although during the dry season Mekong water tends to
become clear, particularly in the upstream reaches;
•
flow conditions - it seems likely that the flows in the main Mekong channel will be too
great to sustain high phytoplankton biomass, even in the dry season. We have insufficient
information to assess whether the conditions in wetlands associated with the main
channel are conducive to phytoplankton growth, but this is more likely to occur.
Table 2a
36
Summary of the transboundary risks from Phnom Penh wastewater discharges on the Vietnam Delta region
Issue
Effect
Effect rating
Likelihood
assessment
Likelihood rating
Transboundary risk
Algal blooms
High – [FRP] >40 ug/L
Mod – [FRP] 10-40 ug/L
Low – [FRP] <10 ug/L
that FRP trigger value
is exceeded during dry
season
Toxicity to fish and
benthic invertebrates
High – [DO] <1.5 mg/L
Mod – [DO] 1.5-6.0 mg/L
Low – [DO] >6.0 mg/L
that DO trigger value is
exceeded during dry
season
• 10% likelihood high
effect
• 80% likelihood moderate
effect
• 95% likelihood of low
effect
• 5% likelihood of
moderate effect
To be determined
Insufficient information
Risk not assessed
Risk not assessed
Adverse effect on fish
movement
upstream,
downstream,
or
onto
floodplains
Increased microbial
contamination causing
sickness
Microbial
sickness due to
primary contact
High – > 0 coliforms/100ml
Low – < 0 coliforms/100ml
Ecological
Eutrophication 1
Toxicity – DO
3
Toxicity – chemicals
4
Ecosystem processes 5
Fish migration 6
Human health
7
Drinking water
Recreation 7
1.
2.
3.
4.
5.
6.
7.
8.
High – >150 faecal
coliforms/100mL
Low – <150 faecal
coliforms/100mL
that coliform trigger
value is exceeded
during any season
that faecal coliform
trigger value is
exceeded during dry
season
Low-moderate
2
Low
Risk not assessed
Risk not assessed 8
Risk not assessed
8
Would prefer to use Chlorophyll-a rather than TP as the indicator, but no data.
More likely low risk since only nutrient concentration used; high turbidity and high flow would also reduce chance of algal problems.
DO measured at one time during the day, need more measurements during dry season and with depth in deep pools, special surveys needed.
Cannot assess risk because no data on heavy metals or toxic organics.
No information to assess risk in the main Mekong River, risk likely to be higher in off-channel wetlands and Tonle Sap Great Lake.
No data, present water quality sampling network will not provide the required information, special survey needed.
Apparently some data on total coliform levels (no data on E. coli), but could not obtain in time available.
Risk likely to be low-moderate, due to large dilution and microbial die-off during transport to Vietnam.
Table 2b
37
Summary of the transboundary risks from Vientiane wastewater discharges on Thailand near Vientiane and downstream
Issue
Effect
Effect rating
Likelihood
assessment
Likelihood rating1
Algal blooms
High – [FRP] >40 ug/L
that FRP trigger value
is exceeded during dry
season
Low2
that DO trigger value is
exceeded during dry
season
Low2
Transboundary risk
Ecological
Eutrophication
Mod – [FRP] 10-40 ug/L
Low – [FRP] <10 ug/L
Toxicity – DO
High – [DO] <1.5 mg/L
Mod – [DO] 1.5-6.0 mg/L
Low – [DO] >6.0 mg/L
Toxicity – chemicals 3
Risk not assessed
Ecosystem processes
To be determined
Risk not assessed
Adverse effect on fish
movement
upstream,
downstream,
or
onto
floodplains
Risk not assessed
Increased microbial
sickness
High – > 0 coliforms/100ml
that coliform trigger
value is exceeded
during any season
Low2
Microbial
sickness due to
primary contact
High – >150 faecal
coliforms/100mL
that faecal coliform
trigger value is
exceeded during dry
season
Low2
Fish migration
4
Human health
Drinking water
Recreation
1.
2.
3.
4.
Low – < 0 coliforms/100ml
Low – <150 faecal
coliforms/100mL
Existing water quality data inadequate to assess the risk in either the immediate location of the wastewater discharge or further downstream, location of water quality sample locations inappropriate –
one site upstream (Vientiane, H011910), one about 300 km downstream (Nakhom Phanom (H013101) – leads to a flawed “experimental design” that has no power to detect impacts due to Vientiane’s
wastewater discharge.
The tentative assessment of low risk is based on the high dilution expected when Vientiane’s wastewater enters the Mekong River – however we recommend MRC investigate further.
Cannot assess risk because no data on heavy metals or toxic organics.
No data, present water quality sampling network will not provide the required information, special survey needed.
Table 2c
38
Summary of the transboundary risks from upstream degraded water on the Vietnam Delta region
Issue
Effect
Transboundary risk
1
Ecological
Eutrophication
Toxicity
Ecosystem function
Human health
Drinking water
Recreation
Agriculture 4
Irrigation
1.
2.
3.
4.
Algal blooms
To be determined
Low
Low
Not determined
2
Increased microbial
sickness
Increased salinity
Risk not assessed 3
Risk not assessed 3
Low
Used the analysis for effects of Phnom Penh (see above).
Used the analysis for effects of Phnom Penh (see above).
Risk likely to be low-moderate, due to large dilution and microbial die-off during transport to Vietnam.
Assessed on the basis of changes in conductivity.
39
In this preliminary assessment of the risk that high algal biomass will be stimulated in
Vietnam by nutrients released from Phnom Penh, we have focused on the existing nutrient
concentrations. Figure 9 shows the mean nutrient concentrations in the Mekong, Bassac and
Prek Thnot Rivers over the period 1995 to 2000. The high variability (RSD approx. 5060%) in the data precludes any meaningful statistical analysis.
Generally, low concentrations of available phosphorus (FRP) were observed (Figure 9a).
For example, mean FRP concentrations ranged from 13 to 37 µg/L, and made up 15-65% of
the Total P concentration. The generally higher FRP concentrations observed in Prek Thnot
River probably reflect the relatively high proportion of wastewater received by this river.
The considerably higher Total-P concentrations noted at the lower four sample locations in
Vietnam (see Figure 6) may be real, but equally may indicate a difference in the analytical
procedures between Vietnam and Cambodia. Most of the available phosphorus discharged
from Phnom Penh would be expected to become associated with SPM, and hence essentially
unavailable for algal growth, by the time it was transported downstream to Vietnam.
Available nitrogen was mostly present as NOx-N (Figure 9b). The mean NOx-N
concentration appeared to be elevated in the Bassac River downstream of Phnom Penh (ca.
230 µg/L cf 160 µg/L); this effect was more noticeable during the dry season.
The range of DIN15/FRP ratios (11-44 mol/mol) suggests that the system is mostly P limited,
a not surprising result given the likelihood that available phosphorus would associate with the
large number of suspended particles known to be present in this system.
(Figure 9a)
Figure 9
(Figure 9b)
(Figure 9c)
Plots of nutrient concentrations in the vicinity of Phnom Penh (a) mean
(s.d.) FRP & Total-P concentrations (µ
µg/L), (b) mean (s.d.) NOx-N &
NH4-N concentrations (µ
µg/L), (c) distribution of dry season FRP
concentrations at Phnom Penh, Ta Khmao and Chau Doc (mg/L).
Table 3 below shows the dry season (Dec-May) FRP concentration ranges adopted to
indicate the potential for low, moderate and high algal biomass to form (assuming that the
light and flow conditions are not limiting phytoplankton growth). Also shown in Table 3 is
the likelihood that these triggers will be exceeded at Phnom Penh and Chau Doc (latter
15
DIN = dissolved inorganic nitrogen = [NOx-N] + [NH4-N]
40
selected as most vunerable transboundary downstream site). Figure 9c contains the data
distributions used to make these assessments.
Table 3
Trigger values for the concentrations of filterable reactive phosphorus (FRP)
that could cause algal problems in the Mekong River, and the likelihood that
these will be exceeded in the vicinity of Phnom Penh
Algal growth
FRP trigger*
Likelihood of trigger being exceeded
potential
Low
Moderate
< 10 µg/L
10-40 µg/L
Phnom Penh
20%
75%
High
> 40 µg/L
5%
Chau Doc
10%
80%
10%
* derived using the method outlined in ANZECC/ARMCANZ (2000).
This analysis suggests there is a slightly higher probability that conditions at Chau Doc could
support moderate to high algal growth compared with the situation at Phnom Penh, but this
probability is still very low.
In summary, our preliminary assessment suggests that on the basis of FRP concentrations
only, there is a low risk that nutrients released from Phnom Penh would cause excessive algal
growth downstream in Vietnam. When one factors in the high turbidity and the generally
high river flows, the risk is negligible.
Nutrient loads have also been used to estimate the potential transboundary issue due to
nutrients discharged from Phnom Penh. In assessing the relative impact of nutrient loads
discharged from Phnom Penh we have assumed:
•
the total annual load transported to Vietnam (via the Bassac River) is 75% of the total
load discharged from Phnom Penh;
•
there is no reduction in the load during transportation from Phnom Penh to Vietnam;
•
there are no additional nutrient inputs downstream of Phnom Penh;
• the annual flow in the Bassac River is 15% of Mekong at Phnom Penh (i.e. 1,800 m3/s).
With these assumptions, it is estimated that the contribution of nutrients from Phnom Penh
adds less that 1% to the loads of Total-P and Total-N transported naturally by the Bassac
River16.
This suggests a very low risk of transboundary nutrient (algal) problems from Phnom Penh,
which is a similar assessment to that derived above on the basis of concentration data.
Toxicity due to low dissolved oxygen concentrations
Low dissolved oxygen (DO) concentration has an adverse effect on many aquatic organisms
(e.g. fish, invertebrates and microorganisms) that depend upon oxygen dissolved in the water
for efficient functioning. It can also cause reducing conditions in sediments, with the
consequent release of previously-bound nutrients and toxicants to the water column where
they may add to existing stresses.
16
Tot-P – Phnom Penh contributes 340 t/y compared with Bassac’s 41,000 t/y; Tot-N (actually DIN
used in calculation) – Phnom Penh contributes 2000 t/y compared with Bassac’s 270,000 t/y.
41
The concentration of DO is highly dependent on temperature, salinity, biological activity
(microbial, primary production) and rate of transfer from the atmosphere. Under natural
conditions, DO will change, sometimes considerably, over a daily (or diurnal) period, and
highly productive systems (e.g. tropical wetlands, dune lakes and estuaries) can become
severely depleted in DO, particularly when these systems are stratified.
Of greater concern is the significant decrease in DO that can occur when organic matter is
added (e.g. from sewage effluent or dead plant material). The depletion of DO depends on the
load of biodegradable organic material and microbial activity, and re-aeration mechanisms
operating. A number of predictive computer models now exist for estimating the DO
depletion in a particular ecosystem type, and so it should be possible to estimate sustainable
loads of biodegradable organic matter for most situations.
The 1992 Australian water quality guidelines (ANZECC, 1992) recommend that dissolved
oxygen should not normally be permitted to fall below 6 mg/L (or 80−90% saturation),
determined over at least one diurnal cycle. Also, the USEPA (2000) has recently reported
DO guidelines for protecting aquatic life (but in seawater). These guidelines recommend that
DO should be above 4.8 mg/L to provide long term protection and above 2.3 mg/L for
juvenile and adult fish survival.
We have used these data to calculate the toxic effects levels associated with DO
concentrations in the Mekong River. The upper and lower limits (or trigger values) have
been taken as the concentrations equivalent to 80% saturation and 20% saturation of water
at ca. 30oC during the dry season (Dec-May) when it is more likely that low DO
concentrations will exist. These are shown in Table 4. Also shown in Table 4 is the
likelihood that these triggers will be exceeded at Phnom Penh, Ta Khmao and Chau Doc (see
Figure 10 for the data distributions used).
This analysis suggests that there is a low probability that adverse effects due to low DO
would exist during the dry season at either Phnom Penh or Chau Doc (Table 4).
Interestingly, there is a higher probability that moderately toxic conditions exist during the
dry season at Ta Khmao (as discussed below).
Table 4
Trigger values for toxic effects due to low DO concentrations in the Mekong
River, and the likelihood that these will be exceeded in the vicinity of Phnom
Penh
Potential
adverse effect
DO trigger
mg/L
Likelihood of trigger being exceeded
Low
>6.0
satn)
(80%
Moderate
High
in range 1.5-6.0 mg/L
<1.5 mg/L (20%
satn)
Phnom Penh
95%
Ta Khmao
25%
Chau Doc
95%
5%
0%
75%
0%
5%
0%
Figure 10
42
Plot of the distribution of dry season DO concentrations at Phnom Penh,
Ta Khmao and Chau Doc.
In summary, this preliminary assessment suggests that there is a low risk that pollution from
Phnom Penh would cause toxicity problems in downstream Vietnam (at Chau Doc) due to
reduced DO concentrations.
On the other hand, the analysis suggests a higher probability of low DO toxicity problems
more locally around Phnom Penh. Low DO values have been regularly measured in the
Bassac River at Ta Khmao, in Prek Kdam and in the Tonle Sap River at Phnom Penh Port
(see Figure 6d). The low DO concentrations are likely to have been caused by organic
pollution from Phnom Penh, since the COD concentrations at both Prek Kdam and Ta
Khmao (and Phnom Penh Port) are noticeably higher than the COD concentrations at the
more upstream Mekong River sites (e.g. Kompong Chan, Kratie) (see Figure 6d). This result
is not in conflict with the lower mean COD concentration measured at Chau Doc (Figure
6d), since it is quite possible that this organic matter was partially decomposed in the several
days it would take the water to reach Chau Doc during the low flow period.
Toxicity due to excessive concentrations of heavy metal and organic contaminants
It was not possible to assess the risk to aquatic organisms due to possible toxic heavy metal
and organic contaminants released to the Mekong River from Phnom Penh, because
insufficient water quality information was available concerning toxicants.
If the MRC wishes to assess this risk it will be necessary to collect information on key
toxicants (see Section 7).
Ecosystem processes
Very few river health programs make any attempt to measure changes to key ecosystem
processes, such as primary production, metabolism, respiration or denitrification. This is
because the methods available to measure these ecosystem indicators are not commonly
available and the interpretation of the results is presently limited by our knowledge of how
unmodified river systems behave. Except for upland streams, it is often difficult to find
unmodified rivers to serve as reference systems.
43
However, given that the Mekong River still appears to be in quite good condition, it is
appropriate that baseline ecosystem process information be collected now. This would then
provide the basis for future assessment of possible changes in the system with the planned
further developments.
Fish migration
It is well known that very extensive migration of fish occurs up and down the Tonle Sap
River, with upstream migration occurring during July to September and downstream
occurring during October to February. We have no information on fish migration in the
Bassac River.
Wastewater discharges from Phnom Penh to the Tonle Sap River could potentially interfere
with both the upstream and downstream migration of fish. At this stage, there is insufficient
water quality data to assess either of these conditions in detail. However, it seems likely that
the greatest risk would be during the dry season, and particularly around March and April,
when the greatest impacts on water quality would occur. The available information suggests
that minimal fish migration occurs at this time.
Water quality in Tonle Sap River is measured at the Phnom Penh Port site, located close to
the city. These data have been used to make a very preliminary assessment of the risk to fish
migration. However, it should be recognised that these data are totally insufficient to allow a
proper risk assessment. Deficiencies include: surface samples only are taken, there is no
sampling across the river or with depth, and no toxicant concentrations are measured.
The data summarised in Figure 6 suggest that the water quality in the Tonle Sap River near
Phnom Penh is slightly degraded (low DO, high COD concentrations). We also have
evidence that the dissolved oxygen concentration during the dry season is considerably
reduced (see Figure 3). These reductions in the surface water DO are unlikely to be
sufficient to cause interference to fish migration, based on the earlier defined DO effect
concentrations. However, it is possible that the DO concentrations at depth could be even
lower and that the situation at times is much worse than suggested by the monthly
monitoring data.
In summary, there are insufficient data to be able to assess the risk that wastewater
discharges from Phnom Penh are causing problems with fish migration in the Tonle Sap
River. However, the available data show that water quality in this river near Phnom Penh is
degraded, particularly during the dry season.
We recommend that a more comprehensive assessment of the risk to fish migration be
undertaken. Almost certainly, this will require a specific study designed to answer this
question. It will never be possible to satisfactorily assess this risk with water quality data
presently being collected.
Human health
Drinking water
There is little information available on the amount of water taken directly from the Mekong
River in downstream Vietnam for drinking. However, for this preliminary risk assessment it
has been assumed that both the Bassac and Mekong channels are used for drinking water. It
is not possible to complete a detailed risk assessment without more detailed information on
this potential use.
Microbial contamination of drinking water presents the greatest risk to human health
(NH&MRC/ARMCANZ, 1996), with the microorganisms of concern including bacteria,
protozoa, toxic algae and viruses. Most guidelines require that safe drinking waters contain
44
no coliform bacteria (including faecal coliform bacteria or E. coli) per 100 mL sample
(WHO, 1984; NH&MRC/ARMCANZ, 1996).
We have been unable to obtain any relevant bacterial water quality data for the Mekong
River in the time available. Therefore, it has not been possible to complete a quantitative
assessment of the risks to human health from drinking water from the river (without
treatment).
However, it is possible to provide a qualitative assessment of the transboundary risks of
human health problems in downstream Vietnam due to faecal contamination from Phnom
Penh. Our assessment is that the risk is likely to be low to moderate. It is expected that very
high bacterial levels will be present in wastewater from Phnom Penh since it receives
essentially no treatment. Two factors, the relative large dilution of any discharges from
Phnom Penh (estimated to be a minimum of ca. 800:1) and the likely dieoff would act to
reduce these bacterial levels during transport downstream to Vietnam (estimated to be
around 3-5 days during low flow periods),
It is recommended that MRC collate all available data relating to bacterial levels in the
Mekong River at Phnom Penh, Chau Doc and Tan Chau, so that a quantitative risk
assessment can be made. If there are insufficient data or these data are suspect, MRC
should commission a special study designed to specifically assess the risk the bacterial
contamination from Phnom Penh is adversely affecting people drinking Mekong water when
it enters Vietnam (specifically at Chau Doc and Tan Chau).
Recreational water
The Mekong River and its tributaries are used extensively for swimming, bathing and
washing. Many countries have developed water quality guidelines aimed at protecting such
protect primary contact activities (WHO, 2001; ANZECC/ARMCANZ, 2000). Most require
that such waters should be sufficiently free from faecal contamination, pathogenic organisms
and other hazards (e.g. poor visibility or toxic chemicals) to protect the health and safety of
the user.
The new Australian and New Zealand guidelines (ANZECC/ARMCANZ, 2000), for example,
recommend that the median bacterial content in samples of water taken over the bathing
season should not exceed:
•
150 faecal coliform organisms/100 mL (minimum of five samples taken at regular
intervals not exceeding one month, with four out of five samples containing less than
600 organisms/100 mL);
•
35 enterococci organisms/100 mL (maximum number in any one sample: 60–
100 organisms/100 mL).
Additionally, these guidelines recommend that recreational waters with temperatures in excess
of 24°C should also be free from pathogenic free-living protozoans.
We have been unable to obtain any relevant bacterial water quality data for the Mekong
River in the time available. Therefore, it has not been possible to complete a quantitative
assessment of the risks to human health from primary contact use of the river.
If MRC decide to go ahead with a more detailed risk assessment related to human health we
recommend that the most recent WHO document on Bathing Water Quality and Human
Health (WHO, 2001) be used for guidance on the risk-based approach that might be used.
45
6.2
Vientiane
Background
Vientiane has a population of 583,000 people distributed over an urban area of 3,920 km2
(Lao National Statistics Book, 2000).
Vientiane does not have an effective sewerage system, a fact of considerable relevance to this
study. All sewage and stormwater is collected either in small open concrete or dirt channels
that exist throughout most of the city, or in the newly-constructed combined stormwatersewage pipe network that now runs through the city center and along the main roads. This
pipe network connects to three main drainage channels that flow into the That LuangSalakham wetland complex (see Figure 5b - J. Lacoursiere, Univ Lund, pers. comm.).
Some of the sewage and stormwater which gathers in the large and deep channel now
running through Nong Chan wetland (behind the Morning Market/Bus station) is pumped to
a complex of six oxidation ponds built some years ago by the European Community. These
each have a treatment capacity of 30,000 equivalent persons. The output from these ponds
also flows to That Luang wetland (J. Lacoursiere, Univ Lund, pers. comm.).
That Luang is a large wetland system (ca. 13 km long) located to the east of Vientiane. It
now receives the entire wastewater load from the city since all direct outflows from the city
to the Mekong River ceased around 1990. Outflow from the wetland drains to the Mekong
via a 50 km long small river.
Since 1990, the size of the wetland has been reduced by drainage and agricultural expansion,
seriously affecting its capacity to treat the continually increasing wastewater load from
Vientiane (J. Lacoursiere, Univ Lund, pers. comm.). Additionally, the quality of the
wastewater entering the wetland has deteriorated over the years as the channels that
transport the wastewater to the wetland have had in-channel vegetation removed and been
concrete-lined17. During the dry season, the wastewater is almost pure sewage, but is more
diluted in the wet season.
We have estimated that the total volume of stormwater and sewage discharged annually to
the Mekong from Vientiane is around 120 million m3; the volume of stormwater is
approximately double that of sewage. This compares with an annual volume of ca. 140,000
million m3 transported by the Mekong River at Vientiane, giving an annual average dilution
of over 1000:1. Even during the dry season when flows in the Mekong are much lower (e.g.
mean flow at Vientiane in March = 1,190 m3/s), the dilution of the sewage is still around
800:1 (assuming most of the wastewater flow is sewage with little stormwater).
Conceptual model
To assess the risk of potential transboundary water quality issues arising because of pollution
from Vientiane, we have adopted the following conceptual model (see Figure 5b):
•
wastewater from Vientiane is discharged downstream of the city to the Mekong via a
wetland and passage through ca. 50 km of a small river. Thus, this wastewater receives
varying levels of treatment depending upon the flow. Unfortunately, we were not able to
obtain any detailed information on the quality of the wastewater from Vientiane in the
time available;
•
the urban regions in Vientiane have little paving, which means that the SPM loads will be
high, particularly during the wet season;
17
Apparently, the Water Quality Laboratory of the Lao Dept of Irrigation (Director: Mr Bon Suk) has
been monitoring the wastewater outflows since early 1990, but we have not been able to obtain any
information in the time available.
46
•
adverse effects due to this wastewater discharges may occur:
(a) in the immediate vicinity of the discharge (i.e. in the Mekong River downstream of
Vientiane),
(b) further downstream in both Thailand and Cambodia.
•
since the Mekong River between Vientiane and Khong Chiam (see Figure 1) is the border
between Lao PDR and Thailand, potential transboundary issues could presumably occur
both in the immediate vicinity of the discharge and further downstream;
•
contaminants discharged into the Mekong River by the city of Vientiane could also
potentially cause problems in Cambodia (see Figure 1).
Issues
As noted in Section 4.2, assessment to the possible transboundary water quality effects will
focus on three environmental values:
•
•
fish migration;
For the first value, ecosystem protection, we have attempted to assess the risk of adverse
changes occurring to three key biological components:
•
eutrophication or algal blooms;
•
toxicity to biota from either low dissolved oxygen concentrations or toxic compounds;
•
ecosystem processes (e.g. changes to light regime with consequent changes to primary
productivity).
The focus of this transboundary assessment is in the region in the immediate vicinity of the
wastewater discharge and further downstream.
Risk assessment
For Phnom Penh (Section 6.1), we were able to use two lines of evidence to provide
information for the assessment of each issue: (a) existing water quality data and comparison
with trigger values to discern any influence of Phnom Penh on the Delta region, and (b) a
comparison of the estimated loads of key contaminants discharged from Phnom Penh with
those transported “naturally” by the Mekong.
Unfortunately, the existing water quality data is of little use in assessing the risk in either the
immediate location of the wastewater discharge or further downstream. The location of
water quality sample locations – one site upstream (Vientiane, H011910) and one about 300
km downstream (Nakhom Phanom (H013101) - leads to a flawed “experimental design” and
one that has no statistical power to detect impacts due to Vientiane’s wastewater discharge.
To assess the transboundary risks from this wastewater discharge, it would be necessary for a
completely different experiment design to be developed, probably involving several sites
upstream coupled with several located downstream of the discharge point (see Section 7).
It has been possible to provide a qualitative assessment of risk using the comparative loads of
key contaminants likely to be discharged to the Mekong from Vientiane. Somewhat
arbitarily, we have assumed the following categories of risk of adverse impacts:
Low risk
If ratio of load from Vientiane to natural load
<5%
Moderate risk If ratio of load from Vientiane to natural load
5-50%
47
High risk
If ratio of load from Vientiane to natural load
The following points are also relevant in making the assessments below:
•
>50%
the distance from Vientiane to the Cambodian border is ca. 600 km;
•
the water temperature is high (ca. 21-34oC) meaning that organic matter discharged from
Vientiane will be relatively rapidly broken down.
Background comments on each of the environmental values made in the previous section will
not be repeated.
Eutrophication
As noted above, there were no statistically meaningful differences in any of the nutrient
concentrations between Vientiane and Nakhom Phanom (see Figure 6c).
Using the estimated nutrient loads discharged from Vientiane (see Table 1), we have been
able to make a qualitative assessment of possible adverse effects. On an annual basis,
Vientiane adds around 1-4% to the loads of Total-P and Total-N transported naturally by the
Mekong River18. Even during the dry season when flows in the Mekong are much lower
(e.g. mean flow at Vientiane in March = 1,190 m3/s), dilution of the wastewater would still
be around 800:1 (assuming most of the wastewater flow is sewage with little stormwater).
This suggests a low risk of transboundary nutrient (algal) problems from Vientiane
wastewater discharges.
Toxicity due to low dissolved oxygen concentrations
As with nutrients, there is inadequate DO data to be able to assess the possible risks from
low DO toxicity problems. However, given the very high dilution ratios (>800:1), it is
reasonable to predict that the transboundary risk from low dissolved oxygen concentrations
will be very low.
Toxicity due to excessive concentrations of heavy metal and organic contaminants
It was not possible to assess the risk to aquatic organisms due to possible toxic heavy metal
and organic contaminants released to the Mekong River from Vientiane, because insufficient
water quality information was available on toxicants. However, given the relatively small
amount of industrialisation in this city, it is reasonable to assume that very small loads of
toxicants are likely to be released.
If the MRC wishes to assess this risk it will be necessary to collect information on key
toxicants (See Section 7).
Ecosystem processes
There is no quantitative information on ecosystem processes occurring in the upper reaches
of the Mekong around Vientiane.
The MRC should investigate the possible introduction of innovative ecosystem process
measures for the Mekong River system.
Fish migration
An assessment of fish migration in the Mekong River by Poulsen & Valbo-Jorgensen (2000)
found that some fish migrate past Vientiane, the direction of travel depending upon the flow
conditions. It is possible, therefore, that wastewater discharges from Vientiane could
18
Tot-P – Vientiane contributes 300 t/y compared with Mekong’s 7,000 t/y; Tot-N (actually DIN used
in calculation) – Vientiane contributes 1,000 t/y compared with Mekong’s 70,000 t/y.
48
potentially interfere with both the upstream and downstream migration of fish in the Mekong
River.
As noted above, there is no water quality data available near the wastewater discharge point
and so it is not possible to assess the condition of the river at this point. This information
will need to be collected if MRC wishes to make an assessment of the risk to fish migration.
It is possible, however, to make a preliminary assessment that the risk is likely to be very
low, based on the high level of dilution (>800:1) of the wastewater even under dry season
lower flow conditions.
In summary, there are no water quality data available to undertake an assessment of the risk
that wastewater discharges from Vientiane are causing problems with fish migration in the
Mekong River. However, on the basis of the very high dilution of this wastewater on
entering the river, it seems likely that this risk would be very low. A comprehensive
assessment of the risk to fish migration would require a specific study designed to answer
this question. It will never be possible to satisfactorily assess this risk with water quality data
presently being collected.
Human health
Drinking water
Little information is available on the amount of water taken directly for drinking from the
Mekong River in the vicinity and downstream of Vientiane’s wastewater discharge.
Additionally, we were unable to obtain any relevant bacterial water quality data for the
Mekong River in this region in the time available.
Therefore, it has not been possible to complete a quantitative assessment of the risks to
human health from drinking water from the river (without treatment).
It is recommended that MRC commission a survey of the behaviour of wastewater
discharges from Vientiane and the possible human health (and ecological) risks in the
vicinity and downstream of the discharge point.
Recreational water
We have assumed that the Mekong River in the vicinity of the wastewater discharge is used
for swimming, bathing and washing. Unfortunately, we were unable to obtain any relevant
bacterial water quality data for the Mekong River in this region in the time available.
Therefore, it has not been possible to complete a quantitative assessment of the risks to
human health from recreational use of the river. See above for recommendation.
6.3
Water quality in the Mekong Delta
Background
The MRC required that this report “assess the degree to which degraded water in Mekong
Delta can be attributed to transboundary transport of poor quality water from upstream”.
The Mekong Delta region of Vietnam is intensively used for agriculture, and has a widespread and well-recognised problem with acid sulfate soils (Minh et al., 1997; MRC, 1997).
Over 40% of the region has these acid sulfate soils, which are of marine origin and contain
high levels of pyrite (FeS2). During the dry season, the soils dry out and crack, which allows
air to penetrate and the pyrite to oxidise. The products of this oxidation, i.e. low pH water
and high concentrations of iron, aluminium and sulfate, are then leached from the soil profile
with the first heavy rains of the wet season (Tin & Willander, 1995), and the acidic and
metal-polluted water enters the irrigation canals and subsequently causes significant
detrimental effect on crops.
49
An additional water quality issue in the delta region occurs during the dry season when low
flows in the Mekong allow high salinity water to penetrate further up the rivers, and prevent
irrigation. There is considerable concern that the possible construction of hydroelectricity
and irrigation dams further up the Mekong could increase this saline intrusion, and adversely
affect even larger areas of irrigated agriculture (MRC, 1997).
This section is concerned with assessing the risk from a possible third water quality issue,
that caused by degraded upstream water reaching the Delta.
Conceptual model
To assess the risk of potential transboundary water quality see (Figure 5c):
•
wastewater discharged from Phnom Penh, mainly via the Bassac River, could potentially
cause water quality problems in the Delta region;
•
agricultural activities in the Mekong catchment upstream of Phnom Penh are unlikely to
influence the quality of the river sufficiently to result in any impacts in the Delta region;
•
however, catchment activities that influenced the Mekong’s flow (e.g. building reservoirs
for hydropower generation and irrigation) could have a major impact on water quantity
and quality in the Delta region;
•
previous studies have shown that serious pollution (acid sulphate soils) is occurring within
the intensive agricultural areas of the Delta region (although this is not a transboundary
issue).
Issues
Assessment of the possible transboundary water quality effects in the Delta region will focus
on three environmental values:
•
•
human health (drinking water & recreational use);
• agricultural use of the water.
Risk assessment
Assessment of risks to the ecology in the Delta caused by degraded upstream water is
equivalent to the assessment that has already been undertaken for Phnom Penh (see Section
6.1). This assessment showed that, despite a deficiency of good data, the discharge of
wastewater from Phnom Penh would most likely result in a low risk of adverse ecological
effects in the Delta region.
Human health
The assessment of risks to human health (drinking water, recreation) in the Delta caused by
degraded upstream water has already been undertaken in Section 6.1. This assessment
showed that, despite a deficiency of good data, the discharge of wastewater from Phnom
Penh would most likely result in a low risk of human health problems.
Agriculture
The main agricultural use of water in the delta region is for irrigation of rice fields. Problems
could arise for this agricultural use if the water from upstream was polluted, the most likely
problem being increases in salinity (conductivity). The long-term water quality data (see
Figure 6a) indicate that the Mekong is low in conductivity and shows no trend towards an
increase in conductivity towards the Delta region.
50
The risk to agricultural water quality from upstream activities is assessed as being very low.
This of course is not the case for pollution from within the Delta region (see below).
The other water quality indicator that could feasibly affect agricultural production in the
Delta region over the long term is the amount of sediment that is deposited on the delta
floodplains. This annual replenishment is needed to continue the sustained high productivity
of these areas. The data available (e.g. SPM loads) allows calculation of the loads of
material transported to the floodplain annually, but not of the amount deposited with each
flow event. Equally, the data available are not good enough to indicate whether the loads of
SPM transported to the Mekong Delta have increased or decreased over the years.
This preliminary assessment indicates that the risk of transboundary water quality issues in
the Delta region due to degraded upstream water (particularly wastewater discharges from
Phnom Penh) is low.
Other issues
As indicated above, there are water quality problems in the agricultural areas of the Mekong
Delta. However, these arise more from issues within the Delta (e.g. acid sulphate soils) than
from upstream.
Also, as noted above in the conceptual model for the Delta region, changes to the Mekong
flows such as could occur by the building of reservoirs for hydropower generation and
irrigation as has been planned (MRC, 1997), could result in a number of problems. These
would include a major impact on the flooding/drying cycle, with consequent increase in
acidity and aluminium toxicity, and on the ingress of saline water from the South China Sea
(MRC, 1997; Joy et al., 1999).
7.
51
Monitoring network required to assess transboundary
issues
7.1
Assessment tools
The tools for risk assessment are a little different from those of ordinary scientific methods,
such as null-hypothesis testing, because natural resource management concerns itself with the
costs of two kinds of error (Table 5). First, decision-makers are keen to avoid declaring
there is an impact when there is none. Second, they are also keen to avoid declaring a
proposal is safe when, in fact, it leads to an unacceptable environmental impact. Typically,
the judgement is based on a statistical test of a null-hypothesis of no impact, which focuses
on just one of these errors, the Type I error rate which, by convention, is set at 0.05.
However, a problem with this approach is that it fails to account for the probability of the
other kind of error, a false negative or Type II error (Mapstone, 1995; Johnson, 1999).
Quantitative ecological risk assessments seek to make an explicit treatment of both kinds of
errors. These include confusion matrices (Table 5 is an example) in which the false positive
and false negative rates are specified explicitly. These tables can then be generalised into
receiver operator curves (Swets et al., 2000) that may be used to evaluate the consequences
of adjusting decision thresholds, and to make risk-weighted decisions that account for the
relative costs of false positive and false negative outcomes. Unfortunately, it is not possible
at this stage to use these tools in assessing the risk of adverse ecological effects in the
Mekong River because of a lack of relevant data.
Table 5
Logic of environmental decisions. Monitoring programs are generally
established to conclude that projects or activities have or have not had an
impact.
True response
Positive result
(impact occurs)
Negative result
(no impact occurs)
Measured response (outcome of test)
Positive result
Correct decision
Negative result
Wrong decision
Probability of reaching correct
decision = 1- α
Type II error
Wrong decision
Correct decision
Type I error
Probability of reaching correct
decision = 1- β
Error risk = α (significance level)
Error risk = β
7.2
Monitoring network design
Many water quality monitoring networks are poorly designed and have little statistical power
to detect likely changes in the indicators that are measured. Common deficiencies include a
lack of a clear objective(s) and, for variable systems, insufficient sample sites and sample
numbers to detect the expected changes.
A well-designed monitoring program should be able to answer the specific question for
which it is established. For example, below we have sought to test the effectiveness of the
present monitoring network around Phnom Penh to answer the question “is there an impact
(25% change) on downstream water quality due to wastewater discharged from Phnom
Penh”?
52
The most sensible design to answer the question of interest to the MRC (are there
unacceptable transboundary impacts due to wastewater discharges from Vientiane and
Phnom Penh?) would have sample locations upstream and downstream (U/D) of the
discharge point(s). For detailed discussions on experimental design the reader is referred to
Mapstone (1995), Underwood (2000) and Quinn & Keough (2001).
Assessment of the present water quality network near Vientiane clearly shows that it is
unable to detect any downstream changes due to wastewater discharges from Vientiane.
There is one sample point upstream at Vientiane and one 300 km downstream, by which time
any changes would have been diluted out or other changes would have occurred to mask any
effects from Vientiane.
The present network is a little better designed to detect changes due to Phnom Penh. For
example, there are 3 sample sites upstream of Phnom Penh (Kratie, Kompong Cham, Phnom
Penh) and potentially 5 sites downstream (Neak Luong & Tan Chau on the Mekong, Ta
Khmao, Koh Khel & Chau Doc on the Bassac). In designs such as this, the monitoring sites
are effectively the replicates for testing the effect of Phnom Penh (Keough & Mapstone,
1995). Hence, the number of sites has the greatest effect on the capacity of the monitoring
program to detect an effect due to the city, and there would be little advantage in increasing
the number of samples taken at each site.
Power analyses of the test of the effect of Phnom Penh on conductivity, SPM and Total-P are
presented in Table 6. We have used data from 3 sites upstream of Phnom Penh and 3 sites in
the Bassac downstream to estimate the variation seen among sites. For this analysis it was
assumed that the Bassac River transports most of the wastewater from Phnom Penh.
Table 6
Analysis of the power of the present water quality monitoring network to detect
changes of greater that 25% in conductivity, SPM and Total-P concentrations due
to wastewater inputs from Phnom Penh.
Indicator
Conductivity
SPM
Tot-P
No.
upstream
sites
Mean
upstream
No.
downstream
sites
Mean
downstream
Power
(1-β)
No. samples
required for
0.80 power
3
3
3
14.9
109
26
3
3
3
11.2
82
20
0.64
0.25
0.06
8
20
618
Two types of results are presented. First, the power of the current design to detect a 25%
change in each of the three indicators is presented. Statistical power is the chance of
detecting the specified effect given the design, and is calculated as 1 minus the Type II error
rate (1-β). Second, the total number of sites that would be required to achieve a statistical
power of 80% is presented. Implicit in this latter calculation is that an equal number of sites
would exist upstream and downstream of the city. The calculations were performed using
the shareware computer program G-Power (Faul & Erdfelder, 1992). The Type I error rate
(α) was set at 0.05 for all calculations.
The analyses (Table 6) show that for conductivity there is a 64% chance that the specified
effect will be detected (i.e. that a change of >25% will not be missed). For SPM, the power
is considerably less with only a 25% probability that such changes will be detected, or
conversely a 75% chance that these changes will be missed. For Total-P, the sample design
has essentially no power (6%) to detect changes of 25% in this indicator.
53
In order to achieve 80% statistical power for conductivity, SPM and Total-P, the respective
total number of sites that would be required are 8, 20 and 618. In other word, to ensure an
80% probability that a 25% change in the Total-P concentrations between upstream and
downstream locations was detected, a total of 618 sites (309 upstream and 309 downstream)
would be needed. The differences in the number of sample sites required for the different
indicators are the result of the different variability of these indicators. In this example,
conductivity had the least inter-site variation, while Total-P had the greatest.
It is clear from the above statistical analysis that the present water quality network has very
little power to detect relatively large (25% or more) changes in water quality indicators
(with the possible exception of conductivity) due to the discharge of wastewater from either
Vientiane or Phnom Penh.
7.3
Towards a new environmental assessment program
A key management objective of the MRC is to ensure that the environmental values of the
Mekong River are not degraded by activities in upstream countries (i.e. that unacceptable
transboundary issues do not arise). This report recommends that the Mekong River
Commission should manage this river system to protect the following environmental values:
•
•
native fisheries production;
•
drinking water;
•
irrigation (mainly for rice);
•
aquaculture;
• recreation and aesthetics.
This report has focused on three key environmental values – riverine and floodplain
ecosystem protection, fish migration and human health – in the belief that if these values are
adequately protected so will the others.
A monitoring program that concentrates solely on physico-chemical indicators (even if it
were better designed) is inadequate for assessing the quality of aquatic ecosystems and the
quality of water for human uses. Clearly, the MRC should consider developing and
implementing a new environmental assessment program.
Approach
A possible approach to develop a new environmental assessment program focused on
transboundary issues would involve the following steps:
•
define all transboundary issues (this report has covered three such issues, but there are
more) – this task must be done in collaboration with member countries;
•
for each transboundary issue (or those given top priority), develop a conceptual model
linking the stressors or drivers and the environmental and human health effects;
•
using these conceptual models, design a statistically robust investigation and monitoring
program to determine each effect (this may require that preliminary investigations be
carried out to provide relevant information to assist in designing the monitoring
program);
•
put in place a regular (3-5 yearly) review of each program to ensure its effectiveness;
•
initiate a targeted training program to ensure each member country has trained personal
to carry out the required monitoring and investigation tasks.
54
What to measure?
The indicators selected to provide information on a particular transboundary issue should
reflect as directly as possible the effect. For example, if the effect being investigated is
eutrophication then direct measures of algae (species composition, biomass, primary
production) are preferred to measuring surrogates such as nutrient concentrations. Equally,
if the objective is to protect ecosystem “health”, then direct measures of key biota (e.g.
macroinvertebrates, fish, algae, bacteria) or ecosystem processes (metabolism, gross primary
production, respiration) should be targeted. That is, measures of both ecosystem structure
and processes should be developed.
An objective method for selecting ecological health indicators that could potentially be used
for the Mekong is the recently completed Stage 3 of the Southeast Queensland Regional
Water Quality Management Strategy (Smith & Storey, 2001). They tested each of the
potential indicators for their sensitivity to a disturbance gradient, in this case to a land use
gradient that ranged from forested, through grazing, cropping, horticulture and urban. Those
indicators that were sensitive to the disturbance gradient were kept and the others were
discarded.
A preliminary list of appropriate indicators for assessing transboundary issues in the Mekong
River basin would include:
•
daily flow;
•
physico-chemical indicators (conductivity, SPM, pH, alkalinity, nutrients (TP, FRP, TN,
NOx-N, NH4-N), DO, TOC, DOC);
•
biological (it would be desirable if MRC initiated a project to determine possible
biological indicators – this should be done in close collaboration with member countries);
•
toxicants (a possible cost-effective program would be to monitor fish tissue for
pesticides, say once every 2 years);
•
monitoring both the flow and quality of wastewater discharges from Phnom Penh and
Vientiane.
This list of indicators should be further developed once the full list of transboundary issues
has been determined.
Where to measure?
The main channel of the Mekong River has been the focus of this preliminary risk assessment
of the three transboundary issues. However, if the MRC is to establish a new environmental
assessment program concerned with a broader range of basin-wide and transboundary issues,
it is likely that other waterways (particularly floodplain and wetlands) will need to be
included, as well as specific activities within the catchment.
Typical study design
In this section, we provide an example of a possible network design that could be introduced
to investigate the downstream effects of wastewater discharges from Phnom Penh and
Vientiane. While these experimental designs focus on investigating physico-chemical
indicators, the principles would also be applicable for other indicators (e.g. biological,
microbiological).
Phnom Penh
For Phnom Penh the question to be tested is: Is there a measurable effect of Phnom Penh
wastewater in the Bassac River and does this effect continue to the Vietnam border?
55
The design we favour would have sites nested within three locations of an
Upstream/Downstream (UD) factor. The locations would be (a) upstream of the city (US),
(b) downstream of the city (DS), and (c) at the border (B) (Figure 11). The sites should be
located relatively close to the area of interest to prevent effects of spurious influences,
perhaps within 20 km of each other for each of the three locations.
Figure 11 shows a possible monitoring program with 6 sites within each location of the UD
design. The above power analysis suggests that this would be the likely numbers of sites
required (at least for conductivity and SPM). It is the number of sites sampled that largely
controls the power of the analyses. Taking more than 12 samples per year, and more than
one sample each sampling time, could reduce the variance among sites within the same
location of the UD design, and increase the power, but only slightly.
Figure 11
Sample design to test effect of wastewater discharged from Phnom Penh
on the Bassac River and downstream at the Vietnam Border.
Two planned comparisons would then be performed. First, a test of US v DS would establish
whether the city was having an effect. Second, a follow-up test of US v B would establish
whether there was an effect at the border. A significant result for both comparisons would be
explained by an effect of the city that persists to the border19. The logical progression of the
statistical tests is shown below.
19
This latter could also be explained by a new effect near the border after the other one had dissipated.
Significant
56
Significant
Persistent
effect of city
Non-Significant
Non-persistent
effect of city
US v B
US v DS
Significant
Non-Significant
US & DS v B
Non-Significant
Effect occurring
after city
No effect
Vientiane
The issues surrounding potential pollution of the Mekong by Vientiane are slightly different
to those around Phnom Penh, but the questions could be answered with a similar sampling
design. Because the Mekong near Vientiane is the border between Thailand and Lao PDR,
any effect of that city on the river would be transborder pollution. A reasonable expectation
might be that any effects of Vientiane’s wastewater discharge are non-detectable a certain
distance downstream of the discharge point20. This distance would not be easily defined (as
was the case with the border of Cambodia and Vietnam), but would have to be based on
expert opinions and negotiations between governments.
If this is acceptable (i.e. that effects from Vientiane’s wastewater should not be detectable x
km downstream from the discharge point), a design fundamentally identical to that
recommended above for Phnom Penh could be employed. Groups of sites (generally around
6) would be located immediately upstream of the city, immediately downstream of the
wastewater discharge point, and at the agreed “no effect” distance downstream. The
numbers of sites required would probably be the same, and the analysis would proceed via
the same design and logical progression outlined above.
We have noted above that the magnitude of flows in the Mekong River is such that there is a
low risk of major water quality effects due to Vientiane. Hence, we recommend that the
MRC first seek to answer the question: “Is there any effect of the city immediately
downstream of the discharge point?”, before progressing to examine sites further
downstream to look for a reduction of this “effect”. Such a preliminary study would require
only two thirds of the sites (US and DS only). If an effect of the city were noted, the extra
sites (B) would then be set up, and the experiment undertaken again.
7.4
Recommendations
We recommend that MRC develop a new and more robust environmental assessment
program designed to identify and assess the risks from a broader range of current and future
transboundary and basin-wide issues. The process to achieve this new assessment program
should be undertaken in collaboration with the member countries and would involve:
•
using the (ecological) risk assessment technique to underpin this process, with the first
task being to scope the full range of existing and possible future transboundary issues
(and their priority);
•
running a number of workshops (involving each country) to develop the conceptual
models and decide upon the target areas, the assessment endpoints and the best indicators
to measure;
20
At this stage we have no information on the behaviour of the wastewater plume when it enters the
Mekong. This will need to be established before a sensible experimental design can be established.
57
•
undertaking a program of short-term, targeted investigations to provide essential
information on specific aspects of the system that will enable the main program to be
better designed;
•
preparing a full program proposal, obtaining funding and implementing the program.
8.
58
Conclusions & Recommendations
Deteriorating water quality in the lower Mekong River basin has been identified as a priority
transboundary issue by each of the four member countries. Three particular transboundary
water quality issues are considered in this report:
1. The potential effects of municipal and industrial wastewater from Phnom Penh on both
downstream Vietnam and fish migration in Tonle Sap River.
2. The potential effects of municipal and industrial wastewater from Vientiane on both
neighbouring Thailand and fish migration in the Mekong River.
3. The influence of upstream water on the degraded water quality in the Mekong Delta.
A risk assessment framework has been used to assess these issues. In particular, the risk of
adverse effects on three key values of the Mekong River – ecosystem health (characterised by
eutrophication, toxicity due to dissolved oxygen and toxicants, and ecosystem processes), fish
migration and human health (drinking, recreation) - have been assessed. The risk to irrigation
water quality was also assessed for the third transboundary issue above.
Risk assessment is concerned with estimating the likelihood or probability of an undesired
event occurring and the consequences if that event does occur. The risk assessment process
seeks to:
•
identify the key (ecological) issues and key stressors;
•
(conceptual model or quantitative ecological model), and from this provide information
on which drivers are most sensitive to management or controls;
•
assess the risks associated with each issue as quantitatively as possible (it is important
here to identify measurable end points for each issue);
•
decide on the confidence that should be placed on the final assessment;
•
assist in establishing a robust monitoring & assessment program;
• identify the key knowledge gaps.
Unfortunately, the data currently available is inadequate to fully assess the risk of
transboundary water quality issues in the Mekong River basin. Assessment of the current
database identified the following deficiencies:
•
Physico-chemical data – the Mekong water quality network is similar to many other such
networks around the world in that it is collecting inadequate data. For example, many of
the indicators currently being measured are inappropriate and should be replaced with
more appropriate indicators. Additionally, samples are being collecting at inappropriate
site locations and, for a number of indicators, at inadequate frequency. In all cases, the
sampling design was such that there was essentially no statistical power in the data to
detect any significant transboundary changes.
•
Toxicant data – the pesticide and heavy metal data were either non-existent or
insufficient to be used to assess transboundary or basin-wide toxicity issues.
•
Biological data – there is no on-going biological monitoring program for the Mekong
River. In the time available we were able to access only a small amount of biological data
relevant to the Mekong, which would include fish, macroinvertebrates, algae and
ecosystem processes. Efforts should be made to collect all published and unpublished
information on the biology and ecology of the Mekong River and its tributaries, and to
prepare a synthesis of this information that summarises current knowledge in this area.
59
•
Urban contaminant loads – the loads of contaminants discharged from the urban centres
of Vientiane and Phnom Penh are poorly known. We estimated likely loads in order to
make a preliminary assessment of the transboundary risks due to discharges. To improve
on the very preliminary risk assessment reported here, a more detailed understanding of
the wastewater systems in each city needs to be developed, and both the quantity and
quality of the wastewater discharges needs to be determined.
A summary of the preliminary risk assessment for the three transboundary water quality
issues is given in the table below. The present risks are low in all cases where they could be
assessed. It was possible to undertake a reasonably quantitative assessment to assess the
risks from eutrophication and the adverse effects of low dissolved oxygen concentrations
caused by wastewater discharges from Phnom Penh. However, for the other effects we were
forced to assess risk on the basis of either a comparison of the loads of contaminants
discharged from Phnom Penh and Vientiane with those transported “naturally” by the
Mekong River, or the degree of dilution achieved on discharge of the wastewater.
Issue
Effect
Ecological
Eutrophication
2
Toxic effects
Ecosystem function
4
Fish migration
3
Algal blooms
To be determined
Adverse effects on fish
movement upstream,
downstream or onto
floodplains
Issue 1
(Phnom Penh)
Issue 2
(Vientiane)
Issue 3
(Delta)
Low-moderate risk1
Low risk
Low risk
Low risk
Low risk
Low risk
Not assessed
Not assessed
Not assessed
Uncertain
Uncertain, likely
to be low risk
Uncertain
Uncertain
Uncertain
Uncertain
Uncertain
Uncertain
Uncertain
Human health5
Recreation
Microbial
sickness
Agriculture
Irrigation
Increased salinity
Drinking water
Low risk
1. More likely low risk since only nutrient concentration were used in the assessment; high turbidity and high flow
would also reduce the chance of algal problems.
2. Risks based on toxic effects due to low dissolved oxygen concentrations. It was not possible to assess toxicity due to
toxicants (heavy metals, pesticides) because of the lack of data.
3. No information is available at present, but should be developed in the future.
4. Lack of data to make assessment, present water quality sampling network cannot provide the required information.
5. Lack of data to make assessment. Risk likely to be low-moderate due to large dilution (also expect significant
microbial die-off during transport to Vietnam in case of Issues 1 & 3).
Another potential transboundary issue not covered in the objectives of this report, but which
appears to require assessment, is the apparent higher salinity (conductivity) in the river Nam
Mun that drains the extensive agricultural region of northern Thailand.
While the assessment reported herein indicates that the transboundary risks due to water
quality are low, this is not the case for local effects. For Phnom Penh in particular, our
preliminary assessment suggests that there are moderate to high risks of adverse ecological
and human health effects in Chaktomuk, Tonle Sap River and the upper reaches of the
60
Bassac River. The relevant Cambodian authorities may wish to further investigate this
situation.
Analysis of the present water quality monitoring network showed that it is unable to detect
any transboundary changes due to the discharge of wastewater from either Vientiane or
Phnom Penh. The network design has insufficient statistical power to detect realistic changes
in physico-chemical water quality. Additionally, since no biological indicators are measured,
there is no possibility of detecting transboundary or basin-wide changes in ecosystem health.
Recommendations
1:
that MRC adopt the risk-based approach to assess transboundary and basin-wide
environmental and human health issues, and to prioritise the management actions
required to reduce the risk due to each important issue.
2:
that the current review of the physico-chemical monitoring network consider in
particular the optimum location of sampling sites, the frequency of sampling, the need
for depth sampling in some cases, the indicators being analysed and the power of the
data collected to detect changes.
3:
that MRC undertake a preliminary risk assessment to identify possible transboundary
or basin-wide toxicity or bioaccumulation problems due to organic contaminants
and/or heavy metals, and if problems are identified, the type of investigations
(including monitoring) should be undertaken to better characterise the risk.
4:
that the MRC establish a project to assess the feasibility of establishing a biological
monitoring program for the Mekong River basin. The following biota should be
considered – fish, macroinvertebrates, algae and macrophytes.
5:
that MRC collect all published and unpublished information on the biology and
ecology of the Mekong River and its tributaries, and prepare a synthesis of this
information that summarises current knowledge in this area.
6:
that MRC obtain a more detailed understanding of the wastewater systems (including
information on the quantity and quality of the wastewater discharges) in the two
major urban centres – Vientiane and Phnom Penh.
that MRC establish a project to undertake a more detailed assessment of the
transboundary ecological and human health risks due to the discharge of wastewater
from both Phnom Penh and Vientiane. Such a project would provide an ideal
opportunity to “train” relevant National Mekong Committee members in the risk
assessment methodology.
that MRC establish a project to investigate the key ecological processes occurring in
the Mekong River basin, including those associated with deep pools in the Mekong
River mainstream. The objective of this project should be to develop a number of
sensitive ecosystem process indicators that can be used to assess the ecological
“health” of the Mekong River.
that MRC develop a new and more robust environmental assessment program
designed to identify and assess the risks from a broader range of current and future
transboundary and basin-wide issues. The process to achieve this new assessment
program should be done in collaboration with the member countries and would
involve:
7:
8:
9:
61
•
using the (ecological) risk assessment process to underpin the process, with the
first task being to scope the full range of existing and possible future
transboundary issues (and their priority);
•
running a number of workshops (involving each country) to develop the conceptual
models and decide upon the target areas, the assessment endpoints and the best
indicators to measure;
•
undertaking a program of short-term, targeted investigations to provide essential
information on specific aspects of the system that will enable the main program to
be better designed;
•
preparing a full program proposal, obtaining funding and implementing the
program.
9.
62
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USEPA (2000). Ambient Aquatic Life Water Quality Criteria for Dissolved Oxygen
(Saltwater): Cape Cod to Cape Hatteras, Office of Water, US Environmental Protection
Agency, Washington DC, pp 49.
WHO (1984). Health Criteria and Supporting Information, Vol. 2 Guidelines for Drinking
Water Quality, World Health Organisation, Geneva.
WHO (2001). Bathing Water Quality and Human Health, Report WHO/SDE/WSH/01.2,
World Health Organization, Geneva, pp 58.
Wolanski, E., Nguyen Ngoc, H., Le Trong, D., Nguyen Huu, N. and Nguyen Ngoc, T.
(1996). Fine-sediment dynamics in the Mekong River Estuary, Vietnam, Estuarine,
Coastal and Shelf Science 43: 565-582.
Wright, J. F., Sutcliffe, D. W. and Furse, M. T. (2000). Assessing the Biological Quality of
Fresh Waters, Freshwater Biological Association, Cumbria, UK.
Appendix A:
Stormwater and Wastewater Pollutant Load Estimates for Phnom
Penh and Vientiane
Associate Professor Tony H F Wong
Department of Civil Engineering
Monash University, Melbourne, Australia
Introduction
Estimates of the mean annual pollutant loads of TSS, TP, TN and BOD derived from
stormwater and wastewater were made for the cities of Phnom Penh and Vientiane.
Pollutant loads generated from urban stormwater runoff were estimated using a continuous
simulation model operating in a daily time step using daily rainfall records for the two cities
(Pochentong – 1985 to 1994; Vientiane – 1990). In addition to the utilisation of local data,
representative daily rainfall distribution for the two cities were selected from reference cities
in Australia (for which long term daily rainfall records are available) and adjusted for their
differences in mean annual rainfalls.
Local rainfall and the adjusted “Australian Reference City” long term rainfall records were
then applied to the urban stormwater quality model MUSIC (Model for Urban Stormwater
Improvement Conceptualisation). The log-normal probability distributions of Event Mean
Concentrations (EMC) of TSS, TP, TN and BOD derived from analysis of world data were
adopted in MUSIC and corresponding EMCs generated stochastically for each daily storm
event.
Pollutant loads generated from wastewater were estimated from published data normalised to
population.
Wastewater
Wastewater contributions to the pollutant load discharge from the cities of Phnom Penh and
Vientiane were estimated from published data1 on loads per capita per day as follows:
•
TSS = 700 mg/L/person with a range of 300 to 1200
•
TP = 12 mg/L/person with a range of 5 to 20
•
TN = 40 mg/L/person with a range of 15 to 90
•
BOD = 250 mg/L/person with a range of 100 to 40
The expected volume of wastewater generated daily per capital in Australian cities is between
150 to 200 L/person/day and corresponding figure for USA cities is approximately
300 L/person/day. For Phnom Penh and Vientiane, a wastewater rate of 200 L/person/day is
recommended in this report.
1
Sundstrom & Klei (1979), Wastewater Treatment, Prentice-Hall, NY.
1
Stormwater
Pollutant Loads
TSS, TP, TN and BOD concentrations in stormwater used in the estimation of stormwater
pollutant loads from the Phnom Penh and Vientiane were based on a comprehensive review of
stormwater quality in urban catchments undertaken by Duncan (1999)2. Analysis by Duncan
(1999) found event mean concentrations of these water quality constituents to be
approximately log-normally distributed.
Table 1 shows the collated Event Mean
Concentrations (EMC) for a range of stormwater quality constituents and their standard
deviations (in log domain).
Table 1:
Mean ± one standard deviation EMC values from analyses of worldwide data
by Duncan (1999) – analyses carried out in the log domain
Water quality parameter
Unit Number of data points and mean EMC (and lower
and upper value of first standard deviation EMC)
All data
150 (51 – 460)
Rainfall < 550 mm
Total Suspended Solids (TSS)
mg/L 247
19
420 (130 – 1300)
Total Phosphorus (TP)
mg/L 206 0.35 (0.15 – 0.84) 13
0.61 (0.28 – 1.3)
Total Nitrogen (TN)
mg/L 139
2.6 (1.4 – 5.1)
13
4.8 (3.2 – 7.4)
Chemical Oxygen Demand (COD)
mg/L 165
80 (36 – 180)
16
170 (92 – 300)
Biological Oxygen Demand (BOD)
mg/L 127
14 (7.2 – 26)
Rainfall Characteristics
Monthly rainfall records for Phnom Penh (1985 to 1993) and Vientiane (1950 to 2000) were
available and the computed mean annual rainfalls in Phnom Penh and Vientiane are 1301 mm
and 1635 mm respectively. The mean annual rainfall in Vientiane corresponding to the period
1985 to 1993 (ie. period of record for Phnom Penh) was 1444 mm (compared to the long-term
mean annual rainfall of 1635 mm) suggesting that the long-term mean annual rainfall for
Phnom Penh, estimated from the 1985 to 1993 record, may be under-estimated by 10% to
15%.
As indicated above, daily rainfall records (Pochentong) for a 10 year period between 1985 to
1994 (mean annual rainfall of 1295 mm) were available for pollutant load estimation from
Phnom Penh. For Vientiane, daily rainfall for 1990 alone, corresponding to an annual total of
1498 mm, was available for pollutant loading export modelling.
2
Duncan, H.P. (1999), Urban Stormwater Quality: A Statistical Overview, Report 99/3, Cooperative
Research Centre for Catchment Hydrology, February 1999.
2
Comparisons of the mean monthly rainfall distributions for Phnom Penh and Vientiane with
that for capital cities in Australia found the most appropriate reference cities to be Darwin and
Perth respectively. Figures 1 and 2 show the normalised monthly rainfall distributions
derived.
Figure 1:
Cumulative distribution of Mean Monthly Rainfall (Vientiane Vs Perth)
Figure 2: Cumulative distribution of Mean Monthly Rainfall (Phnom Vs “Translated”
Darwin)
3
Generated Stormwater Pollutant Loads
Continuous simulations were undertaken to generated urban stormwater runoff and associated
pollutant loads for a 1 km2 representative urban catchment with an assumed fraction
imperviousness of 0.6. This is considered typical of Asian cities.
A stochastic routine was used to assign stormwater pollutant EMCs for each storm event such
that the average and standard deviation of the EMCs over the simulation period is similar to
that listed in Table 1. The results from the use of local rainfall data as well as the “Australian
representative cities” were found to be of similar magnitude and a single set of mean annual
loads was considered sufficient to represent both cities. Table 2 listed the mean annual
pollutant loads expected from a 1 km2 catchment. Figures 3, 4, 5 and 6 show the cumulative
frequency distributions of daily pollutant loads estimated.
The estimated mean annual stormwater runoff volume for a 1 km2 urban catchment in
Vientiane and Phnom Penh approximately 1250 ML/km2/yr.
Table 2:
Estimated Pollutant Loads from Urban Stormwater
Water Quality Constituents
Mean Annual Load (kg/km2/yr)
TSS
300,000
TP
605
TN
3,900
BOD
24,000
4
Figure 3:
Cumulative Probability Curve of Generated TSS daily load (kg/km2)
Figure 4:
Cumulative Probability Curve of Generated TP daily load (kg/km2)
5
Figure 5:
Cumulative Probability Curve of Generated TN daily load (kg/km2)
Figure 6:
Cumulative Probability Curve of Generated BOD daily load (kg/km2)
6

Transboundary Water Quality Issues in the Mekong

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