Effective management for acidic pollution in the canal network of the Mekong Delta of Vietnam: A modeling approach

Journal of Environmental Management 140 (2014) 14e25

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Journal of Environmental Management
journal homepage: www.elsevier.com/locate/jenvman

Effective management for acidic pollution in the canal network
of the Mekong Delta of Vietnam: A modeling approach
Ngo Dang Phong a, c, *, Chu Thai Hoanh b, To Phuc Tuong a, Hector Malano d
a

International Rice Research Institute (IRRI), Los Baños, Philippines
International Water Management Institute (IWMI), Regional Office for Southeast Asia, Lao PDR, Laos
c
University of Agriculture and Forestry, Ho Chi Minh City, Viet Nam
d
Melbourne University, Victoria, Australia
b

a r t i c l e i n f o

a b s t r a c t

Article history:
Received 24 June 2013
Received in revised form
22 October 2013
Accepted 3 November 2013
Available online 12 April 2014

Acidic pollution can cause severe environmental consequences annually in coastal areas overlain with
acid sulfate soils (ASS). A water quality model was used as an analytical tool for exploring the effects of
water management options and other interventions on acidic pollution and salinity in Bac Lieu, a coastal
province of the Mekong Delta. Fifty eight percent of the provincial area is covered by ASS, and more than
three-fourths (approximately 175,000 ha) are used for brackish-water shrimp culture. Simulations of acid
water propagation in the canal network indicate that the combination of opening the two main sluices
along the East Sea of the study area at high tide for one day every week in May and June and widening
the canals that connect these sluices to the West Sea allows for adequate saline water intake and
minimizes the acidic pollution in the study area. On the other hand, canal dredging in the freshwater ASS
area should be done properly as it can create severe acidic pollution.
Ó 2014 Elsevier Ltd. All rights reserved.

Keywords:
Dredging
Salinity
Acidity
Tide
Pollution
Water management
Sluice operation
Coastal acid sulfate soil

1. Introduction
Millions of people living in tidal ecosystems of coastal zones,
especially in South and Southeast Asia, are among the poorest and
most food-insecure because agricultural production is hindered by
seawater intrusion during the dry season. Many of these coastal
zones are also overlain by acid sulfate soils (ASS). Worldwide, about
13 million ha of coastal ASS are located in Asia, Africa and Latin
America (Brinkman, 1982). ASS occupy more than 40% (about 1.6

million ha) of the Mekong River Delta of Vietnam (Minh et al.,
1997).
These ASS contain significant amount of pyrite material. Exposure of this material by excavation, lowering of groundwater or
drainage results in its oxidation and produces high acidity, thus
lowering the pH of the soil and releasing highly toxic elements such
as iron and aluminum (Dent, 1986; Cook et al., 2000). Significant
environmental damage due to changes in land use of coastal

* Corresponding author. International Rice Research Institute (IRRI), Los Baños,
Philippines. Tel.: þ84 1285 295 400; fax: þ84 7103 734 581.
E-mail address: (N.D. Phong).
/>0301-4797/Ó 2014 Elsevier Ltd. All rights reserved.

floodplains with ASS has occurred in Australia (White et al., 1997;
Sammut et al., 1995, 1996a, 1996b); the Netherlands (Pons, 1973);
the Mekong Delta of Vietnam (Tuong et al., 1993; Minh et al.,
1997b); the Pearl River Delta of China (Lin and Melville, 1994);
South Kalimantan, Indonesia (Hamming and van den Eelaart, 1993)
and Finland (Palko and Yli-Halla, 1993).
Rainfall can leach acidic contaminants out of the soil, which in
turn acidify and pollute the receiving waters (Minh et al., 1997).
Acidic pollution of the water causes dramatic changes in the stream
environment (Sammut et al., 1995, 1996b), including many adverse
effects on plants (Dent, 1986; Xuan, 1993), fisheries, domestic water
(White et al., 1997) and corrosion of engineering infrastructure
(White et al., 1996). Surface runoff and sub-flow are the main routes
for draining the acidity from ASS into canals (Minh et al., 2002).
Macdonald et al. (2004) found that runoff from ASS affected the
existing sulfide-rich sediments within an estuarine lake. On the
other hand, Cook et al. (2000) found that acidity in the drains was

mainly coming from agricultural land by groundwater discharge.
They concluded that sub-flow is a more severe hazard than runoff
for acid pollution. Other studies also claimed that the source of acid
loads from agricultural fields entering canal water is groundwater,
leaching from drain bank edges or seepage through drain walls


N.D. Phong et al. / Journal of Environmental Management 140 (2014) 14e25

15

Fig. 1. Soil map of Bac Lieu province, Ca Mau peninsula, Vietnam, with dense canal network in freshwater zone (F) and saline-water zones (S1, S2, S3, B1 and B2).

with a low pH in the range of 3.2e4 (Blunden and Indraratna,
2000).
In the Mekong Delta of Vietnam, reclamation of ASS for agriculture and aquaculture has led to widespread acidic pollution of surface water in the freshwater zone (Tuong, 1993) as well as in the
saline coastal zone (Hoanh et al., 2003; Gowing et al., 2006). Tuong
et al. (2003) showed that inappropriate water management, land
uses of ASS and acidic pollution have led to a 70% reduction in income of the farmers living in the ASS area of Ca Mau peninsula, a
coastal area of the Mekong Delta of Vietnam. On ASS, spoils deposited on canal embankments during construction or dredging may be
oxidized and form a source of acidic pollution (Tuong et al., 1998).
In this study, extensive modeling was used to explore alternative water management practices and other interventions such as
canal widening to reduce acidic pollution in ASS areas. Such
reduction will improve water quality and provide suitable water
environment for both aquaculture and agriculture in the region.
1.1. The study area
The coastal plain of Bac Lieu province, Ca Mau peninsula is the
study area located in the south of the Mekong Delta of Vietnam
(Fig. 1). It is an area with a highly modified environment. The three
most important soil groups in the study area are alluvial soils

located in the northern and eastern parts near the Bassac River, ASS
mainly located in the large depression in the central and western
parts, and saline soils located in the southern and western parts
along the East and West seas. Roughly 90% of the annual rainfall in

Bac Lieu (1800 mm) is concentrated in the rainy season from May to
mid-November. Rice crop is dominant in the north and shrimp
raising is widespread in the south, where salinity is quite common
in canal water (Hoanh et al., 2003). During the dry season from
mid-November to April, freshwater availability for irrigation is a
major constraint to rice production.
The canal network comprises a main canal, the Quan Lo Phung
Hiep (QLPH), which connects the study area to the Bassac River, and
series of canals of different capacity (BWRMBL, 2006, Fig. 1). The
primary canals are perpendicular to the QLPH, at about 4e5 km
apart. Their typical cross-section is 30e50 m wide and 4e10 m
deep. The embankments of primary canals are about 10 m wide.
The secondary canals connect to the primary canals at 1 km
spacing, and their typical cross section is 10e15 m wide and 1.5e
2.0 m deep. The embankments of secondary canals are 7 m wide.
The tertiary canals are spaced at 500 m and connect to secondary
canals. Their typical cross section is 5e8 m wide and 1e2 m deep,
with 5-m wide embankments.
The tide in the East Sea is semi-diurnal (two high waters and
two low waters each day) with high amplitude from 3 to 4 m,
compared with only 0.5e1 m amplitude of diurnal tide (one tidal
cycle per day) in the West Sea.
A series of sluices along the East Sea side is operated for delivery
of saline water taken from the East Sea for shrimp culture in the
central part of Ca Mau peninsula or the western part of Bac Lieu

province (Fig. 1). These sluices are also operated harmonically with
the construction of temporary dams to restrict salinity intrusion
into the agricultural zone in the eastern part of Bac Lieu. The only


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N.D. Phong et al. / Journal of Environmental Management 140 (2014) 14e25

Fig. 2. The VRSAP-ACIDITY model, with ACIDITY in the inset.
Modified from Phong (2008).

freshwater source is diverted from the Bassac River to Ca Mau
peninsula through the QLPH canal. Among these sluices, the two
largest, Ho Phong (HP) and Gia Rai (GR) with 3 Â 8 m and 3 Â 7.5 m
wide, respectively, are playing an important role in controlling
saline-water intake.
The study focuses on Bac Lieu province with 58% of the area
underlain with ASS, and where salinity is found on about
175,000 ha of brackish-water shrimp culture. The province comprises of six land-use zones, F, B1þB2, S1, S2 and S3 delineated from
the existing water regimes and land uses (see Fig. 1 for location).
This paper answers a question raised by provincial agencies: which
management practices and other interventions can be applied to
reduce acidic pollution in the canal network.
2. Methodology
2.1. The model
The study used an ACIDITY module (Fig. 2) coupling with a
hydraulic and salinity model, the VRSAP (Vietnam River System
And Plains) to simulate the temporal and spatial dynamics of
acidity and salinity at a regional scale in the study area. Details of

the ACIDITY and the VRSAP model are summarized as follows:
Using the implicit finite difference scheme to solve the basic
hydraulic Saint-Venant continuity and the momentum equations
and the salinity advection-dispersion equation, the VRSAP model
computes water level, discharge and salinity in each segment of a
complex open canal network subjected to tidal fluctuations (Hoanh
et al., 2001). The model requires two types of input data: (i) the
configuration and dimensions of the river and canal network; and (ii)
hydrological data (water level, discharge and salinity) at boundaries
and initial conditions of segments, nodes and fields. Water level,
discharge and salinity outputted by the model were validated
with observed data in 1996 in the study area (Hoanh et al., 2001).
The ACIDITY module (Phong, 2008) was based on a series of field
and laboratory studies in combination with statistical and GISbased analyses. The module comprises of two main functions to
calculate acid loads into canals, and the acid neutralization of saline

affected canal water in the coastal zones. These functions were not
available in the VRSAP model.
(i) Field experiments were carried out from 1st April to 15th July
2005 at Bac Lieu province to quantify the source and the
dynamic of acidic pollution in a coastal acid sulfate soil area
(Phong, 2008; Phong et al., 2013). Using regression analyses
of time series data, the amount of acid loads transferred from
fields and canal embankments to the canal water could be
quantified from environmental parameters, including cumulative rainfall, types of ASS and age of embankment
deposits (Phong et al., 2013).
(ii) The laboratory experiment namely “titration” (Phong, 2008)
was based on the chemical reaction of seawater on sulfuric
acid with the formation of carbonic acid was described by
Stumm and Morgan (1996). In the experiment, the monitoring pH of a fixed volume of canal saline water sample

(defined as the recipient) when it reacts with consequent
added acid water drops (defined as the titrant in experiment), results in a pH curve (or titration curve) of the canal
water. The experiment was repeated for each combination of
a given set of titrants (pH water from 3 to 7) and recipient
waters (saline water with EC of 0, 10, 20, 30 or 55 dSmÀ1). As
the results of experiment, titration curves allowed the
determination of pH (hence acidity) of the canal water as it
mixed with the inputted acid water.
At each time step of the computation, the ACIDITY module
calculates the acidity (or in term of pH) of canal water at each canal
segment and node (junction of two or more segments) with the
known salinity of canal water computed from the VRSAP and the
input of simulated acidity from canal embankments or fields before
being integrated into the VRSAP model in the next time step (Fig. 2).
The integrative VRSAP-ACIDITY model is capable of simulating
the temporal and spatial variations of water pH (as an indicator of
acidity), salinity and water flow in a coastal canal networks. It was
calibrated with the 2003-data and validated with the 2005-data of a
water quality monitoring network in the study area (Phong, 2008).


N.D. Phong et al. / Journal of Environmental Management 140 (2014) 14e25

17

Fig. 3. The proposed process for management of both salinity and acidity.
Modified from the APWPC of Hoanh et al. (2003).

This study used the VRSAP-ACIDITY model to analyze the impacts of different water management options and other resource
management measures on both salinity and acidity of the canal

water in the study area. Some of these options and measures may
have conflicting influences on water qualities, hence combined
effects could not be assessed without using the model. For examples, canal widening may improve the drainage, reduce pollution
but it also adds new deposits to the canal embankment and increases the acid loads to the canal water, or operating the sluices to
enhance acidity drainage may also reduce the salinity to a level
lower than that required by brackish water shrimp culture.
2.2. Acidity propagation and possible control options
Under current conditions, the following highlights and interventions are influencing acidity propagation in the study area:
 The dredging of canals brings disturbed acid spoils onto canal
embankments and exacerbates acidity in the canal network
(Tuong et al., 2003). However, the effect of dredged acid spoils
along canal embankments as sources of acidity load into canal
water has not been investigated in previous studies (Truong
et al., 1996).
 The acid neutralization capacity for reducing acidity is an
important feature of seawater (Stumm and Morgan, 1996;
Evangelou, 1998). In the study area, saline water from the East
Sea intrudes into the coastal plain and contains high alkalinity,
which implies a potential for acidity reduction (Phong, 2008). This
advantage is taken into account in scenarios of sluice operation.
 Direction of flows in canals in the study area changes during
flood and ebb tides:
 The flow direction from the East Sea to the West Sea through
the canal network in Ca Mau peninsula is caused by the

difference between the high tide amplitude in the East Sea,
ranging from 3 to 4 m, and the low tide amplitude, only 0.5e
1 m in the West Sea (Fig. 3a).
 The dynamics of the flood and ebb tide flows for intake of
saline water or for drainage of excess water through the

sluices along the East Sea has been exploited in sluice operation. These sluices are equipped with hinge gates, thus
opening them for one-way or two-way flow directions can be
easily done at slack tide when the flow has been being slowly
and then changed its direction.
Consequently, it can be advantageous for exploring either one of
three options in sluice operation, canal widening or canal dredging
that affects to water flow, salinity and acidity in canals in the study
area:
1. HP and GR sluices are selected to control salinity in the study area
(Hoanh et al., 2001, 2009). Adjustments in the operation schedule
of these sluices can improve the water flow and saline-water
intake, which could reduce acidity of water in the study area.
2. The expansion or widening of canals facing the West Sea will
increase the flow of canal water from the East Sea toward the
West Sea and hence will affect salinity and acidity propagation
in the canal network.
3. The locations and number of dredged canals in different salinewater or freshwater zones will alter the acidity generation in
those zones, then it will affect to the water quantity and quality
in the study area.
In past years, the main concern of provincial water managers
was how to bring saline water into the study area for shrimp culture
without affecting agricultural production in the freshwater area
(zone F in Fig. 1). Alternatives for salinity management purposes


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N.D. Phong et al. / Journal of Environmental Management 140 (2014) 14e25

Fig. 4. Tide variations and sluice operation schedules. a. Tide variations at Ganh Hao (GH) of East Sea side and at Xeo Ro (XR) of West Sea side in May and June 2003. b. Operation

schedule of Ho Phong (HP) and Gia Rai (GR) sluices in two example scenarios O1 and OT.

were examined using an existing Analytical Process to support
Water Policy Changes (APWPC) suggested by Hoanh et al. (2003).
In this study, step 4 is added to the existing three-step APWPC
for both salinity and acidity management (see the flow diagram of
modified APWPC in Fig. 4):
 Step 1: Land-use investigation by delineating land-use zones
and determining water-quality requirements.
 Step 2: Applying either one or both of the following options in
water-quality management:
(2.a) Exploring sluice operation options and/or
(2.b) Adjusting canal configuration (widening or expansion) in
combination with sluice operation.
Options of sluice operation could be a combination of selected
sluices, number of gates operated at each sluice, days of operation
and control of water flow direction (one way or two ways during
sluice operation).
 Step 3: Checking whether simulated salinity matches with the
requirement. If it does not, return to step 2 to find suitable
options.
 Step 4: Checking whether the water with satisfactory salinity in
step 3 satisfies the acidity requirement. If it does not, return to
step 2 to find suitable options.
In step 3, the salinity at Chu Chi, Pho Sinh, Phuoc Long and Ninh
Quoi stations (locations in Fig. 1) along the QLPH canal is used for
checking the boundary of salinity intrusion.

In step 4, maps of canal water pH are generated to identify the
hot-spots of water pH less than 6, assuming that rice and shrimp

productions are affected when the water pH drops below this
level.

Table 1a
Scenarios of sluice operation (Group 1).a
Scenario Operated sluices
Baseline
OB
HP and GR
For saline intake
O1
HP and GR
O2
HP and GR
O4

HP and GR

OE
OI

HP and GR
HP and GR

OT

HP and GR

For drainage
OD1

HP and GR
OD2

HP and GR

OD3

Sluices in the freshwater
zone. HP and GR are closed.

a

Sluice opening days (*)
Operated as on schedule of 2003 for May.
Closed in June.
One day every week in May and June
Two consecutive days every two weeks
in May and June
Four consecutive days every four weeks
in May and June
Every day in June
Two directions automatically by tide one
day every week in May and June
One day every week on the day with
highest difference in tidal amplitudes
between the East and West seas
One day every week at the lowest tidal
water level
Two consecutive days every two weeks
at the lowest tidal water level

One day a week at ebb tide.

Sluices are operated as in 2003 for other months from January to April.


N.D. Phong et al. / Journal of Environmental Management 140 (2014) 14e25
Table 1b
Scenarios of canal widening combined with sluice operation (Group 2).a
Scenario Canal widening
W1

W2

Widening NTL and QLCC canals connected to HP and GR sluices
(see Fig. 9a) with the same cross section (top width ¼ 50 m, canal
bottom ¼ 2.0 m below mean sea level)
W1 plus widening more secondary canals connected to the West Sea
(see Fig. 9b) with the same cross section (top width ¼ 30 m, canal
bottom ¼ 2.0 m below mean sea level)

a
Operated sluices are the HP and GR. Sluice opening days in these scenarios are
the same as in scenario OT.

Table 1c
Scenarios of dredged canals in different zones (Group 3).a
Scenario

Canal dredging

DF
DB
DS1
DS2

Dredging
Dredging
Dredging
Dredging

canals in zone F (freshwater area)
in zones B1 and B2 (brackish-water area)
canals in zone S1 (saline-water area)
canals in zone S2 (high-saline-water area)

a
Zone locations are shown in Fig. 1. Sluice opening days in these scenarios are the
same as in scenario OT.

2.3. Scenarios for both salinity and acidity management
Acidity propagation in the canal network is investigated with
different options in sluice operation, canal widening or dredging
under three groups of scenarios, 1 to 3 (Table 1a, b and c). A baseline
scenario (OB) is established as a reference to compare with these
scenarios. In this scenario, saline water from the East Sea is taken in
from January to May by sluice operation as in the 2003 records
(Phong, 2008) but it is not taken in June because of acidity problems
in canal water that usually occur at the beginning of the rainy
season. The 2003 hydrological data (water level, salinity, flow) used
for model calibration (Phong, 2008) are applied in all scenarios.

19

intake is the same (See Fig. 3b for sluice operation schedule of O1).
In scenarios OD1, OD2 and OD3, the effect of drainage during ebb
tide in June is considered and no saline water is taken in from HP
and GR during drainage. The same salinity of intake or drainage
water provides the same reduction in acidity but the flow directions in these two cases, reflecting the movements of acid water,
are different. In addition, three other scenarios that focus on the
effect of sluice operation on acidity propagation at the beginning of
the rainy season (OE, OI and OT) are analyzed. In scenario OE, HP
and GR sluices are opened for saline-water intake every day in June.
In scenario OI, HP and GR sluices are opened bi-directionally
automatically by the tide for one day every week in June. In scenario OT (Fig. 3b), HP and GR sluices are opened for saline water
intake in one day a week in May and June when the difference in
tidal amplitudes between the East and West Seas is highest in that
week.
2.5. Group 2: canal widening combined with sluice operation
Since the flow through HP and GR sluices strongly influences
water acidity, enlarging the primary canals that connect these
sluices to secondary canals on the West Sea side of Ca Mau
peninsula can be another alternative for improving acidity conditions. Among the primary canals, the Ninh Thanh Loi (NTL) and the
Quan Lo-Chu Chi (QLCC) are the shortest (20e25 km) canals
(Fig. 9a). The increase in water flow in these canals can be
considered to boost the drainage of acidity in the study area to the
West Sea (Fig. 3a). At present, differences in sectional canal widths
from 25 m to 50 m of these NTL and QLCC canals are causing a
bottleneck of acidity flow to the West Sea (BWRMBL, 2006). In
addition, canal widths of the secondary canals connecting these
canals to the West Sea (Fig. 9b) are also not uniform, varying from

10 m to 30 m. Scenarios W1 with widening of primary canals and
W2 with additional widening of secondary canals are presented in
Table 1b and acidity propagations in these scenarios are presented
in Fig. 9a and b.

2.4. Group 1: operation of HP and GR sluices
2.6. Group 3: dredging canals in different zones
As presented in Table 1a, in scenarios O1, O2 and O3, only one
gate of the HP and GR sluices is operated on a different schedule but
the number of days (4 days every 4-week interval) for saline water

The location and number of dredged canals every year are
important factors in generating acidity (Phong, 2008). In this

Fig. 5. Simulated water pH on 30 June under baseline scenario OB. Note: Ho Phong (HP) and Gia Rai (GR) are the two main sluices in sluice operation.


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N.D. Phong et al. / Journal of Environmental Management 140 (2014) 14e25

Fig. 6. Simulated water pH on 30 June under scenarios of saline water intake. Note: Details of scenarios are presented in Table 1a.

scenario group, the effects of dredged canals in different zones on
acidity generation and propagation in the study area are analyzed.
In each scenario, canal dredging is carried out in one zone only. For
example, in scenarios DF, DB, DS1 and DS2, canal dredging is carried
out in zone F, B1þB2, S1 or S2, respectively, while canals in other
zones remain the same. Dredging in zone S3 is not considered

because dredging in zone S2 in scenario DS2 can represent such
activity in areas with high water salinity. In these scenarios, the
same sluice operation as in the baseline scenario OB is applied.
To compare the effectiveness in reducing the acidity load in
canals (Eff) in different zones (F, B1þB2, S1 and S2) by saline water
in canals in these scenarios, a simple Equation (1) is applied:


N.D. Phong et al. / Journal of Environmental Management 140 (2014) 14e25

Effð%Þ ¼

TALre
100%
TAL

21

(1)

where
 TAL [tons Hþ] is the sum of total acidity load from all canal
embankments (before entering canal water) in zone from the
beginning of December 2002 to the end of June 2003. The total
acidity load into canal is calculated for each canal featured by
the age (number of years after the last dredging) and the ASS
type (severe or medium soil acidity) of the dredged spoils on the
canal embankments (Phong, 2008).
 TALre [tons Hþ] is the sum of total acidity load reduced by saline
water in canals in acid neutralization reactions (Stumm and

Morgan, 1996) during this period.
Although in each scenario the model was run for the period
from December to June, only the simulated water pH and salinity at
the nodes in the canal network on 30 June are presented and discussed in the next section in this paper because acidity on that day
represents the most severe acidic pollution in each year.
3. Results and discussion
3.1. Scenario analysis
3.1.1. The baseline scenario OB
The result of acidity (represented by water pH) propagation in
the baseline scenario OB (Fig. 5) illustrates that, when HP and GR
sluices are closed in June, acidity decreases slightly in zone S3
because of the high salinity water from the East Sea whereas a large
area of severe acidity (water pH 5) is found in the freshwater area
(zone F) and in the western saline part of the study area (zones S1,
B1 and B2). In zones S2 and S3 downstream of the QLPH canal,
except two small spots of severe acidity, water quality in these
zones is better with water pH ! 6.
3.1.2. Group 1: sluice operation
3.1.2.1. Effect of opening HP and GR sluices for saline-water intake.
In scenario O1 of opening HP and GR sluices for one day every week
in May and June (Fig. 6a), canal water with pH ! 6 remained in
narrow areas in zone S3 and small parts of zones S2, B1 and B2
along the QLPH canal. Compared with the baseline scenario OB,
saline water in this scenario is taken in enough to reduce acidity
and maintain higher water pH in these zones until the next intake
of saline water in the following week.
In scenario O2 of opening HP and GR sluices on two consecutive
days every two weeks (Fig. 6b), canal water with pH ! 6 expanded
into broader areas along the QLPH canal in zones S2, S3 and parts of
zones S1, B1 and B2 compared with O1.

In scenario O4, which involves opening HP and GR sluices on
four consecutive days every four weeks in May and June (Fig. 6c), a
greater amount of intake of saline water creates a broader area with
canal water pH ! 6 than in scenario O1 but smaller than in scenario
O2. This expansion indicates that, when saline water is taken in on
four consecutive days, surplus saline water is drained into the West
Sea because the canal system in the study area cannot store all
saline water. As a result, in zones B1 and B2 acidic water with
pH
5 spreads out from the severe acidity spot to other parts in
June. The comparison indicates that, with the same number of
sluice opening days (eight days in May and June), scenario O2 with
two consecutive days every two weeks provides the highest
reduction in canal water acidity in the study area.
In scenario OE involving opening HP and GR sluices every day in
June (Fig. 6d), canal water with pH ! 6 dominates in almost all

Fig. 7. Simulated salinity along QLPH canal on 30 June under scenarios for saline water
intake (a) and drainage (b). Note: Details of scenarios are presented in Table 1a.

zones and eliminates most severe acidity spots except some in zone
S1 and in zone F. Compared with the baseline scenario OB, scenarios O1 to O4 and OE provide higher salinity in the canal system,
especially along the main canal QLPH (Fig. 7). Scenario O1, with
opening HP and GR sluices for one day every week only, provided
lower salinity than in the other scenarios. Therefore, in scenario O1,
the objectives of both controlling salinity intrusion and reducing
acidity can be achieved, whereas, in other scenarios, the acidity
reduction is better but salinity is too high.
3.1.2.2. Effect of sluice operation based on tidal amplitudes.
Fig. 6e shows that, in scenario OT with HP and GR sluices opened to

allow saline water intake for one day every week in May and June
when the difference between tidal amplitudes in the East and West
Seas in that week is highest, canal water pH slightly increases in
zones S1, S2, B1 and B2 compared with that in scenario O1. This
improvement indicates that consideration of tidal variations in
both East and West seas in sluice operation is a potential alternative
in reduction of acidity.
3.1.2.3. Effect of sluice operation without controlling flow direction.
In scenario OI, HP and GR sluices are opened bi-directionally
automatically by the tide for one day every week in June (Fig. 6b).
Canal water area with pH ! 6 becomes broader in zone S3 while
canal water area with pH < 6 still remained in zones S1, S2, B1 and
B2. This situation is explained by the different flow directions
during flood tide and ebb tide in the day of sluice opening, and
therefore saline water does not have enough time to reach other
zones as in scenarios O1, O2 and O4. This result shows that controlling flow direction by sluice operation is very important in
improving canal water quality.
3.1.2.4. Effect of opening sluices for drainage. In scenario OD1 with
drainage toward the East Sea during ebb tide for one day every


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N.D. Phong et al. / Journal of Environmental Management 140 (2014) 14e25

Fig. 8. Simulated water pH on 30 June under scenarios for drainage. b. a. Note: Details of scenarios are presented in Table 1a.

week and without salinity intake (Fig. 8a), acidity is more severe
than in scenario OB. However, in scenario OD2 with drainage on
two consecutive days every two weeks, canal acidity declined

significantly (Fig. 8b). Compared to the baseline scenario OB and
scenario OD1, canal water pH improved significantly in zones B1
and B2 in scenario OD2 and the spots of acidic water pH
5 in
zones S1, S2 and S3 are narrower because the opening of sluices
on two consecutive days provides sufficient time for acidic water
to drain out of the study area and be replaced by freshwater from
the Bassac river through the QLPH canal. As a result, salinity in
scenario OD2 (Fig. 8b) decreased more sharply along the QLPH

canal (below 4 g LÀ1 at Ninh Quoi) than in scenario OD1 (Fig. 8a).
However, a slight salinity intrusion from the West Sea into the
northern part of zone B1 occurs because of more drainage toward the East Sea. In this scenario OD2, the areas with canal
water pH around 6 (5.7e6.3) were broader in zones B1, B2, S2
and S3 (Fig. 8b) but the area with canal water pH ! 6 in zone S3
is smaller than in scenarios O1, O2, O4 and OE (Fig. 6a to d).
Canal water pH ! 7 is suitable for the healthy growth of shrimp
(Brennan et al., 2000), so the sluice operation for the intake of
saline water in scenarios O1 to O4 and OE is more appropriate
for shrimp culture.

Fig. 9. Simulated water pH on 30 June under scenarios of canal widening combined with sluice operation. Note: Details of scenarios are presented in Table 1b.


N.D. Phong et al. / Journal of Environmental Management 140 (2014) 14e25

23

Fig. 10. Simulated water pH on 30 June under scenarios of dredged canals in different zones. Note: Details of scenarios are presented in Table 1c.

The worst case is scenario OD3 when sluices along the freshwater zone (zone F) are operated for drainage toward the East Sea
for one day every week at ebb tide in June (Fig. 8c) while HP and GR
sluices are closed. The results show that drainage through sluices in
freshwater zone F toward the East Sea attracted acid canal water
with pH 5 from zones S1, B1 and B2 into the central part of the
study area. Scenario OD3 also accelerates salinity intrusion from
the East Sea further upstream of the QLPH canal, with its salinity
around 10 g LÀ1 at Ninh Quoi (Fig. 7).
3.1.3. Group 2: canal widening combined with sluice operation
From the above discussion, scenario OT is the most suitable
option for both salinity control and acidity reduction. Therefore,
sluice operation schedule in scenario OT is included in scenarios
W1 (widening only canals connected to HP and GR sluices) and W2
(W1 plus widening more canals connected to the West Sea). Details
of these scenarios are shown in Table 1b.

Compared with scenario OT, scenarios W1 and W2 brought
about a broader area of canal water with pH ! 6 along the newly
widened canals toward the West Sea (Fig. 9a to b) rather than just
along the QLPH canal. In addition, canal water pH in scenario W2
increased more significantly in zones S2, S3, B1 and B2. This
improvement illustrates that sufficient and uniform cross sections
of canals connected to the West Sea are important factors to
improve canal flow and acidity conditions.
3.1.4. Group 3: dredging canals
In general, compared to scenario OB, the epicenters of acidic
pollution do not vary clearly when new canals are dredged in the
freshwater or saline-water zones as in scenarios DF, DB, DS1 and
DS2 (Fig. 10a to d). Hence, the sum of total acidity load in the canal
(TAL) in each zone from the beginning of December to the end of

June in these scenarios is compared in Table 2. The results show
that TAL decreases in the order of zones S1>S2>B1þB2>F in spite

Table 2
Sum of total acidity load into canal (TAL) reduced by saline water (TALre) and effectiveness (Eff %) in acidity reduction under scenarios of canal dredging.
Scenario

Sum of total acidity load (tons Hþ) in each zone
Whole area

DF
DB
DS1
DS2
Average Eff of 4 scenarios

B (B1 þ B2)

F

S1

S2

TAL

TALre

Eff

TAL

TALre

Eff

TAL

TALre

Eff

TAL

TALre

Eff

TAL

TALre

Eff

799.4
802.7
788.0
803.5

678.8

692.4
680.4
693.1

85
86
86
86
86

64.4
55.0
55.0
55.0

19.6
18.8
18.2
18.4

30
34
33
34
33

224.9
237.5
224.9
225.0

217.1
231.1
216.8
217.8

97
97
96
97
97

275.0
275.0
280.1
278.3

270.0
270.3
275.2
273.6

98
98
98
98
98

235.1
235.1

228.0
245.2

172.0.1
172.3
170.2
183.3

73
73
75
75
74

Note: Zone locations are shown in Fig. 1. Whole area is the total area of all zones (F, B1, B2, S1 and S2).
Eff (effectiveness) is computed by Equation (1).


24

N.D. Phong et al. / Journal of Environmental Management 140 (2014) 14e25

of the salinity level in canal water and acidity generation (locations
and number of dredged canals) in dredged zones. This order illustrates other factors such as flow regime could strongly affect to the
spatial distribution of acidity in the zone (Phong, 2008). Van
Breemen (1973) also concluded that pollution could exist in places far away from its source because it was transported by the water
flow.
Nevertheless, the Eff in zones S1 or S2 is nearly double or triple
(74%e98%) that in zone F (only 33%) in all scenarios. This difference
indicates that dredging in the freshwater zone (scenario DF) provides a lower reduction in acidic pollution than dredging in other

saline-water zones (scenarios DB, DS1 and DS2). The results agree
with findings by Evangelou (1998) and Stumm and Morgan (1996)
that saline water provides higher acid neutralization capacity than
freshwater.
3.2. Lessons learnt from scenario analysis
The analysis of saline-water intake scenarios O1, O2, O4 and OE
and drainage scenarios OD1, OD2 and OD3 shows that acidity
propagation as well as salinity intrusion are very sensitive to the
operation of sluices on the East Sea side of the Ca Mau peninsula. In
drainage scenarios OD1, OD2 and OD3, the area of canal water pH
around 6 (5.7e6.3) is broader but the area with canal water pH > 6
is smaller than in scenarios O1 to O4 and OE of saline-water intake.
Furthermore, for more than 60% of shrimp cultivation area that
needs saline water, scenarios O1 to O4 and OE are more appropriate
for both saline-water intake and acidity reduction. Among these
scenarios, scenario O1 of opening HP and GR sluices for one day
every week in May and June can be considered as the most suitable
in this group.
As an extension of scenario O1, scenario OT with sluices opening
in the proper tidal periods when the difference between tide amplitudes in the East Sea and the West Sea is highest shows a more
significant improvement of canal water acidity. Hence, tide amplitudes should be considered in sluice operation to provide an
additional advantage in improving water quality.
A significant reduction in water acidity can be achieved when
combining the operation of the HP and GR sluice gates with the
widening of the canals that connect these sluices to the West Sea as
shown in scenario W2. Shrimp farms in the study area need saline
water with pH around 7; hence, this combination strategy is the
most suitable option for shrimp cultivation in Bac Lieu province.
However, it may cause acidic pollution in the areas along the West
Sea. The trade-off of that scenario should be considered carefully in

water management.
Acidic runoff and seepage flows from the canal embankment
deposits and fields connected to canals are the major sources of
acidity load into the canal network in the study area. The analysis of
scenarios in the third scenario group shows that acidic pollution are
severe when canals are dredged in either freshwater (scenario DF)
or saline-water zones (scenarios DB, DS1 and DS2), and that acidity
can spread far from the dredged canals by water flow. The model
results also show a lower effectiveness of acid neutralization in the
freshwater zone than in the saline-water zone. Hence, the risk of
acidic pollution could be extremely high if canals on ASS are
dredged in the freshwater zone.

dredging canals, or a combination of these interventions on both
salinity and acidity of canal water.
Sluice operation, which is an important intervention in delivering brackish water for shrimp cultivation in the study area and
controlling salinity intrusion upstream of the QLPH canal, is sensitive to acidity propagation in the canal network. Among the
considered scenarios of opening HP and GR sluices, the model results indicate that the opening HP and GR sluices on two consecutive days in two weeks in May and June provides the highest
reduction of acidity concentration in the canal system, especially
along the main canal, QLPH. However, the objective of both managing salinity intrusion to support brackish-water shrimp culture
and reducing acidity can be best achieved by opening HP and GR
sluices for one day every week at the time of the highest difference
between tide amplitudes in the East Sea and the West Sea.
The model is a useful tool to design the canal network and to
determine canal cross sections for altering the water salinity and
acidity in canals as required control. The result of model shows that
widening the selected primary canals to 50 m and the selected
secondary canals to 30 m e in combination with proper sluice
operation analyzed above e helps controlling the water flow and
providing positive effects on reducing acidic pollution in the canal

network. However, a trade-off analysis is required since these interventions may cause acidic pollution in the surrounding areas.
The model also offers a methodology to analyze the effects of
frequency of canal dredging/excavation at different locations, as
well as the properties of the embankment deposits (e.g. age of the
embankment deposits, types of ASS making up the deposits, etc.)
on acidic pollution in the canal network. The results of scenario
analysis point out that acidic pollution could be more severe when
canal dredging/excavation is carried out in freshwater ASS zone
than in saline water ASS zone.
The reduction of acidity in the canal water will decrease the
release of highly toxic elements such as iron, aluminum and sulfuric
acid from the ASS, and limit the adverse effects of acidity on flora
and fauna. It will also improve the suitability of canal water for
domestic use and reduces corrosion of infrastructure. However, it
would take many years to improve significantly the ASS properties
for reducing acidity leached out from these soils by applying only
water management interventions. Therefore in the recent years
local people have been applying lime to neutralize the acidity in
their shrimp and rice fields. This practice has helped to accelerating
the improvement of ASS properties and reducing acidity release.
Acknowledgments
This study was carried out in the framework of the Project CP10,
“Managing Water and Land Resources for Sustainable Livelihoods at
the Interface between Fresh and Saline Water Environments in
Vietnam and Bangladesh”, under the Challenge Program on Water
and Food ( The manuscript was
prepared under the ACIAR-funded project “Climate Change
Affecting Land Use in the Mekong Delta: Adaptation of Rice-based
Cropping Systems”. We also thank Mr. Bill Hardy of IRRI for editing
this paper.

4. Conclusions

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